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Are intercellular junctions, synapses and light-capturing photosynthetic complexes mobile?

Are intercellular junctions, synapses and light-capturing photosynthetic complexes mobile?


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I was reading Cell Biology by Gerald Karp and came across a section which said-

Membrane fluidity makes it possible for clusters of membrane proteins to assemble at particular sites within the membrane and for specialised structures, such as intercellular junctions, light-capturing photosynthetic complexes and synapses.

I would be obliged to know more about the topic.


Not necessarily. Sometimes the membrane localized proteins/complexes are anchored to the cytoskeleton or clustered together which limits their movements. See this post.


Membrane fluidity allows reversible association of membrane compounds, which can be thought of as diluted in a two dimensional fluid; this also implies a crucial property of membrane dynamics, which is the differential association of components into more or less stable molecular groups. Once formed, the more stable will be able to cruise the membrane surface, unchanged. This tells you that certain complexes will, by their stability and membrane fluidity, be in constant motion, and thereby statistically well distributed on the cell surface, a very important feature for many infrastructures (look up "lipids rafts" as a topic for examples). Now, the notion of localization in biology may refer to absolute location (e.g. cell pole), or simply the relative positions of certain compounds. Such is the case for cell junctions, which need to co-localize with certain cytoplasmic compounds, belonging to the cytoskeleton. Relative localization therefore, in this case. Do understand though, that both these infrastructures and photoreceptors need to be well distributed on the cell surface, each for its own reasons (mechanical strength, and optimal photo-capture, respectively). The example of synapses is different, as the exact site of synaptic attachement in between cells plays a role in neural messaging and its regulation. This is, therefore, a different topic, which may involve absolute positioning of infrastructures. However, at these sites, protein movement along membrane surface and ocmplex forming principles remain the same; this is an example of absolute localization.


The Island Pond: Field Notes on the Microscopic Aquatic Life of Vancouver Island

White Crab Spider Killing Bee. Maximum 4X magnification with Samsung Galaxy 4 cell phone camera. White Crab Spider, Samsung Galaxy on Nikon Objective Magnifier,

For photography of small plants, animals and fresh water creatures, there is a need for an intermediate level of magnification between a high camera macro setting (2-4X) and the lowest power available on the microscope (20-40X). Furthermore, it is desirable if high-level macrophotography can be portable: traveling to the flower or the glass of the aquarium rather than the flowers and water critters going to the magnifier.

This gap can be filled by a simple, intermediate-power cell phone magnifier constructed from a microscope lens, a piece of flooring, and a few rubber bands. The lens is a 4X, infinity-corrected Nikon 4X wide-field objective. The mount is a portion of 3/8″ high density fiberboard (HDF) flooring, desirable for its flatness, hardness, and ability to be worked with simple tools. The field of view of a microscope objective lens is small, but this particular lens works well with the very small, high resolution sensor on the cell phone.

On a drill press, the flooring, which is thicker than the length of the objective threads, is drilled approximately half way though with a 1″ wood bit to create a recessed hole for the objective. The remaining thickness of HDF should be equivalent to the length of the objective threads. The latter is then drilled the rest of the way using a bit just slightly smaller than the objective threads. This smaller opening can be carefully rasped and sanded out, maintaining the circular shape, until the objective can be threaded in with gentle pressure, Doing this with a fine circular rasp bit or small sanding drum on the drill press while the mounting plate is moved on the press table will maintain the vertical edges of the hole.The brass threads on the objective, being harder than the fiberboard, will create their own threads in the wall of the mounting hole. The base of the objective should end up flush with the back side of the mounting board. The whole piece is then given a rubbed, wax finish with Briwax or a similar one-step finishing wax. Since the magnifier is intended for use outdoors or around aquaria and ponds, this finish soaks into the porous edges and protects them from moisture.

The phone is then held onto the back of the mounting plate with two broad, sturdy rubber bands I use the ones that hold stalks of broccoli together in the supermarket. If you don’t like broccoli, steal a couple next time you are in the produce aisle.

(Sangunis: Stealing the broccoli itself is shoplifting and will land you in jail, which is a nasty place full of the kind of vertebrates and invertebrates who are NOT quality company. However, stealing the rubber bands, since they are technically part of the packaging, lands in a gray area according to two-footed legalities and will just get you thought to be a bit weird and to be avoided. If you are an old vertebrate, just drool a bit and they will gently escort you outside, hand you a glass of water, and have someone drive you and the rubber bands home. This is the perfect ending to a minor heist.)

On the Samsung Galaxy 4, dial up the magnification to 4X. The image from the objective should almost fully fill the screen with a bit of vignetting at the edge. Focus by moving the whole assembly backwards and forwards through the optimum working distance of the objective, about 1.5 cm. The autofocus on the phone will take care of fine focus, or this function may be turned off. If the phone tends to slip and lose its centering, glue a narrow (

3 mm) strip if thin rubber or leather on either side of the back of the mounting plate to help hold it in place.

This method works well with the Nikon objective ($65 on eBay), which has a flat field and good resolution. There are probably many other objectives, such as the Zeiss Jenas, Tiyoda Planachromats, Polish PZO lenses and even older American Optical 4X infinity-corrected lenses, which may work and are available even more cheaply.

For aquarium pond life photography, the longer working distance of this magnifier/cell phone camera combination allows it to be placed against the aquarium glass, accessing objects within about 1 cm of the inner wall of the aquarium. From this vantage point, still images or videos can readily be taken. The greatest problem arises from trying to hold the assembly steady, especially when pursuing a moving organism. A detachable tripod mounting angle bracket can be added easily. For handheld videos, YouTube has added image stabilization software to its editing package, and this helps greatly in making presentable videos of small moving organisms such as these graceful, bottom-dwelling oligochaete worms:

For still images, the addition of a tripod bracket and focusing rail might also allow stacking of images for non-moving subjects if the autofocus function of the cell phone can be turned off.

A hand-help, portable cell phone magnifier of this power also makes possible new levels of photographic interpretation and creativity. Consider this 10X image of flower petals:and this similar shot of stamens, tweaked with maximum Vibrance and manipulated color balance in Curves:

The possibilities are endless – put your phone and magnifier in your pocket, and go out and explore the world of the very small. You can do it in your garden, on a walk, in the produce or flower department at the supermarket, in an aquarium, or at your local nursery. Create!


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photosynthesis harnesses sunlight to make carbohydrate from CO2

In photosynthesis, energy in sunlight is transformed to chemical energy by converting the C- O bonds in C02 to the C-C and C-H bonds of carbohydrate. The overall reaction-the sum of many independent reactions-can be simplified and written as

photosynthesis electron transfer

photosynthesis is energy demanding

energy demanding series of redox reactions that produce sugar and oxygen from carbon dioxide and water.

autotrophs that manufacture their own carbohydrates from CO2, sunlight, and hydrogen sulfide.

In these organisms a simplified version of the overall reaction for photosynthesis is

oxygen atoms released during plant photosynthesis must come from water

In addition, the reactions responsible for producing O2 only occured in the presence of sunlight, whether or not CO2 was present.

Two distinct sets of reactions in photosynthesis

one that uses light to produce O2 from H20 and one that converts CO2 into sugars.

CO2 is reduced into sugars

CO2 is reduced into sugars

the reactions that reduce carbon dioxide and produce sugar

The calvin cycle depends on light capturing reactions

the calvin cycle does not directly require light, it functions if the light capturing reactions that produce O2 are also occuring.

Light capturing reactions eventually stop producing O2 if the Calvin cycle is not occuring.

Light capturing reactions and the calvin cycle are linked

are linked by a coordinated series of redox reactions.

The light capturing reactions provide high energy molecules that drive the Calvin cycle, which in turn regenerates that ADP, Pi, and NADP+ used by the light capturing reactions.

Light capturing reactions

During the light capturing reactions, electrons are promoted to a high energy state by light. This energy elevation ignites a Chain of electron transport steps that starts with the oxidation of water to form O2 and ends with the reduction of NADP+ to form NADPH.

NADP+ and NADPH are phosphorylated version of NAD+ and NADH used in cellular respiration. Some of the energy released from the redox reaction is also used to produce ATP.

During the Calvin cycle, the electrons in NADPH and the potential energy in ATP are used to reduce CO2 to carbohydrate.

Light capturing reactions and the calvin cycle overview

In the light capturing reactions of photosynthesis, light energy is transformed to chemical energy in the form of ATP and NADPH. The calvin cycle uses the ATP and NADPH to reduce carbon dioxide to sugar and regenerates ADP, Pi, and NADP+ for the light capturing reactions.

Photosynthesis occurs in Chloroplasts

a chloroplast is enclosed by an outer and an inner membrane. The interior of the organelle is dominated buy flattened, membranous sac-like structures called thylakoids, which often occur in interconnected stacks called grana.

The space inside a thylakoid is its lumen.

The fluid filled space between the thylakoids and the inner membrane is the stroma.

Pigments are molecules that absorb only certain wavelengths of light, other wavelengths are either reflected or transmitted. Pigments appear colored because people see only the wavelengths that are not absorbed.

The most abundant pigment in the thylakoid membranes of green plants is chlorophyll, which reflects or transmits green light. As a result, chlorophyll is responsible for the green color of plants, some algae, and many photosynthetic bacteria.

Light capturing reactions begin with the simple act of sunlight striking chlorophyll.

Light capturing reactions begin with the simple act of sunlight striking chlorophyll .

The electromagnatic spectrum

reanges in wavelength from about 400 to about 710 nanometers.

Shorter wavelengths of electromagnetic radiation contain more energy than longe wavelengths.

Thus ther is more energy in blue light than in red.

light exists in discrete packets called photons. Each photon of light has a characteristic wavelength and energy level.

Pigment molecuels absorb the energy of some of these photons. How?

photosynthetic pigments absorb light

When a photon strikes an object, the photon may be absorbed, transmitted, or reflected. A pigment molecule absorbs photons of particular wavelengths. If a pigment absorbs all the visible wavelengths, the pigment appears black because no Visible wavelength of light is reflected back to your eye. If a pigment absorbs many or most of the wavelengths in the blue and green parts of the spectrum but transmits or reflects longer wavelengths, it appears red.

Different Pigments absorb different wavelengths of light

There are two major pigment classes in plant leaves: chlorophylls and cartenoids.

designated chlorophyll a and chlorophyll b, absorb strongly in the blueand red regions of the visible spectrum. The presence of chlorophylls makes plants lookgreen because they mostly reflect green light, which is not absorbed wel l

absorb wavelengths in the blue and green parts of the visible spectrum.Thus, carotenoids appear yellow, orange, or red. The carotenoids found in plants belongto two classes called carotenes and xanthophylls .

Pigments that absorb violet-to-blue and red wavelengths are mosteffective at triggering photosynthesis in alga e

for photosynthesis-the wavelengths that drive the light-capturingreactions. Engelmann’s data indicate that violet-to-blue and red light are the most effective atdriving photosynthesis. Because the chlorophylls absorb these wavelengths, this earlyexperiment showed that chlorophylls are the main photosynthetic pigments .

An absorption spectrum measures how the wavelength of photons influences the amount of light absorbed by a pigment .

In the combined graph, peaks indicate wavelengths where absorbance or photosynthetic activity is high troughsindicate wavelengths where absorbance or photosynthetic activity is low.

Which part of a pigment absorbs light?

chlorophyll a and chlorophyll b are similar in structure. Both have two fundamental parts: a long isoprenoid “tail ” and a “head” consisting of a large ring structure with a magnesium atom in the middle. The tail interacts with proteins embedded in the thylakoid membrane the head is where light is absorbed.

The structure of Beta-carotene, shown in Figure 10.8b, consists of an isoprenoid chainconnecting two rings that are responsible for absorbing light. This pigment is what givescarrots their orange color. A xanthophyll called zeaxanthin, which gives corn kernelstheir bright yellow color, is nearly identical to B-carotene, except that the ring structureson either end of the molecule contain a hydroxyl (-OH) group.

chlorophylls are the main photosynthetic pigments, but carotenoids also absorb light.

Carotenoids are called accessory pigments because they absorb light and pass the energy on to chlorophyll. these accessory pigments can extend the range ofwavelengths that drive photosynthesis by absorbing some of the photons not readily absorbedby chlorophyll.

the primary function of carotenoids is to protect the plan t

To understand why carotenoids are protective, recall that photons-especially the high-energy,short-wavelength photons in the ultraviolet part of the electromagnetic spectrum-contain enoughenergy to knock electrons out of atoms and create free radicals. Free radicals, in turn, triggerreactions that can disrupt and degrade molecules .

Carotenoids “quench” free radicals by accepting or stabilizing unpaired electrons. As a result,they protect chlorophyll molecules from harm. When carotenoids are absent, chlorophyllmolecules are destroyed by free radicals and photosynthesis stops. Starvation and death follow

When light is absorbed, Electrons Enter an Excited State

When a chlorophyll moleculeabsorbs a photon, the photon’s energy is transferred to bonds in the chlorophyll molecule’s n head region. In response, an electron becomes “excited,” or bumped up to a higher energy state .

In chlorophyll, for example, the energy difference between the ground state and state 1 is equal to the energy in a red photon, while the energy difference between state 0 and state 2 is equalto the energy in a blue photon. Thus, chlorophyll can readily absorb red photons and blue photons.

Chlorophyll does not absorb green light well, because there is no discrete step-no difference inpossible energy states for its electrons-that corresponds to the amount of energy in a green photon.

When pigments in chloroplasts absorb photons, about 2 percent of the excited electrons produce fluorescence. The other 98 percent of the energized pigments use their excited electrons to drive photosynthesis.

In the chloroplast thylakoidmembrane, 200-300 chlorophyll molecules and accessory pigments are organized byassociated proteins to form large complexes called photosystems. Most pigment molecules inphotosystems serve as light-gathering antenna pigments that guide energy toward a central reaction center .

When antenna pigments absorb photons, the energy-but not the electron itself-is passed to anearby pigment molecule, where another electron is excited in response. This phenomenon isknown as resonance energy transfer .

resonance energy transfer

Resonance energy transfer is possible only between pigments that are able to absorb differentwavelengths of photons-from those absorbing higher-energy photons to those absorbinglower-energy photons. Proteins organize and tune the absorption potential of antenna pigmentsso that resonance energy is efficiently moved between pigments, as the potential energy dropsat each step.Once the energy is transferred, the original excited electron falls back to its ground state. In thisway, energy is transferred inside the photosystem in a manner that may be likened to thetransfer of sound between tuning forks, or excitement between fans at a sports event during the“wave.” But unlike the stadium wave, most of this resonance energy is directed to a particularlocation in a photosystem, called the reaction center .

When a photon or resonance energy from an antenna pigment reaches the reaction center, theenergy is absorbed by one of two specialized chlorophyll molecules (together called the specialpair). When this pigment is energized, an excited electron is transferred from the special pairpigment to an electron acceptor. As the acceptor becomes reduced, its potential energyincreases. This is a key step in the transformation of light energy: Electromagnetic energy fromsunlight has now been transformed to chemical energy .

Note that in the absence of light, the electron acceptor does not accept electrons. It remains inan oxidized state because the redox reaction that transfers an electron to the electron acceptoris endergonic. But when light excites electrons in chlorophyll to a high-energy state, the reactionbecomes exergonic. In this way, the energy in light transforms an endergonic reaction to an exergonic one.

two photosystem hypothesis

According to the two-photosystem hypothesis, the enhancementeffect occurs because photosynthesis is much more efficient when both photosystems operatetogether .

How Does Photosystem II work

the action often begins when a mobile accessory structure called the light harvesting complex transmits resonsnace energy to an antenna pigment inside the photosystem. From there, resonsnace is relayed by other antenna pigments to transfer energy to the special pair pigments in the reaction center. At this point another type of pigment molecule called pheophytin comes into play.

Structurally pheophytin is identical to chlorophyl except that pheophytin lacks a magnesium atom in its head region.

Unlike other pigments, pheophytin does not become excited by photons or resonance energy-itaccepts excited electrons from the reaction center special pair chlorophylls. The redox reactionbetween pheophytin and special pair pigments is the key step in transforming light energy intochemical energy.Immediately after an excited electron is transferred to pheophytin, however, the oxidizedreaction center special pair pigment becomes an incredibly strong electron acceptor. Whatprevents the electron from being pulled back from pheophytin to the oxidized special pairpigment? The answer is that the electron is quickly shuttled away from the reaction center by aquinone molecule to an electron transport chain (ETC )

When the energy from a single photon excites a special pair chlorophyll in the reaction center ofphotosystem II, the electron is accepted by pheophytin, transferred to a quinone molecule(identified as PQ), and then stepped down in energy along an electron transport chain .

In both structure and function, the thylakoid ETC is similar to components in the mitochondrialETC (Ch. 9, Section 9.5).● Structurally, the photosystem II and mitochondrial ETCs both contain quinones andcytochromes.● Functionally, the redox reactions that occur in both ETCs result in protons being activelytransported from one side of an internal membrane to the other. The resultingproton-motive force drives ATP production via ATP synthase

Two photons excite two electrons to reduce one plastoquinone (PQ), which carries the electronsfrom photosystem 2 along with protons from the stroma. The cytochrome complex oxidizesplastoquinone, releasing the protons into the thylakoid lumen that drive ATP synthesis.When PQ is reduced by picking up two electrons from photosystem II, it carries them throughthe membrane to the lumen side of the thylakoid and delivers them to molecules with a higherredox potential in the cytochrome complex. In this way, PQ shuttles electrons from photosystemII to the cytochrome complex much like ubiquinone shuttled electrons between complexes I or IIand complex III in mitochondria.Similar to the mitochondrial ubiquinone, PQ also picks up protons when it becomes reduced.These protons are pulled from the chloroplast stroma. After being oxidized by the cytochromecomplex, PQ drops off the protons in the thylakoid lumen.The protons transported by PQ result in a high concentration of protons in the thylakoid lumen.The pH in the thylakoid reaches 5 while the pH of the stroma hovers around 8. Because the pHscale is logarithmic (see BioSkills 5), the difference of 3 units means that the concentration ofH+ is 10 x 10 X 10 = 1000 times higher in the lumen than in the stroma. In addition, the stromabecomes negatively charged relative to the thylakoid lumen.The net effect of electron transport, then, is a large proton electrochemical gradient. Thisgradient results in a proton-motive force that, in turn, drives H+ out of the thylakoid lumen andinto the stroma. Proton flow down the electrochemical gradient is an exergonic process that iscoupled to the endergonic synthesis of ATP from ADP and Pi. The stream of protons flowsthrough ATP synthase, causing conformational changes in the enzyme that drive production ofATP .

Since the synthesis of ATP in chloroplasts is initiated by the energy from light, it is calledphotophosphorylation. Although photophosphorylation is similar to the oxidativephosphorylation that occurs in plant and animal mitochondria, there is a key difference in howthis ATP is used. In mitochondria, ATP is exported and fuels many different cellular processes.In chloroplasts, however, the AT P remains within the organelle and is used for the production ofcarbohydrate .

photosystem II obtains electrons by oxidising water

As it turns out, the light energy harvested by photosystem II is responsible for splitting water.When excited electrons are removed from the photosystem II reaction center pigments, theredox potential of the oxidized pigments becomes so strong that enzymes can pull electronsfrom water, pass them to the pigments, and release protons and oxygen .

Photosynthetic organisms thatoxidize water will generate oxygen (02) as a by-product, and thus perform oxygenic (“oxygenproducing”) photosynthesis.

Other organisms that have only a single photosystem do not oxidize water, and thus do notproduce 02. Instead, these organisms use different electron donors, such as H2S in the purplesulfur bacteria, to perform anoxygenic (“no oxygen-producing”) photosynthesis.

heliobacteria have only one photosystem that uses the energy insunlight to promote electrons to a high-energy state. But instead of being passed to an electrontransport chain that pumps protons across a membrane, the excited electrons in heliobacteriaare used to reduce NAD+. When NAD+ gains two electrons and a proton, NADH is produced .

In cyanobacteria and the chloroplasts of eukaryotes, a similar set of light-capturing reactionsreduces a phosphorylated version of NAD+, symbolized NADP+, to yield NADPH. Both NADHand NADPH function as strong reducing agents-that is, because they have beenreduced, they become carriers of electrons that can readily be transferred to reduce othermolecules.

When two photons excite pigments in the reaction center of photosystem I, the excited electrons leave the chlorophyll molecules and pass through a series of iron-sulfur containing proteins until they are accepted by ferredoxin. In an enzyme catalyzed reaction, the two electrons are transferred from ferredoxin to NADP+ to produce NADPH.

1) Antenna pigments absorb photons and pass the energy to the photosystem I reaction center

2) Two electrons ( one for each photon) are excited in reaction center chlorophyll molecules.

3) The reaction center pigments are oxidized and the excited electrons are passed through a series of carriers inside the photosystem, then to a molecule called ferredoxin, and then the enzyme called NADP+ reductase.

4) NADP+ reductase transfers the two electrons and a proton to reduce NADP+ and form NADPH.

Electrons from photosystem I are used to produce NADPH, which is a reducing agent similiar in function to the NADH and FADH2 produced by the citric acid cycle.

Electrons from photosystem II in contrast are used to produce a proton motive foece that drives the synthesis of ATP. The ATP and the reducing power of NADPH, will ultimately be used in the manufacture of sugar.

In combination, photosystems I & II produce chemcial energy stored in ATP and NADPH.

Phostosystems I & II Work together

The process starts when photons excite electrons in photosystem IIs atenna pigments. When the energy in the excited electrons is transferred to the reaction center, a special pair of chlorophyll molecules, ech called P680, passes excited electrons to pheophytin.

Electrons are gradually stepped down in potential energy through redox reactions among a series of quinones and cytochromes. Each reduced plastoquinone (PQ) picks up protons from the stroma and transfer them to the lumen after being oxidized by the cytochrome complex. ATP synthase uses the resulting proton motive force to phosphorylate ADP, creating ATP.

When electrons reach the end of the cytochrome complex, they are passed to a small diffusable protein called plastocyanin (PC). Each reduced PC diffuses through the lumen of the thylakoid and donates one electron to an oxidized reaction center pigment in photosystem 1.

Plastocyanin is critical it forms a phsycal link between the ETC following photosytem II and photosystem I.

The flow of electrons between photosystems, by means of plastocyanin is important because it replaces electrons that are carried away from the special paiur of pigments in the photosystem I reaction center. These special pair chlorophyll molecules are called P700.

The electrons that flow into P700 are eventually excited and transferred to the protein ferredoxin, which passes electrons to the enzyme that catalyzes reduction of NADP+ to NADPH. For each O2 produced by photosystem II, four electrons have been transferred along the Z-Scheme to make two molecules of NADPH.

oxygenic photosynthesis and the evolution of earth

Since ozone O3 is formed from O2 gas, a protective layer of ozone could have arisen in our atmosphere only after the evolution of ogygenic photosynthesis. Without the ozone layer, Earths surface would have been bombarded continually by the searing intensity of ultraviolet radiation, making the evolution of life on land nearly impossible.

As oxygen became more abundant, bacterial cells that evolved the abilityu to use it as an elecron acceptor via cellular respiration flourished. O2 is so electronegative that it creates a huge potential energy drop for the electron transport chains involved in cellular respiration. As a result, organisms that use O2 as an electron acceptor in cellular respiration can produce ATP more efficinetly than can organisms that use other electron acceptors.

CO2 gets into photosynthesizing tissues

through specialized pores stoma ( stomata). Specialized pores bordered by two distinctively shaped cells called guard cells.

An open stoma allows CO2 from the atomosphere to diffuse into uncoated air filled spaced inside the leaf and excess O2 to diffuse out.

Eventually the CO2 diffuses along a concentration gradient into the chloroplasts of photosynthesizing cells. A strong concentration gradient favoring entry of CO2 is maintained by the Calvin cycle, which constantly uses up the CO2 in chloroplasts.

carbon fixation is the addition of carbonb atoms from inorganic carbon dioxide to an organic compound. The process converts CO2 gas to a biologically useful form.

Once carbon atoms are added to an organic compound, they can be used as sources of energy and as building blocks to construct the molecules found in cells.

Carbon fixation is a redox reaction, the carbon atom in CO2 is reduced by attaching it to another carbon.

The calvin cycle fixes CO2

3PGA is the first product of carbon fixation.

ribulose bisphosphate RuBP is the initial reactant.

All photosynetheitc organisms that use the calvin cycle to fix carbon require the CO2 fixing enzyme rubisco

rubisco enzyme is roughly cube shaped and consists of 16 polypeptides that form eight active sites where CO2 is fixed.

Some of these polypeptide subunits are amde in the chloroplast while others are made in the cytoplasm and then imported into the organelle.

Rubisco is thought to be the most abundant enzyme on earth.

Rubisco will catalyze the addition of either O2 or CO2 to RuBP.

Oxygen and carbon dioxide compete at the enzymes active sites which slows the rate of CO2 reduction.

One of the molecules produced from the addition of oxygen to RuBP, 2-phosphoglycolate, is processed in reactions that require ATP and release CO2, regenerating 3PGA.

Part of this pathway occurs in chloroplasts, and part occurs in peroxisomes and mitochondria. The reaction sequence resembles respiration, because it consumes oxygen and produces carbon dioxide.

Rubisco Can React with CO2 or O2

Because photorespiration requires energy and releases fixed CO2, it "undoes" photosynthesis.

When photorespiration occurs, the overall rate of CO2 fixation declines. This does not mean that the plant does not benefit, however. Some of the products from photorespiration are known to be involved in plant signaling and development.

Instead of creating a three carbon molecule as in the calvin cycle, some plant species were able to fix CO2 to produce four carbon molecules.

C4 plants actuallly fix carbon dioxide using both pathways. (to PEP carboxylase in the c4 pathway and to RuBP by rubsico.

PEP carboxylase is common in mesophyll cells near the surface of leaves, while rubisco is found in bundle sheath cells that surround the vascular tissue in the interior of the leaf. d

The reactions that produce sugar from carbon dioxide

depend on the ATP and NADPH produced by the light capturing reactions.

The calvin cycle is a three step process

All three phases of the calvin cycle take place in the stroma of chloroplasts

The number of reactants and products resulting form three turins of the cycle are shown. Of the six G3Ps that are generated during the reduction phase, one is used in the synthesis of other molecules sushc as glucose, and the other five are used to regenerate RuBP. The three RuBPs that are regenerated participate in fixation reactions for additional turns of the cycle.

The Calvin cycle begins when CO2 reacts with RuBP. This phase fixes carbon and produces two molecules of 3PGA, wich is a phosphorylated three carbon organic acid.

the 3PGA is phosphorylated by ATP and then reduced by accepting electrons form NADPH as the phosphate is removed. The product is the phosphorylated three carbon sugar glyceraldehyde-3-phosphate G3P. Some of the G3P that is synthesized is drawn off to produce other organic molecules, like the six carbon sugar glucose.

The rest of the G3P keeps the cycle going by serving as the substrate for the third phase in the cycle: reactions that use additional ATP in the regeneration of RuBP.

One turn of the calvin cycle

fixes one molecule of CO2.

Three turns of the cycle fix three molecules of CO2, yielding one molecule of G3P and three fully regenerated RuBP. Of the six G3Ps that are generated during the reduction phase, one is used in the synthesis of other molecules, such as glucose, and the other five are used to regenerate RuBP. The three RuBPs that are regenerated participate in fixation reaction for additional turns of the cycle.

Each mole of CO2 requires the energy from 3 moles of ATP and 2 moles of NADPH to fix it and reduce it to sugar.

Regulation of Photosynthesis

the presence of light triggers the production of proteins required for photosynthesis.

When sugar supplies are high, the production of proteins required for photosynthesis is inhibited, but the production of proteins required to process and store sugars is stimulated.

Rubisco is activated by regulatory molecules that are produced when light is available, but inhibited in conditions of low CO2 availibility when photorespiration is favored.

What happens to the sugar that is produced by photosynthesis

The products of the calvin cycle enter one of several reaction pathways that result in the production of every organic molecule in the photosynthetic organism. The most importatnt of these reaction sequences uses G3P to produce the monosaccharide glucose, a process called gluconeogenesis.

This glucose is often combined with fructose, which is also made form G3P, to form the disacchardie sucrose.

When photosynthesis is taking place slowly, almost all the G3P that is produced is used to make sucrose. Sucrose is water soluble and readily transported to other parts of the plant. If sucrose is delivered to rapidly growing parts of the plant it is broken down to fuel cellular respiration and growth.

An alternative pathway occurs when photosynthesis is proceeding rapidly and sucrose is abundant. Under these conditions, the glucose molecules are polymerized to form starch, which is stored in the cells of leaves and roots. Starch production occurs inside the chloroplast sucrose synthesis takes place in the cytosol.

Starch acts a temporary sugar storage product

In photosynthesizing cells, starch acts as a temporary sugar storage product. At night, the starch that is stored in leaf cells is borken down to glucose molecules. The glucose is then fed into cellular respiration or used to manufacture sucrose for transport to other parts of the plant. In this way chloroplasts provide sugars for cells througout the plant by day and by night.

Virtually every carbon present in organic molecules, and the energy stored within their bonds, can be traced back to photosynthesis.

Photosynthesis is the staff of life.

Recall that the structure of this membrane consists of a phospholipid bilayer studded with membrane proteins. These proteins are integral, meaning embedded in the bilayer, or peripheral, meaning attached to one surface. Some membrane proteins regulate the transport of substances as part of the primary function of the plasma membrane: to create an environment insise the cell that is different from conditions outside.

The plasma membrane does not exist in isolation, however. Many membrane proteins attach to cytoskeletal elements on the interior surface of the bilayer or to a complex array of extracellular structures, including those attached to the membrnaes of neighboring cells.

The structure and function of the extracellular layer

Most cells secrete products that are assembled into a layer or wall just beyond the membrane. The extracellular material helps define the cells shape and either attaches it to other cells or acts as first line of defense against the outside world.

The structure of cell walls surrounding prokaryotic cells is remarkably different between bacteria and archaea.

In bacteria, cell walls mostly consist of pilymers of the polysacharide peptidoglycan that are connected to one another by peptide bonds.

Archaea do not share any unifying characteristics in thier cell walls apart from the abscence of peptidoglycan. Often the cell walls of these organsism are formed as a dense coat of proteins on the surface of the cell called an S-layer.

Virtually all types of extracellular layers in eukaryotes from the cell walls of algae, fungi, and plants to the extracellular material that surrounds most animal cells have the same fundemental organization.

They are fiber composites: they consist of a cross linked netowkr of long filaments embedded in a stiff surrounding material called the ground substance.

The rods and filaments in a fiber composite are extremely effective at withstanding stretching ans straining forces, or tension. The filaments in the extracellular material of most cells are functionally similar to the steel rodes in reinforced concrete, they resist being pulled or pushed lengthwise.

The stiff ground substance is effective at withstanding pressing forces, called compression. Concrete performs this function in highways, and a gel forming mixture of polysaccharides plays the same role in extracellular material.

Virtually all plant cells are surrounded by a cell wall a fiber composite that is the basis of major industries.

when plant cells first form, they secrete an initial fiber composite called a primary cell wall

the fibrous component of the primary cell wall consists of long strands of the polysaccharide cellulose. These strands are bundled into stout structures termed microfibrils, which are cross linked via hydrogen bonds to other polysaccharide filaments. The microfibrils are synthesized directly into the extracellular space by a complex of enzymes in the plasma membrane, whre they form a crisscrossed network

The spaces between microfibrils are filled with gelatinous polysacchardies such as pectins, the molecules that are used to thicken jams and jellies. Because these polysaccharides are hydrophillic, they attract and large amounts of water, keeping the cell wall moist. The gelatinous components of the cell wall are synthesized in the rough endoplasmic reticulum (ER) and Golgi apparatus and secreted into the extracellular space.

The primary cell wall helps shape a plant cell. Under normal conditions, the concentrations of solutes is higher inside the cell than outside, causing water to enter the cell via osmosis. The incoing water increases the cells volume, pushing the plasma membrane up against the wall. The force exerted by the cell aginst the wall is known as turgor pressure.

Although plant cells exert turgor pressure throughout their lives, it is particularly important in young cells that are actively growing. Young plant cells secrete proteins named expansins into their cell wall. Expansins disrupt the hydrogen bonds that cross link microfibrils to other polymers in the wall, loosening the structure and allowing the microfibrils to slide past one another. Turgor pressure then forces the wall to elongate and expand, allowing for cell growth.

As plant cells mature and stop growing, they may secrete an additional layer of material, a secondary cell wall, between the plasma membrane and the primary cell wall.

The makeup of the secondary cell wall varies from cell to cell in the plant and correlates with each cells function. Cells on the surface of a leaf have secnondary cell wall containing waxes that form a waterproof coating cells that support a plants stem have stiff secondary cell walls the ctonatin a great deal of cellulose.

In cells that form wood, the secondary cell wall also contains lignin a complex polymer that forms an exceptionally rigid network. Thick secondary cell walls of cellulose and lignin help woody plants withstand the forces of gravity and wind.

Most animal cells secrete a fiber composite called the extracellular matrix, or simply ECM. Like the extracellular materials found in other organisms, the ECM provides structural support.

ECM organization follows the same principles observed in the cell walls of algae, fungi, and plants. Ther is a key difference, however: The animal ECM contains much more protein relative to carbohydrate than does a cell wall.

The fibrous component of animal ECM is dominated by glycoproteins named collagen. About a quarter to a third of all the protein in your body is collagen.

The extracellular matrix of animals is a fiber composite

although several types of fibrous proteins are found in the ECM, the most abundant is collagen. After collagen is secreted from the cell, the triple helix proteins can assemble into fibrils and even larger cable like fibers

The spaces between collagnes are filled with a ground substance consisting of proteoglycans. Each individual proteoglycan consists of a core protein attached to many polysaccharides. In some tissues proteoglycans are assembled into even larger complexes

Most ECM proteins are synthesized in the rough ER, processed in the Golgi apparatus, and secreted from the cell via exocytosis. After secretion, the individual proteins may assemble into large structures. For example, groups of collagen triple helixes may coalesce to form collagen fibrils, and bundles of fibrils may link to form even larger fibrous complexes.

Ground substance of the ECM

The ground substance that surrounds collagen and other fibrous components of the EMC contains highly glycosylated, gel-forming proteins called proteoglycans. In addition, secreted proteoglycans may be attached to long polysaccharides synthesized by cellular enzymes in the extracellular space. The resulting huge complexes, such as the one shown in the photo are responsible for the rubber like consitency of cartilage.

Composition of the ECM varies among tissue types

the ECM surrounding lungs contains large amounts of a rubber like protein called elastin, which allows the ECM to expand and contract when you breathe. The structure of a tissues ECM correlates with the function of the tissue.

ECM proteins support cell structure via their attachements to the cell surface.

membrane proteins called integrins bind to extracellular cross linking proteins, including laminins, which in turn bind to other components of the ECM

The intracellular portions of the integrins bind to proteins that are connected tot he cytoskeleton, effectively linking the cytoskeleton and ECM. This linkage is critical. Besides keeping individual cells in place, it helps adjacent cells adhere to each other via their common connection to the ECM.

Signalling pathways monitor the cytoskeleton-ECM linkage

Cells monitor the cytoskeleton-ECM linkage via signaling pathways. When integrins bind to the ECM, they transmit signals that inform the cell it is in the right place and properly anchored. If this linkage breakes down, the signals are not transmitted and cells normally die as a result.

Materials and structures that bind cells together are particularly importnat in epithelia, tissues that form external and internal surfaces. Epithelia function as barriers between the external and internal environments of plants and animals. In animals, epithlia also serve as gatekeepers that regulate the transport of substances, such as the absorption of water and nutrients across the epithelia of the intestines.

The adhesive structures that hold cells together vary among organisms

Indirect Cell to Cell Attachments

The extracellular space between the walls of adjacent plant cells sandwhich a central, the middle lamella, which conisits primarily of gelatinous pectins. Because the lamella is continuous with the primary cell walls of adjacent cells, it serves to glue them together. The two cell walls are like slices of bread, andthe middle lamella is like a layer of peanut butter. If the enzymes degrade the middle lamella, as they do when flower petals and leaves detach and fall, the adjacent cells separate.

In animals integrins in the plasma membranes of cells will form connections between their cytoskeletal structures and the extracellular matrix. By interacting with the same network of ECM components, multiple cells both within and between different tissues through the ECM are particularly important in reinforcing these interactions.

Direct Cell to Cell Attachments

In contrast to such indirect intercellular connections, in animals, where cell walls do not exist, a varitey of membrane proteins allow for direct cell to cell attachment in epithelia and other tissues.

Tight junctions form waterproof seals. A tight junctions is a cell to cell attachment composed of specialized proteins in the plasma membranes of adjacent animal cells. As the drawing in FIgure 11.8b indicates, long chains of these proteins form on the surface of a cell and attach to the same proteins on adjacent cells. The tight interactions between these proteins will pull the membranes of the two cells very close together. The resulting structure resemble a quilt, wher the proteins "stitch" the membranes of two cells together. In cells, the structure forms a water tight seal that prevent solutions from flowing through the space between two cells.

A strong cell to cell attachment particularly common in animal epithelial cells and certain muscle cells. In their structure and function, desmosomes are analogous to the rivets that hold pieces of sheet metal together.

Desmosomes comprise linking proteins and cytosolic anchoring proteins. The linking proteins span the membrane and directly connect adjacent cells and thier anchoring proteins located on the inner faces of each cell membrane. Cytoskeletal intermediate filaments help reinforce desmosomes by attaching to the intracellular anchoring proteins. In this way desmosomes help form a continuous structural support system between all the cells in the tissue.

adhering to other cells of the same tissue type

In both plants and animals, direct connections between cells in the same tissue help the cells to work in a coordinated fashion. One way of accomplishing this is to have channels in the membranes of adjacent cells, allowing the cells to communicate via the diffusion of cytosolic ions and small molecules from cell to cell.

Ions and small molecules are just two of many different forms of signals that convey information between cells. How cells respond to this exchange of information depends on the type of cell and the type of signal, but there are two general mechanisms

1) Signals may regulate gene expression, altering which proteins are produced and wich are not

2) signals may activate or inactivate particular proteins that already exist in the cell, often those involved in metabolsim, membrane transport, secretion, and the cytoskeleton.

Gap Junctions Connect Animal Cells via Protein Channels

In many animal tissues, structures called gap junctions connect adjacent cells. In a gap junction, specialized proteins assemble in the membrnaes of adjacent cells, creating interconnected channels that allow water, ions, and small molecules such as amino acids, sugars, and nucleotides to move between the cells.

Gap junctions are communication portals. They can help adjacent cells coordinate their activities by allowing the rapid passage of regulatory ions or small molecules. In the muscle cells of your heart, for example, a flow of ions through gap junction acts as a signal that coordinates contractions.

Plasmodesmata Connect Plant Cells via Membrane Lined Channels

In plants, gaps through cell walls allow direct connections between the cytoplasm of adjacent cells. At these connection, named plasmodesmata, the plasma membrane and cytoplasm of the two cells are continuous. Tubular extenstion from the smooth ER run through these membrane lined channels.

Like gap junctions, plasmodesmata are communication portals through the plasma membrnae. In plants, the plasma membrane separates most tissues into tow independent corridors

1) the symplast which is a continuous network of cytoplasm connected by plasmodesmata

2) the apoplast which is the region outside the plasma membrane. The apoplast consists of cell walls, the middle lamella, and air spaces. Small molecuels can move through plant tissues in either of these compartments without ever crossing a membrane.

Cell to Cell Signaling in Multicellular organisms

Biologists have classified many types of signaling molecules that keep distant tissues in touch. One, type, neurotransmitters, may open or close ion channels in the plasma membrane of distant cells, changing the electrical properties of the membrane. This type of signal is responsible for the transmission of information through the nervous system allowing you brain to control the movements of the rest of your body.

Hormones are information carrying molecules that are secreted by plant and animal cells into bodily fluids and act on distant target cells. Hormones are usually small molecules and inlcude certain peptides, steriods, and even gases. Although hormones are typically present in minute concentrations, they have a large impact on the activity of target cells.

Hormones and other types of cell to cell signaling molecules deliver their message by binding to receptor molecules. The key characteristic of this interaction is that it changes the shape, or conformation of the receptor. A signal receptor, then, is a protein that changes its shape and activity after binding to a signaling molecule. This change in shape is how a message is passed from the signaling molecule to its receptor.

Most lipid soluble signaling molecules can diffuse across the hydrohobic region of the membrane and enter the cytosol of their target cells. The receptors for these molecules exist inside the cell.

Large or hydrophillic signaling molecules are lipid insoluble, and most cannot cross the plasma membrane. To affect a target cell, the have to be recognized at the cell surface. Their receptors are usually located in the plasma membrane.

Receptors are dynamic. The number of receptors in a particular cell may decline if hormonal stimulation occurs at high levels over a long time. The ability of a receptor to bind tightly to a signaling molecule may also decline in response to intensive stimulation. As a result , the sensitivity of a cell to a particular hormone may change over time.

Receptors can be blocked. Many drugs are used to block the interaction between hormones and their receptors. For example, certain beta blocker drugs will prevent adrenaline from binding to its receptor on heart cells.

Once a cell receives a signal , it has to process the signal to initiate a response. This step happens in one of two ways depending on whether the receptors are located in the cytosol or at the membrane surface.

Processing Lipid Soluble Signaling Molecules

Steriod hormones such as estrogens and cortisol are examples of lipid soluble signaling molecules. Because they are hydrophobic, most lipid soluble signaling molecules must be carried through the bloodstream by hydrophilic proteins. After reaching their target cells, these signaling molecules are released from the carrier proteins, diffuse through the plasma membrane, and enter the cytosol. Often a hormone receptor complex is formed in the cytosol and then transported to the nucleus, where it triggers changes in gene expression. By altering the expression of genes the cell produces different proteins that will directly affect the function or shapre of the cell.

Processing Lipid Insoluble Signaling Molecules

Hormones that cannot diffuse across the plasma membrane and enter the cytosol do not directly participate in intracellular activities, like changin gene expression. Instead, the signal that arrives at the surface of the cell has to produce an intracellular signal, the processing step is indirect.

When a signaling molecule binds at the cell surface, it triggers signal transduction, the conversion of a signal from one form to another. A long and often complex series of events ensues, collectively called a signal transduction pathway.

signal transduction pathways

In a cell, signal transduction converts an extracellular signal to an intracellular signal.

When a hormone arrives at the cell surface, the message it transmits may be amplified as the signal changes form. An increased number of intracellular signals also makes it possible for hormones to affect different molecules in the cell.

In cells, signal transduction begins at the plasma membrane amplification and diversification of the signal takes place inside the cell. This may occur in a variety of ways, depending on the mechanism of signal transduction. In general, the arrival of a single signaling molecule results in a secondary signal that involves many ions or molecules that can affect several different cellular activities.

For example, when liver cells are stimulated by adrenaline to release glucose into the bloodstream, the signal is amplified by the production of numerous small molecules called second messengers. The signal then diversifies to activate enzymes that break down glycogen into glucose, inhibit enzymes that synthesize glycogen, and produce new enzymes that make glucose.

Two major types of signal transduction systems that are distinguished based on how they are initiated.

1) G-protein-coupled receptors initiate the production of intracellular second messengers, which then amplify and diversify the signal.

2) Enzyme-linked receptors activate a series of proteins inside the cell, through the addition of phosphate groups. The number and type of proteins activated lead to the amplification and diversification of the signal.

Signal Transduction via G-Protein Coupled Receptors

Many signal receptors span the plasma membrane and are closely associated with peripheral proteins inside the cell called G Proteins. When G proteins are activated by a signal receptor, they often trigger a key step in signal transduction: the production of a second messanger, a small non protein signaling molecule or ion that elicits an intracellular response to the first messenger.

G proteins link the receipt of an extracellular signal to the production of an intracellular signal.

got their name because their activity is regulated by the type of nucleotide they are bound to: either

guanosine triphosphate (GTP)

guanosine diphosphate (GDP)

G proteins are activated when they bind GTP they are inactivated when a phosphate group (negative charge) is removed from GTP to form GDP.

The G protein will remain inactive until the GDP is replaced with a new GTP.

GTP is a nucleoside triphosphate that is similar in structure to adenosine triphosphate (ATP)

Nucleoside triphosphates have high potential energy becuase their three phosphate groups have four negative charges close together.

When GTP bind to a G protein, the addition of the negative charges alters the proteins shape. Changes in shapre produce changes in activity.

How do G protein coupled receptors work?

STEP 1: A signaling molecule arrives and binds to a receptor in the plasma membrane. The receptor is a transmembrane protein whose intracellular portion is coupled to a G protein composed of multiple subunits. The G protein is anchored by a lipid tail to the cytosolic side of the cell membrnae. The lipid anchor permits the G protein to diffuse laterally in the membrane.

STEP2: In response to binding of the signaling molecule, the receptor changes shape and activates its G protein. Specifically, the receptor kicks out the GDP from the inactive G protein, allowing GTP to bind to the protein. When GTP is bound, the G protein will change shape radically: The active GTP binding subunit splits off.

STEP3: The active G protein subunit interacts with a nearby enzyme that is embedded in the plasma membrane. This interaction stimulates the enzyme to catalyze production of a second messenger.

Second messengers are effective because they are small and therefore can diffuse rapidly to spread the signal throughout the cell. In addition, they can be produced quickly in large quantities. This characteristic is important. Because the arrival of a single signaling molecule can stimulate the production of many second messengers, the signal transduction event amplifies the original signal.

Several types of small molecules and ions act as second messengers in cells.

Several second messengers activate protein kinases, enzymes that activate or inactivate other proteins by adding a phosphate group to them.

Second messengers arent restricted to a single role, the same second messenger can initiate dramatically different events in the same cell or in different cell types receiving the same signaling molecule.

More than one type of second messenger may be involved in triggering a cells response to the same extracellular signaling molecule.

Signal Transduction via Enzyme Linked Receptors via Receptor Tyrosine Kinases RTKs

Enzyme linked receptors transduce hormonal signals by directly catalyzing a reaction inside the cell.

STEP1: A hormone binds to two subunits of an RTK and cuase them to form a dimer.

STEP2: The conformational change in the RTK turns on its catalytic activity, allowing RTK to phosphorylate itself at tyrosine residues using ATP inside the cell.

STEP3: Step 3 Proteins inside the cell bind to the phosphorylated RTK, forming a bridge between the receptor and a lipid anchored peripheral membrane protein called Ras, which is a single subunit G protein. Bridge formation activates Ras by causing it to exchange its bound GDP for a GTP.

STEP4: When Ras is activated, it triggers the phosphorylation and activation of a protein kinase.

STEP5: The Ras-activated kinase catalyzes the phosphorylation and activation of a second kinase, which then phosphorylates and activates a third kinase. The third kinase triggers the cell response by phosphorylating additional proteins.

The sequence of protein modifications that culminates in a cell response.

Since these cascades are often initiated by mitogens signaling molecules that activate cell division, the three numbered kinases in Figure 11.16 are called mitogen activated protein kinases (MAPKs)

Although the change in MAPK conformation after it is phosphorylated is very subtle, it has a dramatic effect in MAPK catalytic activity.

In general, intracellular signals initiated by G-Protein coupled receptors result in the production of second messengers, while enzyme linked receptors, like RTKs, drive phosphorylation cascades. Although sometimes the opposite is true.

A signal transduction event has two results

1) It converts an extracellular message into an intracellular message,

2) in some cases it amplifies and diversifes the original message to elicit a large and multifaceted response in the cell.

Recall that when adjacent cells share information through cell gaps two general categories of response may occur: a change in gene expression or a change in the activity of proteins that already exist in the cell. The same holds true for responses to messages carried by signaling molecuels.

Cells have built in systems for turning off intracellular signals. Although many different mechanisms may be used most signal transduction systems are exquisitely sensitive to small changes in the concentration of signaling molecules or the number and activity of signal receptors. As a result, they trigger a rapid response and can be shut down quickly.

For example once an activated G protein turns on a downstream enzyme, the bound GTP is hydrolyzed by the G protein to GDP and Pi. This reaction changes the G proteins conformation and returns the protein to its inactive state. Activation of its downstream target stops, and production of the second messenger ceases. To produce a high concentration of second messengers, the pool of inactive G proteins must be continuously reactivated by the signal receptor to keep the process going. Otherwise, the signal transduction system quickly shuts down.

Phosphorylation cascades are also sensitive to the continuing presence of external signaling molecules. If stimulation of a receptor tyrosine kinase ends, enzymes called phosphatases will remove the phosphate groups from components of the phosphorylation cascade, causing the dignal transduction to cease.

The presence of second messengers in the cytosol is also short lived. For example, pumps in the membrane of the smooth ER return cytosolic calcium ions to storage in the ER lumen, and enzymes called phosphodiesterases convert active cAMP and cGMP (see Table 11.1) to inactive AMP and GMP, respectively. If the production of second messengers is halted, then they are quickly cleared from the cytosol and the signal transduction stops.

To appreciate what happens when a signal transduction system does not shut down properly, let’s return to the phosphorylation cascade illustrated in Figure 11.16. Recall that Ras is active when it is bound to GTP, but it is deactivated when it hydrolyzes GTP to GDP and Pi. If this hydrolysis activity were defective, however, Ras would remain active and continue stimulating the cascade even when the external signal is no longer present.

One environmental factor that is closely monitored by populations of unicellular organisms is the density of the population. The use of signaling pathways to respond to population density in prokaryotic and eukaryotic microbes is referred to as guorum sensing. The name was inspired by the observation that cells of the same species may undergo dramatic changes in activity when their numbers reach a threshold, or quorum. Quorum sensing is based on signaling molecules that are secreted by cells and diffuse through the environment. The response to these molecules depends on the species. In bacteria, quorum sensing is often used to help glue a community of microbes to a surface in a biofilm (Ch. 2_6, Section 26.1), such as the plaque that forms on your teeth. Quorum sensing is also involved in light emission (bioluminescence) by certain bacteria. For example, bacterial species including Vibrio Fischeri are actively cultured in the light organs of the bobtail squid after reaching a certain density, they express enzymes that catalyze a light-producing reaction (see the chapter 18 Case Study). Quorum sensing allows unicellular organisms to communicate and coordinate their activities. When it occurs, these cells take on some of the characteristics of multicellular organisms. For example, quorum sensing via a G-protein-coupled receptor causes the free-living cells (amoebae) of the slime mold Dictyostelium to aggregate into multicellular mounds (Figure 11.18). Amazingly, the slug-like body that is formed from one of these aggregates can crawl across a surface and eventually organize itself into a fruiting body that releases spores into the air

splitting of preexisting cells

Early studies revealed two fundamentally different ways that nuclei divide before cell division: meiosis and mitosis.

In animals meiosis leads tot he production of sperm and eggs, which ar the male and female reproductive cells termed gametes.

Mitosis leads to the production of all other cell types referred to as somatic cells.

Mitosis and meiosis are usually accompanied by cytokinesis the division of the cytoplasm into two distinct cells. When cytokinesis is complete, a so called parent cell has given rise to two daughter cells.

The basic steps in cellular replication

3)dividing the cytoplasm to create two complete cells.

consists of single long DNA double helix that is wrapped around proteins called histones, in a highly organized manner.

A region of DNA in a chromosome that codes for a particular protein or ribonucleic acid RNA

Each chromosome is replicated. As mitosis starts the chromsomes condense into compact structures that can be moved around the cell efficiently. Then one copy of each chromosome is distributed to each of two daughter cells.

The two chromatids are joined along their entire length by proteins called cohesins. Once mitosis begins, however these connections are removed except for those at a specialized region of the chromsome called the centromere.

each of the double stranded DNA copies in a replicated chromosome

chromatid copies that remain attached at their centromere

Cells alternate between M phase and Interphase

M phase- occurs when cells are in the process of separating their chromosomes.

Interphase- the rest of the time when the cell is not in M phase. The chromsomes uncoil into the extremely long, thin structures shown in Figure 12.1 and longer appear as individual threads. The cell is either grwoing and preparing to divide or fulfilling its specialized function in a multicellular individual. Cells actually spend most of their time in interphase.

interphase consisting of G1, S, and G2 phases

In multicellular organisms, cells perform their functional roles mostly during G1 phase. G1 is also the period when the cell "decides" to begin replication and transistions to S phase.

Before mitosis can take place, a cell uses G2 phase to prepare for M phase. The time spent in both G1 and G2 allows the cell to grow and replicate organelles so it will be able to divide into two cells that can function normally.

M phase typically consists of two distinct events: the division of the nucleus and the division of the cytoplasm. Mitosis divides the replicated chromosomes to form two daughter nuclei with indentical chromsomes and genes. Cytokensis usually follows mitosis and divides the cytoplasm of the parent cell to form two daughter cells.

chromosomes are replicated during S phase, and the cell then enters G2 phase. During M phase, the replicated chromosomes are partitioned to the two daughter cells. Each duaghter cell contain the same number of chromosomes that the parent cell had.

consist of DNA wrapped around globular histone proteins. This DNA histone complex is called chromatin.

During interphase, the chromatin of each chromosome is in relaxed or less condensed state.

the cell contains replicated chromsomes before mitosis. Each chromosome now consists of two sister chromatids. Each chromatid contains one long DNA double helix and sister chromatids represent exact copies of the same genetic information.

Mitosis begins when chromatin condenses to form a much more compact structure.

During mitosis the two sister chomatids separate to form independent daughter chromosomes. One copy of each chromosome goes to each of the two daughter cells.

Biologists have identified five subphases within mitosis based on distinctive events that occur: prophase, prometaphase, metaphase, anaphase, and telophase.

Mitosis begins with the events of prophase. when chromosomes condense into compact structures. Individual chromosomes first become visible in the light microscope during prophase.

Prophase is also marked by the formation of the spindle apparatus. The spindle apparatus is structure that produces mechanical forces that

- move replicated chromsomes during early mitosis

-pull chromatids apart in late mitosis

consists of microtubules, components of the cytoskeleton.

are composed of alpha-tubulin and beta-tubulin dimers

they have a plus end and a minus end, meaning they are asymmetric

the plus end isthe site where microtubule growth normally occurs. Microtubule disassembly is more frequent at the minus end

microtubules originate from microtubule organizing centers MTOCs. MTOCs define the two poles of the spindle apparatus and produce large numbers of microtubules, whose plus ends grow outward through the cytoplasm. Although the nature of the MTOC varies among plants, animals, fungi, and other eukaryotic groups, the spindle apparatus has the same function.

a structure that contains a pair of centrioles

During S phase, the single centrosome replicates along with the DNA. At the start of prophase, the two centrosomes move to opposite sides of the nucleus to begin forming the spindle appartus. Some of these microtubules extend from each spindle pole and overlap with one another ehse are called polar microtubules.

In many eukaryotes, once chromosomes have condensed, the nuclear envelop disintigrates. Removal of the envelope allows the cytoplasmic microtubules to attach to chromosomes at specialized structures called kinetochores. These events definet the start of prometaphase.

Each sister chromatid has its own kinetochore, which is assembled at the centromere. Because the centromere is also the attachement site for chromatids, the result is two kinetochores on opposite sides of each replicated chromsome. The microtubules attached to these structures are called kinetochore microtubules.

Early in prometaphase, kinesin and dynin motors attached to the knetochores "walk" the chromsomes up and down microtubules. This process is similar to the way the same motors transport vesicles and organelles along microtubules. When the chromosomes reach the plus ends of the microtubules, the kinetochore proteins secure their attachment.

Eventually each chromosome will have its two kinetochores attached to microtubules that originate from opposite sides of the spindle apparatus. The chromosomes are then pushed and pulled by microtubules and motor proteins until they reach the middle of the spindle.

Once all the chromosomes have migrated to the middle of the spindle, the mitotic cell enters metaphase. At this point, the chromosomes are lined up on an imaginary plane between the two spindle poles called the metaphase plate.

Formation of the spindle apparatus is now complete. The polar microtubules that extend from ech spindle pole overlap in the middle of the cell, thereby forming a pole to pole connection. Each chromosome is held by kinetochore microtubules reaching out from opposite poles and exerting the same amount of tension, or pull. The spindle poles are held in place partly because of astral microtubules that extend from the MTOCs and interact with proteins on the plasma membrane.

The polarized growth and disassembly of the kinetochore microtubules contributes to the alignment of chromosomes at the metaphase plate. The slow disassembly of the minus ends at the MTOCs is balanced by the slow growth of the plus ends at the kinetochores. Because the sister chromatids of each chromosome are connected to opposite poles, a tug of war between the poles begins during metaphase.

At the start of anaphase the cohesins that hold sister chromatids together at the centromeres are cleaved by an enzyme. Because the chromatids are under tension, each replicated chromosome is pulled apart, creating two independent daughter chromosomes. By definition, this separation of chromatids instantly double the number of chromosomes in the cell.

Two types of movement occur during anaphase. First, the daughter chromosomes move to opposite poles via the attachemnt of kinetochore proteins to the shrinking kinetochore microtubules. Second the two poles of the spindle are pushed and pulled farther apart. The push comes from motor proteins in overlapping polar microtubules, which force the poles away form each other. The pull comes from different motors on the plasma membrane, which walk along on the astral microtubules and drag the poles to opposite sides of the cell.

The separation of replicated chromosomes to opposite poles is a critical step in mitosis because it ensures that each daughter cell receives the same complement of chromosomes. When anaphase is complete, two complete sets of chromosomes are fully separated, each set identical to taht of the parent cell before chromosome replication.

During telophase the nuclear envelope re-forms around each set of chromosomes, and the chromosomes begin to condense. Once two independent nuclei have formed mitosis is complete.

How do Chromsomes move during anaphase

The exact and equal partitioning of genetic material to the two daughter nuclei is the most fundamental aspect of mitosis. To understand how sister chromatids separate and move to opposite sides of the spindle, biologists have focused on the role of kinetochore microtubules. How do these microtubules pull chromatids apart?

During mitosis, the microtubules originating from the spindle poles are highly dynamic. Rapid growth and disassembly ensures that some of the microtubules will be able to attach to kinetochores with their plus ends. Others Will be stabilized by different proteins in the cytoplasm and become polar or astral microtubules.

These observations suggest two hypotheses for the movement of chromosomes during anaphase. The simpler hypothesis is that kinetochore microtubules stop growing at their plus ends but remain attached to the kinetochores. As the minus ends disassemble at the spindle poles, the chromosomes would be reeled in like hooked fish. An alternative hypothesis is that the chromosomes move along microtubules that are being disassembled at their plus ends at the kinetochores. In this case, each chromosome would be like a yo-yo running up a string into your hand.

Kinetochores are linked to retreating ends

The kinetochore is a complex of many proteins that attaches the centromere region of the chromosome to one or more microtubules.

The kinetochore is a complex of many proteins that attaches the centromere region of the chromosome to one or more microtubules.

cytokinesis Results in Two Daughter Cells

At this point, the chromosomes have been replicated in S phase and distributed to opposite sides of the spindle via mitosis. Now it’s time to divide the cell into two daughter cells that contain identical copies of each chromosome. If these cells are to survive, however, the parent cell must also ensure that more than just chromosomes make it into each daughter cell. While the cell was in interphase, the cytoplasmic contents, including the organelles, increased in number or volume. During cytokinesis (Figure 12.5 steps 7 and 8), the cytoplasm divides to form two daughter cells, each with its own nucleus and complete set of organelles. In most types of cells, cytokinesis directly follows mitosis. In plant cells, polar microtubules left over from the spindle apparatus help define and organize the region where the new plasma membranes and cell walls will form. Vesicles from the Golgi apparatus carry components for a new cell wall to the middle of the dividing cell. These vesicles are moved along the polar microtubules via motor proteins. In the middle of what was the spindle, the vesicles start to fuse and form a flattened, sac-like structure called the cell plate (Figure 12.8a). The cell plate continues to grow as new vesicles fuse with it. Eventually, the cell plate contacts and fuses with the existing plasma membrane, dividing the cell into two daughter cells.

In animals and many other eukaryotes cytokenesis begins with the formation of a cleavage furrow. the furrow appears when a ring of overlapping actin filaments starts to contract just insidethe plasma membrnae, in the middle of what used to be the spindle. This contraction is caused by myosin motor proteins that bind to the actin filaments and use ATP to slide the filaments past one antoher.

As myosin moves the actin filaments, the ring shrinks and tightens. Because the ring is attached to the inside of the plasma membrane, the contracting ring pulls the membrane with it. As a result, the plasma membrane is drawn inward. Myosin continues to slide the actin filaments past each other, tightening the ring further until the plasma membrane fuses and cell division is complete. Chromosome separation and cytoplasmic division are common requirements for all organisms, not just eukaryotes. What is known about cell division in prokaryotes? Is the process of cell division in your cells similar to that in bacteria?


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Course topics

DNA, environment, gene, species, population, system, adaptation, energy, ecosystem, photosynthesis, plants, plant, cell, phenotype, allele, human, organisms, embryo, mutation, genetic, reproduction, evolution, inheritance, crossing-over, eutrophication, blood, antigen, genotype, homozygote, heterozygote, transpiration, variation, hemoglobin, respiration, diseases, individual, life, matter, herbivore, recombination, meiosis, collagen, bacteria, humans, antibodies, insulin, phagocytosis, mutations, capillaries, organic compounds, pituitary gland, gluconeogenesis, root, asexual reproduction, photoautotroph, autotrophs, hormones, hypothalamus, sex, natural selection, zygote, translation, transcription, amnion, nucleotide, uracil, receptor, nerve impulse, synapse, inorganic compounds, RNA, competition, predation, herbivory, transport, invertebrates, vertebrates, homeostasis, osmosis, glucose, clone, autotroph, cells, predator, sperm cell, egg cell, testes, theory, skeletal, cartilage, osteoblasts, bone marrow, gametes, enzymes, proteins, pesticides, biomass, microorganisms, cell division, interphase, mitosis, groups, agglutination, pasteurization, biotechnology, lysosomes, granulocytes, tissues, cambium, cuticle, meristem, growth, vitamins, digestion, protease, nervous system, nerve cell, axon, myelin sheath, dendrites, peripheral nervous system, circulatory, circulatory system, pulse, biocenosis, pancreas, regulation, thyroid gland, metabolic, leaf, eukaryotes, alternation of generations, malaria, chromosomal aberration, genetic recombination, mutagens, deletion, monosomy, trisomy, endocrine glands, glucagon, gonads, symbiosis, membranous labyrinth, generation, birds, organ, adaptations, zooplankton, tRNA, necrosis, nucleus, chromosome, solenoid, chromatin, homologous chromosomes, karyotype, engineering, applications, mitotic division, cytosine, thymine, hormone, stimulus, earth, cytoplasm, AIDS, environmental resources, ecological niche, animal food, plant food, xylem, phloem, pollution, poisons, environmental pollution, natural resources, animals, climate, atmosphere, prokaryotes, glacier, protoctists, production, fertilization, food chain, national park, ecosystems, mRNA, nucleosome, codon, gene expression, polymerase, glycogen, habitat, heredity, ozone layer, phytoplankton, placenta, scale, water, polyp, oxygen, carbon dioxide, haploidy, food, fungi, spontaneous generation, composition, consumer, chemoautotroph, decomposer, mammals, sterilisation, bacterial, neurone, neurotransmitter, allantois, animal, antibiotics, pH, aerobic respiration, nucleotides, replication, Escherichia coli, heterotrophs, chloroplasts, mitochondria, specialization, corona radiata, zona pellucida, reproductive, the reproductive system, semen, primary spermatocyte, spermatogenesis, ejaculation, oogenesis, cellular membrane, cortical granules, germinal epithelial cell, oogonium, primary oocyte secondary oocyte, secondary spermatocyte, spermatids, chromatids, centromere, acrocentric chromosome, metacentric chromosome, submetacentric chromosome, telocentric chromosome, pachytene, genetic map, chromosomal, the chromosomal theory of inheritance, substance, femur, the skeletal system, humerus, fibula, tibia, patella, ankle, discs, osteoclasts, osteocytes, intercellular, ossification, chordin, epiphyses, phalanges, tarsals, metatarsals, movements, auxins, tropisms, nastic movements, taxes, fototropism, geotropism, chemotropism, thigmotropism, photonastic movements, termonastic movements, seismonastic movement, responsiveness, uses, ultrasound, methane, organic matter, calorific value, cyanobacteria, bioreactor, industrial, biofiltration, detergents, denitrification, nitrificatio, division, cell cycle, metaphase, prophase, anaphase, karyokinesis, cytokinesis, crossing over, binary fission, Rh, galactose, T cells, opsonisation, galactosamine, inheritance of blood groups in humans, filtration, fermentation, maturation, somatotrophin, somatostatin, asepsis, DNA recombination, transgenic organism, malting, milling, whirling, brewing, lysozyme, immune system, amoeboid movement, lysosomal enzymes, pathogenic factor, plasma cells, B cells, B lymphocytes, T lymphocytes, immune, The human immune system, cutin, protoplast, systematics, plant tissues, race, environmental variation, deoxyribonucleic acid, fruit, germination, seed, endosperm, cotyledon, seedling, juvenile phase, vegetative phase, sexual maturity, embryonic root, embryonic radicle, embryonic plumule, cotyledons, dicots, nutrients, anemia, avitaminosis, enzyme, amylase, lipase, neuron, cell body, Schwann cell, node of Ranvier, autonomic nervous system, synapses, terminal dendrites, sympathetic nervous system, parasympathetic nervous system, artery, vein, aorta, blood pressure, automatism of the heart, pericardium, biosphere, biogeochemical cycles, biogeochemical, cycles, hypoglycaemia, glycogenolysis, lipolysis, lipogenesis, acromegaly, pituitary dwarfism, hyperglycaemia, alcohol, addiction, alcoholism, stimulants, hallucinogens, cigarettes, alcoholic psychosis, cannabinol, drug addiction, inhalants, narcotic analgesics, nicotinism, sedatives, withdrawal symptoms, drugs, effects of drugs, flower, cellulose, ovule, spore, photosynthetic pigments, rhizoids, sporangium, vegetative reproduction, changes, inversion, substitution, point mutation, duplication, DNA polymerase III, induced mutations, insertion, gene mutation, proliferation, sickle-cell disease, spontaneous mutations, unscheduled DNA synthesis, UDS, mutations as changes in DNA, factor, agglutinogens, agglutinins, transfusion, haemolysi, rhesus, twins, Down’s syndrome, Turner’s syndrome, pedigree chart, Edwards’ syndrome, Patau’s syndrome, heterozygotes, homozygotes, genetic diseases, metabolism, ovaries, glands, target cells, exocrine glands, posterior pituitary, anterior pituitary, thymus gland, adrenal cortex, adrenal medulla, menstruation, sex hormones, ovulation, estrogens, progesterone, FSH, LH, endometrium, chorionic gonadotrophin, HCG, follicle-stimulating hormone, luteinizing hormone, microorganism, ecological succession, phototroph, autotrophic organism, cooperation, protocooperation, commensalism, mycorrhizas, mutualism, eye, ear, orbit, lens, cochlea, middle ear, pinna, semi-circular canals, retina, iris, cornea, vitreous humor, blind spot, cones, rods, aqueous humor, auditory ossicles, bony labyrinth, choroid, ciliary body, conjunctiva, endolymph, Eustachian tube, external auditory canal, eye chambers, fovea, lachrymal gland, organ of Corti, perilymph, saccule, sclera, vestibule, fish, convergent evolution, eurytopic species, stenotopic species, ubiquistic species, haploid, gametophyte, sporophyte, antheridia, archegonia, diploid spore, double fertilization, sperm nucleus, vegetative cell, generative cell, prothallus, plant reproduction, mineral, mineral water, nutrition, trace elements, macro-elements, ultra-trace elements, chlorosis, mineralization of plants, euchromatin, heterochromatin, DNA double helix, histones, loop domain, metaphase chromosome, diploid set of chromosomes, nucleus as a store of genetic material, other, base, nitrogenous base, denaturation, restriction endonuclease, sequencing, other applications of genetic engineering, information, behavior, communication, nature, genetic information, transmitter, taxis, tropism, plant hormone, phytochrome, warning signals, alarm signals, animals mating behavior, transmitters, origin, ribosomes, bacterial cell, biogenesis, chemical evolution, coacervate, liposome, microsphere, endosymbiosis, nucleic acidic, HIV, virus, antibody, genome, hydrolases, macrophage, lymphocyte, Kaposi’s sarcoma, lymphoma, immunodeficiency, (hiv), human immunodeficiency virus (HIV), density, interspecific, intraspecific, individual organism, population size predation predator prey, competition., stem, substances, organic, vascular tissue, accumulation, primary phloem, secondary phloem, primary xylem, secondary xylem, secondary stem structure, secondary root structure, growth of a plant in girth, environmental, disease, contamination, pesticide, introduced species, decibel, dB, pathogenicity, DDT, dichloro-diphenyl-trichloroethane, indigenous species, carcinogen, history, extinction, chlorophyll, mollusks, annelids, arthropods, amphibians, reptiles, dinosaurs, sponges, psilophytes, birds mammals, the history of life on Earth, runner, crop, crop rotation, herbicide, weed, cultivation, humus, winter crop, spring crop, rhizome, hydroponics, crop production, peristalsis, bile, pepsinogen, emulsification, alimentary canal, waste, resources, recycling, conservation, prevention, natural environment, nature conservation, reintroduction, environment pollution, household waste, species conservation, manufacturing process, anticodon, termination site, operator, operon, promoter, polymerase binds, triplet code, The gene as a structural and functional unit of DNA, muscles, connective tissue, skeletal muscles, creatine phosphate, free fatty acids, myoglobin, muscle relaxation, extensor, flexor, relaxed muscle, contracted muscle, tropomyosin, troponin, laws, gene pool, speciation, bottleneck effect, hybridisation, immigration, sexual selection, founder effect, genetic drift, geographical isolation, laws of evolution and speciation, sound, light, smell, taste, sensory organs, chemoreceptors, baroreceptors, proprioceptive, exteroreceptors, interoreceptors, pheromones, ozone, greenhouse, emissions, radiation, ultraviolet, ultraviolet radiation, UV, ozone hole, chlorofluorocarbons, ozonosphere, nerve, central nervous system, nerve center, brain, enamel, kidneys, limb, vertebral column, feather, gills, eardrum, auditory bones, axial skeleton, extra-embryonic membranes, mammary glands, skin gland, skull uric acid, vertebra, epidermis, endodermis, guttation, vascular bundles, pericycle, primary cortex, stele, effort, stethoscope, physical effort, erythropoietin, muscle pump, sphygmomanometer, systolic pressure, aerobic metabolism, anaerobic metabolism, diastolic pressure, functioning, trachea, chitin, parenchyma, appendages, digestive glands, flame cell, haemocyanin, haemolymph, hydrostatic skeleton, Malpighian tubule, mesogloea, metamere metanephridium, ommatidium, protonephridium, water vascular system, optimum, atp, stroma, ADP, thylakoids, photolysis, limiting factor, NADPH, NADP, secondary, principles, aneuploidy, allosome, autosome, sexual characteristics, primary sexual characteristics, tertiary sexual characteristics, the principles of sex inheritance in humans, adrenaline, cellular respiration, extracellular fluid, blood plasma, thermoregulation, noradrenaline, Homeostasis, thyroid hormones, source, chromosomes, amino acids, Homologous chromosomes, alleles, mutations as a source of variation in organisms, refrigerator, antibiotic, spores, protecting, thermometer, manometer, autoclave, aseptic conditions, gram-positive bacterium, freezer, spoilage, inspiration, diaphragm, lung, respiratory system, respiratory, The respiratory system, expiration, pleura, protists, protoctista, photosynthetic pigment, eukaryote, thallus, chemo-autotroph, fruiting body, haustorium, heterotroph, mycelium mycorrhiza, photo-autotroph, pilus, symbiont, protists and fungi, chemical, amino acid, condensation, dry weight, hydrolysis, membrane receptor, chemical composition of cells, mucus, plankton, echolocation, electroreception, osmotic pressure, osmoregulation, compensation depth, electroreceptors, prey, chains, food chains, food web, detritivore, trophic level, arteries, blood vessels, vessels, arterioles, colloidal osmotic pressure, endothelium, veins, venules, plate, land, lungs, resistance, uric acid, aerodynamic force, chemoreception, chitinous exoskeleton, cholera, typhoid fever, macrophages, microbiology, immune serum, treponema pallidum, pneumonic plague, zoonoses, bacterial diseases, size, resolution, condenser, magnification, eyepiece, objective lens, electron microscope, transmission electron microscope, scanning electron microscope, dehydratation, ocular lens, fine adjuster, Light microscope, Microscope stage, objective lens stage, microscopes, microscopes and the size of cells, dendrite, stimuli, excitability, action potential, effector, end plate, neuromuscular junction, motor neurone, neuromodulator, relay neurone, resting potential, sensory neurone, across, diffusion, gradient, membranes, hemolysis, plasmolysis, facilitated diffusion, passive transport, simple diffusion, active transport, transport across membranes, predators, ionizing radiation, population density, parental care, abiotic factors, biotic factors, denaturation of protein, ultra-violet radiation, population range, individual and the population, acids, ribosome, nucleic acids, rRNA, adenine, Nucleotide, guanine, superhelix, nucleic, multiplication, assembly, viruses, bacteriophages, phages, bacterial viruses, capsid, droplet infection, inhalatory infection, lysis, release of a virus, viral adsorption, viral replication, virion, viroids, ovary, yolk, oviduct, womb, oviparous, spermatophore, air chamber, chorion, embryonic water, extra-embryonic membranes ontogeny, ovoviviparity, viviparity, reproduction in vertebrates, erythrocytes, cardiomyocytes, ectoderm, endoderm, leukocytes, mesoderm, myofibrils, platelets, thick filament, basement membrane, animal tissues, theory of evolution, creationism, Darwin, colonization, preservation, Charles Darwin and the theory of evolution, excretory system, kidney, plasma, fluids, nephron, transplantation, systemic, ADH, antidiuretic hormone, hemodialysis, reabsorption, filtration process, attack, heart, factors, myocardial infarction, arterial hypertension, atherosclerotic plaque, cerebrovascular disease, Chlamydia, intermittent claudication, ischaemic heart disease, thrombus, protista, protozoa, parasitic, moulting, ascariasis, amoebic movement, amoebae, invasive eggs, toxocariasis, enterobiasis, cysticercosis, proglottids, schizogony, schizonts, amoebiasis, parasitic diseases, chickenpox, measles, mumps, tuberculosis, vaccine, vaccination, rubella, rabies, poliomyelitis, smallpox, anthrax, disinfection, cowpox, BCG, polyvalent vaccines, vaccinations, absorption, enterocyte, portal vein, portal system, micelle, mesentery, glicogenolisis, catalyst, gas exchange, transformations, specificity, catalysis, proteases, lipases, nucleases, amylases, activation energy, metabolic transformations in a cell, fat, protein, anaerobic respiration, cellular, oxygen debt, cell respiration, cellular respiration and energy production, ecology, biome, biotope, its, transformation, vector, Transcription, ligase, conjugation, plasmids, introns, exons, hybridization, hydrogen bonds, retrovirus, PCR, genetic engineering and its applications in biotechnology, sexual reproduction, runners, budding, thallus fragmentation, reproduction and variation, prokaryote, parasites, circulation of matter, cell culture, photoautotrophs, chemoautotrophs, saprotrophs, bacterial growth, implantation, umbilical cord, pregnancy, notochord, yolk sac, cleavage, gastrulation, morula, blastocyst, allantoic cavity, amniotic cavity, gastrula, development of the human embryo, organism, classification, taxonomy, artificial systems, binomial nomenclature, natural systems, systems of classification, taxonomic units, classification of organisms, egg, reproductive organs, hermaphrodite, copulatory organs, gonochorism, heterogeny, semen sexual reproduction, oviparity, parthenogenesis, reproduction in invertebrates, biodiversity, trait, homeothermy, biocoenosis, species protection, structure, cell structure, plant cell, animal cell, plasma membrane, cell compartment, cellulose cell wall, vacuoles, granules, endoplasmic reticulum, cytosol, Golgi apparatus, structure of plant and animal cells, modification, differentiated cell, artificial selection, cloning, polyploid, in vitro fertilization, cord blood, genetic modification of organisms, haemoglobin carbamate, lymphocytes, monocytes, oxyhaemoglobin phagocytes, hepatitis, asthma, viral, hepatitis B, renal failure, jaundice, hepatitis A, droplet-respiratory route, Heine-Medin disease, pandemic, hepatitis D, Poliovirus, viral diseases, penicillin, bacteriophage, endospore, antisepsis, antiseptic activities, aseptic activities, spectrum of antibiotic, bacterial resistance, resistance plasmid, resistance gene, nosocomial infections, saprophytic organisms, antiseptics, antiseptics and antibiotics, tissue, cell specialization, single-cell organisms, simple colony organisms, complex colony organisms, multicellular tissueless organisms, multicellular tissued organisms, fertility, chloroplast, peat land, epiphyte, autotrophic organisms, carnivorous, carnivorous plants, reflex, conditioned reflex, monosynaptic reflex, Pavlov’s dog, polysynaptic reflex, reflex action, reflex arc, sucking reflex, unconditioned reflex, voluntary response, carnivore, omnivore, feeding, nectar, saprophages, omnivory, scavenger, rotoctists, carnivory, nutritional specialization, society, medicine, science, technology, migration, fossil, Homo sapiens, bipedalism, prosimians, australopithecines, Homo erectus, Homo habilis, human evolution, locus, according, diploidy, dominant alleles, recessive alleles, test cross, mendel, heredity according to Mendel, liver, external environment, internal environment, oxytocin, TSH, TRH, beta cells, dynamic equilibrium, homeostatic mechanism, negative feedback, alpha-cells, positive feedback


Are intercellular junctions, synapses and light-capturing photosynthetic complexes mobile? - Biology

If the dominant trait occurs in 19% of the population, then p2 + 2pq = 0.19. Since p2 + 2pq + q2 = 1 when Hardy-

Weinberg is applicable, then q2 = 1 – p2 + 2pq = 1 – 0.19 = 0.81. If q2 = 0.81, q = 0.9 and p = 0.1. Homozygous

dominant individuals are represented by p2 , so p2 = 0.1 × 0.1 = .01, or 1%.

E. Auxin (IAA, indoleacetic acid) stimulates growth in most plant tissues by increasing cell wall plasticity. When

cell walls are relaxed in this way, water entering by osmosis causes the cell to elongate, resulting in growth.

Gibberellins, together with auxins, also stimulate growth. Abscisic acid is a growth inhibitor, delaying growth

of buds and promoting seed dormancy. Cytokinins stimulate cell division. Ethylene promotes fruit ripening by

stimulating the breakdown of cell walls. Carotene is a plant pigment, responsible for the orange color in carrots.

B. Although both mitochondria and chloroplasts generate ATP, that is not evidence for their endosymbiotic origin.

The other choices, however, are evidence for the theory. The two lipid bilayers suggest that one bilayer came

with the endosymbiont while the second bilayer was generated by the host as the endosymbiont entered (as in

endocytosis). Mitochondria and chloroplasts resemble most prokaryotes with respect to their chromosomes (single,

circular DNA molecules without histones, but with nucleotide sequences that suggest a common ancestor with

certain bacteria) and with respect to their ribosomes (structurally similar to bacteria but different from eukaryotes).

The divisions of mitochondria and chloroplasts occur independently of their host eukaryotic cell divisions and the

divisions resemble the cell divisions of prokaryotes (binary fission).

A. During a typical menstrual cycle (when no egg is implanted on the uterine wall), a decline in FSH and LH

results in the breakdown of the corpus luteum. When the corpus luteum deteriorates, its estrogen and

progesterone production stops. In response, the endometrium, which is maintained by those hormones, begins to

slough off, generating the menstrual phase. Also, menopause begins when the ovaries stop producing estrogen.

B. Aphids feed on sugar solutions which are transported through the phloem. The transport of sugar through

phloem, according to the pressure-flow hypothesis, occurs by bulk flow forced forward by hydrostatic pressure.

Because of this pressure, fluid is forced entirely through the aphid’s digestive tract. In contrast, tracheids and

vessels, two kinds of xylem cells, transport water and dissolved minerals.

E. A protein is a polymer made up of individual amino acids (the monomers). Similarly, starch is a polymer

made up of individual glucose units.

B. A thick cuticle, sunken stomata, and an endodermis with Casparian strips are clear adaptations to water

conservation. During winter and early spring, most water is locked up in snow and effectively unavailable. Only

in late spring and early summer, after the snow melts, is the water available. During most of the remainder of the

year, drought conditions prevail over most of the terrain.

E. The endometrium is the lining of the uterus and is entirely of maternal origin. The placenta consists of both

maternal tissue and embryonic tissue. The chorion, the yolk sac, and the amnion are all extraembryonic membranes,

cells of embryonic origin that are outside of the embryo. The chorion is the outer membrane that surrounds the

embryo in mammals, it grows into the endometrium to form the embryonic part of the placenta. The amnion

encloses the amniotic cavity, which contains amniotic fluid that cushions the developing embryo. In birds and

reptiles, the yolk sac encloses the yolk which provides nutrients to the developing embryo.

D. Because mature red blood cells in mammals do not contain nuclei, no DNA is available for analysis.

C. Because the population size remains constant, each breeding pair must leave two offspring, replacing the two

parents that produced them. A more accurate number would be slightly higher than 2 to account for deaths (from

disease or accidents) of individuals before they reach reproductive age. In some human populations, the number

A. The processed mRNA found in the cytoplasm codes directly for the protein. Making a DNA complement

(cDNA) of this mRNA using reverse transcriptase allows for the insertion of a DNA segment that codes only

for mRNA exons. When translated, this mRNA will yield the correct polypeptide. In contrast, the unprocessed

mRNA transcript found in the nucleus contains introns that the bacteria cannot remove. An unprocessed mRNA

will result in the production of an incorrect polypeptide or no polypeptide at all. Similarly, the original DNA

segment that coded for the protein contains introns that the bacteria cannot remove.

D. Since Turner syndrome individuals have only one X chromosome, a Barr body (an inactive second X

chromosome) does not form. Turner syndrome results when nondisjunction produces a gamete missing a sex

chromosome. The union of this gamete (egg or sperm) with a normal egg or sperm bearing an X chromosome

produces a Turner syndrome zygote.

B. The genomes for the nematode C. elegans, the yeast (fungus) Saccharomyces, the fruit fly Drosoophila, and

the mustard plant Arabidopsis have all been sequenced. They serve as model organisms for the study of molecular

genetics and developmental biology. The pea Pisum, is an historically important plant that Mendel used to study

genetics. However, its current use as a tool for molecular study is limited.

A. Since the cuckoo chick is raised in the absence of adult cuckoos and without ever hearing the cuckoo song,

the song must develop as a result of instinct. It is innate, or under genetic control.

A. Gametes are produced by the gametophyte. Since the gametophyte is haploid, haploid gametes must be produced

by mitosis. Alternation of generations occurs when there is multicellularity in both haploid and diploid stages of the

life cycle. Fertilization occurs between stages II and III when the gametes fuse to form the zygote. Meiosis occurs

between stages IV and V, when the diploid sporophyte produces haploid spores.

E. Parenchyma cells are typically undifferentiated, with spherical, elongated, or many-sided polygon shapes and

thin (primary) cell walls. They are usually loosely arranged with evident intercellular spaces. Parenchyma cells

often serve a storage function (for water or starch) or carry out photosynthesis.

D. Xylem cells are thick-walled cells that transport water. At maturity they are dead, and only the cell wall

remains. There are two kinds of xylem, tracheids and vessel elements. Tracheids are long and narrow with tapered

ends. Water passes from one tracheid to the next tracheid through pits in the cell walls where the tapered ends of

adjacent tracheids overlap. Vessels are shorter and have larger diameters with non-tapered ends. Water passes

from one vessel to the next through perforations, areas devoid of any cell wall.

D. The circular arrangement of vascular bundles is typical of a herbaceous dicot stem. In a monocot stem,

vascular bundles are generally scattered throughout the pith. In a mature woody dicot, secondary tissue would be

present and appear as rings of xylem that are produced seasonally. In roots, an endodermis would be present.

The transfer of alleles or genes from one population to another. Migration into or out of a population may be responsible for a marked change in allele frequencies.

The act of leaving one's own country to settle permanently in another. Emigrating "from" somewhere

The exiting from a region

The migration seen as the settling in one region. Immigrating "to" a placee

D. RuBP carboxylase fixes O2 as well as CO2. The Calvin cycle occurs when CO2 is combined with RuBP. When

O2 combines with RuBP, photorespiration occurs. As O2 concentration increases, more O2 and less CO2 is fixed.

E. A CO2 uptake of less than zero means that CO2 is being released. This occurs when the CO2 concentration is

so low that photosynthesis cannot be supported and cellular respiration begins.

E. Transformation is the process that describes the absorption of DNA by bacteria that is subsequently expressed.

Bacteria can also acquire foreign DNA through viruses (transduction) or from other bacteria (conjugation).

Do all cells have a nucleus?

All cells have hereditary material (DNA), but not all cells have a membrane bound nucleus.

-In eukaryotic cells, the cell nucleus serves to protect the DNA of the organism

-Prokaryotic cells do not have a centralized nucleus and do not have many of the other cell organelles that eukaryotic cells have with the exceptions of ribosomes. They instead have a nucleoid region

Neurodegenerative diseases characterized by excessive apoptosis

Alzheimer's, Parkinson's, and Huntington's diseases

If a scientist wants to detach a peripheral membrane protein from the exterior of a cell membrane, what would be the best method to do so?

Change the salt concentration

-Peripheral membrane proteins are held in place by electrostatic interactions and hydrogen bonding. They are generally hydrophilic. Changing the salt concentration or the pH would disrupt both of these types of bonds and release the peripheral membrane protein from the cell membrane.

How are integral proteins extracted?

A detergent is added. Usually a hydrophobic detergent will destroy the membrane and expose the hydrophobic integral protein.

This used ammonia, methane, water and hydrogen sealed in a sterile arrangement of tubes and flasks with connecting loops.

Three different methods for particles to get through the cell membrane

1. Simple diffusion: particles are able to move directly through the phospholipid bilayer-- very small and uncharged particles

2. Facilitated diffusion: particles are able to cross the membrane but with the help of integral proteins that span the length of the cell membrane

3. Active transport: Occurs when particles are pumped or forced across the membrane against their concentration gradient. This transport requires ATP or energy.

Muscle cells and microfilaments

Muscles are made of long chains of cytoskeleton comprised of two filaments- actin and myosin. Of these, actin is a microfilament, while myosin is a motor protein. If actin degenerates, then our muscles would not contract.

Occurs when an inhibitor is able to prevent the enzyme from binding with the reactant by binding to the enzyme at a site away from the active site, and change the enzyme's conformation so it cannot bind to the reactant.

Occurs when the inhibitor competes directly with the reactant at the active site, and this substrate takes the place of the reactant and prevents the reaction from occuring

Small short "hairs" called fimbriae on the surface of bacteria that can be used in the exchange of genetic material between bacteria and in cell adhesion.

A long "tail" made of flagellin that provides locomotion to a bacterial cell

A receptor protein on the surface of a cell

Are only found on gram-positive bacteria and help keep the cell wall rigid

Amount of CO2 and resulting rate of photosynthesis

As a plant performs photosynthesis, the amount of Co2 present should decrease over time as the plant consumes the carbon to make glucose.

Is glycolysis exergonic or endergonic?

It requires the use of energy when the glucose molecule is broken into two pyruvates. The two steps in which ATP is used can be considered endergonic however, overall glycolysis produces energy to be consumed by cells. If energy is released, then the reaction is exergonic.

What type of microscope is used to view the following?

Transmission electron microscope

You can tell that this is a micrograph was taken with a transmission electron microscope because it is a very magnified 2D image of a single bacterla cell, which is very small. A scanning electron microscope would produce a 3D image

Compound light microscope

This is a 100x magnification compound light micrograph of Meissner’s corpuscle at the tip of a dermal papillus. These images often need to be stained with a colored dye to make them visible

This is another name for dissection microscopes, which only offer low magnification to observe the surface of a specimen

Transmission electron microscope

This is a transmission electron micrograph (TEM) of poliovirus, each measuring just 30 nm across. Notice how the TEM micrograph is flat, 2D, and extremely magnified.

This is a photo of a human lymphocyte nucleus from fluorescence microscopy. Fluorescence microscopes produce colorful images by dying the specimen with fluorophores and illuminating them with a specific wavelength of light. Notice how the image is brightly colored, with parts of the nucleus marked green and red with different fluorophores.

Scanning electron microscope

This is a scanning electron micrograph (SEM) of normal circulating human blood. Notice that the SEM micrograph is a 3D image at an extremely high magnification, allowing you to study the morphology and surface of the specimen.

This is a feature that will eventually develop into a part of the spinal discs.

-Complete digestive systems

-Triploblasts with bilateral symmetry

-They are often parasitic and contain a thick protective out layer known as the cuticle

Examples: round worms, hook worms, and C elegans

-Complete digestive system

-Triploblasts with bilateral symmetry

-Coelomates with segmented bodies

-Closed circulatory systems

Examples: earthworms and leeches

-Complete digestive systems

-Triploblasts with bilateral symmetry

-Coelomates with open circulatory systems (except for cephalopods with closed circulatory systems)

Examples: clams, snails, squids, and octupuses

-Complete digestive system

-Triploblasts with radial symmetry as adults

-Coelomates with open circulatory systems

-Deuterostomes (like chordata)

-Bilaterally symmetrical as larvae and radial as adults

Examples: starfish, sea urchins, and sea cucumbers

-Do not have a complete digestive system

-Have a gastrovascular cavity in which two way digestion takes place, rather than the one way digestion through an alimentary canal.

-Triploblasts with bilateral symmetry

Examples: flatwormms, tapeworms, and flukes

Fungi cell wall are made of glucans and chitin

-Only organisms to contain both in its cell walls.

refers to immunity where antibodies are generated by the individual themselves in response to a perceived immune threat

Passive immunity refers to immunity where antibodies are generated by one individual and then transferred to another. When a mother breastfeeds her newborn infant and transfers her antibodies to it in the process, it is passive immunity

Natural immunity refers to when an immune response is generated by natural means (as opposed to an artificial method, such as the use of a vaccine).

Artificial immunity refers to when an immune response is generated by artificial means, such as in vaccination where antigenic material is intentionally introduced to cause an immune response.

Permanent immunity refers to the same concept as secondary response in immunity: having been previously infected by a certain antigen, such as a bacteria or virus, the body will be able to quickly recognize and mount an immune response to the same antigen (much faster than during the initial exposure which results in the primary response).

A reflex is the involuntary, rapid response to a stimulus. This does not relate to the sustained contraction of muscles

Most reflex arcs in humans synapse directly in the spinal cord, rather than integrating in the brain first (allowing for a faster response time). An example of a reflex is the knee-jerk/patellar reflex, which you may remember from a checkup at the doctor: when the patellar tendon below the knee is tapped, the leg reflexively kicks outward.

Tetanus describes a continued state of muscle contraction during which a muscle does not relax. During tetanus, the frequency of action potentials is so high that tension is maintained throughout the muscle. Tetanus can also be used to describe the infection caused by the bacteria Clostridium tetani (often associated with rusty metallic objects), which causes muscle spasms of the jaw (hence the term “lockjaw”) that can spread across the body.

Refraction in biology refers to the refractory period, the time after an action potential during which a neuron will not respond to new stimulus – a muscle cell would not be able to maintain contraction during refraction Once the Na+/K+ pumps of the cell return ions to their resting potential balance, the refractory period will end and the neuron can once again respond to an action potential. Refractory periods can be absolute or relative – during an absolute refractory period, a second stimulus cannot generate another action potential no matter how powerful it is but during a relative refractory period, a sufficiently powerful stimulus can cause an action potential to occur.

Activation in biology can refer generally to the initiation of a biological process, or in immunology to the triggering of proliferation, differentiation, and maturation of defensive cells (e.g. the activation of T-lymphocytes by antigen presenting cells).

a type of simple muscle response caused by one action potential, which produces a single contraction and then complete relaxation. Since the muscle relaxes before another contraction is produced and not sustained continuously. In contrast to twitch contractions, tetanic contractions (tetanus) involve action potentials so frequent that the contraction is maintained before relaxation can occur, resulting in a sustained contracted state.

Beta cells in the pancreas

Beta cells secrete insulin, which functions to lower blood glucose levels.

G cells secrete the peptide hormone gastrin, which passes into the blood and stimulates the parietal cells of the stomach to secrete acid (HCl) for digestion.

Spermatogonia of the testes

Spermatogonia are located in the seminiferous tubules of the testes and undergo mitosis to produce the diploid primary spermatocytes.

a catecholamine, a class of peptidehormones. While the catecholamines are water-soluble, they are not steroids or otherwise derived from cholesterol. Epinephrine is released from the adrenal medulla and is sometimes referred to as adrenaline. It functions in “fight or flight” response and raises blood glucose levels. It causes vasoconstriction to internal organs and the skin, but causes vasodilation to the skeletal muscles and increases the respiratory and heart rate.

a mineralocorticoid, which are a class of steroid hormones. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum Aldosterone (released from the adrenal cortex) acts on the distal convoluted tubule and collecting duct of the kidney to increase reabsorption of Na + and excretion of K + . This leads to passive reabsorption of water in the nephron, which causes blood volume and blood pressure to rise.

a glucocorticoid, which are a class of steroidhormones. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum Cortisol is released from the adrenal cortex and primarily raises blood glucose levels. It is a stress hormone.

a gonadal steroid hormone. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum Testosterone is primarily produced by the interstitial cells of the testicles. Testosterone functions in spermatogenesis and is responsible for male secondary sex characteristics.

a gonadal steroid hormone. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum

Progesterone is produced by the ovaries (later in pregnancy, the placenta also produces progesterone) and functions in the menstrual cycle and the development and maintenance of the endometrial wall and fetus. Birth control pills frequently use high doses of progesterone (or progesterone and estrogen together) to cause negative feedback that suppresses LH and FSH levels, which in turn prevents ovulation from occurring.

A marine fish is hypoosmostic to its environment, meaning that it is less salty than the concentrated saltwater surrounding it. Therefore, it will constantly lose water to the environment. To make up for this, the marine fish must constantly drink water. It also rarely urinates to not waste any water, and it secretes the salts it acquires from constantly drinking.

In contrast, freshwater fish are hyperosmotic, or saltier than their environment. Therefore, water will constantly flow into the fish. The fish must constantly urinate to get rid of the excess water. It also rarely drinks, and absorbs salt through its gills to maintain homeostasis.

Fish in freshwater environments:

  1. Are hyperosmotic relative to their environment
  2. Drink very little water
  3. Salt enters the gills via active transport
  4. Produce large volume of urine

In contrast, fish in saltwater environments (i.e. marine fish):

  1. Are hypoosmotic relative to their environment
  2. Constantly drink
  3. Salt leaves the gills via active transport
  4. Produce low volume of urine

The blastopore or the opening in the archenteron (the primitive gut that forms during gastrulation) gives rise to the anus

Cleavage: radial and indeterminate

Coelom formation: folds of archenteron form coelom

Fate of blastopore: Blastopore forms the anus

Cleavage: spiral and determinate

Coelom formation: solid masses of mesoderm split and form coelom

Fate of the blastopore: Blastopore forms the mouth

Eventually form the placenta, but the villi are finger-like sections that burrow into the wall of the uterus near the mother's blood vessel

Disposes of wastes, and forms part of the umbilical cord to carry waste away from the embryo and towards the mother's blood vessels

A thin sac that surrounds the embryo and produces amniotic fluid to provide cushion for growing embryo

In placental mammals the yolk sac is completely empty and contains no yolk. Instead, one of its major functions is to aid in the formation of developing red blood cells.

Sertoli cells are cells located in the male testes and are primarily important for nourishing spermatozoa. Action of Sertoli cells are activated by follicle stimulating hormone (FSH).

The endometrium is the mucous membrane lining of the uterus that is shed during mammalian menstruation. During pregnancy, the developing embryo is implanted within the walls of the endometrium. This helps to shelter the embryo/fetus and will eventually give rise to the placenta.

In complete dominance, one allele completely masks the expression of the other (recessive) allele.

An example of complete dominance is Huntington’s disease, a degenerative nervous system disorder. If an individual possesses one allele for Huntington’s, then the condition will occur regardless of the other allele.

n incomplete dominance, neither allele is fully expressed For example, if a flower possessed one allele for red and a second allele for white, the resulting outcome would be pink if these alleles showed incomplete dominance.

The presence of the AB blood group indicates that both alleles are being expressed simultaneously. In AB type blood, both A and B antigens are expressed on the surface of red blood cells (as a result, no antibodies against either antigen are found in the plasma, and individuals with AB type blood are therefore universal recipients – then can receive blood from any other blood type). Another hypothetical example of codominance: if a flower possessed one allele for red and a second allele for white, and these two alleles were codominant, the resulting outcome would be a flower that had patches of both red and white color.

To confirm the genetic similarity or difference between organisms, which of the following biotechnology processes should be used?

Gel electrophoresis

Gel electrophoresis is a biotechnology process that allows for the separation of DNA, RNA, or proteins on the basis of size and charge (shorter molecules move further). If multiple samples are loaded, they can be compared to determine genetic similarities and differences

In the process of gel electrophoresis of DNA, the DNA is first cut up into pieces using a restriction enzyme. It is then loaded into an agarose gel under an electric field for the separation of DNA based on charge and size (the negatively charged DNA moves towards the positive anode, away from the negative cathode). The DNA is subsequently distributed by size and can be compared to the size of known standard samples and other samples from different sources for comparison. After electrophoresis, the DNA can then be sequenced, or probed to identify the location of a specific sequence of DNA.

Vectors are the vehicles used to transfer foreign genetic material into a cell.

Cloning is the biotechnology process by which the DNA of an organism is copied and maintained separately. It can refer to gene cloning, in which a gene of interest from an organism is replicated and then typically inserted into a plasmid and then introduced into another organism such as bacteria where the plasmid will be replicated so that multiple copies of the gene or genes will be available. It can also refer to the duplication of an entire organism.

Phrenology is a defunct field of study that aimed to deduce a person’s mental abilities and personality based on the shape and measurements of their skull. Phrenology is now considered a pseudoscience, and has little to no scientific validity.

A cladogram is a diagram that shows the evolutionary relationship among organisms based on either morphological characteristics (e.g. differences in physical structures, such as the presence of a notochord or the presence of fins) or molecular characteristics (e.g. differences in DNA sequence). A cladogram is not a process in biotechnology, and is not specific enough to confirm genetic differences between individual organisms

Polymerase Chain Reaction

Polymerase chain reaction (commonly abbreviated as PCR) is a biotechnology process that uses a synthetic primer, nucleotides, and a polymerase enzyme to clone DNA in a way that can rapidly amplify it. This technique is not used as a diagnostic tool therefore the answer choice is incorrect. PCR consists of three main steps:

  1. Denaturation (>90C)
  2. Addition of primers + Annealing (

DNA fingerprinting is a technique used to identify individuals (e.g. in paternity and forensic cases) based on aspects of their DNA unique to them such as short tandem repeats (STR’s). Since the number of STR’s tends to vary significantly in the population, the DNA of an individual (e.g. a suspect in a crime) can be compared to the DNA of a sample (e.g. blood left at the scene of a crime) for a positive match.

Northern blotting is a technique to identify fragments of known RNA sequence in a large population of RNA. First, the fragments of RNA containing the known sequence are put through electrophoresis to separate them by size and charge. Next, the RNA strands are separated into single strands (usually with NaOH) and then the single-stranded fragments are transferred to a nitrocellulose membrane. At this point a probe is added which will hybridize to the known sequence of RNA and mark it with some visual tag, usually fluorescence.

Southern blotting is similar to Northern blotting, but is used on DNA instead of RNA.

similar technique for proteins

Mnemonic for remembering lab techniques

S = Southern blotting -> DNA = D
N = Northern blotting -> RNA = R
O = O = O (nothing)
W = Western blotting -> protein = P

is a general condition describing a situation where the genome has an extra or missing chromosome number, often caused by nondisjunction. If nondisjunction were to occur during meiosis II, and a pair of sister chromatids failed to separate, the resulting two gametes would be produced: one that has an extra chromosome (n + 1) and one that is missing a chromosome (n – 1). As a result, after fertilization, a zygote with aneuploidy would result

Tetraploid refers to the number of sets of chromosomes, specifically four sets (4n). Cells with more than two sets of homologous chromosomes (such as triploids and tetraploids) are said to exhibit polyploidy which is common in plants.

Disomic refers to the state of having two sets of chromosomes – it can be thought of as interchangeable with the term diploid. The disomic state is standard in humans, and is not the result of nondisjunction during meiosis II and subsequent fertilization

A mix between salt and fresh water, which would be found in an estuary. An estuary is a specific area where freshwater meets seawater. A mangrove swamp often grows near an estuary and is characterized by a mix of salt and freshwater

In commensalism, a form of symbiosis, one of the two organisms benefits while the other remains unaffected. Examples of commensalism include barnacles and whales (the barnacle gets wider feeding opportunities as a result of being attached to the whale, while the whale is unaffected).

Allelopathy is the production of biochemicals by an organism that influences the growth, survival, and reproduction of other organisms. Allelopathy is a form of interference competition, which occurs directly between individuals via aggression. In interference competition, other individuals are directly prevented from physically establishing themselves on a shared habitat.

Exploitation competition is a type of competition that occurs indirectly through depletion of a common resource. For example, lions and cheetahs face exploitation competition in Africa as both hunt for a common resource: the gazelle. If cheetahs were more successful and ate all the gazelles, lions would suffer from depletion of the food resource.

Apparent competition is a type of competition that occurs between two species preyed upon by the same predator. For example, say a species of spider and a species of beetle are both hunted by owls and the amount of spiders suddenly increased. This would lead to survival of more owls (due to the increased food resource of spiders), which would in turn hunt more of the beetles, ultimately decreasing their overall number.

Intraspecific competition is a type of competition that occurs between members of the same species.

The deciduous forest biome is characterized by cold (but not particularly harsh) winters, warm summers, and moderate levels of precipitation. It has deciduous trees that shed their leaves during the winter, not coniferous trees Due to the shedding of leaves, the soil in deciduous forests is rich. This biome is characterized by vertical stratification (plants and animals live on the ground, in low branches, and high in treetops).

The savanna biome is characterized by warm temperature year-round, with some small seasonal variation. There is very little precipitation in terms of rainfall, and the dry season can last many months each year. Plants in this biome consist of grasses and scattered trees with small leaves. Animals in this biome consist primarily of large plant-eating mammals (e.g. zebras) and their predators (e.g. hyenas).

The tundra biome is characterized by cold winters (to the point that the top layer of soil freezes). In the summer, the top layer thaws, but deeper soil (permafrost) remains frozen year-round. The summers are still relatively cold (generally average less than 50° F), and there is very little precipitation or vegetation Plants in this biome consist of shrubs, grasses, mosses, and lichens (permafrost restricts the growth of plant roots). Animals in this biome include musk oxen, caribou, arctic hares, and arctic foxes.

The taiga biome (sometimes referred to as boreal forests), located south of the tundra biome, is the largest terrestrial biome. It is characterized by very long, harsh winters and precipitation in the form of heavy snow, along with short rainy and humid summers. The primary form of vegetation is coniferous forests.

The chaparral biome is characterized by highly seasonal precipitation, with rainy winters and dry summers. The scattered vegetation in this biome consists primarily of shrubs, grasses, and herbs. Animals include deer and goats. The chaparral biome is found along the California coastline, and many California fires happen here.

True statements about plant cell organelles

-Most of the cytoplasmic volume is occupied by a single vacoule

-Plant cell walls are composed of cellulose and function in maintaing cell shape

-Adjacent plant cells contain channels allowing for intercellular communication.

-Plant cells have mitochondria, but do not have a centriole:: they use mitochondria to convert the glucose they produce into ATP.

-Plant cells have both a cell wall and a cell membrane

Which of the following is a reason for why the intracellular binding of the steroid hormone testosterone is slow acting?

Steroids up regulate genes which must be transcribed and translated.

-They act as transcription factors. There is only an effect seen once the mRNA has been translated to protein, which is a slow process.

-Steroid hormones bind directly to the DNA and do not require second messengers

-Steroid hormones are non-polar and pass through the membrane

They aid in providing cell-cell adhesion and mechanical stability

Form a seal to prevent the passage of material between cells

All for passage of ions and small molecules while preventing the cytoplasm of adjacent cells from mixing

Intercalated discs in the heart

Narrow tunnels between plant cells that allow for exchange of material through cytoplasm around a narrow tube of the ER known as the desmotubule

What protein are the microfilaments of the cytoskeleton composed of?

Arranges to make up microtubules

A regulatory protein in skeletal muscle cells that prevents myosin from binding to actin.

Arranges to make up intermediate filaments

-A parasite that infects other cells and uses their machinery to survice

-The viral coats are made up of protein subunits called capsomeres, forming the capsid.

-It has no cell wall, no plasma membrane, nor any organelles.

-Can replicate using the lysogenic or lytic cycles

In the lysogenic cycle the virus binds to the host and inserts its viral DNA into the host cells’ DNA chromosome. The viral DNA will be replicated whenever chromosomal DNA is replicated. The virus is considered dormant and does not harm the host while in the lysogenic stage.

In the lytic cycle the virus attaches to a host, inserts its DNA into that host, and takes over the host cell’s machinery. This includes making many copies of viral DNA and translating viral proteins. The many virions then break out of the host cell, destroying the host cell in the process.

C3 photosynthesis is called C3 because CO2 will become a three carbon (C3) compound. This is conventional photosynthesis, and is what I discussed above. We can contrast this with C4 photosynthesis as well as CAM photosynthesis. C4 and CAM photosynthesis are both techniques that are used to prevent photorespiration.

This method is called C4 photosynthesis because the CO2 molecule integrates into and becomes a four carbon compound first, before it bonds to RuBisCO later on.

PEP carboxylase has even lower affinity for O2 compared to RuBisCO, so even in the presence of O2, it is very unlikely to bind to oxygen. This is an advantage.

The enzyme PEP carboxylase takes CO2 and converts it into oxaloacetate. Oxaloacetate quickly turns into malic acid. Both oxaloacetate and malic acid are four carbon compounds, hence the name C4.

The malic acid will be transferred from the mesophyll cells where PEP carboxylase reaction has occurred, to the bundle sheath cells.

The bundle sheath cells are located in a different area in the leaf anatomy (they surround the vascular bundles of plants), where O2 concentration is much lower.

Here the malic acid can be decarboxylated to release CO2. The CO2 can now undergo the conventional Calvin cycle with RuBisCO, in an environment where O2is not as prevalent, and RuBisCO has low risk of photorespiration.

The C4 photosynthesis process isolates CO2 spatially. Spatial isolation means that CO2 is transported to a different location (a different space) to prevent photorespiration. C4 photosynthesis transports the CO2 to the bundle sheath cells.

CAM plants have a different method of isolation. CAM plants use temporal isolation. Temporal isolation is an isolation based on timing, as a means of preventing photorespiration. There is no spatial separation: the processes occur in the same part of the leaf, but the plant does different processes at different times.

During the day, CAM plants close stomata to prevent excessive loss of water, via transpiration, evaporation out of the stomates (keep in mind this will also limit new gases like O2 from entering the plant). At night, CAM plants have their stomata open, allowing CO2 to enter into the leaf. The same enzyme in C4 photosynthesis is used in CAM photosynthesis: PEP carboxylase will fixate CO2into a four carbon molecule of oxaloacetate which converts into malic acid. In contrast, to the C4 pathway, rather than shuttle the malic acid to a different part of the leaf, the malic acid will be stored in a vacuole, for later use.

During the day, the malic acid will be shuttled out of the vacuole, CO2 will be decarboxylated from the malic acid, and the typical Calvin cycle will occur in a low O2 environment (stomates are closed). During the day the sun shines brightly, and ATP as well as NADPH are being produced plentifully.


Contents

In many small organisms such as bacteria, quorum sensing enables individuals to begin an activity only when the population is sufficiently large. This signaling between cells was first observed in the marine bacterium Aliivibrio fischeri, which produces light when the population is dense enough. [10] The mechanism involves the production and detection of a signaling molecule, and the regulation of gene transcription in response. Quorum sensing operates in both gram-positive and gram-negative bacteria, and both within and between species. [11]

In slime moulds, individual cells known as amoebae aggregate together to form fruiting bodies and eventually spores, under the influence of a chemical signal, originally named acrasin. The individuals move by chemotaxis, i.e. they are attracted by the chemical gradient. Some species use cyclic AMP as the signal others such as Polysphondylium violaceum use other molecules, in its case N-propionyl-gamma-L-glutamyl-L-ornithine-delta-lactam ethyl ester, nicknamed glorin. [12]

In plants and animals, signaling between cells occurs either through release into the extracellular space, divided in paracrine signaling (over short distances) and endocrine signaling (over long distances), or by direct contact, known as juxtacrine signaling (e.g., notch signaling). [13] Autocrine signaling is a special case of paracrine signaling where the secreting cell has the ability to respond to the secreted signaling molecule. [14] Synaptic signaling is a special case of paracrine signaling (for chemical synapses) or juxtacrine signaling (for electrical synapses) between neurons and target cells.

Synthesis and release Edit

Many cell signals are carried by molecules that are released by one cell and move to make contact with another cell. Signaling molecules can belong to several chemical classes: lipids, phospholipids, amino acids, monoamines, proteins, glycoproteins, or gases. Signaling molecules binding surface receptors are generally large and hydrophilic (e.g. TRH, Vasopressin, Acetylcholine), while those entering the cell are generally small and hydrophobic (e.g. glucocorticoids, thyroid hormones, cholecalciferol, retinoic acid), but important exceptions to both are numerous, and a same molecule can act both via surface receptors or in an intracrine manner to different effects. [14] In animal cells, specialized cells release these hormones and send them through the circulatory system to other parts of the body. They then reach target cells, which can recognize and respond to the hormones and produce a result. This is also known as endocrine signaling. Plant growth regulators, or plant hormones, move through cells or by diffusing through the air as a gas to reach their targets. [15] Hydrogen sulfide is produced in small amounts by some cells of the human body and has a number of biological signaling functions. Only two other such gases are currently known to act as signaling molecules in the human body: nitric oxide and carbon monoxide. [16]

Exocytosis Edit

Exocytosis is the process by which a cell transports molecules such as neurotransmitters and proteins out of the cell. As an active transport mechanism, exocytosis requires the use of energy to transport material. Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic portion of the cell membrane by passive means. Exocytosis is the process by which a large amount of molecules are released thus it is a form of bulk transport. Exocytosis occurs via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.

In exocytosis, membrane-bound secretory vesicles are carried to the cell membrane, where they dock and fuse at porosomes and their contents (i.e., water-soluble molecules) are secreted into the extracellular environment. This secretion is possible because the vesicle transiently fuses with the plasma membrane. In the context of neurotransmission, neurotransmitters are typically released from synaptic vesicles into the synaptic cleft via exocytosis however, neurotransmitters can also be released via reverse transport through membrane transport proteins.

Forms Edit

Autocrine Edit

Autocrine signaling involves a cell secreting a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on that same cell, leading to changes in the cell itself. [17] This can be contrasted with paracrine signaling, intracrine signaling, or classical endocrine signaling.

Paracrine Edit

In paracrine signaling, a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse over a relatively short distance (local action), as opposed to cell signaling by endocrine factors, hormones which travel considerably longer distances via the circulatory system juxtacrine interactions and autocrine signaling. Cells that produce paracrine factors secrete them into the immediate extracellular environment. Factors then travel to nearby cells in which the gradient of factor received determines the outcome. However, the exact distance that paracrine factors can travel is not certain.

Paracrine signals such as retinoic acid target only cells in the vicinity of the emitting cell. [18] Neurotransmitters represent another example of a paracrine signal.

Some signaling molecules can function as both a hormone and a neurotransmitter. For example, epinephrine and norepinephrine can function as hormones when released from the adrenal gland and are transported to the heart by way of the blood stream. Norepinephrine can also be produced by neurons to function as a neurotransmitter within the brain. [19] Estrogen can be released by the ovary and function as a hormone or act locally via paracrine or autocrine signaling. [20]

Although paracrine signaling elicits a diverse array of responses in the induced cells, most paracrine factors utilize a relatively streamlined set of receptors and pathways. In fact, different organs in the body - even between different species - are known to utilize a similar sets of paracrine factors in differential development. [21] The highly conserved receptors and pathways can be organized into four major families based on similar structures: fibroblast growth factor (FGF) family, Hedgehog family, Wnt family, and TGF-β superfamily. Binding of a paracrine factor to its respective receptor initiates signal transduction cascades, eliciting different responses.

Endocrine Edit

Endocrine signals are called hormones. Hormones are produced by endocrine cells and they travel through the blood to reach all parts of the body. Specificity of signaling can be controlled if only some cells can respond to a particular hormone. Endocrine signaling involves the release of hormones by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. In humans, the major endocrine glands are the thyroid gland and the adrenal glands. The study of the endocrine system and its disorders is known as endocrinology.

Juxtacrine Edit

Juxtacrine signaling is a type of cell–cell or cell–extracellular matrix signaling in multicellular organisms that requires close contact. There are three types:

  1. A membrane ligand (protein, oligosaccharide, lipid) and a membrane protein of two adjacent cells interact.
  2. A communicating junction links the intracellular compartments of two adjacent cells, allowing transit of relatively small molecules.
  3. An extracellular matrixglycoprotein and a membrane protein interact.

Additionally, in unicellular organisms such as bacteria, juxtacrine signaling means interactions by membrane contact. Juxtacrine signaling has been observed for some growth factors, cytokine and chemokine cellular signals, playing an important role in the immune response.

Cells receive information from their neighbors through a class of proteins known as receptors. Receptors may bind with some molecules (ligands) or may interact with physical agents like light, mechanical temperature, pressure, etc. Reception occurs when the target cell (any cell with a receptor protein specific to the signal molecule) detects a signal, usually in the form of a small, water-soluble molecule, via binding to a receptor protein on the cell surface, or once inside the cell, the signaling molecule can bind to intracellular receptors, other elements, or stimulate enzyme activity (e.g. gasses), as in intracrine signaling.

Signaling molecules interact with a target cell as a ligand to cell surface receptors, and/or by entering into the cell through its membrane or endocytosis for intracrine signaling. This generally results in the activation of second messengers, leading to various physiological effects. In many mammals, early embryo cells exchange signals with cells of the uterus. [22] In the human gastrointestinal tract, bacteria exchange signals with each other and with human epithelial and immune system cells. [23] For the yeast Saccharomyces cerevisiae during mating, some cells send a peptide signal (mating factor pheromones) into their environment. The mating factor peptide may bind to a cell surface receptor on other yeast cells and induce them to prepare for mating. [24]

Cell surface receptors Edit

Cell surface receptors play an essential role in the biological systems of single- and multi-cellular organisms and malfunction or damage to these proteins is associated with cancer, heart disease, and asthma. [25] These trans-membrane receptors are able to transmit information from outside the cell to the inside because they change conformation when a specific ligand binds to it. By looking at three major types of receptors: Ion channel linked receptors, G protein–coupled receptors, and enzyme-linked receptors).

Ion channel linked receptors Edit

Ion channel linked receptors are a group of transmembrane ion-channel proteins which open to allow ions such as Na + , K + , Ca 2+ , and/or Cl − to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter. [26] [27] [28]

When a presynaptic neuron is excited, it releases a neurotransmitter from vesicles into the synaptic cleft. The neurotransmitter then binds to receptors located on the postsynaptic neuron. If these receptors are ligand-gated ion channels, a resulting conformational change opens the ion channels, which leads to a flow of ions across the cell membrane. This, in turn, results in either a depolarization, for an excitatory receptor response, or a hyperpolarization, for an inhibitory response.

These receptor proteins are typically composed of at least two different domains: a transmembrane domain which includes the ion pore, and an extracellular domain which includes the ligand binding location (an allosteric binding site). This modularity has enabled a 'divide and conquer' approach to finding the structure of the proteins (crystallising each domain separately). The function of such receptors located at synapses is to convert the chemical signal of presynaptically released neurotransmitter directly and very quickly into a postsynaptic electrical signal. Many LICs are additionally modulated by allosteric ligands, by channel blockers, ions, or the membrane potential. LICs are classified into three superfamilies which lack evolutionary relationship: cys-loop receptors, ionotropic glutamate receptors and ATP-gated channels.

G protein–coupled receptors Edit

G protein-coupled receptors are a large group of evolutionarily-related proteins that are cell surface receptors that detect molecules outside the cell and activate cellular responses. Coupling with G proteins, they are called seven-transmembrane receptors because they pass through the cell membrane seven times. [29] Ligands can bind either to extracellular N-terminus and loops (e.g. glutamate receptors) or to the binding site within transmembrane helices (Rhodopsin-like family). They are all activated by agonists although a spontaneous auto-activation of an empty receptor can also be observed. [29]

G protein-coupled receptors are found only in eukaryotes, including yeast, choanoflagellates, [30] and animals. The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. G protein-coupled receptors are involved in many diseases.

There are two principal signal transduction pathways involving the G protein-coupled receptors: cAMP signal pathway and phosphatidylinositol signal pathway. [31] When a ligand binds to the GPCR it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor (GEF). The GPCR can then activate an associated G protein by exchanging the GDP bound to the G protein for a GTP. The G protein's α subunit, together with the bound GTP, can then dissociate from the β and γ subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the α subunit type (Gαs, Gαi/o, Gαq/11, Gα12/13). [32] : 1160

G protein-coupled receptors are an important drug target and approximately 34% [33] of all Food and Drug Administration (FDA) approved drugs target 108 members of this family. The global sales volume for these drugs is estimated to be 180 billion US dollars as of 2018 [update] . [33] It is estimated that GPCRs are targets for about 50% of drugs currently on the market, mainly due to their involvement in signaling pathways related to many diseases i.e. mental, metabolic including endocrinological disorders, immunological including viral infections, cardiovascular, inflammatory, senses disorders, and cancer. The long ago discovered association between GPCRs and many endogenous and exogenous substances, resulting in e.g. analgesia, is another dynamically developing field of the pharmaceutical research. [29]

Enzyme-linked receptors Edit

Enzyme-linked receptors (or catalytic receptors) are transmembrane receptor that, upon activation by an extracellular ligand, causes enzymatic activity on the intracellular side. [34] Hence a catalytic receptor is an integral membrane protein possessing both enzymatic, catalytic, and receptor functions. [35]

They have two important domains, an extra-cellular ligand binding domain and an intracellular domain, which has a catalytic function and a single transmembrane helix. The signaling molecule binds to the receptor on the outside of the cell and causes a conformational change on the catalytic function located on the receptor inside the cell. Examples of the enzymatic activity include:

Intracellular receptors Edit

Steroid hormone receptor Edit

Steroid hormone receptors are found in the nucleus, cytosol, and also on the plasma membrane of target cells. They are generally intracellular receptors (typically cytoplasmic or nuclear) and initiate signal transduction for steroid hormones which lead to changes in gene expression over a time period of hours to days. The best studied steroid hormone receptors are members of the nuclear receptor subfamily 3 (NR3) that include receptors for estrogen (group NR3A) [37] and 3-ketosteroids (group NR3C). [38] In addition to nuclear receptors, several G protein-coupled receptors and ion channels act as cell surface receptors for certain steroid hormones.

When binding to the signaling molecule, the receptor protein changes in some way and starts the process of transduction, which can occur in a single step or as a series of changes in a sequence of different molecules (called a signal transduction pathway). The molecules that compose these pathways are known as relay molecules. The multistep process of the transduction stage is often composed of the activation of proteins by addition or removal of phosphate groups or even the release of other small molecules or ions that can act as messengers. The amplifying of a signal is one of the benefits to this multiple step sequence. Other benefits include more opportunities for regulation than simpler systems do and the fine- tuning of the response, in both unicellular and multicellular organism. [15]

In some cases, receptor activation caused by ligand binding to a receptor is directly coupled to the cell's response to the ligand. For example, the neurotransmitter GABA can activate a cell surface receptor that is part of an ion channel. GABA binding to a GABAA receptor on a neuron opens a chloride-selective ion channel that is part of the receptor. GABAA receptor activation allows negatively charged chloride ions to move into the neuron, which inhibits the ability of the neuron to produce action potentials. However, for many cell surface receptors, ligand-receptor interactions are not directly linked to the cell's response. The activated receptor must first interact with other proteins inside the cell before the ultimate physiological effect of the ligand on the cell's behavior is produced. Often, the behavior of a chain of several interacting cell proteins is altered following receptor activation. The entire set of cell changes induced by receptor activation is called a signal transduction mechanism or pathway. [39]

A more complex signal transduction pathway is shown in Figure 3. This pathway involves changes of protein–protein interactions inside the cell, induced by an external signal. Many growth factors bind to receptors at the cell surface and stimulate cells to progress through the cell cycle and divide. Several of these receptors are kinases that start to phosphorylate themselves and other proteins when binding to a ligand. This phosphorylation can generate a binding site for a different protein and thus induce protein–protein interaction. In Figure 3, the ligand (called epidermal growth factor, or EGF) binds to the receptor (called EGFR). This activates the receptor to phosphorylate itself. The phosphorylated receptor binds to an adaptor protein (GRB2), which couples the signal to further downstream signaling processes. For example, one of the signal transduction pathways that are activated is called the mitogen-activated protein kinase (MAPK) pathway. The signal transduction component labeled as "MAPK" in the pathway was originally called "ERK," so the pathway is called the MAPK/ERK pathway. The MAPK protein is an enzyme, a protein kinase that can attach phosphate to target proteins such as the transcription factor MYC and, thus, alter gene transcription and, ultimately, cell cycle progression. Many cellular proteins are activated downstream of the growth factor receptors (such as EGFR) that initiate this signal transduction pathway. [ citation needed ]

Some signaling transduction pathways respond differently, depending on the amount of signaling received by the cell. For instance, the hedgehog protein activates different genes, depending on the amount of hedgehog protein present. [ citation needed ]

Complex multi-component signal transduction pathways provide opportunities for feedback, signal amplification, and interactions inside one cell between multiple signals and signaling pathways. [ citation needed ]

A specific cellular response is the result of the transduced signal in the final stage of cell signaling. This response can essentially be any cellular activity that is present in a body. It can spur the rearrangement of the cytoskeleton, or even as catalysis by an enzyme. These three steps of cell signaling all ensure that the right cells are behaving as told, at the right time, and in synchronization with other cells and their own functions within the organism. At the end, the end of a signal pathway leads to the regulation of a cellular activity. This response can take place in the nucleus or in the cytoplasm of the cell. A majority of signaling pathways control protein synthesis by turning certain genes on and off in the nucleus. [40]

In unicellular organisms such as bacteria, signaling can be used to 'activate' peers from a dormant state, enhance virulence, defend against bacteriophages, etc. [41] In quorum sensing, which is also found in social insects, the multiplicity of individual signals has the potentiality to create a positive feedback loop, generating coordinated response. In this context, the signaling molecules are called autoinducers. [42] [43] [44] This signaling mechanism may have been involved in evolution from unicellular to multicellular organisms. [42] [45] Bacteria also use contact-dependent signaling, notably to limit their growth. [46]

Signaling molecules used by multicellular organisms are often called pheromones. They can have such purposes as alerting against danger, indicating food supply, or assisting in reproduction. [47]

Short-term cellular responses Edit

Brief overview of some signaling pathways (based on receptor families) that result in short-acting cellular responses
Receptor Family Example of Ligands/ activators (Bracket: receptor for it) Example of effectors Further downstream effects
Ligand Gated Ion Channels Acetylcholine
(Such as Nicotinic acetylcholine receptor),
Changes in membrane permeability Change in membrane potential
Seven Helix Receptor Light(Rhodopsin),
Dopamine (Dopamine receptor),
GABA (GABA receptor),
Prostaglandin (prostaglandin receptor) etc.
Trimeric G protein Adenylate Cyclase,
cGMP phosphodiesterase,
G-protein gated ion channel, etc.
Two Component Diverse activators Histidine Kinase Response Regulator - flagellar movement, Gene expression
Membrane Guanylyl Cyclase Atrial natriuretic peptide,
Sea urching egg peptide etc.
cGMP Regulation of Kinases and channels- Diverse actions
Cytoplasmic Guanylyl cyclase Nitric Oxide(Nitric oxide receptor) cGMP Regulation of cGMP Gated channels, Kinases
Integrins Fibronectins, other extracellular matrix proteins Nonreceptor tyrosine kinase Diverse response

Regulating gene activity Edit

Brief overview of some signaling pathways (based on receptor families) that control gene activity
Frizzled (Special type of 7Helix receptor) Wnt Dishevelled, axin - APC, GSK3-beta - Beta catenin Gene expression
Two Component Diverse activators Histidine Kinase Response Regulator - flagellar movement, Gene expression
Receptor Tyrosine Kinase Insulin (insulin receptor),
EGF (EGF receptor),
FGF-Alpha, FGF-Beta, etc (FGF-receptors)
Ras, MAP-kinases, PLC, PI3-Kinase Gene expression change
Cytokine receptors Erythropoietin,
Growth Hormone (Growth Hormone Receptor),
IFN-Gamma (IFN-Gamma receptor) etc
JAK kinase STAT transcription factor - Gene expression
Tyrosine kinase Linked- receptors MHC-peptide complex - TCR, Antigens - BCR Cytoplasmic Tyrosine Kinase Gene expression
Receptor Serine/Threonine Kinase Activin(activin receptor),
Inhibin,
Bone-morphogenetic protein(BMP Receptor),
TGF-beta
Smad transcription factors Control of gene expression
Sphingomyelinase linked receptors IL-1(IL-1 receptor),
TNF (TNF-receptors)
Ceramide activated kinases Gene expression
Cytoplasmic Steroid receptors Steroid hormones,
Thyroid hormones,
Retinoic acid etc
Work as/ interact with transcription factors Gene expression

Notch signaling pathway Edit

Notch is a cell surface protein that functions as a receptor. Animals have a small set of genes that code for signaling proteins that interact specifically with Notch receptors and stimulate a response in cells that express Notch on their surface. Molecules that activate (or, in some cases, inhibit) receptors can be classified as hormones, neurotransmitters, cytokines, and growth factors, in general called receptor ligands. Ligand receptor interactions such as that of the Notch receptor interaction, are known to be the main interactions responsible for cell signaling mechanisms and communication. [52] notch acts as a receptor for ligands that are expressed on adjacent cells. While some receptors are cell-surface proteins, others are found inside cells. For example, estrogen is a hydrophobic molecule that can pass through the lipid bilayer of the membranes. As part of the endocrine system, intracellular estrogen receptors from a variety of cell types can be activated by estrogen produced in the ovaries.

In the case of Notch-mediated signaling, the signal transduction mechanism can be relatively simple. As shown in Figure 2, the activation of Notch can cause the Notch protein to be altered by a protease. Part of the Notch protein is released from the cell surface membrane and takes part in gene regulation. Cell signaling research involves studying the spatial and temporal dynamics of both receptors and the components of signaling pathways that are activated by receptors in various cell types. [53] [54] Emerging methods for single-cell mass-spectrometry analysis promise to enable studying signal transduction with single-cell resolution. [55]

In notch signaling, direct contact between cells allows for precise control of cell differentiation during embryonic development. In the worm Caenorhabditis elegans, two cells of the developing gonad each have an equal chance of terminally differentiating or becoming a uterine precursor cell that continues to divide. The choice of which cell continues to divide is controlled by competition of cell surface signals. One cell will happen to produce more of a cell surface protein that activates the Notch receptor on the adjacent cell. This activates a feedback loop or system that reduces Notch expression in the cell that will differentiate and that increases Notch on the surface of the cell that continues as a stem cell. [56]


A scientist wants to track a specific molecule formed during oxidative phosphorylation. He places a special radioactive tag on the carbons of pyruvate molecules that will only become active once pyruvate is decarboxylated. Between glycolysis and the citric acid cycle, the tag becomes active and the scientist is able to observe the molecule of interest. Which molecule is the scientist tracking?

A. ATP
B. Ethanol
C. FADH2
D. Acetyl CoA
E. lactate

[D]: Acetyl CoA

Correct. Do not be overwhelmed by the experimental format presented in the question. Instead, isolate the relevant pieces of information that are presented to you: the molecule is formed during oxidative phosphorylation, specifically in between glycolysis and the citric acid cycle. As the question reveals, this is the step of pyruvate decarboxylation, where pyruvate is decarboxylated and is converted into acetyl CoA. This process also produces NADH and CO2. Answering this question correctly requires no special knowledge about the experiment itself – only what pyruvate is converted into.


Are intercellular junctions, synapses and light-capturing photosynthetic complexes mobile? - Biology

The migration seen as the settling in one region. Immigrating "to" a placee

D. RuBP carboxylase fixes O2 as well as CO2. The Calvin cycle occurs when CO2 is combined with RuBP. When

O2 combines with RuBP, photorespiration occurs. As O2 concentration increases, more O2 and less CO2 is fixed.

E. A CO2 uptake of less than zero means that CO2 is being released. This occurs when the CO2 concentration is

so low that photosynthesis cannot be supported and cellular respiration begins.

E. Transformation is the process that describes the absorption of DNA by bacteria that is subsequently expressed.

Bacteria can also acquire foreign DNA through viruses (transduction) or from other bacteria (conjugation).

Do all cells have a nucleus?

All cells have hereditary material (DNA), but not all cells have a membrane bound nucleus.

-In eukaryotic cells, the cell nucleus serves to protect the DNA of the organism

-Prokaryotic cells do not have a centralized nucleus and do not have many of the other cell organelles that eukaryotic cells have with the exceptions of ribosomes. They instead have a nucleoid region

Neurodegenerative diseases characterized by excessive apoptosis

Alzheimer's, Parkinson's, and Huntington's diseases

If a scientist wants to detach a peripheral membrane protein from the exterior of a cell membrane, what would be the best method to do so?

Change the salt concentration

-Peripheral membrane proteins are held in place by electrostatic interactions and hydrogen bonding. They are generally hydrophilic. Changing the salt concentration or the pH would disrupt both of these types of bonds and release the peripheral membrane protein from the cell membrane.

How are integral proteins extracted?

A detergent is added. Usually a hydrophobic detergent will destroy the membrane and expose the hydrophobic integral protein.

This used ammonia, methane, water and hydrogen sealed in a sterile arrangement of tubes and flasks with connecting loops.

Three different methods for particles to get through the cell membrane

1. Simple diffusion: particles are able to move directly through the phospholipid bilayer-- very small and uncharged particles

2. Facilitated diffusion: particles are able to cross the membrane but with the help of integral proteins that span the length of the cell membrane

3. Active transport: Occurs when particles are pumped or forced across the membrane against their concentration gradient. This transport requires ATP or energy.

Muscle cells and microfilaments

Muscles are made of long chains of cytoskeleton comprised of two filaments- actin and myosin. Of these, actin is a microfilament, while myosin is a motor protein. If actin degenerates, then our muscles would not contract.

Occurs when an inhibitor is able to prevent the enzyme from binding with the reactant by binding to the enzyme at a site away from the active site, and change the enzyme's conformation so it cannot bind to the reactant.

Occurs when the inhibitor competes directly with the reactant at the active site, and this substrate takes the place of the reactant and prevents the reaction from occuring

Small short "hairs" called fimbriae on the surface of bacteria that can be used in the exchange of genetic material between bacteria and in cell adhesion.

A long "tail" made of flagellin that provides locomotion to a bacterial cell

A receptor protein on the surface of a cell

Are only found on gram-positive bacteria and help keep the cell wall rigid

Amount of CO2 and resulting rate of photosynthesis

As a plant performs photosynthesis, the amount of Co2 present should decrease over time as the plant consumes the carbon to make glucose.

Is glycolysis exergonic or endergonic?

It requires the use of energy when the glucose molecule is broken into two pyruvates. The two steps in which ATP is used can be considered endergonic however, overall glycolysis produces energy to be consumed by cells. If energy is released, then the reaction is exergonic.

What type of microscope is used to view the following?

Transmission electron microscope

You can tell that this is a micrograph was taken with a transmission electron microscope because it is a very magnified 2D image of a single bacterla cell, which is very small. A scanning electron microscope would produce a 3D image

Compound light microscope

This is a 100x magnification compound light micrograph of Meissner’s corpuscle at the tip of a dermal papillus. These images often need to be stained with a colored dye to make them visible

This is another name for dissection microscopes, which only offer low magnification to observe the surface of a specimen

Transmission electron microscope

This is a transmission electron micrograph (TEM) of poliovirus, each measuring just 30 nm across. Notice how the TEM micrograph is flat, 2D, and extremely magnified.

This is a photo of a human lymphocyte nucleus from fluorescence microscopy. Fluorescence microscopes produce colorful images by dying the specimen with fluorophores and illuminating them with a specific wavelength of light. Notice how the image is brightly colored, with parts of the nucleus marked green and red with different fluorophores.

Scanning electron microscope

This is a scanning electron micrograph (SEM) of normal circulating human blood. Notice that the SEM micrograph is a 3D image at an extremely high magnification, allowing you to study the morphology and surface of the specimen.

This is a feature that will eventually develop into a part of the spinal discs.

-Complete digestive systems

-Triploblasts with bilateral symmetry

-They are often parasitic and contain a thick protective out layer known as the cuticle

Examples: round worms, hook worms, and C elegans

-Complete digestive system

-Triploblasts with bilateral symmetry

-Coelomates with segmented bodies

-Closed circulatory systems

Examples: earthworms and leeches

-Complete digestive systems

-Triploblasts with bilateral symmetry

-Coelomates with open circulatory systems (except for cephalopods with closed circulatory systems)

Examples: clams, snails, squids, and octupuses

-Complete digestive system

-Triploblasts with radial symmetry as adults

-Coelomates with open circulatory systems

-Deuterostomes (like chordata)

-Bilaterally symmetrical as larvae and radial as adults

Examples: starfish, sea urchins, and sea cucumbers

-Do not have a complete digestive system

-Have a gastrovascular cavity in which two way digestion takes place, rather than the one way digestion through an alimentary canal.

-Triploblasts with bilateral symmetry

Examples: flatwormms, tapeworms, and flukes

Fungi cell wall are made of glucans and chitin

-Only organisms to contain both in its cell walls.

refers to immunity where antibodies are generated by the individual themselves in response to a perceived immune threat

Passive immunity refers to immunity where antibodies are generated by one individual and then transferred to another. When a mother breastfeeds her newborn infant and transfers her antibodies to it in the process, it is passive immunity

Natural immunity refers to when an immune response is generated by natural means (as opposed to an artificial method, such as the use of a vaccine).

Artificial immunity refers to when an immune response is generated by artificial means, such as in vaccination where antigenic material is intentionally introduced to cause an immune response.

Permanent immunity refers to the same concept as secondary response in immunity: having been previously infected by a certain antigen, such as a bacteria or virus, the body will be able to quickly recognize and mount an immune response to the same antigen (much faster than during the initial exposure which results in the primary response).

A reflex is the involuntary, rapid response to a stimulus. This does not relate to the sustained contraction of muscles

Most reflex arcs in humans synapse directly in the spinal cord, rather than integrating in the brain first (allowing for a faster response time). An example of a reflex is the knee-jerk/patellar reflex, which you may remember from a checkup at the doctor: when the patellar tendon below the knee is tapped, the leg reflexively kicks outward.

Tetanus describes a continued state of muscle contraction during which a muscle does not relax. During tetanus, the frequency of action potentials is so high that tension is maintained throughout the muscle. Tetanus can also be used to describe the infection caused by the bacteria Clostridium tetani (often associated with rusty metallic objects), which causes muscle spasms of the jaw (hence the term “lockjaw”) that can spread across the body.

Refraction in biology refers to the refractory period, the time after an action potential during which a neuron will not respond to new stimulus – a muscle cell would not be able to maintain contraction during refraction Once the Na+/K+ pumps of the cell return ions to their resting potential balance, the refractory period will end and the neuron can once again respond to an action potential. Refractory periods can be absolute or relative – during an absolute refractory period, a second stimulus cannot generate another action potential no matter how powerful it is but during a relative refractory period, a sufficiently powerful stimulus can cause an action potential to occur.

Activation in biology can refer generally to the initiation of a biological process, or in immunology to the triggering of proliferation, differentiation, and maturation of defensive cells (e.g. the activation of T-lymphocytes by antigen presenting cells).

a type of simple muscle response caused by one action potential, which produces a single contraction and then complete relaxation. Since the muscle relaxes before another contraction is produced and not sustained continuously. In contrast to twitch contractions, tetanic contractions (tetanus) involve action potentials so frequent that the contraction is maintained before relaxation can occur, resulting in a sustained contracted state.

Beta cells in the pancreas

Beta cells secrete insulin, which functions to lower blood glucose levels.

G cells secrete the peptide hormone gastrin, which passes into the blood and stimulates the parietal cells of the stomach to secrete acid (HCl) for digestion.

Spermatogonia of the testes

Spermatogonia are located in the seminiferous tubules of the testes and undergo mitosis to produce the diploid primary spermatocytes.

a catecholamine, a class of peptidehormones. While the catecholamines are water-soluble, they are not steroids or otherwise derived from cholesterol. Epinephrine is released from the adrenal medulla and is sometimes referred to as adrenaline. It functions in “fight or flight” response and raises blood glucose levels. It causes vasoconstriction to internal organs and the skin, but causes vasodilation to the skeletal muscles and increases the respiratory and heart rate.

a mineralocorticoid, which are a class of steroid hormones. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum Aldosterone (released from the adrenal cortex) acts on the distal convoluted tubule and collecting duct of the kidney to increase reabsorption of Na + and excretion of K + . This leads to passive reabsorption of water in the nephron, which causes blood volume and blood pressure to rise.

a glucocorticoid, which are a class of steroidhormones. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum Cortisol is released from the adrenal cortex and primarily raises blood glucose levels. It is a stress hormone.

a gonadal steroid hormone. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum Testosterone is primarily produced by the interstitial cells of the testicles. Testosterone functions in spermatogenesis and is responsible for male secondary sex characteristics.

a gonadal steroid hormone. Steroid hormones are synthesized from cholesterol in the smooth endoplasmic reticulum

Progesterone is produced by the ovaries (later in pregnancy, the placenta also produces progesterone) and functions in the menstrual cycle and the development and maintenance of the endometrial wall and fetus. Birth control pills frequently use high doses of progesterone (or progesterone and estrogen together) to cause negative feedback that suppresses LH and FSH levels, which in turn prevents ovulation from occurring.

A marine fish is hypoosmostic to its environment, meaning that it is less salty than the concentrated saltwater surrounding it. Therefore, it will constantly lose water to the environment. To make up for this, the marine fish must constantly drink water. It also rarely urinates to not waste any water, and it secretes the salts it acquires from constantly drinking.

In contrast, freshwater fish are hyperosmotic, or saltier than their environment. Therefore, water will constantly flow into the fish. The fish must constantly urinate to get rid of the excess water. It also rarely drinks, and absorbs salt through its gills to maintain homeostasis.

Fish in freshwater environments:

  1. Are hyperosmotic relative to their environment
  2. Drink very little water
  3. Salt enters the gills via active transport
  4. Produce large volume of urine

In contrast, fish in saltwater environments (i.e. marine fish):

  1. Are hypoosmotic relative to their environment
  2. Constantly drink
  3. Salt leaves the gills via active transport
  4. Produce low volume of urine

The blastopore or the opening in the archenteron (the primitive gut that forms during gastrulation) gives rise to the anus

Cleavage: radial and indeterminate

Coelom formation: folds of archenteron form coelom

Fate of blastopore: Blastopore forms the anus

Cleavage: spiral and determinate

Coelom formation: solid masses of mesoderm split and form coelom

Fate of the blastopore: Blastopore forms the mouth

Eventually form the placenta, but the villi are finger-like sections that burrow into the wall of the uterus near the mother's blood vessel

Disposes of wastes, and forms part of the umbilical cord to carry waste away from the embryo and towards the mother's blood vessels

A thin sac that surrounds the embryo and produces amniotic fluid to provide cushion for growing embryo

In placental mammals the yolk sac is completely empty and contains no yolk. Instead, one of its major functions is to aid in the formation of developing red blood cells.

Sertoli cells are cells located in the male testes and are primarily important for nourishing spermatozoa. Action of Sertoli cells are activated by follicle stimulating hormone (FSH).

The endometrium is the mucous membrane lining of the uterus that is shed during mammalian menstruation. During pregnancy, the developing embryo is implanted within the walls of the endometrium. This helps to shelter the embryo/fetus and will eventually give rise to the placenta.

In complete dominance, one allele completely masks the expression of the other (recessive) allele.

An example of complete dominance is Huntington’s disease, a degenerative nervous system disorder. If an individual possesses one allele for Huntington’s, then the condition will occur regardless of the other allele.

n incomplete dominance, neither allele is fully expressed For example, if a flower possessed one allele for red and a second allele for white, the resulting outcome would be pink if these alleles showed incomplete dominance.

The presence of the AB blood group indicates that both alleles are being expressed simultaneously. In AB type blood, both A and B antigens are expressed on the surface of red blood cells (as a result, no antibodies against either antigen are found in the plasma, and individuals with AB type blood are therefore universal recipients – then can receive blood from any other blood type). Another hypothetical example of codominance: if a flower possessed one allele for red and a second allele for white, and these two alleles were codominant, the resulting outcome would be a flower that had patches of both red and white color.

To confirm the genetic similarity or difference between organisms, which of the following biotechnology processes should be used?

Gel electrophoresis

Gel electrophoresis is a biotechnology process that allows for the separation of DNA, RNA, or proteins on the basis of size and charge (shorter molecules move further). If multiple samples are loaded, they can be compared to determine genetic similarities and differences

In the process of gel electrophoresis of DNA, the DNA is first cut up into pieces using a restriction enzyme. It is then loaded into an agarose gel under an electric field for the separation of DNA based on charge and size (the negatively charged DNA moves towards the positive anode, away from the negative cathode). The DNA is subsequently distributed by size and can be compared to the size of known standard samples and other samples from different sources for comparison. After electrophoresis, the DNA can then be sequenced, or probed to identify the location of a specific sequence of DNA.

Vectors are the vehicles used to transfer foreign genetic material into a cell.

Cloning is the biotechnology process by which the DNA of an organism is copied and maintained separately. It can refer to gene cloning, in which a gene of interest from an organism is replicated and then typically inserted into a plasmid and then introduced into another organism such as bacteria where the plasmid will be replicated so that multiple copies of the gene or genes will be available. It can also refer to the duplication of an entire organism.

Phrenology is a defunct field of study that aimed to deduce a person’s mental abilities and personality based on the shape and measurements of their skull. Phrenology is now considered a pseudoscience, and has little to no scientific validity.

A cladogram is a diagram that shows the evolutionary relationship among organisms based on either morphological characteristics (e.g. differences in physical structures, such as the presence of a notochord or the presence of fins) or molecular characteristics (e.g. differences in DNA sequence). A cladogram is not a process in biotechnology, and is not specific enough to confirm genetic differences between individual organisms

Polymerase Chain Reaction

Polymerase chain reaction (commonly abbreviated as PCR) is a biotechnology process that uses a synthetic primer, nucleotides, and a polymerase enzyme to clone DNA in a way that can rapidly amplify it. This technique is not used as a diagnostic tool therefore the answer choice is incorrect. PCR consists of three main steps:

  1. Denaturation (>90C)
  2. Addition of primers + Annealing (

DNA fingerprinting is a technique used to identify individuals (e.g. in paternity and forensic cases) based on aspects of their DNA unique to them such as short tandem repeats (STR’s). Since the number of STR’s tends to vary significantly in the population, the DNA of an individual (e.g. a suspect in a crime) can be compared to the DNA of a sample (e.g. blood left at the scene of a crime) for a positive match.

Northern blotting is a technique to identify fragments of known RNA sequence in a large population of RNA. First, the fragments of RNA containing the known sequence are put through electrophoresis to separate them by size and charge. Next, the RNA strands are separated into single strands (usually with NaOH) and then the single-stranded fragments are transferred to a nitrocellulose membrane. At this point a probe is added which will hybridize to the known sequence of RNA and mark it with some visual tag, usually fluorescence.

Southern blotting is similar to Northern blotting, but is used on DNA instead of RNA.

similar technique for proteins

Mnemonic for remembering lab techniques

S = Southern blotting -> DNA = D
N = Northern blotting -> RNA = R
O = O = O (nothing)
W = Western blotting -> protein = P

is a general condition describing a situation where the genome has an extra or missing chromosome number, often caused by nondisjunction. If nondisjunction were to occur during meiosis II, and a pair of sister chromatids failed to separate, the resulting two gametes would be produced: one that has an extra chromosome (n + 1) and one that is missing a chromosome (n – 1). As a result, after fertilization, a zygote with aneuploidy would result

Tetraploid refers to the number of sets of chromosomes, specifically four sets (4n). Cells with more than two sets of homologous chromosomes (such as triploids and tetraploids) are said to exhibit polyploidy which is common in plants.

Disomic refers to the state of having two sets of chromosomes – it can be thought of as interchangeable with the term diploid. The disomic state is standard in humans, and is not the result of nondisjunction during meiosis II and subsequent fertilization

A mix between salt and fresh water, which would be found in an estuary. An estuary is a specific area where freshwater meets seawater. A mangrove swamp often grows near an estuary and is characterized by a mix of salt and freshwater

In commensalism, a form of symbiosis, one of the two organisms benefits while the other remains unaffected. Examples of commensalism include barnacles and whales (the barnacle gets wider feeding opportunities as a result of being attached to the whale, while the whale is unaffected).

Allelopathy is the production of biochemicals by an organism that influences the growth, survival, and reproduction of other organisms. Allelopathy is a form of interference competition, which occurs directly between individuals via aggression. In interference competition, other individuals are directly prevented from physically establishing themselves on a shared habitat.

Exploitation competition is a type of competition that occurs indirectly through depletion of a common resource. For example, lions and cheetahs face exploitation competition in Africa as both hunt for a common resource: the gazelle. If cheetahs were more successful and ate all the gazelles, lions would suffer from depletion of the food resource.

Apparent competition is a type of competition that occurs between two species preyed upon by the same predator. For example, say a species of spider and a species of beetle are both hunted by owls and the amount of spiders suddenly increased. This would lead to survival of more owls (due to the increased food resource of spiders), which would in turn hunt more of the beetles, ultimately decreasing their overall number.

Intraspecific competition is a type of competition that occurs between members of the same species.

The deciduous forest biome is characterized by cold (but not particularly harsh) winters, warm summers, and moderate levels of precipitation. It has deciduous trees that shed their leaves during the winter, not coniferous trees Due to the shedding of leaves, the soil in deciduous forests is rich. This biome is characterized by vertical stratification (plants and animals live on the ground, in low branches, and high in treetops).

The savanna biome is characterized by warm temperature year-round, with some small seasonal variation. There is very little precipitation in terms of rainfall, and the dry season can last many months each year. Plants in this biome consist of grasses and scattered trees with small leaves. Animals in this biome consist primarily of large plant-eating mammals (e.g. zebras) and their predators (e.g. hyenas).

The tundra biome is characterized by cold winters (to the point that the top layer of soil freezes). In the summer, the top layer thaws, but deeper soil (permafrost) remains frozen year-round. The summers are still relatively cold (generally average less than 50° F), and there is very little precipitation or vegetation Plants in this biome consist of shrubs, grasses, mosses, and lichens (permafrost restricts the growth of plant roots). Animals in this biome include musk oxen, caribou, arctic hares, and arctic foxes.

The taiga biome (sometimes referred to as boreal forests), located south of the tundra biome, is the largest terrestrial biome. It is characterized by very long, harsh winters and precipitation in the form of heavy snow, along with short rainy and humid summers. The primary form of vegetation is coniferous forests.

The chaparral biome is characterized by highly seasonal precipitation, with rainy winters and dry summers. The scattered vegetation in this biome consists primarily of shrubs, grasses, and herbs. Animals include deer and goats. The chaparral biome is found along the California coastline, and many California fires happen here.

True statements about plant cell organelles

-Most of the cytoplasmic volume is occupied by a single vacoule

-Plant cell walls are composed of cellulose and function in maintaing cell shape

-Adjacent plant cells contain channels allowing for intercellular communication.

-Plant cells have mitochondria, but do not have a centriole:: they use mitochondria to convert the glucose they produce into ATP.

-Plant cells have both a cell wall and a cell membrane

Which of the following is a reason for why the intracellular binding of the steroid hormone testosterone is slow acting?

Steroids up regulate genes which must be transcribed and translated.

-They act as transcription factors. There is only an effect seen once the mRNA has been translated to protein, which is a slow process.

-Steroid hormones bind directly to the DNA and do not require second messengers

-Steroid hormones are non-polar and pass through the membrane

They aid in providing cell-cell adhesion and mechanical stability

Form a seal to prevent the passage of material between cells

All for passage of ions and small molecules while preventing the cytoplasm of adjacent cells from mixing

Intercalated discs in the heart

Narrow tunnels between plant cells that allow for exchange of material through cytoplasm around a narrow tube of the ER known as the desmotubule

What protein are the microfilaments of the cytoskeleton composed of?

Arranges to make up microtubules

A regulatory protein in skeletal muscle cells that prevents myosin from binding to actin.

Arranges to make up intermediate filaments

-A parasite that infects other cells and uses their machinery to survice

-The viral coats are made up of protein subunits called capsomeres, forming the capsid.

-It has no cell wall, no plasma membrane, nor any organelles.

-Can replicate using the lysogenic or lytic cycles

In the lysogenic cycle the virus binds to the host and inserts its viral DNA into the host cells’ DNA chromosome. The viral DNA will be replicated whenever chromosomal DNA is replicated. The virus is considered dormant and does not harm the host while in the lysogenic stage.

In the lytic cycle the virus attaches to a host, inserts its DNA into that host, and takes over the host cell’s machinery. This includes making many copies of viral DNA and translating viral proteins. The many virions then break out of the host cell, destroying the host cell in the process.

C3 photosynthesis is called C3 because CO2 will become a three carbon (C3) compound. This is conventional photosynthesis, and is what I discussed above. We can contrast this with C4 photosynthesis as well as CAM photosynthesis. C4 and CAM photosynthesis are both techniques that are used to prevent photorespiration.

This method is called C4 photosynthesis because the CO2 molecule integrates into and becomes a four carbon compound first, before it bonds to RuBisCO later on.

PEP carboxylase has even lower affinity for O2 compared to RuBisCO, so even in the presence of O2, it is very unlikely to bind to oxygen. This is an advantage.

The enzyme PEP carboxylase takes CO2 and converts it into oxaloacetate. Oxaloacetate quickly turns into malic acid. Both oxaloacetate and malic acid are four carbon compounds, hence the name C4.

The malic acid will be transferred from the mesophyll cells where PEP carboxylase reaction has occurred, to the bundle sheath cells.

The bundle sheath cells are located in a different area in the leaf anatomy (they surround the vascular bundles of plants), where O2 concentration is much lower.

Here the malic acid can be decarboxylated to release CO2. The CO2 can now undergo the conventional Calvin cycle with RuBisCO, in an environment where O2is not as prevalent, and RuBisCO has low risk of photorespiration.

The C4 photosynthesis process isolates CO2 spatially. Spatial isolation means that CO2 is transported to a different location (a different space) to prevent photorespiration. C4 photosynthesis transports the CO2 to the bundle sheath cells.

CAM plants have a different method of isolation. CAM plants use temporal isolation. Temporal isolation is an isolation based on timing, as a means of preventing photorespiration. There is no spatial separation: the processes occur in the same part of the leaf, but the plant does different processes at different times.

During the day, CAM plants close stomata to prevent excessive loss of water, via transpiration, evaporation out of the stomates (keep in mind this will also limit new gases like O2 from entering the plant). At night, CAM plants have their stomata open, allowing CO2 to enter into the leaf. The same enzyme in C4 photosynthesis is used in CAM photosynthesis: PEP carboxylase will fixate CO2into a four carbon molecule of oxaloacetate which converts into malic acid. In contrast, to the C4 pathway, rather than shuttle the malic acid to a different part of the leaf, the malic acid will be stored in a vacuole, for later use.

During the day, the malic acid will be shuttled out of the vacuole, CO2 will be decarboxylated from the malic acid, and the typical Calvin cycle will occur in a low O2 environment (stomates are closed). During the day the sun shines brightly, and ATP as well as NADPH are being produced plentifully.


Contents

In many small organisms such as bacteria, quorum sensing enables individuals to begin an activity only when the population is sufficiently large. This signaling between cells was first observed in the marine bacterium Aliivibrio fischeri, which produces light when the population is dense enough. [10] The mechanism involves the production and detection of a signaling molecule, and the regulation of gene transcription in response. Quorum sensing operates in both gram-positive and gram-negative bacteria, and both within and between species. [11]

In slime moulds, individual cells known as amoebae aggregate together to form fruiting bodies and eventually spores, under the influence of a chemical signal, originally named acrasin. The individuals move by chemotaxis, i.e. they are attracted by the chemical gradient. Some species use cyclic AMP as the signal others such as Polysphondylium violaceum use other molecules, in its case N-propionyl-gamma-L-glutamyl-L-ornithine-delta-lactam ethyl ester, nicknamed glorin. [12]

In plants and animals, signaling between cells occurs either through release into the extracellular space, divided in paracrine signaling (over short distances) and endocrine signaling (over long distances), or by direct contact, known as juxtacrine signaling (e.g., notch signaling). [13] Autocrine signaling is a special case of paracrine signaling where the secreting cell has the ability to respond to the secreted signaling molecule. [14] Synaptic signaling is a special case of paracrine signaling (for chemical synapses) or juxtacrine signaling (for electrical synapses) between neurons and target cells.

Synthesis and release Edit

Many cell signals are carried by molecules that are released by one cell and move to make contact with another cell. Signaling molecules can belong to several chemical classes: lipids, phospholipids, amino acids, monoamines, proteins, glycoproteins, or gases. Signaling molecules binding surface receptors are generally large and hydrophilic (e.g. TRH, Vasopressin, Acetylcholine), while those entering the cell are generally small and hydrophobic (e.g. glucocorticoids, thyroid hormones, cholecalciferol, retinoic acid), but important exceptions to both are numerous, and a same molecule can act both via surface receptors or in an intracrine manner to different effects. [14] In animal cells, specialized cells release these hormones and send them through the circulatory system to other parts of the body. They then reach target cells, which can recognize and respond to the hormones and produce a result. This is also known as endocrine signaling. Plant growth regulators, or plant hormones, move through cells or by diffusing through the air as a gas to reach their targets. [15] Hydrogen sulfide is produced in small amounts by some cells of the human body and has a number of biological signaling functions. Only two other such gases are currently known to act as signaling molecules in the human body: nitric oxide and carbon monoxide. [16]

Exocytosis Edit

Exocytosis is the process by which a cell transports molecules such as neurotransmitters and proteins out of the cell. As an active transport mechanism, exocytosis requires the use of energy to transport material. Exocytosis and its counterpart, endocytosis, are used by all cells because most chemical substances important to them are large polar molecules that cannot pass through the hydrophobic portion of the cell membrane by passive means. Exocytosis is the process by which a large amount of molecules are released thus it is a form of bulk transport. Exocytosis occurs via secretory portals at the cell plasma membrane called porosomes. Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell.

In exocytosis, membrane-bound secretory vesicles are carried to the cell membrane, where they dock and fuse at porosomes and their contents (i.e., water-soluble molecules) are secreted into the extracellular environment. This secretion is possible because the vesicle transiently fuses with the plasma membrane. In the context of neurotransmission, neurotransmitters are typically released from synaptic vesicles into the synaptic cleft via exocytosis however, neurotransmitters can also be released via reverse transport through membrane transport proteins.

Forms Edit

Autocrine Edit

Autocrine signaling involves a cell secreting a hormone or chemical messenger (called the autocrine agent) that binds to autocrine receptors on that same cell, leading to changes in the cell itself. [17] This can be contrasted with paracrine signaling, intracrine signaling, or classical endocrine signaling.

Paracrine Edit

In paracrine signaling, a cell produces a signal to induce changes in nearby cells, altering the behaviour of those cells. Signaling molecules known as paracrine factors diffuse over a relatively short distance (local action), as opposed to cell signaling by endocrine factors, hormones which travel considerably longer distances via the circulatory system juxtacrine interactions and autocrine signaling. Cells that produce paracrine factors secrete them into the immediate extracellular environment. Factors then travel to nearby cells in which the gradient of factor received determines the outcome. However, the exact distance that paracrine factors can travel is not certain.

Paracrine signals such as retinoic acid target only cells in the vicinity of the emitting cell. [18] Neurotransmitters represent another example of a paracrine signal.

Some signaling molecules can function as both a hormone and a neurotransmitter. For example, epinephrine and norepinephrine can function as hormones when released from the adrenal gland and are transported to the heart by way of the blood stream. Norepinephrine can also be produced by neurons to function as a neurotransmitter within the brain. [19] Estrogen can be released by the ovary and function as a hormone or act locally via paracrine or autocrine signaling. [20]

Although paracrine signaling elicits a diverse array of responses in the induced cells, most paracrine factors utilize a relatively streamlined set of receptors and pathways. In fact, different organs in the body - even between different species - are known to utilize a similar sets of paracrine factors in differential development. [21] The highly conserved receptors and pathways can be organized into four major families based on similar structures: fibroblast growth factor (FGF) family, Hedgehog family, Wnt family, and TGF-β superfamily. Binding of a paracrine factor to its respective receptor initiates signal transduction cascades, eliciting different responses.

Endocrine Edit

Endocrine signals are called hormones. Hormones are produced by endocrine cells and they travel through the blood to reach all parts of the body. Specificity of signaling can be controlled if only some cells can respond to a particular hormone. Endocrine signaling involves the release of hormones by internal glands of an organism directly into the circulatory system, regulating distant target organs. In vertebrates, the hypothalamus is the neural control center for all endocrine systems. In humans, the major endocrine glands are the thyroid gland and the adrenal glands. The study of the endocrine system and its disorders is known as endocrinology.

Juxtacrine Edit

Juxtacrine signaling is a type of cell–cell or cell–extracellular matrix signaling in multicellular organisms that requires close contact. There are three types:

  1. A membrane ligand (protein, oligosaccharide, lipid) and a membrane protein of two adjacent cells interact.
  2. A communicating junction links the intracellular compartments of two adjacent cells, allowing transit of relatively small molecules.
  3. An extracellular matrixglycoprotein and a membrane protein interact.

Additionally, in unicellular organisms such as bacteria, juxtacrine signaling means interactions by membrane contact. Juxtacrine signaling has been observed for some growth factors, cytokine and chemokine cellular signals, playing an important role in the immune response.

Cells receive information from their neighbors through a class of proteins known as receptors. Receptors may bind with some molecules (ligands) or may interact with physical agents like light, mechanical temperature, pressure, etc. Reception occurs when the target cell (any cell with a receptor protein specific to the signal molecule) detects a signal, usually in the form of a small, water-soluble molecule, via binding to a receptor protein on the cell surface, or once inside the cell, the signaling molecule can bind to intracellular receptors, other elements, or stimulate enzyme activity (e.g. gasses), as in intracrine signaling.

Signaling molecules interact with a target cell as a ligand to cell surface receptors, and/or by entering into the cell through its membrane or endocytosis for intracrine signaling. This generally results in the activation of second messengers, leading to various physiological effects. In many mammals, early embryo cells exchange signals with cells of the uterus. [22] In the human gastrointestinal tract, bacteria exchange signals with each other and with human epithelial and immune system cells. [23] For the yeast Saccharomyces cerevisiae during mating, some cells send a peptide signal (mating factor pheromones) into their environment. The mating factor peptide may bind to a cell surface receptor on other yeast cells and induce them to prepare for mating. [24]

Cell surface receptors Edit

Cell surface receptors play an essential role in the biological systems of single- and multi-cellular organisms and malfunction or damage to these proteins is associated with cancer, heart disease, and asthma. [25] These trans-membrane receptors are able to transmit information from outside the cell to the inside because they change conformation when a specific ligand binds to it. By looking at three major types of receptors: Ion channel linked receptors, G protein–coupled receptors, and enzyme-linked receptors).

Ion channel linked receptors Edit

Ion channel linked receptors are a group of transmembrane ion-channel proteins which open to allow ions such as Na + , K + , Ca 2+ , and/or Cl − to pass through the membrane in response to the binding of a chemical messenger (i.e. a ligand), such as a neurotransmitter. [26] [27] [28]

When a presynaptic neuron is excited, it releases a neurotransmitter from vesicles into the synaptic cleft. The neurotransmitter then binds to receptors located on the postsynaptic neuron. If these receptors are ligand-gated ion channels, a resulting conformational change opens the ion channels, which leads to a flow of ions across the cell membrane. This, in turn, results in either a depolarization, for an excitatory receptor response, or a hyperpolarization, for an inhibitory response.

These receptor proteins are typically composed of at least two different domains: a transmembrane domain which includes the ion pore, and an extracellular domain which includes the ligand binding location (an allosteric binding site). This modularity has enabled a 'divide and conquer' approach to finding the structure of the proteins (crystallising each domain separately). The function of such receptors located at synapses is to convert the chemical signal of presynaptically released neurotransmitter directly and very quickly into a postsynaptic electrical signal. Many LICs are additionally modulated by allosteric ligands, by channel blockers, ions, or the membrane potential. LICs are classified into three superfamilies which lack evolutionary relationship: cys-loop receptors, ionotropic glutamate receptors and ATP-gated channels.

G protein–coupled receptors Edit

G protein-coupled receptors are a large group of evolutionarily-related proteins that are cell surface receptors that detect molecules outside the cell and activate cellular responses. Coupling with G proteins, they are called seven-transmembrane receptors because they pass through the cell membrane seven times. [29] Ligands can bind either to extracellular N-terminus and loops (e.g. glutamate receptors) or to the binding site within transmembrane helices (Rhodopsin-like family). They are all activated by agonists although a spontaneous auto-activation of an empty receptor can also be observed. [29]

G protein-coupled receptors are found only in eukaryotes, including yeast, choanoflagellates, [30] and animals. The ligands that bind and activate these receptors include light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and vary in size from small molecules to peptides to large proteins. G protein-coupled receptors are involved in many diseases.

There are two principal signal transduction pathways involving the G protein-coupled receptors: cAMP signal pathway and phosphatidylinositol signal pathway. [31] When a ligand binds to the GPCR it causes a conformational change in the GPCR, which allows it to act as a guanine nucleotide exchange factor (GEF). The GPCR can then activate an associated G protein by exchanging the GDP bound to the G protein for a GTP. The G protein's α subunit, together with the bound GTP, can then dissociate from the β and γ subunits to further affect intracellular signaling proteins or target functional proteins directly depending on the α subunit type (Gαs, Gαi/o, Gαq/11, Gα12/13). [32] : 1160

G protein-coupled receptors are an important drug target and approximately 34% [33] of all Food and Drug Administration (FDA) approved drugs target 108 members of this family. The global sales volume for these drugs is estimated to be 180 billion US dollars as of 2018 [update] . [33] It is estimated that GPCRs are targets for about 50% of drugs currently on the market, mainly due to their involvement in signaling pathways related to many diseases i.e. mental, metabolic including endocrinological disorders, immunological including viral infections, cardiovascular, inflammatory, senses disorders, and cancer. The long ago discovered association between GPCRs and many endogenous and exogenous substances, resulting in e.g. analgesia, is another dynamically developing field of the pharmaceutical research. [29]

Enzyme-linked receptors Edit

Enzyme-linked receptors (or catalytic receptors) are transmembrane receptor that, upon activation by an extracellular ligand, causes enzymatic activity on the intracellular side. [34] Hence a catalytic receptor is an integral membrane protein possessing both enzymatic, catalytic, and receptor functions. [35]

They have two important domains, an extra-cellular ligand binding domain and an intracellular domain, which has a catalytic function and a single transmembrane helix. The signaling molecule binds to the receptor on the outside of the cell and causes a conformational change on the catalytic function located on the receptor inside the cell. Examples of the enzymatic activity include:

Intracellular receptors Edit

Steroid hormone receptor Edit

Steroid hormone receptors are found in the nucleus, cytosol, and also on the plasma membrane of target cells. They are generally intracellular receptors (typically cytoplasmic or nuclear) and initiate signal transduction for steroid hormones which lead to changes in gene expression over a time period of hours to days. The best studied steroid hormone receptors are members of the nuclear receptor subfamily 3 (NR3) that include receptors for estrogen (group NR3A) [37] and 3-ketosteroids (group NR3C). [38] In addition to nuclear receptors, several G protein-coupled receptors and ion channels act as cell surface receptors for certain steroid hormones.

When binding to the signaling molecule, the receptor protein changes in some way and starts the process of transduction, which can occur in a single step or as a series of changes in a sequence of different molecules (called a signal transduction pathway). The molecules that compose these pathways are known as relay molecules. The multistep process of the transduction stage is often composed of the activation of proteins by addition or removal of phosphate groups or even the release of other small molecules or ions that can act as messengers. The amplifying of a signal is one of the benefits to this multiple step sequence. Other benefits include more opportunities for regulation than simpler systems do and the fine- tuning of the response, in both unicellular and multicellular organism. [15]

In some cases, receptor activation caused by ligand binding to a receptor is directly coupled to the cell's response to the ligand. For example, the neurotransmitter GABA can activate a cell surface receptor that is part of an ion channel. GABA binding to a GABAA receptor on a neuron opens a chloride-selective ion channel that is part of the receptor. GABAA receptor activation allows negatively charged chloride ions to move into the neuron, which inhibits the ability of the neuron to produce action potentials. However, for many cell surface receptors, ligand-receptor interactions are not directly linked to the cell's response. The activated receptor must first interact with other proteins inside the cell before the ultimate physiological effect of the ligand on the cell's behavior is produced. Often, the behavior of a chain of several interacting cell proteins is altered following receptor activation. The entire set of cell changes induced by receptor activation is called a signal transduction mechanism or pathway. [39]

A more complex signal transduction pathway is shown in Figure 3. This pathway involves changes of protein–protein interactions inside the cell, induced by an external signal. Many growth factors bind to receptors at the cell surface and stimulate cells to progress through the cell cycle and divide. Several of these receptors are kinases that start to phosphorylate themselves and other proteins when binding to a ligand. This phosphorylation can generate a binding site for a different protein and thus induce protein–protein interaction. In Figure 3, the ligand (called epidermal growth factor, or EGF) binds to the receptor (called EGFR). This activates the receptor to phosphorylate itself. The phosphorylated receptor binds to an adaptor protein (GRB2), which couples the signal to further downstream signaling processes. For example, one of the signal transduction pathways that are activated is called the mitogen-activated protein kinase (MAPK) pathway. The signal transduction component labeled as "MAPK" in the pathway was originally called "ERK," so the pathway is called the MAPK/ERK pathway. The MAPK protein is an enzyme, a protein kinase that can attach phosphate to target proteins such as the transcription factor MYC and, thus, alter gene transcription and, ultimately, cell cycle progression. Many cellular proteins are activated downstream of the growth factor receptors (such as EGFR) that initiate this signal transduction pathway. [ citation needed ]

Some signaling transduction pathways respond differently, depending on the amount of signaling received by the cell. For instance, the hedgehog protein activates different genes, depending on the amount of hedgehog protein present. [ citation needed ]

Complex multi-component signal transduction pathways provide opportunities for feedback, signal amplification, and interactions inside one cell between multiple signals and signaling pathways. [ citation needed ]

A specific cellular response is the result of the transduced signal in the final stage of cell signaling. This response can essentially be any cellular activity that is present in a body. It can spur the rearrangement of the cytoskeleton, or even as catalysis by an enzyme. These three steps of cell signaling all ensure that the right cells are behaving as told, at the right time, and in synchronization with other cells and their own functions within the organism. At the end, the end of a signal pathway leads to the regulation of a cellular activity. This response can take place in the nucleus or in the cytoplasm of the cell. A majority of signaling pathways control protein synthesis by turning certain genes on and off in the nucleus. [40]

In unicellular organisms such as bacteria, signaling can be used to 'activate' peers from a dormant state, enhance virulence, defend against bacteriophages, etc. [41] In quorum sensing, which is also found in social insects, the multiplicity of individual signals has the potentiality to create a positive feedback loop, generating coordinated response. In this context, the signaling molecules are called autoinducers. [42] [43] [44] This signaling mechanism may have been involved in evolution from unicellular to multicellular organisms. [42] [45] Bacteria also use contact-dependent signaling, notably to limit their growth. [46]

Signaling molecules used by multicellular organisms are often called pheromones. They can have such purposes as alerting against danger, indicating food supply, or assisting in reproduction. [47]

Short-term cellular responses Edit

Brief overview of some signaling pathways (based on receptor families) that result in short-acting cellular responses
Receptor Family Example of Ligands/ activators (Bracket: receptor for it) Example of effectors Further downstream effects
Ligand Gated Ion Channels Acetylcholine
(Such as Nicotinic acetylcholine receptor),
Changes in membrane permeability Change in membrane potential
Seven Helix Receptor Light(Rhodopsin),
Dopamine (Dopamine receptor),
GABA (GABA receptor),
Prostaglandin (prostaglandin receptor) etc.
Trimeric G protein Adenylate Cyclase,
cGMP phosphodiesterase,
G-protein gated ion channel, etc.
Two Component Diverse activators Histidine Kinase Response Regulator - flagellar movement, Gene expression
Membrane Guanylyl Cyclase Atrial natriuretic peptide,
Sea urching egg peptide etc.
cGMP Regulation of Kinases and channels- Diverse actions
Cytoplasmic Guanylyl cyclase Nitric Oxide(Nitric oxide receptor) cGMP Regulation of cGMP Gated channels, Kinases
Integrins Fibronectins, other extracellular matrix proteins Nonreceptor tyrosine kinase Diverse response

Regulating gene activity Edit

Brief overview of some signaling pathways (based on receptor families) that control gene activity
Frizzled (Special type of 7Helix receptor) Wnt Dishevelled, axin - APC, GSK3-beta - Beta catenin Gene expression
Two Component Diverse activators Histidine Kinase Response Regulator - flagellar movement, Gene expression
Receptor Tyrosine Kinase Insulin (insulin receptor),
EGF (EGF receptor),
FGF-Alpha, FGF-Beta, etc (FGF-receptors)
Ras, MAP-kinases, PLC, PI3-Kinase Gene expression change
Cytokine receptors Erythropoietin,
Growth Hormone (Growth Hormone Receptor),
IFN-Gamma (IFN-Gamma receptor) etc
JAK kinase STAT transcription factor - Gene expression
Tyrosine kinase Linked- receptors MHC-peptide complex - TCR, Antigens - BCR Cytoplasmic Tyrosine Kinase Gene expression
Receptor Serine/Threonine Kinase Activin(activin receptor),
Inhibin,
Bone-morphogenetic protein(BMP Receptor),
TGF-beta
Smad transcription factors Control of gene expression
Sphingomyelinase linked receptors IL-1(IL-1 receptor),
TNF (TNF-receptors)
Ceramide activated kinases Gene expression
Cytoplasmic Steroid receptors Steroid hormones,
Thyroid hormones,
Retinoic acid etc
Work as/ interact with transcription factors Gene expression

Notch signaling pathway Edit

Notch is a cell surface protein that functions as a receptor. Animals have a small set of genes that code for signaling proteins that interact specifically with Notch receptors and stimulate a response in cells that express Notch on their surface. Molecules that activate (or, in some cases, inhibit) receptors can be classified as hormones, neurotransmitters, cytokines, and growth factors, in general called receptor ligands. Ligand receptor interactions such as that of the Notch receptor interaction, are known to be the main interactions responsible for cell signaling mechanisms and communication. [52] notch acts as a receptor for ligands that are expressed on adjacent cells. While some receptors are cell-surface proteins, others are found inside cells. For example, estrogen is a hydrophobic molecule that can pass through the lipid bilayer of the membranes. As part of the endocrine system, intracellular estrogen receptors from a variety of cell types can be activated by estrogen produced in the ovaries.

In the case of Notch-mediated signaling, the signal transduction mechanism can be relatively simple. As shown in Figure 2, the activation of Notch can cause the Notch protein to be altered by a protease. Part of the Notch protein is released from the cell surface membrane and takes part in gene regulation. Cell signaling research involves studying the spatial and temporal dynamics of both receptors and the components of signaling pathways that are activated by receptors in various cell types. [53] [54] Emerging methods for single-cell mass-spectrometry analysis promise to enable studying signal transduction with single-cell resolution. [55]

In notch signaling, direct contact between cells allows for precise control of cell differentiation during embryonic development. In the worm Caenorhabditis elegans, two cells of the developing gonad each have an equal chance of terminally differentiating or becoming a uterine precursor cell that continues to divide. The choice of which cell continues to divide is controlled by competition of cell surface signals. One cell will happen to produce more of a cell surface protein that activates the Notch receptor on the adjacent cell. This activates a feedback loop or system that reduces Notch expression in the cell that will differentiate and that increases Notch on the surface of the cell that continues as a stem cell. [56]


18.13 The Fruit

A fruit is the seed-bearing structure in angiosperms formed from the ovary after flowering.

Fruits are the means by which angiosperms disseminate seeds. Edible fruits, in particular, have propagated with the movements of humans and animals in a symbiotic relationship as a means for seed dispersal and nutrition in fact, humans and many animals have become dependent on fruits as a source of food. Accordingly, fruits account for a substantial fraction of the world’s agricultural output, and some (such as the apple and the pomegranate) have acquired extensive cultural and symbolic meanings.

In common language usage, “fruit” normally means the fleshy seed-associated structures of a plant that are sweet or sour, and edible in the raw state, such as apples, bananas, grapes, lemons, oranges, and strawberries. On the other hand, in botanical usage, “fruit” includes many structures that are not commonly called “fruits”, such as bean pods, corn kernels, tomatoes, and wheat grains. The section of a fungus that produces spores is also called a fruiting body.

The outer, often edible layer, is the pericarp, formed from the ovary and surrounding the seeds, although in some species other tissues contribute to or form the edible portion. The pericarp may be described in three layers from outer to inner, the epicarp, mesocarp and endocarp.

Fruit that bears a prominent pointed terminal projection is said to be beaked.

A fruit results from maturation of one or more flowers, and the gynoecium of the flower(s) forms all or part of the fruit. Gynoecium (from Ancient Greek γυνή, gyne, meaning woman, and οἶκος, oikos, meaning house) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower it consists of (one or more) pistils and is typically surrounded by the pollen-producing reproductive organs, the stamens, collectively called the androecium. The gynoecium is often referred to as the “female” portion of the flower, although rather than directly producing female gametes (i.e. egg cells), the gynoecium produces megaspores, each of which develops into a female gametophyte which then produces egg cells.

Inside the ovary/ovaries are one or more ovules where the megagametophyte contains the egg cell. After double fertilization, these ovules will become seeds. The ovules are fertilized in a process that starts with pollination, which involves the movement of pollen from the stamens to the stigma of flowers. After pollination, a tube grows from the pollen through the stigma into the ovary to the ovule and two sperm are transferred from the pollen to the megagametophyte. Within the megagametophyte one of the two sperm unites with the egg, forming a zygote, and the second sperm enters the central cell forming the endosperm mother cell, which completes the double fertilization process. Later the zygote will give rise to the embryo of the seed, and the endosperm mother cell will give rise to endosperm, a nutritive tissue used by the embryo.

As the ovules develop into seeds, the ovary begins to ripen and the ovary wall, the pericarp, may become fleshy (as in berries or drupes), or form a hard outer covering (as in nuts). In some multiseeded fruits, the extent to which the flesh develops is proportional to the number of fertilized ovules. The pericarp is often differentiated into two or three distinct layers called the exocarp (outer layer, also called epicarp), mesocarp (middle layer), and endocarp (inner layer). In some fruits, especially simple fruits derived from an inferior ovary, other parts of the flower (such as the floral tube, including the petals, sepals, and stamens), fuse with the ovary and ripen with it. In other cases, the sepals, petals and/or stamens and style of the flower fall off. When such other floral parts are a significant part of the fruit, it is called an accessory fruit. Since other parts of the flower may contribute to the structure of the fruit, it is important to study flower structure to understand how a particular fruit forms.

Figure 18.16: The development sequence of a typical drupe, the nectarine (Prunus persica) over a 7.5 month period, from bud formation in early winter to fruit ripening in midsummer.)1. Bud formation can be observed on new growth on the plant (early winter) 2. Flower buds clearly formed and leaves start to develop (early spring, ≈ 3 months) . 3. Flowers fully develop and are pollinated by wind or insects (early spring, ≈ 3½ months). 4. If successfully pollinated, flowers die back and incipient fruit can be observed leaves have quickly grown to provide tree with food and energy from photosynthesis (mid-spring, ≈ 4 months). 5. Fruit is well developed and continues to grow (late spring, ≈ 5½ months). 6. Fruit fully ripens to an edible form to encourage spreading of seed contained within by animals (midsummer, ≈ 7½ months)

There are three general modes of fruit development:

  • Apocarpous fruits develop from a single flower having one or more separate carpels, and they are the simplest fruits.
  • Syncarpous fruits develop from a single gynoecium having two or more carpels fused together.
  • Multiple fruits form from many different flowers.

Plant scientists have grouped fruits into three main groups, simple fruits, aggregate fruits, and composite or multiple fruits. The groupings are not evolutionarily relevant, since many diverse plant taxa may be in the same group, but reflect how the flower organs are arranged and how the fruits develop.

18.13.1 Reproductin of Ferns

Ferns are vascular plants differing from lycophytes by having true leaves (megaphylls), which are often pinnate. They differ from seed plants (gymnosperms and angiosperms) in reproducing by means of spores and they lack flowers and seeds. Like all land plants, they have a life cycle referred to as alternation of generations, characterized by alternating diploid sporophytic and haploid gametophytic phases. The diploid sporophyte has 2n paired chromosomes, where n varies from species to species. The haploid gametophyte has n unpaired chromosomes, i.e. half the number of the sporophyte. The gametophyte of ferns is a free-living organism, whereas the gametophyte of the gymnosperms and angiosperms is dependent on the sporophyte.

The life cycle of a typical fern proceeds as follows:

  1. A diploid sporophyte phase produces haploid spores by meiosis (a process of cell division which reduces the number of chromosomes by a half).
  2. A spore grows into a free-living haploid gametophyte by mitosis (a process of cell division which maintains the number of chromosomes). The gametophyte typically consists of a photosynthetic prothallus.
  3. The gametophyte produces gametes (often both sperm and eggs on the same prothallus) by mitosis.
  4. A mobile, flagellate sperm fertilizes an egg that remains attached to the prothallus.
  5. The fertilized egg is now a diploid zygote and grows by mitosis into a diploid sporophyte (the typical fern plant).

Ferns typically produce large diploids with stem, roots, and leaves and on fertile leaves called sporangium, spores are produced. The spores are released and germinate to produce short, thin gametophytes that are typically heart-shaped, small and green in color. The gametophytes or thallus, produce both motile sperm in the antheridia and egg cells in separate archegonia. After rains or when dew deposits a film of water, the motile sperm are splashed away from the antheridia, which are normally produced on the top side of the thallus, and swim in the film of water to the antheridia where they fertilize the egg. To promote out crossing or cross-fertilization the sperm is released before the eggs are receptive of the sperm, making it more likely that the sperm will fertilize the eggs of the different thallus. A zygote is formed after fertilization, which grows into a new sporophytic plant. The condition of having separate sporophyte and gametophyte plants is called alternation of the generations. Other plants with similar reproductive means include the Psilotum, Lycopodium, Selaginella and Equisetum.

18.13.2 Reproductin of Bryophytes

The bryophytes, which include liverworts, hornworts and mosses, reproduce both sexually and vegetatively. The gametophyte is the most commonly known phase of the plant. All are small plants found growing in moist locations and like ferns, have motile sperm with flagella and need water to facilitate sexual reproduction. These plants start as a haploid spore that grows into the dominant form, which is a multicellular haploid body with leaf-like structures that photosynthesize. Haploid gametes are produced in antheridia and archegonia by mitosis. The sperm released from the antheridia respond to chemicals released by ripe archegonia and swim to them in a film of water and fertilize the egg cells, thus producing zygotes that are diploid. The zygote divides by mitotic division and grows into a sporophyte that is diploid. The multicellular diploid sporophyte produces structures called spore capsules. The spore capsules produce spores by meiosis, and when ripe, the capsules burst open and the spores are released. Bryophytes show considerable variation in their breeding structures and the above is a basic outline. In some species each gametophyte is one sex while other species produce both antheridia and archegonia on the same gametophyte which is thus hermaphrodite.


Watch the video: Synapses and Signal Propagation (November 2022).