Most cell membranes are therefore strengthened and supported by a framework of proteins, attached to the membrane via transmembrane proteins. For plants, yeasts, and bacteria, the cell’s shape and mechanical properties are conferred by a rigid cell wall—a fibrous layer of proteins, sugars, and other macromolecules that encases the plasma membrane.

By contrast, the plasma membrane of animal cells is stabilized by a meshwork of filamentous proteins, called the cell cortex, that is attached to the underside of the membrane.

The cortex of the human red blood cell has a relatively simple and regular structure and has been especially well studied. Red blood cells are small and have a distinctive flattened shape. The main component of their cortex is the dimeric protein spectrin, a long, thin, flexible rod about 100 nm in length. Spectrin forms a lattice that provides support for the plasma membrane and maintains the cell’s biconcave shape. The spectrin network is connected to the membrane through intracellular attachment proteins that link spectrin to specific transmembrane proteins.

Proteins similar to spectrin, and to its associated attachment proteins, are present in the cortex of most animal cells. But the cortex in these cells is especially rich in actin and the motor protein myosin, and it is much more complex than that of red blood cells. Whereas red blood cells need their cortex mainly to provide mechanical strength as they are pumped through blood vessels, other cells also use their cortex to selectively take up materials from their environment, to change their shape, and to move. as we discuss in Chapter 17. In addition, cells also use their cortex to restrain the diffusion of proteins within the plasma membrane