Cell envelope, its modifications, ribosomes and inclusion bodies

Most prokaryotes, especially bacteria, have a three-layered cell envelope: outer glycocalyx, a cell wall and an inner plasma membrane, which together act as one protective unit.

The glycocalyx differs between bacteria: a loose sheath is called the slime layer, while a thick, tough covering is called the capsule.

Based on how the envelope reacts to Gram's stain, bacteria are divided into Gram-positive (take up the stain) and Gram-negative (do not).

The bacterial cell wall decides the cell's shape and is strong enough to stop the cell from either bursting or collapsing.

Mesosomes are formed when the plasma membrane folds inwards into vesicles, tubules and lamellae. Their roles include forming the cell wall, replicating DNA and distributing it to daughter cells, respiration and secretion; they also increase the surface area of the membrane and its enzyme content.

Cyanobacteria also have chromatophores, membranous extensions into the cytoplasm that carry the photosynthetic pigments.

A bacterial flagellum has three parts: filament (the longest part, extending outward), hook and basal body. Pili (long tubes of a special protein) and fimbriae (short bristle-like fibres) are surface structures that play no part in motility; in some bacteria fimbriae help the cell cling to rocks in streams or to host tissue.

Prokaryotic ribosomes are 70S, built from a 50S and a 30S subunit, and are attached to the plasma membrane. Several ribosomes reading one mRNA form a chain called a polyribosome or polysome.

Inclusion bodies store reserve material free in the cytoplasm, with no membrane around them; examples are glycogen, phosphate and cyanophycean granules. Blue-green, purple and green photosynthetic bacteria also have gas vacuoles.