- They have no membrane bound nucleus and organelles.
- They include bacteria, blue-green algae, mycoplasma & PPLO (Pleuro Pneumonia Like Organisms).
- They are generally smaller and multiply more rapidly than the eukaryotic cells.
- They vary in shape & size. E.g. Bacteria have 4 basic shapes: Bacillus, Coccus, Vibrio and Spirillum.
1. Cell Envelope
It is a chemically complex protective covering.
It is made of 3 tightly bound layers.
- Glycocalyx: Outer layer. Its composition and thickness vary in different bacteria. It may be a slime layer (loose sheath) or capsule (thick & tough).
- Cell wall: Middle layer. Seen in all prokaryotes except mycoplasma. It gives shape to the cell and provides a structural support to prevent the bacterium from bursting or collapsing.
- Plasma membrane: Inner layer. It is semi-permeable in nature and interacts with the outside. This is structurally similar to that of the eukaryotes.
- Gram positive: They take up and retain the gram stain.
- Gram negative: They do not retain the gram stain.
2. Mesosomes & Chromatophores
Mesosome is formed by the infoldings of plasma membrane.
It includes vesicles, tubules & lamellae.
Functions: Mesosomes help
- In cell wall formation.
- In DNA (chromosome) replication.
- In distribution of chromosomes to daughter cells.
- In respiration and secretion processes.
- To increase the surface area of the plasma membrane and enzymatic content.
- It is formed of non-membranous (naked) circular genomic DNA (single chromosome/ Genetic material) & protein.
- Many bacteria have small circular DNA (plasmid) outside the genomic DNA. It gives some unique phenotypic characters (e.g. resistance to antibiotics) to bacteria.
- These are thin filamentous extensions from the cell wall of motile bacteria. Their number and arrangement are varied in different bacteria.
- Bacterial flagellum has 3 parts – filament, hook and basal body.
- The filament is the longest portion and extends from the cell surface to the outside.
5. Pili and Fimbriae
- These are surface structures that have no role in motility.
- Pili (sing. Pilus) are elongated tubular structures made of a special protein (pilin).
- Fimbriae are small bristle like fibres sprouting out of the cell. In some bacteria, they help to attach the bacteria to rocks in streams and to the host tissues.
- They are associated with plasma membrane of prokaryotes.
- They are about 15 nm by 20 nm in size.
- They are made of 2 subunits - 50S & 30S (Svedberg’s unit). They together form 70S prokaryotic ribosomes. (S= sedimentation coefficient; a measure of density & size).
- Function: Ribosomes are the site of translation (protein synthesis). Several ribosomes may attach to a single mRNA to form a chain called polyribosomes (polysome). Ribosomes translate the mRNA into proteins.
7. Inclusion Bodies
- These are non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells.
- E.g. phosphate granules, cyanophycean granules and glycogen granules, gas vacuoles etc.
- Gas vacuoles are found in blue green and purple and green photosynthetic bacteria.