Prokaryotic cell and its ultra-structure :
The prokaryotic (Gr., pro= primitive, karyon= nucleus) cells are small, simple and most primitive. In these cells nuclear material is not bound by a definite nuclear membrane. Organisms like bacteria and cyanobacteria (blue-green algae) show prokaryotic organization. The average diameter of a bacterial cells is about 1 . There are four forms of bacteria:
i) Cocci (singular-Coccus) are spherical or ovoid in shape.
ii) Bacilli (singular-Bacillus) are rod- shaped.
iii) Vibrios (Singular-vibrio) are comma-shaped.
iv) Spirilla (singular-Spirillum) are long twisted bacteria.
The cyanobacteria or blue-green algae are photosynthetic prokaryotes. They may be filamentous or non-filamentous. Many cyanobacteria fix atmospheric nitrogen. They are found in symbiotic (Anabaena) as well as in non-symbiotic forms (Nostoc).
Motile bacteria possess one or more fine thread-like appendages celled flagella (singular-flagellum), which help on locomotion.
The prokaryotic cell shows following structure :
1. Cell envelope :
It consists of three basic layers, namely glycocalyx, cell wall and cell membrane (plasma membrane). Together these three layers acts as a single protective covering.
a. Glycocalyx : It is the outermost layer made up of macromolecules which help in adhesion of bacteria. Some bacteria have a loose sheath called slime layer. Others may have a thick and tough covering called capsule. The slime layer and capsule are made up of polysaccharides and may contain proteins.
b. Cell wall : It is present below the glycocalyx. It provides definite shape and gives strong structural support. Chemically, the cell wall material is peptidoglycan or murein in eubacteria and pseudo-peptidoglycan in archaebacteria. The cell walls of different bacteria show different types of reactions to Gram's stain.
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c. Plasma membrane : It is the innermost covering of the cell envelope chemically composed of lipids and proteins. The plasma membrane separates the interior of the cell form its environment, serves as a barrier and helps in intercellular communication. The bacterial plasma membrane is the site for critical metabolic processes like respiration, photosynthesis of lipids and cell wall constituents, etc. The membrane also holds receptor molecules that help bacteria to detect and respond to chemicals in their surroundings.
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2. Cytoplasm :
Cytoplasm is a semi-fluid ground substance or matrix present inner to the cell membrane. It contains a variety of inorganic and organic compounds which provide a suitable chemical environment for cellular activities. The cytoplasm does not show cytoplasm streaming and lacks membrane-bound organelles such as mitochondria, endoplasmic reticulum, Golgi bodies and lysosomes. However, certain membranous structures like mesosomes and chromatophores are observed besides inclusion bodies.
a. Mesosomes : These are invaginations of plasma membrane into the cell in the form of vesicles, tubules and lamellae. The exact function of mesosomes is still not known. It is believed that mesosomes are involved in the function of cell wall and in the DNA replication and its distribution to the daughter cells. They also help in respiration.
b. Chromatophores : These are usually found in photosynthetic bacteria and cyanobacteria. They contain different types of pigments such as bacteriochlorophyllus, bacteriophaeophytin abd carotenoids.
c. Inclusion Bodies : These are a kind of storage granules that lie free in the cytoplasm and are of two types.
i) Organic inclusions : These include cyanophycean starch granules and glycogen granules.
ii) Inorganic inclusions : These include phosphate and sulphur granules.
3. Ribosomes :
The cytoplasmic matrix of a prokaryotic cell possesses dense particles celled ribosomes. They are made up of RNA and protein. The prokaryotic ribosomes are of 70S type. Each 70S is composed of two sub-units: 50S and 30S. The ribosomes help in protein synthesis.
4. Genetic Material (Nucleoid) :
The prokaryotes do not have membrane-bound nucleus. The genetic material consists of a long, double-stranded, circular DNA molecule known as prokaryotic chromosome. It is highly looped and coiled with associated proteins. These proteins differ from the histone proteins of a eukaryotic cell. The circular DNA molecule is connected to the plasma membrane through the mesosome.
5. Plasmids :
The prokaryotic cell may possess one or more additional self replicating molecules of circular DNA or mini chromosomes. These extra chromosomal DNA are called plasmid. Plasmids can carry out functions, such as antibiotic resistance and fertility.