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4: Cells: structure and function (mostly Bacterial) - Biology

4: Cells: structure and function (mostly Bacterial) - Biology


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Bio 440: Prokaryotic/bacterial cell structure: Ch 4 Tortora

See lecture handout diagrams


I. Characteristics of cells

  • -growth and division
  • -metabolism
  • -DNA and RNA
  • -ribosomes for protein synthesis
  • -synthesis of cytoplasmic membranes
  • -more…


II. Viruses are NOT cells

  • Viruses are “acellular”: Viruses lack the characteristics of cells (violate all the characteristicslisted above) and are therefore not considered cellular. Viruses are considered acellular microbial agents.
  • Viruses are obligate intracellular parasites: Viruses are obligate intracellular parasites as they canonly replicate within a host cell. (note some cellular bacteria are also obligate intracellular parasites for example Rickettsia and Chlamydia)


III. Prokaryotic cell structure: see diagrams from lecture

  • -“pro” – before karyon; nut/kernel ~nucleus
  • -prokaryotes lack a membrane-bound nucleus and other membrane bound organelles
  • -unicellular
  • -first cells to evolve ~3.8 bya ( bya= billion years ago)
  • -size: “Unusual Giants”
  • E. coli ~ 1.0μm x 2.0 μm
  • 1985: Eupulopiscium fishelsoni ~600 μm
  • 1997: Thiomargarita namibiensis ~750μm
  • -prokaryotes are found in Domain Bacteria and Domain Archaea
  • -our discussion will focus on Domain Bacteria

IV. Cytoplasmic Membrane: ALL CELLS HAVE A CYTOPLASMIC MEMBRANE (aka CELL MEMBRANE/PLASMA MEMBRANE)

see handout diagram, textbook.


A. structure: phospholipid bilayer and proteins based on fluid mosaic model. Hydrophobic core
made of HC tails of fatty acid residues of phospholipids (review). Hydrophilic heads associate
with water molecules fig.


B. functions: semi- or selectively permeable membrane, controls movement of substances into
and out of cell. Site for electron transport chain and photosystems in some
bacteria and of flagellar base in flagellated bacteria and more
-location of membrane transport proteins essential for moving large polar or
charged substances across membrane


C. More on transport later…


D. Damage to cytoplasmic membrane may kill bacteria


E. Cytoplasmic membranes are relatively weak and are vulnerable to osmotic lysis
Vulnerability of cytoplasmic membranes: osmosis and osmotic lysis
-osmosis: diffusion of water from area of high water concentration to area of low water
concentration through selectively permeable membrane (e.g. cytoplasmic membrane)
-effects of isotonic, hypertonic and hypotonic solutions on cells (aka isoosmotic, hyperosmotic, hypoosmotic):
-Osmotic lysis:
-most bacteria live in hypotonic environments

-concentration of solutes inside cell is higher than concentration of solutes
outside cell, consequently concentration of water outside cell is greater than
concentration of water inside cell, consequently…
-net flow of water will be from outside cell into cell across cytoplasmic
membrane consequently

> water pressure inside cell continues to increase
until cytoplasmic membrane bursts, cell undergoes “lysis”. This process is called
“osmotic lysis” and will kill bacterium.


F. How do bacteria prevent osmotic lysis? Most common solution is formation of “cell wall” (see
next section)

V. Cell walls of Domain Bacteria


A. components of bacterial cell wall: peptidoglycan (“pg”)


1. only members of Domain Bacteria synthesize peptidoglycan

2. function: prevention of osmotic lysis; shape of bacterium

3. Peptidoglycan structure: alternating covalently linked
-N-acetylglucosamine (NAG or G) and
-N-acetylmuramic acid (NAM or M; only found in Domain Bacteria!) with
-tetrapeptide “tails”

4. peptide cross-links essential for strength of pg; crosslinks formed by bacterial enzymes, “transpeptidases” (aka bacterial PBP Penicillin Binding Proteins)

a. beta-lactam antibiotics (ex penicillin, ampicillin, amoxicillin) irreversibly bind the bacterial transpeptidases so they cannot form peptide crosslinks in pg, thus weakening pg and leading to osmotic lysis
of growing bacteria. Beta-lactams are not effective at killing bacteria in “stationary stage’ ie bacteria which are not actively growing, nor are they active against bacteria lacking cell walls ex Mycoplasma.

b. Penicillin was discovered by Sir Alexander Fleming

c. Vancomycin also interferes with crosslinking of pg, leading to osmotic lysis of bacteria (different mechanism than beta-lactam antibiotics however)


5. Lysozyme (also discovered by Alexander Fleming) is an enzyme found in tears, saliva, sweat. Lysozyme cleaves the covalent glycosidic bonds between NAM and NAG, weakening cell wall, leading to osmotic lysis of some bacteria.


B. Different types of Bacterial cell walls


1. Gram- positive cell walls: thick layer of peptidoglycan; teichoic and lipoteichoic acids; some have additional carbohydrates; some have cell wall proteins (ex M protein of Streptococcus pyogenes)

2. Gram-negative cell walls: thin layer of peptidoglycan connected by lipoproteins to outer membrane. Space between cell membrane and outer membrane is called the “periplasmic space”

a. outer membrane components:
i. lipopolysaccharide aka “LPS”, “endotoxin”; when released from dying gram- negative bacteria, triggers massive cytokine release leading to vasodilation and increased capillary permeability and “DIC” disseminated intravascular coagulation (blood clots form). Hypotension, decreased tissue perfusion, multiple organ system failure, shock (endotoxic shock) and death may result as a consequence (“endotoxemia). LPS bind to host cell leukocyte Tolllike receptors to trigger cytokine flood (more later).LPS is also used in “serotyping” gram-negative bacteria ( “O” somatic antigens; more later). LPS found only in gram-negative bacteria.
ii. phospholipids
iii. porins: protein channels through which hydrophilic substances can
cross outer membrane


b. outer membrane functions: prevents diffusion of secreted enzymes; protects
bacterium against toxic substances (ex some antibiotics such as penicillin,
lysozyme, bile)


3. Acid fast cell wall: e.g.,Mycobacterium tuberculosis, leprae: peptidoglycan covered by
mycolic acid-lipid bilayer. Creates hydrophobic barrier against antibiotics , chemicals, stains,
drying. Nutrients slow to pass barrier therefore these bacteria are very slow growing (difficult to
culture and perform antibiotic sensitivity testing). Patients infected with Mycobacterium are often
on long term antimicrobial therapy (months, years!) Protein porins in mycolic acid-lipid layer
permit passage of some hydrophilic substances. Described as a ‘waxy or lipid-rich” cell wall.
Requires special staining procedure (acid-fast stain see p109 in textbook).


4. Some Bacteria lack cell walls, e.g., Mycoplasm, Chlamydia and “L-forms”. If a patient suffers
from infection with such bacteria, treatment with beta-lactam antibiotics would have no effect as
these bacteria lack the target of the antibiotics. (recall Mycoplasma “steal” cholesterol from their
animal hosts to incorporate into their cell membranes to strengthen membranes in absence of cell
wall)


5. Domain Archaea: Archaea do not synthesize true peptidoglycan-
External Structures of Prokaryotic cells


VI. Glycocalyx= “sugar cup”, a “sticky” covering found on some prokaryotes


Two types of glycocalyces: capsules (tightly attached) and slime layers (loosely attached).
Capsule/Slime layers: outermost layer, covers cell wall, produced by many but not all bacteria.
Structure: Usually polysaccharides, some exceptions (Bacillus anthracis produces
capsule of poly-D-glutamic acid)
-weakly antigenic, the “K” antigens of Enterobacteriaceae


Function:
• prevents desiccation/drying out
• adherence to surfaces (oral streptococci make sticky capsule/slime
layer which permits adherence to tooth surfaces),
• antiphagocytic : inhibits phagocytosis by leukocytes , essential for
pathogenicity of many bacteria


VII. Flagella (plural; singular=flagellum) Read as homework

  • function: motility.
  • structure= flagellin protein subunits make up 20nm diameter filament, attached to hook and basal
  • body . Basal body consists of protein rings and shaft embedded in cell wall/cell membrane (more
  • in lab). Flagellar proteins act as antigens (trigger antibody production). “H” or “Hauch” antigens
  • of Enterobacteriaceae are the flagellar antigens used in serotyping (eg. E . coli O157:H7).
  • Flagellar arrangements: monotrichous( single polar flagellum), amphitrichous (both ends),
  • lophotrichous (tuft), peritrichous (flagella all over) - Flagella rotates similar to boat propeller
  • Chemoreceptors located in cell membrane permit bacteria to detect concentration gradients of
  • chemicals in environment. Chemotaxis is movement in response to chemical gradients. Positive
  • chemotaxis: movement in direction of increasing concentration gradient ex nutrient molecules.
  • Negative chemotaxis: movement down concentration gradient ex toxin molecules
  • Axial filaments or endoflagella : spirochetes are spiral shaped bacteria. Examples are Treponema
  • pallidum (causes syphilis) and Borrelia burgdorferi (causes Lyme Disease). These bacteria have
  • bundles of endoflagella attached at both ends of their cells covered by an outer sheath forming an
  • axial filament . Rotation of the endoflagella causes axial filament to rotate around spirochete,
  • permitting the bacteria to “corkscrew” through their environment, often thick mucous blankets,
  • perhaps even through tissues.

VIII. Pili and fimbriae

  • function: attachment.
  • structure= pilin protein subunits form hollow tubes projecting from surface of cell.
  • adhesins: Specific proteins in pili called adhesins permit attachment to surfaces in environment, including host cells. Adhesins often bind to specific receptors on host cell surfaces
  • fimbriae: usually numerous, relatively short, used to attach to surfaces in environment, including
  • other cells ex Neisseria gonorrhoeae uses fimbriae to attach to cells of host mucous membranes.
  • Reports have suggested Neisseria can change the types of adhesins expressed on its fimbriae so it
  • can first attach to mucosal cells of genital tract, then to cells of oral region, then cells of eye. What would happen if a mutation inhibited production of fimbriae by Neisseria?
  • Role of fimbriae in Biofilms
  • sex pilus (gram -negative bacteria aka conjugation pilus, “F” pilus): attaches one
  • bacteria to another, facilitates exchange of genetic information/DNA. example.
  • Involved in transfer of antibiotic resistance genes between bacteria.
  • Inside the bacterial cell


IX. Cytoplasm and internal structures: 90% water. Contains chromosome, plasmids, ribosomes, enzymes, nutrients, waste products, inclusion bodies


X. Chromosome

most bacteria have single, circular double stranded DNA chromosome ( a few have linear chromosomes). DNA carries genetic information. DNA base sequence determines amino acid sequence of proteins. (more later -antibiotics fluoroquinolones such as ciprofloxacin used to treat anthrax victims are bacterial DNA gyrase inhibitors; these antibiotics prevent “relaxation” of supercoiled bacterial DNA required for DNA replication and transcription (other DNA gyrase inhibitors include nalidixic acid and novobiocin)


XI. Plasmids

extrachromosomal, circular, self-replicating DNA. Frequently carry “extra” genetic
information example antibiotic resistance genes (“R” or resistance plasmids). May be passed
from one bacterium to another resulting in spread of antibiotic resistance. Conjugative plasmids
carry genes for synthesis of sex pili and proteins involved in transfer of bacterial DNA from
donor to recipient (more later in genetics)


XII. Ribosomes

70S ribosomes (compared to larger 80s cytoplasmic ribosomes of eukaryotes)

  • -structure: 2 subunits, 50S and 30S; made of ribosomal RNA/rRNA and ribosomal proteins
  • -site of protein synthesis
  • -S-Svedberg unit, used to express sedimentation rates using ultracentrifuges.
  • -70S bacterial ribosomes are the target of many antibiotics examples tetracycline,
  • chloramphenicol, macrolides (erythromycin, azithromycin) aminoglycosides ex gentamicin,
  • kanamycin. These antibiotics inhibit protein synthesis by bacteria

XIII. Endospores

resistant, dormant /resting structures, protect bacterium’s DNA, under harsh
conditions. Layers of protein, peptidoglycan, high calcium ion contenet and dipicolinic acid, low water
contenet. .Bacillus and Clostridium are endospore formers. Endospores can germinate to produce new
metabolically active, replicating vegetative cells. If inhaled, endospores of Bacillus anthracis will
germinate in lungs causing pneumonia and may spread throughout body (usually lethal). Unfavorable conditions trigger vegetative cells to sporulate and produce endospores. Contain very little water and dipicolinic acid (heat resistance)
See: anthrax and clostridial diseases


XIV. Inclusions (not on exam 1)


XV. Cytoskeleton read (not on exam 1)


Addition: Homework transport of substances across membranes and eukaryotic cells-read sections in
textbook
Transport of substances across cell membranes is presented following discussion of eukaryotic cells in the
lecture PowerPoint, however a few notes regarding specialized transport in bacteria follow:
Reference: Protein Secretion Systems in Gram-negative bacteria Source p63-65 Prescott, Harley and
Klein’s Microbiology Wiley et al ed 2008 McGraw Hill Publ
Type III protein secretion systems of some gram-negative bacterial pathogens: these systems
permit injection of virulence factors (ex toxins) into host target cells. Structurally complex,
similar to hypodermic syringe/needle; mutation in genes for Type III secretion may have permitted
evolution of bacterial flagella.
Type IV secretion systems: used to transport proteins AND to transfer DNA during conjugation;
components form syringe-like structure similar to Type III system


Bio 440 Eukaryotic Cells: Ch 4 Tortora


I. Eukaryotic microorganisms: organisms with membrane bound nucleus.
Domain Eukarya
Kingdoms: ( out-of-date" Protista" "algae", "protozoa"), Fungi, Plantae, Animalia


II. Evolution of endomembrane system
Primitive prokaryotic cell: in-folding of cell membrane--> nuclear membrane, endoplasmic reticulum, Golgi body, vesicles, lysosomes. Compartmentalization of functions.


III. Nucleus

A. Nuclear membrane with pores
B. Chromosomes and reproduction: multiple, linear chromosomes of double-stranded DNA split into
coding exons and non-coding introns. DNA associated with histone proteins.
Eukaryotes which reproduce sexually normally are “diploid”, ie cells contain 2 copies of each
chromosome, (one copy of each chromosome donated by each parent). Therefore there are usually 2 copies
of each gene, and the genes may not be identical. Haploid gametes are formed during sexual reproduction,
containing one copy of each chromosome. 2 haploid gametes fuse to form a diploid zygote (fusion of
gamete nuclei form zygote nucleus). Some eukaryotic microbes can reproduce asexually and have haploid
cells. Some organisms can reproduce sexually or asexually (ex fungi) and therefore may have either
diploid or haploid cells. HOMEWORK read mitosis and meiosis in textbook

IV. Endoplasmic reticulum

A. Continuous w/ nuclear membrane
B. 2 types

1. RER=Rough Endoplasmic Reticulum: “studded” with ribosomes -synthesis of proteins destined for export, incorporation into membranes or delivery to other organelles
2. SER= Smooth endoplasmic reticulum -lipid synthesis, detoxification


V. Golgi Body

receive proteins/lipids from ER via transport vesicle; processing and shipping to final
destination (secretion, membranes, other organelles)


VI. Lysosomes and Peroxisomes

Lysosomes (animal cells): vesicles filled with hydrolytic enzymes. May fuse with phagosomes to hydrolyze nutrient molecules or destroy invading microorganisms (phagocytosis). Peroxisomes :vesicles containing peroxidase/catalase and other enzymes. Plants: oxidize fats. Animals: oxidize amino acids.


VII. Ribosomes

80S cytoplasmic ribosomes ; sites of protein synthesis. Free in cytoplasm or fixed to RER. In mitochondria and chloroplasts, 70S-like ribosomes.


VIII. Mitochondria

all aerobic respiring eukaryotes (exception: Giardia lacks mitochondria)
A. “Powerhouse” of cell, site of ATP generation via aerobic respiration
C6H12O6 + 6O2--> 6CO2 + 6 H2O + Energy (heat + ATP)
B. Evolved from primitive prokaryotic cell? see Theory of Endosymbiosis/Endosymbiotic Theory


IX. Chloroplast: photosynthetic plants and algae

A. Sites of oxygenic photosynthesis: 6CO2 + 6H2O--light energy--> C6H12O6 + 6O2
B. Evolved from primitive cyanobacteria? Theory of Endosymbiosis/Endosymbiotic Theory

X. Theory of Endosymbiosis/Endosymbiotic

primitive nucleated cell phagocytizes (“eats”)
primitive aerobic respiring bacterium. Bacterium becomes an endosymbiont, living within host cell,
generates ATP for host, host provides protection for bacterium. Bacterium eventually evolves into
mitochondrion. Similar story for chloroplast evolution. Primitive nucleated cell with mitochondria
phagocytize primitive photosynthetic cyanobacterium. Bacterium becomes endosymbiont, evolves into
chloroplast.


Evidence to support Theory of Endosymbiosis:
1. mitochondria (mito.) and chloroplast (chloro) are self replicating-divide independently of host
cell
2. mito. and chloro. have own self-replicating, circular chromosomes similar to prokaryotic
chromosomes
3. mito/chlor. size similar to bacteria
4. membrane arrangement of mito./chlor. fit theory of bacterium engulfed in phagosome
5. mito./chlor. have ribosomes similar to prokaryotic 70S ribosomes and are inhibited by
antibiotics targeting 70S ribosomes
6. mito/chloro. ribosomal RNA sequences similar to Bacteria rRNA sequences.


XI. Cytoskeleton

A. Microtubules of tubulin subunits: mitotic spindles, flagella and cilia (protein dynein associated w/fl & cilia).
B. Microfilaments of actin subunits: cytoplasmic streaming; pseudopodia formation f amoeba and slime molds
C. Intermediate fibers: variety ex keratin: rigidity


XII. Appendages


A. Flagella: motility. Structurally very different from bacterial flagella. Microtubules (9 doublets + 2
central) covered with cell membrane, flex/beat (do not rotate/turn like bacterial flagella). ex Protozoa
B. Cilia: very similar to flagella except shorter, more numerous. Protozoa ciliates
Ciliated epithelium of respiratory tract important part of mucociliary escalator; destroyed by viruses,
smoking, predisposes to bacterial respiratory infections. Ciliated epithelium of oviduct important for
moving egg to uterus. Pathogens such as Chlamydia and Neiserria gonorrhoeae cause destruction of cilia,
results in ectopic pregnancy, sterility.


XIII. Cell wall: provides shape, resists turgor pressure/prevents osmotic lysis

A. Animals lack cell wall; some protozoa have protein layer called pellicle
B. Fungi and algae have cell wall
1. fungi cell wall: may contain chitin, polymer of N-acetylglucosamine (NAG) nitrogencontaining
polysaccharide
2. some algal cell walls and some fungal walls contain cellulose


XIV. Cell membrane phospholipid bilayer with proteins based on fluid mosaic model. Proteins may move
laterally. Consistency of membrane is like thin layer of oil. Primary function is to control movement of
substances into and out of cell


A. Homework: Movement of substances across membranes text

1. nonpolar substances may cross: diffusion, passive process
-Oxygen, carbon dioxide, ethanol and medium-length fatty acids may diffuse across
membrane. (A small amount of water may also diffuse through phospholipid bilayer )
2. water may cross rapidly via water-transmitting pores (aquaporins”)
-osmosis, passive. Know lysis, plasmolysis and when each happens
- “osmosis” and “tonicity” (know)
3. Hydrophilic substances ie charged or polar substances, may not cross membrane unless assisted
by transport proteins ex protein pores/carrier proteins/”permeases”
4. Note: whenever substances are moved against their concentration gradient( that is from an area
of low concentration to an area of high concentration), energy must be expended.

passive transport: substance moved from area / Transport requiring energy/active transport
of high concentration to low. No NRG req’d. / (energy=ATP, proton/chemical gradient)
1. Simple diffusion / 4. Active transport: Substance moved from low to
2. Osmosis / high concentration. Specific protein carriers. sugars,
amino acids, vitamins
3. Facilitated diffusion: /
- protein mediates diffusion. / 5. group translocation: (primarily in bacteria;) --
Specific channels/pores or / -substance to be transported is modified
carrier proteins. / as it crosses membrane. ex glucose

B. Specialized transport: Animal cells/ protozoa lacking cell walls: endocytosis (pinocytosis and
phagocytosis) and exocytosis; require cytoskeleton rearrangements and energy expenditure
endocytosis :taking in material in membrane in-folding ->vesicles


-: engulf cell/solid= phagocytosis engulf liquid =pinocytosis
ex phagocytic cells of immune system: monocytes-macrophages and
neutrophils: recognize “foreign” bacterial invaders, attach, phagocytize
and destroy via phagosomes and lysosomes
-also receptor mediated endocytosis
-exocytosis: expel/secrete substances from cell via vesicles


What Are Prokaryotic Cells? Structure, Function, and Definition

Prokaryotes are single-celled organisms that are the earliest and most primitive forms of life on earth. As organized in the Three Domain System, prokaryotes include bacteria and archaeans. Some prokaryotes, such as cyanobacteria, are photosynthetic organisms and are capable of photosynthesis.

Many prokaryotes are extremophiles and can live and thrive in various types of extreme environments including hydrothermal vents, hot springs, swamps, wetlands, and the guts of humans and animals (Helicobacter pylori).

Prokaryotic bacteria can be found almost anywhere and are part of the human microbiota. They live on your skin, in your body, and on everyday objects in your environment.


Introduction

Close your eyes and picture a brick wall. What is the wall's basic building block? It is a single brick. Like a brick wall, cells are the building blocks that make up your body.

Your body has many kinds of cells, each specialized for a specific purpose. Just as we use a variety of materials to build a home, the human body is constructed from many cell types. For example, epithelial cells protect the body's surface and cover the organs and body cavities within. Bone cells help to support and protect the body. Immune system cells fight invading bacteria. Additionally, blood and blood cells carry nutrients and oxygen throughout the body while removing carbon dioxide. Each of these cell types plays a vital role during the body's growth, development, and day-to-day maintenance. In spite of their enormous variety, however, cells from all organisms—even ones as diverse as bacteria, onion, and human—share certain fundamental characteristics.

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    Structure of Bacterial Cell (With Diagram)

    Bacteria (sing. bacterium) are unicellular prokaryotic microorganisms which divide by binary fission. They do not possess nuclear membrane and the nucleus consists of a single chromosome of cir­cular double-stranded DNA helix (Fig. 1.1).

    These are long filamentous, cytoplasmic appen­dages, 12-30 μm in length, protruding through the cell wall and contain contractile protein flagellin. These are organs of locomotion.

    Fimbriae or pili:

    These are thin, short filaments (0.1-1.5 μm x 4 to 8 nm) extruding from the cytoplasmic membrane, also called pili. They are made of protein (pilin).

    It is an outer covering of thin jelly-like material (0.2 μm in width) that surrounds the cell wall. Only some bacterial species possess capsule. Capsule is usually made of polysaccharide (e.g. pneumococcus), occasionally polypeptide (e.g. anthrax bacilli) and hyaluronic acid (e.g. streptococcus).

    It is a tough and rigid structure of peptidoglycan with accessory specific materials (e.g. LPS, teichoic acid etc.) surrounding the bacterium like a shell and lies external to the cytoplasmic membrane. It is 10-25 nm in thickness. It gives shape to the cell.

    The single circular double-stranded chromosome is the bacterial genome. Other structures include cytoplasmic membrane, mesosomes, ribosomes and cytoplasmic inclusions. Unlike eukaryotes cytoplasm does not contain ribosome, Golgi, cytoskeleton.


    GK Questions & Answers Cell its Structure and Functions

    All living organisms are made up of cells. It is the basic, structural, and functional unit of life. Also, it is known as "Building blocks of life". Cell biology is the study of cells. It consists of organelles that have different functions and helps the cell to survive. Multicellular organisms are made up of various cells. Let us solve a quiz based on the Cell its structure and functions.

    1. Who discovered cell in 1665?

    Explanation: In 1665, the cell was first discovered by Robert Hooke. The cell has a rich and interesting history that has ultimately provided the way to various scientific advancements of today.

    2. Name an Organelle which serves as a primary packaging area for molecules that will be distributed throughout the cell?

    Explanation: Golgi apparatus is also known as Golgi complex or Golgi body. It is an organelle that serves as a primary packaging area for molecules that will be distributed throughout the cell. It is located in the cytoplasm next to the endoplasmic reticulum and near the nucleus of the cell.

    3. Name the outermost boundary of the cell?

    Explanation: Plasma Membrane is the outermost boundary of the cell. It is also known as the cell membrane. It is the membrane found in all cells that separate the interior of the cell from the outside environment. A cell wall is attached to the plasma membrane on its outside surface in bacterial and plant cells.

    4. Name the process in which the ingestion of material by the cells is done through the plasma membrane?

    Explanation: Endocytosis is the process in which the ingestion of material by the cells is done through the plasma membrane. Or we can say that it is the process of actively transporting molecules into the cell by engulfing it with its membrane.

    5. Which among the following sentence is not correct about the organelles?

    a) They are found in all Eukaryotic cells.

    b) They are found in multicellular organisms.

    c) They coordinate to produce the cell.

    d) They are small sized and mostly internal.

    Explanation: Organelles are found in all Eukaryotic cells. They coordinate to produce the cell. They are small-sized and mostly internal. Organelle, any of specialised structures inside a cell that perform a specific function (e.g. mitochondria, ribosomes, endoplasmic reticulum). Organelles in unicellular organisms are the equivalent of organs in multicellular organisms.

    6. Name the process in which the passage of water goes from a region of higher concentration to a region of lower concentration through a semi permeable membrane?

    Explanation: Osmosis is the process in which the passage of water goes from a region of higher concentration to a region of lower concentration through a semi-permeable membrane.

    7. Name an organism that contains a single chromosome and cell division occurs through fission or budding?

    Explanation: Prokaryotes contain a single chromosome and cell division occurs through fission or budding. The usual method of prokaryote cell division is termed binary fission. The prokaryotic chromosome is a single DNA molecule that first replicates, then attaches each copy to a different part of the cell membrane.

    8. Name the process in which the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium?

    Explanation: Exocytosis is the process in which the membrane of a vesicle can fuse with the plasma membrane and extrude its contents to the surrounding medium.

    9. The jelly-like substance present inside the cell is known as:

    Explanation: Cytoplasm is the jelly-like substance present inside the cell and contains other organelles. It is a thick solution that fills each cell and is enclosed by the cell membrane.

    10. Blue-green Algae are:

    Explanation: Blue-green Algae or Cyanobacteria are Prokaryotes. They lack membrane-bound organelles and nuclei.


    Stains: Meaning, Purpose and Components| Microorganisms

    A stain is any colouring organic compound that, when combined with another substance, imparts a colour to that substance. The terms ‘dyes’ and ‘stains’ are often used interchangeably by biologists, but they are not the same. The term ‘dye’ is used to refer to a colouring agent that is used for general purposes, whereas the term ‘stain’ is used to refer to that dye which is used for biological purposes.

    Most of the stains used, particularly for bacteria, are aniline dyes, so called because their derivation from aniline (C6H5NH2). The most commonly used aniline dyes are crystal violet, methylene blue, basic fuchsin, safranin, eosin, etc.

    Purpose of Staining:

    Staining is done for any or all of the following three purposes:

    (a) To see organism better:

    Staining enables to see the organism better in contrast with background.

    (b) To differentiate one organism from another:

    Some microorganisms take colour under the given staining conditions, some do not. Such differences are particularly evident in staining procedures which are therefore called “differential stains”, the most common differential stains being the Gram- stain and the acid-fast stain,

    (c) To determine particular structures:

    There are special stains which react only with certain structures of the organism, e.g., spores, cell wall, nuclei, or others. This is why an organism stained with a cell wall-stain shows only the presence or absence of its cell wall.

    Structural Components (Nature) of Stains:

    Stains (dyes) usually have complex molecular structure and are chiefly benzene derivatives. A stain consists of three constituents: the organic compound containing a benzene ring, the chromophore, and the auxochrome. Thus a stain (Fig. 17.1) may be defined chemically as an organic compound containing both chromophore and auxochrome groups linked to its benzene ring.

    The ability of a stain to bind macromolecular cellular components such as proteins or nucleic acids depends on the electrical charge found on the chromogen portion, as well as on the cellular component to be stained.

    For convenience, when benzene of an organic colourless solvent binds to the nitro group of chromophore, it results in a yellow coloured compound called trinitrobenzene in which three hydrogen atoms in the benzene molecule are replaced by three nitro groups.

    Trinitrobenzene is a chromogen but not a stain. If however, another hydrogen atom is replaced by an auxochrome group, such as OH, the compound known as picric acid (trinitrohydroxybenzene) is formed. The picric acid is capable of ionization or electrolytic dissociation to form salt that binds to opposite-charged biological substance (Fig. 17.2).

    Thus the picric acid, which is yellow in colour, is a stain (dye). The colour of picric acid is due to the chromophoric nitro group (NO2), and its staining property is due to the auxochromic hydroxyl group (OH), which imparts the molecule the property of ionization or electrolytic dissociation.

    Chromophore and Auxochrome:

    Chromophore (Gk. chroma = colour phoros = to bear) is such a group with multiple bond that is associated with a compound and produces colour in that compound. It was Otto N. Witt (1876) who first designated the chromophore. Chromophore containing molecule is called chromogen.

    The most effective chromophores are nitroso (NO), nitro (NO2), azo (N = N), p-Quinoid, o-Quinoid, etc. The chemical structure of a chromophores is given in Fig. 17.3. The presence of any one of the chromophores in a molecule is usually sufficient to produce colour. Thus nitrobenzene is pale-green, azobenzene is orange-red, p-quinones are yellow, and o- quinones are orange or red.

    Auxochromes (Gk. auxein = to increase, chroma = colour) are the groups, while not producing colour themselves, are able to intensify the colour when present in a molecule together with a chromophore. The most effective auxochromes are: —OH, NH2, —NHR. —NR2, CI, and CO2H.

    Mordant and Its Function:

    Mordant is a substance that forms an insoluble compound with a stain and helps to fix the colour to the cell components. Some stains never stain the cells or its components unless treated with a mordant. The mordant becomes attached to the cell or its components and then combines with the stain to form an insoluble colour complex. This complex is called a lake.

    Commonly used mordants are the oxides of aluminium, iron, and chromium. Alizarin is an example of a stain that imparts colour only in collaboration with a mordant. It gives different colours when used with different mordents. It gives red colour with aluminium and tin salts, brownish red colour with a chromium mordant, and black-violets with an iron mordant.

    Classification (Types) of Stains:

    The stains (dyes) can be classified in various ways on the basis of their origin, purpose of use, staining activity, and charge on their surface.

    Different classifications of stains are the following:

    1. Classification Based on Origin:

    On the basis of their origin the stains can be classified as natural or synthetic.

    These stains are obtained from natural resources directly as natural products. Haematoxylin and carmine are good examples. Haematoxylin is obtained from the heart wood of a tree (Haematoxylon campechianum), whereas carmine is obtained from a cochineal female insect. The natural dyes are used mainly for histological purposes.

    Synthetic stains are artificially produced mainly from fractionation and recombination of coal-tar products hence popularly are called coal-tar dyes. The latter are used mainly for the bacterial stain preparations. Important synthetic stains are safranin, fast green, amiline blue, methylene blue, crystal violet, eosine, acid fuchsin, orange-G, etc.

    2. Classification Based on Purpose of Use:

    Stains can be categorized as under on the basis of the purpose of their use.

    (i) Direct or general stains:

    Aniline dyes are able to stain bacteria directly. Exceptions are bacterial spores, e.g., of Bacillus spp. and the bacteria that have waxy coaling on their cell wall, e.g., Mycobacterium spp.

    These are the dyes which stain only the background, e.g., nigrosin or India ink used either for observing mucilaginous covering enveloping bacteria (capsules) or certain spores of fungi or cells of unicellular animals.

    These tains are used for special purposes, to stain particular parts of the organism such as spores, metachromatin granules, flagella, nuclei, etc.

    (iv) Differential stains:

    These stains are those which enable one to differentiate two different groups of bacteria in a mixture, for instance, gram-positive and gram-negative.

    3. Classification Based on Staining Activity:

    On the basis of staining activity, the stains can be classified as nuclear stains, cytoplasmic stains, and histological stains.

    Nuclear stains are acidic in nature and stain the chromatin materials only. Examples are carmine, haematoxylin, etc.

    (ii) Cytoplasmic stains:

    These stains are basic in nature and stain the cytoplasm and its inclusions. Fast green, aniline blue, erythrosine, eosin, orange-G, etc. are the examples.

    (iii) Histological stains:

    Histological stains are those that specifically stain some particular tissues in the sections. Safranin stains the lignified and suberized cell walls.

    4. Classification based on Charge:

    On the basis of the charge which the stain (dye) molecules possess, they are categorized as acidic, basic, and neutral.

    Acidic stains (dyes) are acidic in nature because they possess negative (anionic) charge on their surface on ionization. Acid fuchsin, eosin, and picric acid are examples.

    Basic stains (dyes) are basic in nature because they possess positive (cationic) charge on their surface on ionization. Fast green, aniline blue, methylene blue, crystal violet, safranin, etc. are examples.

    (iii) Neutral stains (dyes):

    Neutral stains (dyes) are formed by the combination of acidic and basic stains in aqueous form. The colouring matter in neutral stains is present in both the anionic and the cationic groups. Therefore, these dyes are neither acidic nor basic. Neutral red is an example.

    Acidic and Basic Stains (Dyes):

    All stains (dyes) used to stain bacteria are synthetic products because they are artificially produced mainly from fractionation and recombination of coal-tar (aniline) and hence are generally called coal-tar dyes or aniline dyes. Although the synthetic stains (dyes) vary greatly in their chemical nature and staining properties they are, for practical purposes, often divided as acidic (dyes) and basic stains (dyes).

    The acidic stains (dyes) are anionic (negative) and ionize to impart a negative charge on the chromogen portion. An acidic stain (dye), therefore, has a strong affinity for the cationic (positive) constituents of the cell.

    These stains (dyes) are used to stain the cytoplasmic components because the proteins, the positively charged cytoplasmic components, readily bind to and accept the colour of the negatively charged chromogen of these stains. Acid fuchsin, eosin, picric acid, etc. are examples. Picric acid, for example, produces an anionic chromogen on ionization as illustrated.

    The basic stains (dyes) are cationic (positive) and ionize to provide a positive charge on the chromogen portion. A’ basic stain (dye), therefore, has a strong affinity for the anionic (negative) constituents of the cell.

    These stains (dyes) are used to stain the negative charged cellular components (e.g., nucleic acids) because they readily bind and accept the colour of the positively charged cationic chromogen of a basic stain. Methylene blue, crystal violet, safranin, etc. are the basic dyes. Methylene blue is actually a salt (methylene blue chloride) and produces a cationic chromogen as illustrated.

    Basic stains (dyes) are more commonly used for bacterial staining. The presence of a negative charge on the bacterial surface acts to repel most acidic stains and thus prevents their penetration into the bacterial cell.

    3. Salts of Acidic and Basic Stains (Dyes):

    Use of terms ‘acidic’ and ‘basic’ stains (dyes) does not mean that the stains in question are free acids or free bases. The free colour acids and bases are often insoluble in water and rarely possess appreciable staining action, i.e., the colours do not “stick”. The salts of these- compounds, on the other hands, are more soluble, penetrate better, and stain permanently, and they are the true stains.

    An acidic stain is the salt of a colour acid, whereas a basic stain is the salt of a colour base. In other words, acid dyes owe their coloured properties to the anion and the basic dye to the cation. However, important salts of acidic and basic stains are shown in Fig. 17.4.


    Cell Structure and Functions Class 8 Extra Questions Science Chapter 8

    Cell Structure and Functions Class 8 Extra Questions Very Short Answer Questions

    Question 1.
    What is the basic, functional and structural unit of life?
    Answer:
    Cell

    Question 2.
    Which cell does not have nucleus?
    Answer:
    Red blood cell

    Question 3.
    What is the name of the organism which can be seen only with the help of microscope?
    Answer:
    Microorganisms

    Question 4.
    What are the organisms which are composed of many cells packed together called?
    Answer:
    Multicellular organisms

    Question 5.
    What is the name of the power house of the cell?
    Answer:
    Mitochondria

    Question 6.
    Name one example of prokaryotic cell.
    Answer:
    Blue-green algae

    Question 7.
    Amoeba and Paramecium belong to which category of organisms?
    Answer:
    Unicellular

    Question 8.
    Which instrument is used to observe cells?
    Answer:
    Micrbscope

    Question 9.
    Why we do not sense any pain when we cut nails and hair?
    Answer:
    Nails and hair are made up of dead cells.

    Question 10.
    What is the name of living substance present in cell?
    Answer:
    Protoplasm

    Question 11.
    What is the other name of cell membrane?
    Answer:
    Plasma membrane

    Question 12.
    What is the name of thread-like structure present in nucleoplasm?
    Answer:
    Chromatin

    Question 13.
    What is the name of cell which has a well-defined nucleus?
    Answer:
    Eukaryotic cell

    Question 14.
    Name the two types of cell.
    Answer:
    Prokaryotic cells and eukaryotic cells.

    Cell Structure and Functions Class 8 Extra Questions Short Answer Questions

    Question 1.
    What is the function of cell wall?
    Answer:
    Cell wall is a tough, rigid layer that surrounds some types of cells (plants and some bacterial cells). The major function of the cell wall is to provide rigidity, tensile strength, structural support, protection against mechanical stress and infection.

    Question 2.
    Name the parts of the nucleus and state its function.
    Answer:
    Nucleus consists of three main parts—nuclear membrane, nucleoplasm and nucleolus. Nucleus plays an important role during cell division. It also controls the activities of the cell.

    Question 3.
    What is an organ?
    Answer:
    The structure that contains more than one type of tissues and is visible to the naked eyes are called organs.

    Question 4.
    What do you mean by unicellular and multicellular organisms?
    Answer:
    Organisms which consists of only one cell are called unicellular organisms while the organisms made up of more than one cell are called multicellular organisms.

    Question 5.
    Give a brief description of nucleus.
    Answer:
    Nucleus is a dense round body found in the centre of an animal cell and mostly on the periphery of the plant cell. The nucleus controls all the activities in a cell.

    Question 6.
    Name the following:

    • Controls the function of a cell.
    • Selectively allows things to get in and out of the cell.
    • Transfer characters from parents to offsprings.

    Question 7.
    What is nucleolus?
    Answer:
    A spherical body present at the centre of the nucleus is called the nucleolus.

    Question 8.
    What is endoplasmic reticulum?
    Answer:
    It is the system of complex folded network of membranous tubes which connects nuclear membrane with the plasma membrane. They allow movement of substances within the cell.

    Question 9.
    What are cell organelles?
    Answer:
    The tiny components present in the cytoplasm are called cell organelles.

    Question 10.
    What is cell membrane?
    Answer:
    The thin and delicate membrane surrounding the cell cytoplasm is called cell membrane.

    Cell Structure and Functions Class 8 Extra Questions Long Answer Questions

    Question 1.
    Differentiate between
    (a) Cell wall and cell membrane
    (b) Leucoplast and chloroplast
    (c) Vacuole in a plant cell and an animal cell
    (d) A tissue and an organ
    Answer:
    (a)

    Cell wall Cell membrane
    (i) It is present in only plant cells. (i) It is present in both plant and animal cells.
    (ii) It is rigid, thick structure. (ii) It is delicate, thin structure.
    (iii) It is completely permeable to ordinary molecules. (iii) It is selectively permeable to molecules.
    (iv) It is metabolically inactive and non­living. (iv) It is metabolically active and living.

    Leucoplast Chloroplast
    (i) It is colourless plastid. (i) It is green plastid.
    (ii) It is found in underground parts of plants like, roots, and underground modified stems. (ii) It is found in green parts of plants like leaves, stem and sepals.
    (iii) It help in storage of food. (iii) It helps in photosynthesis.

    Vacuoles in plants Vacuoles in animals
    (i) Plant cell vacuoles are large in size. (i) Animal cell vacuoles are smaller in size.
    (ii) Usually a large central vacuole is found. (ii) Many vacuoles are found.
    (iii) It is usually permanent structure. (iii) It is mostly temporary structure.

    Tissue Organ
    It is made of similar cells.
    Example: Muscle tissue, connective tissue, nerve tissue, etc.
    It is made of similar tissues. Example: Heart, lung, stomach, etc.

    Question 2.
    What are the main functional regions of a cell? Explain.
    Answer:
    Main functional regions of a cell are:

    • Plasma membrane: This is the membrane which makes the outer boundary of the cells. It is very thin, delicate and selectively permeable.
    • Cytoplasm: Cytoplasm is viscous, transparent jelly-like substance of the cell. It contains cell organelles.
    • Nucleus: Nucleus controls the working of the cell. It is a dense oval body lying in the protoplasm of the cell.

    Question 3.
    Define cell membrane and state its functions.
    Answer:
    Cell membrane or plasma membrane is a thin, delicate membrane surrounding the cytoplasm. Following are the functions of cell membrane:

    • It separates the cells from one another and also separates the cells from the surrounding medium.
    • It gives a definite shape to the cell.
    • Being porous, it allows the movement of substances from both inside and outside the cells.
    • Its porous structure helps in regulating the movement of materials through the cells.

    Question 4.
    Define nucleus and state its major parts.
    Answer:
    Nucleus is a dense round body found in the centre of an animal cell and mostly on the periphery of the plant cell. The nucleus controls all the activities in the cell like digesting movement of substances with¬in tj^ie cell. Nucleus also controls the process of cell division. This is the reason nucleus is also known as the ‘brain of the cell’.
    Nucleus consists of four major parts. They are:

    Cell Structure and Functions Class 8 Extra Questions Higher Order Thinking Skills

    Question 1.
    Which organism is more efficient in its functioning—unicellular or multicellular? Why?
    Answer:
    Multicellular organisms are more efficient in its functioning because labour is divided among the cells and have great capacity to survive than unicellular organisms.

    Question 2.
    What would happen if animals have cell wall?
    Answer:
    All parts of the animal would become rigid which will make their movement of limbs and body parts difficult.

    Question 3.
    Cells consist of many organelles, yet we do not call any of these organelles as structural and functional unit of living organisms. Explain.
    Answer:
    Organelles cannot function outside the cell as an independent unit. They can perform their functions only when they are within the living cells.

    Question 4.
    Why plant cells need cell walls?
    Answer:
    As plants cannot move, they need protection against variations in temperature, high wind speed, atmo¬spheric moisture, etc.

    Cell Structure and Functions Class 8 Extra Questions Value-Based Questions

    Question 1.
    In a cell, every cell organelle perform its own function. They coordinate the functions of cell.

    • Can a cell work if any of its organelle stops working?
    • What does a cell represent in a real life?
    • What values do we get from the above paragraph?
    • Cell would not work if any of its organelle stops working.
    • Cell represent our society where various people do their work to maintain it.
    • We should perform our duty regularly and honestly for the betterment of the whole world.

    Question 2.
    Shyam being very inquisitive, didn’t listen to the teacher’s instruction properly. He took the thin layer of onion peel with a drop of water on the glass slide. He placed the coverslip on it and start observing the slide under a microscope. But he was unable to see the cells of onion peel properly.

    • What do you think he might have forgotten?
    • What is the function of a dye?
    • What values do you get from the above incidence?
    • He might have forgotten using dye.
    • Dye stains the cell and its component and make them visible.
    • We must always follow teacher’s instruction. In laboratory we must not hurry and keep patience.

    Activities and Projects
    Question 1.
    Visit a laboratory for senior secondary students in your school or in a neighbouring school. Learn about the functioning of a microscope in the laboratory. Also observe how a slide is observed under the microscope.
    Answer:
    Do it yourself.

    Question 2.
    Talk to the senior biology teacher in your school or a neighbouring school. Find out if there are diseases which are passed on from parents to the offspring. Find out how these are carried and also if these diseases can be treated. For this you can also visit a doctor.
    Answer:
    Diseases which are passed from parents to offspring are colour blindness, haemophilia, thalassemia, etc. Most of these diseases cannot be treated. (For more information search internet).

    Question 3.
    Visit an agriculture extension centre in your area. Find out about genetically modified (GM) crops. Prepare a short speech for your class on this topic.
    Answer:
    Genetically Modified (GM) crops have a great role to play in Indian agriculture, when we need more from lesser resources. The transgenics have the potentiality to resist biotic and abiotic stresses and result in increased productivity in addition to better nutritional quality. The hue and cry on the impact of GM crops on biodiversity has created hypes regarding economic, social and ethical concerns. Though several workers have contradicted the fears expressed regarding the risks and hazards of GM crops, concerns on the safety of a GM foods are still creating controversies. India must avoid taking extreme decisions and has to develop a symbiotic relationship between the public and private sectors, to use new technological inputs to complement the traditional methods for making an ‘Evergreen Revolution’.

    Question 4.
    Find out about Bt cotton from an agriculture expert (or from envfor.nic.in/divisions/csnrv/ btcotton/bgnote.pdf). Prepare a short note on its advantages/disadvantages.
    Answer:
    Bt cotton is a Genetically Modified Organism (GMO) cotton variety, which produces an insecticide to bollworm. It is produced by Monsanto. Strains of the bacterium Bacillus thuringiensis produce over 200 different Bt toxins, each harmful to different insects. Most notably, Bt toxins are insecticidal to the larvae of moths and butterflies, beetles, cotton bollworms and ghtu flies but are harmless to other forms of life. The gene coding for Bt toxin has been inserted into cotton as a transgene, causing it to produce this natural insecticide in its tissues. In many regions, the main pests in commercial cotton are lepidopteran larvae, which are killed by the Bt protein in the genetically modified cotton they eat.
    Advantages:

    • It eliminates the need to use large amounts of broad-spectrum insecticides to kill these pests.
    • It spares natural insect predators in the farm ecology.
    • It kills only targeted pest.

    Disadvantages:

    • By cotton is ineffective against many cotton pests such as plant bugs, stink bugs, and aphids.
    • GM seeds are expensive.
    • GM crops cannot be use as fodder for cattles.
    • Effectiveness up to 120 days, after that the toxin producing efficiency of the Bt gene drastically reduces.

    I. Multiple Choice Questions (MCQs)
    Choose the correct option.
    Question 1.
    Nucleus is separated from cytoplasm by
    (a) nuclear membrane
    (b) nucleoplasm
    (c) organs
    (d) cell membrane

    Question 2.
    The liquid material in the nucleus is
    (a) chromosomes
    (b) bacteria
    (c) nucleoplasm
    (d) nucleolus

    Question 3.
    Tissues combine to form
    (a) nucleus
    (b) cells
    (c) organism
    (d) organs

    Question 4.
    Cells present in living organism differ in
    (a) numbers
    (b) shape
    (c) size
    (d) all of these

    Question 5.
    Cells which lack nuclear membrane are
    (a) eukaryotic cells
    (b) prokaryotic cells
    (c) single cells
    (d) multicells

    Question 6.
    The control centre of all the activities of a cell is
    (a) nucleus
    (b) nucleoplasm
    (c) cytoplasm
    (d) organelles

    Question 7.
    The coloured organelles which are found in plants only are
    (a) chlorophyll
    (b) plastids
    (c) vacuoles
    (d) WBC

    Question 8.
    Genes are located in
    (a) chrpmosomes
    (b) plastids
    (c) cytoplasm
    (d) lysosome

    Question 9.
    A group of similar cells combine to form
    (a) tissue
    (b) organ
    (c) organisms
    (d) organelles

    Question 10.
    The organism containing only a single cell is called
    (a) unicellular organism
    (b) multicellular organism
    (c) organelle
    (d) all of these

    Question 11.
    Cell walls is found in
    (a) plant cells only
    (b) animal cells only
    (c) both (a) and (b)
    (d) none of them

    Question 12.
    The empty blank looking structures in the cytoplasm is
    (a) vacuoles
    (b) plastids
    (c) plasma membrane
    (d) nucleus

    Question 13.
    The other name of cell membrane is
    (a) plasma membrane
    (b) cell wall
    (c) nuclear membrane
    (d) none of these

    Question 14.
    The basic structural and functional unit of all living organism is
    (a) cell
    (b) cell wall
    (c) cell membrane
    (d) chloroplasts

    Question 15.
    Chromosomes are found in
    (a) nucleus
    (b) nucleolus
    (c) nucleoplast
    (d) vacuole
    Answer:
    1. (a)
    2. (c)
    3. (d)
    4. (d)
    5. (b)
    6. (a)
    7. (b)
    8. (a)
    9. (a)
    10. (a)
    11. (a)
    12. (a)
    13. (a)
    14. (a)
    15. (a)

    II. Fill in the Blanks
    Fill in the blanks with suitable word/s.
    1. Cells are the __________ unit of all living organisms.
    2. __________ cells are branched.
    3. Amoeba has __________ shape.
    4. The __________ cell transfers the messages.
    5. __________ is a group of tissues.
    6. Protoplasm is __________ inside the cell.
    7. Eukaryotes cells have a well-developed __________.
    8. The jelly-like substance found between the nucleus and the cell-membrane is __________
    9. Nucleus is separated from the cytoplasm by __________.
    10. An __________ is a group of organs.
    11. __________ are present in plant cells but in animal cells.
    12. __________ is the process of making food in plants.
    13. __________ is absent in animal cell.
    14. The kitchen of plant cell is __________.
    15. __________ is the power house of the cell.
    Answer:
    1. basic
    2. Muscle
    3. irregular
    4. nerve
    5. Organ
    6. viscous fluid
    7. nucleus
    8. cytoplasm
    9. nuclear membrane
    10. organ system
    11. Plastids, absent
    12. Photosynthesis
    13. Cell wall
    14. plastid
    15. Mitochondria

    III. Match the following
    Match the items given in column I suitably with those given in column II

    Answer:
    1. (e)
    2. (a)
    3. (b)
    4. (c)
    5. (d)
    6. (h)
    7. (g)
    8. (f)
    9. (j)
    10. (i)

    IV. True or False
    State whether the given statements are true or false.
    1. Cytoplasm is not a part of cell.
    2. Cell wall surrounds the cell membrane.
    3. A group of similar cells is called tissue.
    4. Tissues combine to form an organism.
    5. Cell are of different sizes.
    6. Chloroplasts are blue in colour.
    7. Plastid occur in plant cells only.
    8. Chromosomes contain basic hereditary units called genes.
    9. Nucleus is the control unit of cell.
    10. The shape and size of cell is related to its function.
    11. Organs make up tissues.
    12. The cytoplasm and nucleoplasm make up the protoplasm.
    13. The smallest unit of life is chloroplast.
    14. Unicellular organisms have one-celled body.
    15. The cell wall is living in nature.
    Answer:
    1. False
    2. True
    3. True
    4. False
    5. True
    6. False
    7. True
    8. True
    9. True
    10. True
    11. False
    12. True
    13. False
    14. True
    15. False


    Measuring Biological Responses with Automated Microscopy

    Kenneth A. Giuliano , . D. Lansing Taylor , in Methods in Enzymology , 2006

    Prospectus

    Systems cell biology will be applied to the whole continuum of the drug discovery and development process. Cellular models of disease are being created to better understand the selected targets, along with the systems response of the cell models to optimize selection of lead compounds. In addition, cytotoxicity profiling, using the SCB approach, will provide a critical “filter” for prioritizing lead compounds and could evolve into a predictive tool. Patient sample profiling at the cell and tissue levels will become an important approach in defining patient subpopulations for more focused clinical trials and could also become a valuable cell‐based diagnostic method. SCB will also have a major impact on basic biomedical research where complex processes such as the differentiation of stem cells are clearly a systems challenge. A major effort will be focused on creating powerful reagents that measure and manipulate specific biochemical/ molecular events in a reversible manner, such as protein–protein interactions in relation to other dynamic cellular processes. In addition, a new generation of informatics tools will allow understanding of the complex systems responses of cells to various manipulations.


    Antibiotics: Function and Bacterial Interaction

    Antibiotics work effectively by killing bacterial microorganisms, referred to as bactericidal or by inhibiting the growth of bacterial microorganisms, referred to as bacteriostatic.

    Penicillin is an example of a bactericidal antibiotic and acts by killing bacteria cells. Penicillin is able to differentiate between healthy human cells and bacteria cells because many bacteria cells have cell walls while human cells do not have cell walls. The cells walls in bacteria are termed cell wall: peptidoglycan. The peptidoglycan builds powerful walls around the bacteria to protect them and deter extrinsic matter or particles to enter them. Penicillin is able to inadvertently destroy peptidoglycan during cell division. When the bacteria cells divide, small holes open up in the peptidoglycan during division, are refilled with peptidoglycan and close back up to make the new cells.

    Penicillin is able to latch onto the open peptidoglycan during this process and disrupt the filling in of the hole, preventing the bacteria from closing its wall and completing cell division. This creates altering pressure between the inside of the wall and the surrounding fluid and the bacteria cells in effect, burst. If the cells do not immediately burst, the bacteria cells will continue to grow without dividing or developing new cell wall, the wall gets weaker, and eventually ruptures (lysis).


    Other antibiotics work by inhibiting the protein synthesis or nucleic acid synthesis. Tetracycline is a bacteriostatic antibiotic that binds to ribosomes in bacteria, preventing the cell from making proteins and stunting further growth.

    Consideration must be made when choosing antibiotics to treat diseases as bacteria have a different type of ribosome (70S) to humans (80S), and will only work to target the bacteria. Narrow spectrum antibiotics target specific reaction in particular microorganisms whereas broad spectrum antibiotics will have an effect on more general features affecting a wide range of pathogens.


    How It Gets Its Shape

    Some people have corkscrew curly hair, while others have thick, straight, shiny hair. The natural appearance of hair is attributed to the shape of the hair. The amount of natural curl that a hair has is determined by its cross-sectional shape. Straight hair has a mostly circular circumference. Strands of curly or kinky hair are flat. The more circular the hair shaft, the straighter the hair. The flatter the shaft, the curlier the hair.

    The cross-sectional shape if a hair also determines the amount of shine that the hair has. Straighter hair is shinier because sebum from the sebaceous gland can travel down the hair more easily. The kinkier the hair, the more difficulty the sebum has traveling down the hair, and the more dry and dull the hair looks.

    And your hair can change color, texture, and thickness (and its location can change—too much in some areas and too little in others) over time as you get older.