Characteristics of Prokaryotic cells

 BASIC COMPONENTS OF PROKARYOTIC AND EUKARYOTIC CELL





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PROKARYOTIC CELL

Prokaryotic cells are the most primitive cells and have simple structural organization.

It has a single membrane system. They include bacteria, viruses, blue-green algae, mycoplasmas, rickettsias, spirochetes etc. Cyanobacteria or blue green algae are the largest and most complex prokaryote, in which photosynthesis of higher plants type have evolved. Prokaryotes are included in the kingdom Monera and the super kingdom Prokaryota.


 The Prokaryotes have the following characters:

1. The size of prokaryotic cells ranges between 1 to 10 µm. They occur in a variety of forms.

2. Prokaryotic cell consists of three main components:

(I) Outer covering: It is composed of inner cell or plasma membrane, middle cell wall and outer slimy capsule.

a. Cell membrane: Cell membrane made up of lipids and proteins, is thin and flexible and controls the movement of molecules across the cell. Respiratory enzymes are carried by it for energy releasing reactions. Mesosomes, the in-folds of plasma membrane bears respiratory enzymes and these are considered analogous to mitochondria of eukaryotic cells. Similarly, the pigments and enzymes molecules that absorb and convert the light into chemical energy in photosynthetic cells are also associated with the plasma membrane’s in-folds called photosynthetic lamella.


These lamellae are analogous to the chloroplast of eukaryotic cells. Plasma membrane plays role in replication and division of nuclear material. Since the in-folds remain continuous with the cell membrane, they are not considered as separate compartments. Thus, prokaryotic cell is non compartmentalized.




b. Cell wall : It is a rigid or semi-rigid non-living structure that surrounds the cell membrane and its thickness ranges between 1.5 to 100 µm. Chemically it is composed of peptidoglycans. . Some bacteria such as mycoplasmas lack cell wall.



c. Slimy capsule: A gelatinous coat outside the cell wall is the slimy capsule. It is composed of largely of polysaccharides and sometimes it may have polypeptides and other compounds also. It protects the cell against desiccation, virus attacks, phagocytosis and antibiotics.


Read: Differentiation of Antigenicity, Immunogenicity and Antigenicity


(II) Cytoplasm: Prokaryotic cytoplasm contains proteins, lipids, glycogen and inorganic ions along with enzymes for biosynthetic reactions and ribosomes, tRNA and mRNA for protein synthesis. Prokaryotic cytoplasm has some special features as follows:


a. It lacks cell organelles like endoplasmic reticulum, mitochondria, Golgi apparatus, Centrosomes, vacuoles, Lysosomes, microfilaments, intermediate filaments and microtubules.


b. The only cytoplasmic organelle found in prokaryotic cells is the ribosomes. They are smaller than eukaryotic ribosomes i.e., 70S and lie free in the cytoplasm. They form poly-ribosomes at the time of protein synthesis. They are the sites of protein synthesis.



c. Like eukaryotic cells, the cytoplasm of prokaryotic cell does not show streaming movement or cyclosis.


d. Gas vacuoles are also formed in some prokaryotic cells.


e. The cell does not show phagocytosis, pinocytosis and exocytose, substances enter and leave the cell through the cell membrane.


f. They may contain deposits of polysaccharides or inorganic phosphates.


Read:  General characteristics of Fungi 


(III) Nucleoid: Nuclear envelope is absent in prokaryotic cell and the genetic material lies directly into the cytoplasm. Such nuclear material is known as nucleoid.

Nucleoid consists of greatly coiled single pro-chromosome. It shows the following special features:

a. A short and simple pro-chromosome is present which is attached at least at one point on cell membrane.

b. Mostly there is single copy of chromosome, the prokaryotic cell is haploid.

c. The DNA is naked as it is not associated with basic histone proteins. It is double stranded, helical and circular.

d. The amount of DNA is lesser than eukaryotic cell and it codes fewer proteins.

Replication of DNA is continuous throughout the cell cycle. Transcription and transl, ation occurs in cytoplasm and processing of mRNA is not required.

e. The processes like meiosis, gamete formation or fertilization are absent. Conjugation is seen in some bacteria.

f. Mitotic apparatus absent.

g. There is no nucleolus.

h. Cell membrane folds or mesosomes help to segregate the replicated products of chromosomes into daughter cells.


3. Plasmids: In some prokaryotic cells, in addition to nucleoid, a small circular double stranded DNA molecule is present. It is called plasmid. Plasmids have 1000 to 30,000 base pairs and they generally encode proteins required by the organism to resist antibiotic and other toxic material.


4. Flagellum: It is a whip like locomotory structure found in many bacteria. It is 150Å thick and 10 to 15µm long. As the flagellum does not have any surrounding membrane, it grows at the tip.



It has two main parts: Filament and basal body.

(i) Filament- Filament extends out of cell into the medium and it is composed of many intertwined spiral chains of the subunits of a protein called flagellin. Flagellin differs from actins or tubulin.



(ii) Basal Body- The basal body attaches the flagellum to the cell and generates the force to rotate it. It is composed of many components and numerous proteins. It has two parts: shaft and hook.


5. Pili: These are short, rod like non-motile processes or fimbriae present on many bacteria. These are formed of pilin protein. They are usually less than 10 nm thick.

They help in attachment of bacteria to surfaces or food or to one another. Tubular sex. Pili are present in some bacteria.


Prokaryotic cells have all the biochemical mechanisms required to synthesize complex organic materials from simple organic precursors necessary for life. Thus, inspite of being simple in structure prokaryotes are more versatile in their synthetic activities than eukaryotes.



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