Active transport also exists, in which some ions and molecules can be taken into or out of the cell against the normal concentration gradient, and accumulated on the side of the membrane where they already are in excess. Such backward flow leads to a state of higher free energy, and is thermodynamically nonspontaneous. The energy to drive active transport comes from ATP. We shall see passive and active transport in more detail with eucaryotes.

All but a very few bacteria have a cell wall, 100 to 800 thick. The wall provides rigid mechanical protection, but is not a barrier to molecular diffusion. It is built from glycopeptide, which is a polymer of glucose derivatives that is cross-linked by short polypeptide chains. Lipids also are present in the wall in the form of lipid-peptide combinations, or lipopeptides. The bacterial cell wall often is surrounded by yet another protective coating, the capsule. This is a gelatinous outer layer made from short-chain sugar polymers.

Right: Bacterial cells of M. paratuberculosis growing in clumps with a rough, waxy cell wall.