Self-Sustaining Chemical Systems:
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.