Prokaryotic Cells

Prokaryotic cells

The best-known examples of prokaryotic cells are bacteria, blue-green algae, PPLO, and Mycoplasma. Prokaryotic cells multiply more rapidly than eukaryotic cells and are also smaller in size. Their basic shapes are rod-like (bacillus), comma-shaped (vibrio), spherical (coccus), and spiral (spirillum). Despite exhibiting great diversity in shapes, their basic organization remains similar. Except for Mycoplasma, all prokaryotes have a cell wall surrounding their cell membrane. The cytoplasm contains a non-membrane-bound nucleus, containing naked genetic material. Apart from genomic DNA additional circular DNA is also present in prokaryotes known as Plasmid DNA. This plasmid DNA is responsible for imparting unique phenotypes to bacteria. The plasmid DNA also offers antibiotic resistance to the bacterium, which is helpful in case of bacterial transformation.  Except for Ribosomes, no other organelle is seen in prokaryotes. 

A prokaryotic cell envelope is composed of a tightly bound triple-layered structure. The outermost part of this triple layer is the glycocalyx followed by a cell wall and then the plasma membrane. Each layer has a specific function. Based on the distinctions in the cell envelopes, bacteria are classified into 2 groups namely Gram-positive and Gram-negative. Due to the differences in cell envelops, the bacteria which respond to the Gram stain are regarded as positive whereas the ones that do not take up the stain, are known as Gram-negative.

The composition of glycocalyx differs among different bacterial groups. In some groups, it appears as a loose slimy layer while in others it is present as a thick and sturdy capsule. The cell wall maintains the shape of the cell and provides resistance to the bacterium against disintegration or bursting. The plasma membrane establishes the connection of cells with the outside world. It is selectively permeable in both prokaryotes and eukaryotes.

A unique characteristic of Prokaryotes is the presence of Mesosomes, which are the infoldings of the plasma membrane into the cell. These infoldings can be tubular, lamellar, or vesicular in appearance. They are responsible for cell wall formation, DNA replication, respiration, and secretion processes. In cyanobacteria, other cell membrane modifications are seen which extend into the cytoplasm. They contain pigments and are known as Chromatophores.

Motile bacterial cells possess thin filamentous extensions arising from the cell wall, known as Flagella. A single flagellum consists of a filament, a hook, and a basal body.  Apart from flagella, Pili and Fimbriae are also found on the bacterial surface although they have no role to play in motility. Pili are elongated structures made up of proteins and Fimbriae are bristle-like structures erupting from the cell surface. They are known to help the bacteria attach to host tissues or surfaces. 

The prokaryotic plasma membrane encloses ribosomes. These ribosomes consist of 2 subunits namely 50s (15nm) and 30s (20nm). Together they form the 70s ribosomal subunit. Ribosomes are known as the site of protein synthesis. They often attach themselves to a single mRNA and form a chain called polyribosome which eventually translates mRNA into proteins.

Prokaryotic cells store their unused or reserved material in specialized non-membranous structures called inclusion bodies which are present in the cytoplasm. Examples of inclusion bodies include glycogen granules, phosphate granules, and cyanophycean granules.  In blue-green and purple photosynthetic bacteria, gas vacuoles are also present. 

Figure 2: Features of a Prokaryotic cell.
 

Prokaryotic Cells

1. Cell wall present (bacteria) or absent (mycoplasma)

2. A prokaryotic cell is a single membrane system.

3. Cell membrane bears respiratory enzymes.

4. Mesosomes are formed by infolding of cell membrane.

5. Cytoplasm lacks membrane bound organelles

6. Ribosomes (non membrane bound organelle) are70S, lie free in cytoplasm.

7. There are no streaming movements of cytoplasm.

8. Photosynthetic lamellae i.e., thylakoids (if present) occur free in the cytoplasm.

9. Sap vacuoles are lacking. Gas vacuoles may occur.

10. Transcription and translation occur in the cytoplasm.

11. Protein synthesis takes place in cytoplasm only.

12. Cytoskeleton absent.

13. Nuclear material is not enclosed by nuclear envelope and lies directly in cytoplasm. It is called nucleoid.

14. There is no nucleolus.

15. DNA is closed and circular and without histone core (Polyamines may be present in place of histones)

16. DNA occurs in the cytoplasm only.

17. Plasmids and pili occur in many prokaryotic cells.

18. Flagella, if present, are singlet fibres (9 + 0) and are formed of a protein flagellin.

19. Mitotic spindle is not formed in cell division (Amitotic).

20. Sexual reproduction absent (recombination is present in bacteria)

21. e.g., Bacteria, blue-green algae and mycoplasmas.