Cell Structure and Organization — Biology STD 11 Science — Question

General characteristics of prokaryotic cell:
1. Prokaryotic cells are primitive type of cells.

2. It does not have membrane bound cell organelles (like endoplasmic reticulum, Golgi complex, mitochondria, etc.) and well-defined nucleus (nuclear membrane is absent).

3. Genetic material is in the form of nucleoid.

4. Cell envelope:
a. Prokaryotic cell has chemically complex protective cell envelope having glycocalyx, cell wall and plasma membrane.
b. In some bacteria, glycocalyx occurs in the form of a slime layer (loose sheath). Other bacteria may have a thick and tough covering called capsule. It helps in protection of bacterial cell.

5. Cell wall:
The Gram-positive bacteria show presence of peptidoglycan layer in the cell wall and Gram-negative bacteria show presence of murein in the cell wall. It gives mechanical strength to the cell.

6. Cell membrane:
a. It is the innermost covering of the cell envelope, chemically composed of lipids and proteins.
b. It helps in intercellular communication.
c. Cell membrane shows infoldings called mesosomes which help in cell wall formation, cellular respiration and DNA replication.
d. The cyanobacteria show longer extensions called as chromatophores which carry photosynthetic pigments.

7. In motile bacteria either cilia or flagella are found. Both are driven by rotatory movement produced by basal body (which works as motor) of flagellum. Other parts of flagellum are filament and hook.

8. Some other surface projections are the tubular pili (which help in inter-cellular communication) and fimbriae (for clinging to support).

9. Ribosomes:
Bacterial cell cytoplasm contains dense particles called ribosomes which help in protein synthesis. Ribosomes are 70S type (composed of a larger sub-unit 50S and + smaller sub-unit 30S).