Introductory Microbiology (1+1)

Ribosomes and Cellular reserve materials

• The most notable structures in the bacterial cytoplasm are the ribosomes which are involved in protein synthesis. Their number varies with rate of protein synthesis. The greater the rate of protein synthesis, the greater is the number of ribosomes. These are ribo-nucleo protein particles, have a diameter of 200 A0 and are characterized by their sedimentation properties.
• The bacterial ribosomes are referred to as 70s ribosomes (S=svedbery unit, the unit of sedimentation). These ribosomes, during active protein synthesis are associated with the mRNA and such associations are called polysomes.

Cellular reserve materials
• A variety of reserve materials are found in the prokaryotic cells and are termed as granular cytoplasmic inclusions. Of these, starch, glycogen and polyhydroxy butyric acid are important. Poly ?-hydroxy butyric acid is found only in prokaryotes. For example, glycogen or starch in clostridia, while poly ß-hydroxy butyric acid in many pseudomonads. Generally, prokaryotes do not store nitrogenous organic materials except blue green algae which accumulate nitrogenous reserve material as cyanophycin.
• Many prokaryotes accumulate volutin (metachromatic) granules due to presence of inorganic polyphosphate which accumulate under phosphate starvation conditions. Sulphur bacteria, such as photosynthetic purple sulphur bacteria and filamentous non-photosynthetic bacteria accumulate S during hydrogen sulphide production. In BGA, contain Thylakoids involved in photosynthesis. In certain BGA, and purple bacteria contain polyhedral structures called carboxysomes involved in carbon dioxide fixation by carboxy-dismutase enzyme, site of CO2 fixation.