Q.What are porins? What role do they play in diffusion?

Bacteria are single-celled organisms built on a small number of basic shapes: cocci (spherical), bacilli (rod-shaped) and spirilla (spiral). Most bacteria keep one of these shapes throughout their life, but some, called pleomorphic bacteria, can vary in shape rather than showing one fixed form - Rhizobium, the nitrogen-fixing bacterium found in the root nodules of legumes, is a commonly cited example of a pleomorphic bacterium.

Bacteria are also classified by the number and arrangement of the whip-like flagella on their surface, which provide motility. A cell with no flagella is atrichous; one with a single flagellum at one pole is monotrichous; one with flagella at both poles is amphitrichous; one with a tuft of flagella clustered at one pole is lophotrichous; and one with flagella spread over its entire surface is peritrichous. A 'peritrichous bacillus', for example, is simply a rod-shaped cell whose flagella cover its whole surface rather than being confined to one end or pole. …