Prokaryotic cells

Prokaryotic cells are found in bacteria, blue-green algae, mycoplasma and PPLO (pleuro-pneumonia-like organisms). They are usually small and multiply very fast. Bacteria come in four shapes: bacillus (rod), coccus (round), vibrio (comma) and spirillum (spiral).

There is no nuclear membrane. The genetic material is a single, circular, naked DNA lying in the cytoplasm. Many bacteria also have small extra rings of DNA called plasmids, which can give them special traits such as resistance to antibiotics. Apart from ribosomes, there are no membrane-bound organelles.

Most bacteria are wrapped in a three-layered cell envelope:

• Glycocalyx (outermost): a loose slime layer in some, a thick, tough capsule in others.
• Cell wall: gives shape and strong support, stopping the cell from bursting.
• Plasma membrane (innermost).

Bacteria that take up the Gram stain are Gram-positive; those that do not are Gram-negative.

Some special parts:

• Mesosomes: infoldings of the plasma membrane that help make the wall, copy DNA, respire and secrete. (A memory aid: M for mesosome does some of the energy work that M for mitochondria does in bigger cells.)
• Chromatophores: membranes with pigments in cyanobacteria, for photosynthesis.
• Flagella: whip-like tails for swimming, each with a filament, a hook and a basal body.
• Pili and fimbriae: fine surface hairs that help bacteria stick to surfaces or to each other; they are not for swimming.

Bacterial ribosomes are 70S (made of a 50S and a 30S part), about 15 nm by 20 nm, attached to the plasma membrane. Several ribosomes reading one messenger RNA together form a polysome. Reserve food is stored as inclusion bodies (phosphate granules, cyanophycean granules, glycogen granules); some bacteria also have gas vacuoles to float.