Prokaryotic Cells
- They have no membrane-bound nucleus and organelles.
- They include bacteria, blue-green algae, mycoplasma, and PPLO (Pleuro Pneumonia Like Organisms).
- They are generally smaller and multiply more rapidly than eukaryotic cells.
- They vary in shape and size. E.g., Bacteria have four basic shapes: Bacillus, Coccus, Vibrio, and Spirillum.
Cell Organelles in Prokaryotic Cells
1. Cell Envelope
It is a chemically complex protective covering made of three 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 and tough).
- Cell Wall: Middle layer. Seen in all prokaryotes except mycoplasma. It gives shape to the cell and provides structural support to prevent the bacterium from bursting or collapsing.
- Plasma Membrane: Inner layer. It is semi-permeable and interacts with the outside. Structurally similar to that of eukaryotes.
Based on the types of cell envelopes and response to Gram staining, bacteria are two types:
- Gram Positive: They take up and retain the Gram stain.
- Gram Negative: They do not retain the Gram stain.
2. Mesosomes and Chromatophores (Membranous Structures)
Mesosome: Formed by infoldings of the plasma membrane. It includes vesicles, tubules, and lamellae.
Functions: Mesosomes help in:
- Cell wall formation.
- DNA (chromosome) replication.
- Distribution of chromosomes to daughter cells.
- Respiration and secretion processes.
- Increasing the surface area of the plasma membrane and enzymatic content.
Chromatophores: Pigment-containing membranous infoldings in some prokaryotes (e.g., cyanobacteria).
3. Nucleoid
- Formed of non-membranous (naked) circular genomic DNA (single chromosome/genetic material) and protein.
- Many bacteria have small circular DNA (plasmid) outside the genomic DNA. It gives unique phenotypic characters (e.g., resistance to antibiotics) to bacteria.
4. Flagella
- Thin filamentous extensions from the cell wall of motile bacteria. Their number and arrangement vary in different bacteria.
- Bacterial flagellum has three 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
- Surface structures that have no role in motility.
- Pili (sing. Pilus): Elongated tubular structures made of a special protein (pilin).
- Fimbriae: Small bristle-like fibers sprouting out of the cell. In some bacteria, they help attach the bacteria to rocks in streams and to host tissues.
6. Ribosomes
- Associated with the plasma membrane of prokaryotes.
- About 15 nm by 20 nm in size.
- Made of two subunits: 50S and 30S (Svedberg’s unit). Together, they form 70S prokaryotic ribosomes. (S = sedimentation coefficient; a measure of density and 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
- Non-membranous, stored reserve material seen freely in the cytoplasm of prokaryotic cells.
- E.g., phosphate granules, cyanophycean granules, glycogen granules, gas vacuoles, etc.
- Gas vacuoles: Found in blue-green and purple and green photosynthetic bacteria.