Prokaryotic Cell and its Ultrastructure

Since our 7th or 8th, we have been hearing the word prokaryotic cells and if you've taken Biology for your 11th and 12th you will still continue to hear it. You might have not spent time reading it because it might seem too boring in the textbook, so it make it simple in today's article we are going to be learning about prokaryotic cell. 

What is Prokaryotic Cell ?  

Prokaryotic cells are small, simple and the most primitive type of cells. These were the first to come into existence 3.5 billion years ago. The size of these cells ranges from 100nm to 500mm. They are known for your their rapid rate of multiplication which is more than eukaryotic cells. 

Examples of prokaryotic cells are - Blue-Green Algae (BGA), Mycoplasma, Rickettsiae, Spirochaetes, etc.. There two ways by which these cells can be classified - On the basis of size and Gram stain. If the cells pick up gram stain that means they are gram positive and the rest are gram negative. 

There are four forms which are as follows:
  1. Cocci - These are spherical or ovoid shaped found in groups of two or in chains of many cells. 
  2. Bacilli - These are rod shaped which are single at start but once they start to divide they form may clusters of two. If there are more than two cells in the cluster, they are recognised as Streptobascilli. 
  3. Vibrios - These cells are slightly curved which makes them look like a comma (,). 
  4. Spirilla - These cells are long and twisted. Sometimes they coil like a screw. 

Ultrastructure of Prokaryotic Cell:

Though prokarytoic cell may be a simple, primitive cell, it has a complex structure. The structure consists of - Cell envelope, cytoplasm, inclusion bodies, genetic material, plasmids, flagella and fimbriae or pili


We will looking at each component of the prokaryotic cell in the following headings. 

Cell Envelope:

The prokaryotic cell has a chemically complex cell envelope. The layers of the envelope are arranged one upon another. There are three basic layers of the cell envelope - Glycocalyx (Outermost), cell wall and cell membrane (Innermost). Each layers has its own function but altogether they act as a single protective covering

Glycocalyx:

Glycoclalyx is the outermost layer of the cell envelope. It is made up of macromolecules which helps in sticking to the neighbouring cells. In some bacteria, glycocalyx may be made up of slime layers or a capsule. 
  • Slime layer - This layer helps in sticking to the neighbouring cells, prevents the cells from losing moisture (Desiccation) and provides nutrients and water to the cell. 
  • Capsule - Capsule is formed by polysaccharides and gives mechanical support and protection to the cell. They may or may not be immunogenic (substances that produces an immune response). 
Slime layer as well as the capsule may also contains proteins. Different chemical composition and thickness is seen in different bacteria. Overall functions of glycocalyx are as follows: 
  1. Prevents desiccation. 
  2. Protects from phagocytes. 
  3. Protects from toxic chemicals and drugs. 
  4. Protects from viruses. 
  5. Highly specific and immunogenic (substances that produces an immune response). 

Cell Wall:

The cell wall is the second layer of the cell envelope below the glycocalyx. It is rigid and a solid covering which gives shape and structural support to the cell. The rigidity comes because of the presence of macromoecule peptidoglycan

The cell wall prevents osmosis and also protects from bursting or collapsing when in hypotonic solution. Though it can be destructed by hydrolysis by action of lysozymes or by preventing the cross linking by the help of various antibiotics. 

The thickness and the composition of the cell wall differs in gram +ve and gram -ve bacteria. In gram +ve bacteria the cell wall is 20-80 nm thick composing tightly bound of teichoic acids. They are made up of peptidoglycan

In gram -ve bacteria the cell wall is wavy, double layered and 8-12 nm thick. The outer layer also called the outer membrane consists of lipopolysaccharides, lipids and proteins (Porins). Porins help in making a pathway for the entry and exit of hydrophilic low molecules weight substances. The inner layer is made up of peptidoglycan (Gives the cells its structural framework), proteins, non-cellulose carbohydrates, lipids, amino acids, etc.. 

Cell Membrane:

Cell membrane also known as plasma membrane is a living, ultrathin (6 to 8 nm thick) and dynamic. It is a selectively permeable membrane. It is the basic characteristic of each cell. Few carrier molecules are embedded in the membrane which helps to control the flow of specific molecules by binding to them and transporting it in a specific direction. 

It shows fluid mosaic model which comprises of lipids (phospholipids, glycolipids and cholestrol molecules) and protein molecules. They are amphipathic in nature i.e, having hydrophilic as well as hydrophobic ends. Few oligosaccharides and water molecules are also present. 

Functions of cell membrane:
  1. It separated the cells from the adjacent cells. 
  2. Allows only specific ions or molecules to enter or leave the cell. 
  3. It checks if there is not much loss of essential substances from the cell. 
  4. They ensure active uptake of nutrients. 
  5. Since there are no cell organelles, the essential enzymes are attached to the membrane which helps the membrane to participate in different biochemical reactions. 
  6. Takes part in metabolic processes like photosynthesis or chemosynthesis, respiratio (Aerobic or anaerobic), lipid synthesis, etc.. 
  7. Acts as a barrier between the environment and the interior of the cell. 
  8. Helps in intercellular communication. 
  9. Receptor molecules have bacteria to find and respond to chemicals in the surrounding. 

Cytoplasm:

Cytoplasm is a semi-fluid ground substance which is interior to the cell membrane. It provides suitable environment for cellular activities because of the presence of inorganic and organic compounds. The cytoplasm is where all the metabolic activities take place. 

Cytoplasm does not show presence of cytoplasmic streaming and lacks membrane-bound organelles like mitochondria, endoplasmic reticulum, golgi bodies and lysosomes. They show presence of mesosomes, chromatophores and inclusion bodies

Mesosomes:

Mesosomes are extensions or invagination (cavity or a pouch) of the plasma membrane in the form of vesicles, tubules and lamellae. These are commonly found in gram -ve bacteria. Even though they were discovered years ago their exact function is still not know. 

Scientist have postulated (assumed) that mesosomes are involved in the following:
  1. In the formation of cell wall. 
  2. Replicating chromosomes and distributing it to the daughter cells. 
  3. In secretory processes.
  4. Helps in increasing the surface area and enzymatic content of the plasma membrane. 
  5. Helps in respiration. 
Chromatophores:

Chromatophores are internal membrane system which contains pigments like bacteriochlorophylls, bacteriophaeophytin and carotenoids which are necessary for different functions. They are commonly found in photosynthetic bacteria and cyanobacteria

In nitrifying (nitrogen fixing) bacteria, these membranes form round flattened or tubular vesicles by mass of fragments which increases the surface of the membrane for large metabolic activity. 

Inclusion Bodies:

Inclusion bodies also known as storage granules are reserved food materials stored in the form of granules. These are not bounded by any membrane and are free. Eg, cyanophycean granules, phosphate granules, glycogen granules, etc.. 

Some granules have a single non-unit layer membrane, 2 to 4 nm thick surrounding them, eg., poly-β-hydroxybutyrate granules. There are three types of granules on the basis of their nature - Gas vacuoles, inorganic inclusions and food reserves

● Gas vacuoles:
       They store gas.
       They have no covering of their own but each vesicle is surrounded by single, non-unit, no-lipid, protein membranes.
       These are found in cyanobacteria, purple and green photosynthetic bacteria and a few other free-floating aquatic forms. The gas vacuoles helps the bacteria float. 
       These are in the form of small hollow, cylindrical masses of gas vesicles. These are non-permeable to atmospheric gases. 
● Inorganic inclusions:
        They are also known as metachromatic granules because they have the ability to take different colours from basic dyes. 
       There are two major inorganic inclusions - Phosphate granules or volutin granules and sulphur granules. 
       Phosohate/volutin granules are refractile (can change the direction) granules of metaphosphate polymers. They act as a storage reservoir for phosphate. 
       Sulphur granules are found in the bacteria which live in sulphur rich environment. They are formed when bacteria uses hydrogen sulphide as electron donor during photosynthesis. They get accumulated in periplasmic space (space between the outer and the inner membrane) or in special cytoplasmic globules.
● Food reserves - These are in the form of glycogen and poly-β-hydroxybutyrate granules. 

Ribosomes:

Ribosomes are dense particles made up of RNA and protein. They are chemically and structurally complex. Ribosomes are of 70S type. It has two sub-units - Larger 50S and smaller 30S which makes up 70S. Here, 'S' is denoted as sedimentation coefficient or Svedberg's number. 

They have a measurement of 14 to 15 nm X 20 nm and are found in the cytoplasm and/or plasma membrane. Their main function is to synthesize proteins. The ribosomes on the plasma membrane synthesizes proteins that releases out of the cell and the ribosomes present in the cytoplasm synthesize proteins required enzymes for the reactions taking place inside the cell. 

Many ribosomes are attached to single mRNA which forms a complex known as polyribosomes or polysomes. Polysomes helps to translate the message from mRNA into proteins this way it creates several copies of the same protein. 

Genetic Material (Nucleoid):

The nucleoid contains the genetic material of the cell and is scattered in the cytoplasm of the cell. It shows no membrane bound, well-defined nucleus. The nucleoid composes DNA which is not packed in chromosome but in the nucleoid. The length of the DNA is more than that of the cell (250-700 times), because of this length the DNA is looped and coiled with the help of proteins in the nucleoid. Histone is absent. 

Small traces of RNA and proteins may also be present in the nucleoid. Proteins in prokaryotic nucleoid is different form that of eukaryotic cell. In eukaryotes, the nucleoid is usually associated with mesosomes but in the case of prokaryotes it is attached to the plasma membrane. 

Hence, it is concluded that DNA is attached to the cell membrane where the cell membrane helps to distribute the duplicated DNA into daughter cells during division. Additional accessory DNA strand is seen in some bacteria called plasmid

Plasmid:

Plasmids are extra chromosomes which self replicate from DNA. Its main function is to give antibiotic resistance and fertility

Out of all the plasmids, some might be bacteriophage (viral) DNA which may be formed directly from the chromosomes and other may be formed by separated parted of the normal genome of the same or a foreign cell and the combining with the main chromosome. 

Flagella:

Many prokaryotic cells are motile in nature and contains flagella for locomotion of the cells. The prokaryotic flagella is longer than that of eukaryotic flagella. Its ogranisation is simple with 15 to 20 nm in diameter and 20 μm

The flagella shows three parts under the electron microscope - Filament, Hook and Basal Body
● Filament:
       It is the longest part of the flagella. 
       ◑ It is a hollow, rigid and tubular structure. 
       ◑ It extends from the top surface of the cell.
       ◑ It is 1 to 70 nm in length and 20 nm in diameter. 
       ◑ It is made up of protein called flagellin which are arranged in a spiral manner. 
       ◑ The filament is inserted in a curved hook which is the middle section and is attached to the basal body. 
● Hook:
       It is the middle, curved, tubular structure. 
       ◑ It is made up of different subunits of protein. 
       ◑ It is the thickest part of the flagella. 
       ◑ It connects the basal body and the filament. 
● Basal Body:
       It is the most complex part of the flagella. 
       ◑ In gram -ve bacteria there are four basal rings connected to the central rod. 
       ◑ In gram +ve bacteria there are two basal rings connected to the central rod. 
       ◑ The inner ring is connected to the plasma membrane and the outer ring is connected to the peptidoglycan membrane. 

The flagella is not of whiplash type. The three parts of the flagella are fitted in such a way that the filament can rotate at 360° and move the body in the opposite direction so that the cell is pushed forward. 

Classification of bacteria on the basis of flagella:
  1. Atrichous - Flagella is absent. 
  2. Monotrichous - Only one flagellum present. 
  3. Aphitrichous - One flagellum present on either sides. 
  4. Cephalotrichous - One or more flagella present on one side. 
  5. Lophotrichous - Flagella on both sides in groups. 
  6. Petritrichous - Numerous flagella present on the entire surface of the cell. 

FAQ:

1.  What are 5 examples of prokaryotic cells ?
Ans: Examples of prokaryotic cells are - Blue Green Algae, Mycoplasm, Rickettsiae, Spirochaetes, Escherichia Coli Bacterium. 

2. What are the 3 main features of a prokaryotic cell ?
Ans: The three main features of prokaryotic cell - No well defined nucleus, nucleoid associated with plasma membrane and longer flagella. 

3. Is bacteria a prokaryotic cell ?
Ans: Bacteria are made up of prokaryotic cells. Prokaryotic cell is also known as bacterial cell. 

4. What is unique about prokaryotic DNA ? 
Ans: The DNA in prokaryotic cell is not embedded in a nucleus instead it is free-floating and less complex. 

5. What structures does a prokaryotic cell have ? 
Ans: Prokaryotic cell consists of the following structures - Cell Envelope, Cytoplasm, Ribosomes, Genetic Material, Plasmid and Flagella. 

6. Do prokaryotic cells have a cell wall ? 
Ans: Yes, prokaryotic cells do have a cell wall. 

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