A eucaryotic cell membrane is thicker than that of bacteria, around 90 . To a first approximation the lipid-bilayer unit membrane described in Chapter 21, with a covering of protein on both sides, is a good model (see above). As was mentioned in Chapter 21, some proteins appear to extend all the way through the membrane, and the lipid must be exposed to the surface in places. The cell membrane is a selective barrier that passes some molecules in and out, and excludes others.

The free permeability to , , , and other small uncharged molecules suggests the existence of pores, as drawn at the left above. From the rates at which molecules of different sizes penetrate the membrane, the pores are thought to be approximately 8 in diameter and to occupy one twentieth of a percent of the total surface area. Some cations can pass through the pores but anions cannot, which implies that the rim of a pore might contain negative charges such as carboxyl groups.