The bacterial cell membrane plays the vital role of separating the bacterium from its environment. Without such a barrier, a cell existing as a local concentration of ordered molecules and reactions would be impossible. The membrane is a lipid-protein bilayer 70 thick. lt is somewhat simpler than that of eucaryotes, and resembles the unit-membrane model seen in Chapter 21. The membrane is freely perme able to water, but not to simple ions or charged molecules, or neutral molecules larger than glycerol.

The membrane controls the contents of the cell. Water and small neutral molecules can enter and leave by free diffusion. Other specific ions and molecules can diffuse across the membrane with the aid of carrier molecules, in the process of passive transport. Although carrier molecules are necessary to make the penetration of the membrane possible, passive transport still represents a diffusion along a concentration gradient, from the side of the membrane with an excess of the molecule or ion, to the side where it is in short supply.

Right: Click on cell to view labels for structural components.