Does osmosis take place in prokaryotic cells?

Does osmosis take place in prokaryotic cells?

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As far as I know, osmosis occurs in Eukaryotic cells, and I'm wondering if it could take place in prokaryotic cells too.

Osmosis works across every cell membrane along a concentration gradient as its a physico-chemical principle. Water can cross the membrane (or cell wall), while the substance dissolved in it (for example salts) can not. Because eukaryotic cells only have a cell membrane, they will burst eventually, while bacteria (and also plant cells) have a more rigid cell wall, which will mostly prevent bursting. However the influx (or outflux) of water creates a pressure which is called turgor pressure. How this works is shown below (figure from here), bacterial cells and plant cells work pretty much the same way:

The force driving osmosis exists in any system with an imbalance of solute molecules across a semipermeable membrane.

Think of a concentration gradient as an electrical potential, where high concentration is negative charge and low concentration is positive charge. In the case of electricity, its the magnetic force which causes the interactions leading to charge equilibrium.

In osmosis, there is no communication between solute molecules by an 'osmotic force'. Osmosis is a result of entropy. In chemistry, a separation of solute concentrations is called a chemical potential. As entropy increases in the system, this chemical potential is decreased, either endo or exothermically.

Here's a chart showing the change in heat for the dissolution of substances. Negative numbers mean a system loses heat (enthalpy) and is exothermic. Cold packs use ammonium nitrate, which absorb heat as they are dissolved in water.

The processes of osmosis, entropy are fundamental to our universe, not to a specific domain of life.

Enthalpy change of solution for some selected compounds hydrochloric acid -74.84 ammonium nitrate +25.69 ammonia -30.50 potassium hydroxide -57.61 caesium hydroxide -71.55 sodium chloride +3.87 potassium chlorate +41.38 acetic acid -1.51 sodium hydroxide -44.51 Change in enthalpy ΔHo in kJ/mol in water at 25°C[1]

Prokaryotic Cells

The Earth has formed 4.5 billion years ago and on the Earth, which arose the first life form in the form of Prokaryotic cells. These unicellular creatures are primordial and are building blocks of multicellular organisms. Life is supposed to have originated from the oceans which is why embryos of terrestrial and aerial animals still have gill clefts in some phases of their ontogenic development. It took 3 million years for the first cell to have existed on the earth. Prokaryotes cells are extremely simple in their structure. If we split the word ‘PROKARYOT’, we get two words- Pro, meaning Primitive and Karyon, which means the nucleus. The Prokaryotic cells are not as complex as the eukaryotic structures. They did not have a true nucleus and the genetic material was suspended in the cytoplasm called a nucleoid. Example – bacteria.

Structure of Prokaryotic Cell:

Cell Envelope:

The cell envelope is the outer covering of the cell and gives shape to the cell and protects the cell organelles. It consists of the following 3 layers:

It is found in some of the bacterial cells and is mainly composed of the macromolecules. It protects the contents of the cell and is of two forms: The Capsule and The Slime Layer. The Capsule is thick, strong and provides mechanical support to the cell. It is immunogenic in nature. And because of their thick skin, they sometimes get on people’s nerves. Not literally but this capsule is so strong that it can withstand the attack of the White Blood Cells. Moving on, the capsule is a layer made by the firm gathering of polysaccharides which is a distinguishing character between the capsule and the slime layer. The Slime layer is also known as loose sheath because here the glycoprotein molecules are loosely arranged. This layer helps to maintain moisture in the cell.

The cell wall is normally absent in the Prokaryotic cells. However, if it is present, it is made up of poly peptidoglycans. Peptidoglycans are only found in the cell walls of bacteria. It helps in shape maintenance and in osmosis and transporting nutrients in and out of the cells. Peptidoglycans are alternating units of N- acetylglucosamine and N- acetylmuramic acid. They help in the transportation process, for certain nutrients, these bacteria have to use another way. If the nutrients are too large to be taken inside through the pores, certain enzymes are used. These enzymes convert the nutrients into smaller or simpler substances that can be easily taken in by the cell. And the cytoplasm is responsible for the secretion of these ‘Exoenzymes’. However, certain bacteria do not have cell walls. These bacteria use certain proteins as a protective covering. And sometimes this helps in getting the antibiotics. Smart technology there!

Plasma Membrane:

The Plasma Membrane is the innermost covering and is made of amphipathic molecules. That means that these molecules have hydrophilic and hydrophobic ends. Most of these molecules are the proteins, lipids and cholesterols. Universally, The Fluid Mosaic Model has been accepted as a structure of the plasma membrane. This model represents the membrane to be like a sea of lipids with protein icebergs floating on it and in it. The proteins which are completely submerged as known as intrinsic proteins and the ones that are outside the lipid layers are known as the extrinsic proteins. Some protein passes through the lipid layers and they are known as tunnel proteins. Their positions affect their solubility in the lipids. This shall be discussed later. Thus based on the plasma membrane’s mosaic-like structure, Seymour Jonathan Singer and Garth L. Nicolson put forth the FLUID MOSAIC MODEL in the year 1972. The plasma membrane is extremely important for all the life forms. It not only separates the contents but also helps in the exchange of materials and helps in the uptake of the nutrients that are essential to the cell. Numerous activities to take place in the cell membrane.

Prokaryotic cells are primitive cells and hence do not show well-defined membrane-bound organelles like the ones in the eukaryotic cells. But there are some membrane-bound organelles and these are mesosomes and certain pigment containing chromatophores.

These are formed by the invaginations of the plasma membrane or the cell membrane. Invaginations are nothing but certain infoldings. These mesosomes are mostly seen in the gram-negative bacteria and can be of the form of tubules, vesicles and lamellae.

They help to form the cell wall and increase its surface area. They especially help in respiration since the respiratory enzymes are associated with them. In eukaryotes, these enzymes are present in the mitochondria. Mesosomes also help in the replication of the DNA. There are two types of mesosomes. Septal: the ones which extend towards the centre of the cell. and Lateral: the ones that are peripheral.


The cyanobacteria like Nostoc and Anabaena have chromatophores consisting of the pigments that are necessary for photosynthesis.

The cytoplasm consists of water, enzymes, salts and other compounds. It is like a semi-liquid structure and does not show cytoplasmic streaming or cyclosis. It appears to be granular because of ribosomes and inclusion bodies.

Inclusion Bodies:

  • Organic Inclusion Bodies: These are phosphate granules, poly beta-hydroxybutyrate granules, carboxysomes, cyanophycean granules, glycogen, gas vacuoles and more. The gas vacuoles give buoyancy to the aquatic plants. This helps in the process of photosynthesis since the plants can trap sunlight from the atmosphere.
  • Inorganic inclusion Bodies: These are Phosphate and sulphur granules. They are known as metachromatic granules due to their ability to take various colours. The phosphate granules store phosphate. The sulphur granules are formed when H2 S is used as a hydrogen donor.

In prokaryotic cells, the ribosomes are of a 70S type. These have small 30S subunit and large 50S subunit. The small subunit consists of rRNA of 16s type and the large subunit have 23S and 5S type. Subunits are long RNA with proteins on them. And these subunits help in the protein synthesis by locking in together with each other. The main function of the ribosomes is as mentioned earlier, protein synthesis.

As the name prokaryotic suggests, a true nucleus is absent in these kinds of cells. The circular and double-stranded DNA is known as the genome. The histone proteins that are present in the eukaryotic cells are absent in the prokaryotes. The basic function of the histone proteins is to hold the DNA together and it affects the gene regulation. There are 11 types of histone proteins viz. H2A, H2B, H3K4, H3K9, H3K27, H3K36, H4K5, H4K8, H4K12, H4K16, H4K20, each having a different composition. The DNA is 1 micrometre long and is attached to the plasma membrane through the mesosomes. The DNA has 3000-4000 genes. A looped domain can be seen which is a structure formed by the tightly coiled structure of the DNA. This looped domain is held in position by RNA molecules.

The extrachromosomal, self-replicating units are known as plasmids. The plasmids are circular and have a double-stranded DNA. Plasmids serve as agents for gene transfer and hence are used in Recombinant DNA Technology as vectors or vehicles for carrying proteins. They have anti-biotic, metal and drug resistance. For bacterial fertility, episomes which are a type of plasmid are highly important. Episomes have a self-replication capacity.

Other Structures:

Flagellum, used for locomotion Pili, used in the mating process. Fimbrae, used for the clinging of the cells and Spinae, the appendages used by the cell for the adjustment to the external environmental conditions like temperature, pH, salinity, etc are some of the structure present in the Prokaryotic cells.

Photosynthesis in Prokaryotes

The two parts of photosynthesis—the light-dependent reactions and the Calvin cycle—have been described, as they take place in chloroplasts. However, prokaryotes, such as cyanobacteria, lack membrane-bound organelles (including chloroplasts). Prokaryotic photosynthetic organisms have infoldings of the plasma membrane for chlorophyll attachment and photosynthesis (Figure 1). It is here that organisms like cyanobacteria can carry out photosynthesis.

Figure 1 A photosynthetic prokaryote has infolded regions of the plasma membrane that function like thylakoids. Although these are not contained in an organelle, such as a chloroplast, all of the necessary components are present to carry out photosynthesis. (credit: scale-bar data from Matt Russell)


The reverse process of moving material into a cell is the process of exocytosis. Exocytosis is the opposite of the processes discussed in the last section in that its purpose is to expel material from the cell into the extracellular fluid. Waste material is enveloped in a membrane and fuses with the interior of the plasma membrane. This fusion opens the membranous envelope on the exterior of the cell, and the waste material is expelled into the extracellular space (Figure 4). Other examples of cells releasing molecules via exocytosis include the secretion of proteins of the extracellular matrix and secretion of neurotransmitters into the synaptic cleft by synaptic vesicles.

Figure 4. In exocytosis, vesicles containing substances fuse with the plasma membrane. The contents are then released to the exterior of the cell. (credit: modification of work by Mariana Ruiz Villareal)

A summary of the cellular transport methods discussed is contained in Table 1, which also includes the energy requirements and materials transported by each.

Table 1. Methods of Transport, Energy Requirements, and Types of Material Transported
Transport Method Active/Passive Material Transported
Diffusion Passive Small-molecular weight material
Osmosis Passive Water
Facilitated transport/diffusion Passive Sodium, potassium, calcium, glucose
Primary active transport Active Sodium, potassium, calcium
Secondary active transport Active Amino acids, lactose
Phagocytosis Active Large macromolecules, whole cells, or cellular structures
Pinocytosis and potocytosis Active Small molecules (liquids/water)
Receptor-mediated endocytosis Active Large quantities of macromolecules
Exocytosis Active Waste materials, proteins for the extracellular matrix, neurotransmitters

In Summary: Endocytosis and Exocytosis

Cells perform three main types of endocytosis. Phagocytosis is the process by which cells ingest large particles, including other cells, by enclosing the particles in an extension of the cell membrane and budding off a new vesicle. During pinocytosis, cells take in molecules such as water from the extracellular fluid. Finally, receptor-mediated endocytosis is a targeted version of endocytosis where receptor proteins in the plasma membrane ensure only specific, targeted substances are brought into the cell.

Exocytosis in many ways is the reverse process from endocytosis. Here cells expel material through the fusion of vesicles with the plasma membrane and subsequent dumping of their content into the extracellular fluid.

Role of Prokaryotes in Ecosystems

Prokaryotes are ubiquitous: There is no niche or ecosystem in which they are not present. Prokaryotes play many roles in the environments they occupy. The roles they play in the carbon and nitrogen cycles are vital to life on Earth.

Prokaryotes and the Carbon Cycle

Carbon is one of the most important macronutrients, and prokaryotes play an important role in the carbon cycle (Figure 2). Carbon is cycled through Earth’s major reservoirs: land, the atmosphere, aquatic environments, sediments and rocks, and biomass. The movement of carbon is via carbon dioxide, which is removed from the atmosphere by land plants and marine prokaryotes, and is returned to the atmosphere via the respiration of chemoorganotrophic organisms, including prokaryotes, fungi, and animals. Although the largest carbon reservoir in terrestrial ecosystems is in rocks and sediments, that carbon is not readily available.

A large amount of available carbon is found in land plants. Plants, which are producers, use carbon dioxide from the air to synthesize carbon compounds. Related to this, one very significant source of carbon compounds is humus, which is a mixture of organic materials from dead plants and prokaryotes that have resisted decomposition. Consumers such as animals use organic compounds generated by producers and release carbon dioxide to the atmosphere. Then, bacteria and fungi, collectively called decomposers, carry out the breakdown (decomposition) of plants and animals and their organic compounds. The most important contributor of carbon dioxide to the atmosphere is microbial decomposition of dead material (dead animals, plants, and humus) that undergo respiration.

In aqueous environments and their anoxic sediments, there is another carbon cycle taking place. In this case, the cycle is based on one-carbon compounds. In anoxic sediments, prokaryotes, mostly archaea, produce methane (CH4). This methane moves into the zone above the sediment, which is richer in oxygen and supports bacteria called methane oxidizers that oxidize methane to carbon dioxide, which then returns to the atmosphere.

Figure 2. Prokaryotes play a significant role in continuously moving carbon through the biosphere. (credit: modification of work by John M. Evans and Howard Perlman, USGS)

Prokaryotes and the Nitrogen Cycle

Nitrogen is a very important element for life because it is part of proteins and nucleic acids. It is a macronutrient, and in nature, it is recycled from organic compounds to ammonia, ammonium ions, nitrate, nitrite, and nitrogen gas by myriad processes, many of which are carried out only by prokaryotes. As illustrated in Figure 3, prokaryotes are key to the nitrogen cycle. The largest pool of nitrogen available in the terrestrial ecosystem is gaseous nitrogen from the air, but this nitrogen is not usable by plants, which are primary producers. Gaseous nitrogen is transformed, or “fixed” into more readily available forms such as ammonia through the process of nitrogen fixation. Ammonia can be used by plants or converted to other forms.

Another source of ammonia is ammonification, the process by which ammonia is released during the decomposition of nitrogen-containing organic compounds. Ammonia released to the atmosphere, however, represents only 15 percent of the total nitrogen released the rest is as N2 and N2O. Ammonia is catabolized anaerobically by some prokaryotes, yielding N2 as the final product. Nitrification is the conversion of ammonium to nitrite and nitrate. Nitrification in soils is carried out by bacteria belonging to the genera Nitrosomas, Nitrobacter, and Nitrospira. The bacteria performs the reverse process, the reduction of nitrate from the soils to gaseous compounds such as N2O, NO, and N2, a process called denitrification.

Figure 3. Prokaryotes play a key role in the nitrogen cycle. (credit: Environmental Protection Agency)

Practice Questions

Which of the following statements about the nitrogen cycle is false?

  1. Nitrogen fixing bacteria exist on the root nodules of legumes and in the soil.
  2. Denitrifying bacteria convert nitrates (NO3 − ) into nitrogen gas (N2).
  3. Ammonification is the process by which ammonium ion (NH4 + ) is released from decomposing organic compounds.
  4. Nitrification is the process by which nitrites (NO2 − ) are converted to ammonium ion (NH4 + ).

Think about the conditions (temperature, light, pressure, and organic and inorganic materials) that you may find in a deep-sea hydrothermal vent. What type of prokaryotes, in terms of their metabolic needs (autotrophs, phototrophs, chemotrophs, etc.), would you expect to find there?

Prokaryotic versus Eukaryotic Gene Expression

To understand how gene expression is regulated, we must first understand how a gene becomes a functional protein in a cell. The process occurs in both prokaryotic and eukaryotic cells, just in slightly different fashions.

Because prokaryotic organisms lack a cell nucleus, the processes of transcription and translation occur almost simultaneously. When the protein is no longer needed, transcription stops. When there is no mRNA present, no protein can be made. As a result, the primary method to control what type and how much protein is expressed in a prokaryotic cell is through the regulation of DNA transcription into RNA. All the subsequent steps happen automatically. When more protein is required, more transcription occurs. Therefore, in prokaryotic cells, the control of gene expression is almost entirely at the transcriptional level.

Eukaryotic cells, in contrast, have intracellular organelles and are much more complex. Recall that in eukaryotic cells, the DNA is contained inside the cell’s nucleus and that is where it is transcribed to produce mRNA. The newly synthesized mRNA is transported out of the nucleus into the cytoplasm, where ribosomes translate the mRNA to produce protein. The processes of transcription and translation are physically separated by the nuclear membrane transcription occurs only within the nucleus, and translation only occurs outside the nucleus in the cytoplasm. The regulation of gene expression can occur at any stage of the process (Figure 1):

  • Epigenetic level: regulates how tightly the DNA is wound around histone proteins to package it into chromosomes
  • Transcriptional level: regulates how much transcription takes place
  • Post-transcriptional level: regulates aspects of RNA processing (such as splicing) and transport out of the nucleus
  • Translational level: regulates how much of the RNA is translated into protein
  • Post-translational level: regulates how long the protein lasts after it has been made and whether the protein is processed into an active form
Figure 1 Eukaryotic gene expression is regulated during transcription and RNA processing, which take place in the nucleus, as well as during protein translation, which takes place in the cytoplasm. Further regulation may occur through post-translational modifications of proteins.

The differences in the regulation of gene expression between prokaryotes and eukaryotes are summarized in Table 1.

Table 1: Differences in the Regulation of Gene Expression of Prokaryotic and Eukaryotic Organisms

RNA transcription occurs prior to protein translation, and it takes place in the nucleus. RNA translation to protein occurs in the cytoplasm.

RNA post-processing includes addition of a 5′ cap, poly-A tail, and excision of introns and splicing of exons.

Isotonic Solution

An isotonic solution (for example, the ECF) has the same osmotic pressure as the ICF. Under these conditions, water passes back and forth across the semipermeable membrane to keep the cell in equilibrium with the surroundings. Adding more solute particles to the ECF changes the osmotic gradient so that the osmotic pressure increases inside the cell and more water flows out across the membrane. If not stopped, the cell will wrinkle and eventually shrivel up and die. Conversely, more solute particles in the ICF causes more water to rush into the cell which may cause it to burst.

Plant cells have a cell wall surrounding the plasma membrane. The effect of an isotonic solution is the same but not as obvious because of the rigid wall. There are observable changes in hypertonic and hypotonic solutions, however, if there is a sufficient difference in the osmotic gradient.

The image above shows what happens to red blood cells in hypertonic, isotonic, and hypotonic solutions.

IV. Post-test to identify whether students have corrected their misconceptions

Identify each statement as CORRECT or INCORRECT . Change each incorrect statement to a correct one.

1. Osmosis can take place if a dead cell is placed in a solution hypotonic to the cell contents. Correct

2. Water movement into a cell through aquaporin channels requires energy investment by the cell.

Incorrect – Water moves into a cell through aquaporin channels by diffusion, which does not require energy investment by the cell.

3. A solution of distilled water is hypotonic.

Incorrect – A solution of distilled water is hypotonic to water containing solute molecules.

4. Diffusion is due to random movements of molecules. Correct

What is Osmosis?

Osmosis is a natural phenomenon taking place regularly in all living beings. It refers to the movement of water molecules from a higher water potential area to lower water potential area through a semi-permeable membrane. Since osmosis occurs along the concentration gradient, it does not use energy. Hence, it is a passive process.

Figure 01: Osmosis

Osmosis is the primary process that facilitates the water movements of cells via the cell membrane in both plant and animal cells. Since the cell membrane is a selectively permeable membrane, it allows selected molecules to pass through it. Therefore, only via osmosis, water molecules and solvent molecules transport in and out the cell in order to balance the solute concentration inside and the outside the cell.

Most prokaryotic cells are much smaller than eukaryotic cells. Although they are tiny, prokaryotic cells can be distinguished by their shapes. The most common shapes are helices, spheres, and rods (see Figure below).

Prokaryotic Cell Shapes. The three most common prokaryotic cell shapes are shown here.

Plasma Membrane and Cell Wall

Like other cells, prokaryotic cells have a plasma membrane (see Figure below). It controls what enters and leaves the cell. It is also the site of many metabolic reactions. For example, cellular respiration and photosynthesis take place in the plasma membrane.

Most prokaryotes also have a cell wall. It lies just outside the plasma membrane. It gives strength and rigidity to the cell. Bacteria and Archaea differ in the makeup of their cell wall. The cell wall of Bacteria contains peptidoglycan, composed of sugars and amino acids. The cell wall of most Archaea lacks peptidoglycan.

Prokaryotic Cell. The main parts of a prokaryotic cell are shown in this diagram. The structure called a mesosome was once thought to be an organelle. More evidence has convinced most scientists that it is not a true cell structure at all. Instead, it seems to be an artifact of cell preparation. This is a good example of how scientific knowledge is revised as more evidence becomes available. Can you identify each of the labeled structures?

Cytoplasm and Cell Structures

Inside the plasma membrane of prokaryotic cells is the cytoplasm. It contains several structures, including ribosomes, a cytoskeleton, and genetic material. Ribosomes are sites where proteins are made. The cytoskeleton helps the cell keep its shape. The genetic material is usually a single loop of DNA. There may also be small, circular pieces of DNA, called plasmids. (see Figure below). The cytoplasm may contain microcompartments as well. These are tiny structures enclosed by proteins. They contain enzymes and are involved in metabolic processes.

Prokaryotic DNA. The DNA of a prokaryotic cell is in the cytoplasm because the cell lacks a nucleus.

Extracellular Structures

Many prokaryotes have an extra layer, called a capsule, outside the cell wall. The capsuleprotects the cell from chemicals and from drying out. It also allows the cell to stick to surfaces and to other cells. Because of this, many prokaryotes can form biofilms, like the one shown in Figure below. A biofilm is a colony of prokaryotes that is stuck to a surface such as a rock or a host&rsquos tissues. The sticky plaque that collects on your teeth between brushings is a biofilm. It consists of millions of bacteria.

Most prokaryotes also have long, thin protein structures called flagella (singular, flagellum). They extend from the plasma membrane. Flagella help prokaryotes move. They spin around a fixed base, causing the cell to roll and tumble. As shown in Figure below, prokaryotes may have one or more flagella.

Bacterial Biofilm. The greatly magnified biofilm shown here was found on a medical catheter (tubing) removed from a patient&rsquos body.

Variations in the Flagella of Bacteria. Flagella in prokaryotes may be located at one or both ends of the cell or all around it. They help prokaryotes move toward food or away from toxins.


Many organisms form spores for reproduction. Some prokaryotes form spores for survival. Called endospores, they form inside prokaryotic cells when they are under stress. The stress could be UV radiation, high temperatures, or harsh chemicals. Endospores enclose the DNA and help it survive under conditions that may kill the cell. Endospores are commonly found in soil and water. They may survive for long periods of time.

Watch the video: Prokaryotic vs. Eukaryotic Cells Updated (November 2022).