For compounds to enter cells from the outside they must somehow penetrate the membrane (also outer membrane in gram-negative bacteria, and the cell wall if present FIGURE 11-1 Biological membranes. Viewed in cross section, all cell membranes share a characteristic trilaminar appearance. This erythrocyte was stained with osmium tetroxide and viewed with an electron microscope. The plasma membrane appears as a three-layer structure, 5 to 8 nm (50 to 80 Å) thick. The trilaminar image consists of two electron-dense layers (the osmium, bound to the inner and outer surfaces of the membrane) separated by a less dense central region.

Cell membranes consist of lipid bilayers

Some simple rules about movement of molecules across cellular membranes: Cell membranes represent physical barriers to the surrounding environment. But they are not like a concrete wall and also not like the thin membrane in a soap-bubble. It is a much more complex barrier that lets certain selected compounds through in a strictly controlled manner. Macromolecules and charged smaller molecules do not generally pass through membranes passively (by diffusion). This for example means that organisms cannot use proteins directly as food. They are instead partly degraded before uptake. Some uncharged smaller molcules can enter cells by passive diffusion. Many types of molecules can enter cells by various types of active transport systems. These processes are compound-specific and may or may not require energy consumption (i. e ATP).

The detailed structures of membranes are very complicated