Passage VII

All cells have a thin outer layer called the plasma membrane. This membrane separates the cell's cytoplasm from the surrounding environment (see Figure 1).

Two models of the structure of the plasma membrane are proposed.

Model 1

Phospholipids are the most abundant type of molecule in the plasma membrane. These molecules consist of hydrophobic (water-repelling) tails composed of hydrocarbons and a hydrophilic (water-attracting)head containing a phosphate group (see Figure 2).

The phospholipids orient themselves to form a bilayer, with the tails on the inside of the bilayer and the heads forming the outer surfaces of the bilayer. A single layer of proteins coats both the inner and outer surfaces of the plasma membrane, rigidly holding the phospholipids in place. Small, protein-coated pores in the phospholipid bilayer allow small water-soluble molecules to pass in and out of the cell (see Figure 3).

Model 2

Model 2 agrees with Model 1 about the abundance and orientation of phospholipids in the bilayer; however, the way proteins are distributed differs. Some proteins are located in the inner half of the plasma membrane, some in the outer half. Transmembrane proteins span from the outside of the cell through the bilayer to the inside of the cell.

The proteins are distributed unevenly, giving the plasma membrane a rough appearance (see Figure 4). Since the phospholipids are not held in place by a layer of proteins, the plasma membrane is highly fluid, allowing phospholipids and proteins to move within the plasma membrane.

Figures 3 and 4 adapted from William T. Keeton and James L. Gould, Biological Science, 4th ed. 1986 by W. W. Norton & Company, Inc.