3.4–3.7: Cell membranes are gatekeepers.

Like gatekeepers, cell membranes control the movement of material into and out of the cell.

Just as skin covers our bodies, every cell of every living thing on earth is enclosed by a plasma membrane, a two-layered membrane that holds the contents of a cell in place and regulates what enters and leaves the cell. Plasma membranes are thin (a stack of a thousand would be only as thick as a single hair) and flexible, and in photos or diagrams the membranes often resemble simple plastic bags, holding the cell contents in place. This image is a gross oversimplification, however. Membranes are indeed thin and flexible, but they are far from simple: a close look at a plasma membrane will reveal that its surface is filled with pores, outcroppings, channels, and complex molecules floating around within the two layers of the membrane itself (FIGURE 3-9).

Figure 3.9: More than just an outer layer. The plasma membrane performs several critical functions beyond simply enclosing a cell’s interior contents.

Cells are perpetually interacting with their external environment. And in these interactions, the plasma membranes must perform several critical functions beyond simply holding a cell’s interior contents. The plasma membrane must function in ways that accomplish the following.

• The membrane enables the cell to take in food and nutrients and dispose of waste products.
• It allows the cell to take in water.
• It allows the cell to build and export molecules needed elsewhere in the body.
• It mediates communications with the external environment and other cells and adhesion to other cells or surfaces.
• Like a border control checkpoint, it controls the flow of molecules into and out of the cell.

The foundation of all plasma membranes is a layer of lipid molecules all packed together. These are a special type of lipid, called phospholipids, which, as you’ll recall from Section 2-13, have what appear to be a head and two long tails. The head consists of a molecule of glycerol linked to a molecule containing phosphorus (FIGURE 3-10). This head region is said to be polar, because the electrons are not shared equally among the atoms, leading to regions of partial positive and partial negative charge. As you learned in Chapter 2, water is also a polar molecule and, for this reason, other polar molecules mix easily with water. Molecules that can mix with water are described as hydrophilic (“water-loving”) molecules. The two tails of the phospholipid are long chains of carbon and hydrogen atoms. Because the electrons in these bonds are shared equally, the carbon-hydrogen chains are nonpolar. And because they are nonpolar, these tails do not mix with water and are said to be hydrophobic (“water-fearing”). The chemical structure of phospholipids gives them a sort of split personality: their hydrophilic head region mixes easily with water, while their hydrophobic tail region does not mix with water.

Figure 3.10: Good membrane material. The phospholipid bilayer of the plasma membrane prevents fluid from leaking out of the cell.

The split personality of phospholipids makes them good membrane material. Once a large number of phospholipids are packed together with all of their heads facing one way and their tails the other, we have a sheet with one side that is hydrophilic and one that is hydrophobic. In the cell’s plasma membrane, two of these sheets of phospholipids are arranged so that the hydrophobic tails are all in contact with one another and the hydrophilic heads are in contact with the watery solution outside and inside the cell (see Figure 3-10). This arrangement gives us another way to describe the structure of the plasma membrane: as a phospholipid bilayer.

The phospholipids are not locked in place in the plasma membrane; they just float around their side of the bilayer. They cannot pop out of the membrane or flop from one side to the other, because their hydrophobic tails always line up away from any watery solution. Just as similarly charged sides of two magnets push away from each other, so do the hydrophobic tails in the center of the membrane push away from and avoid coming into contact with water molecules. Because the center part of the bilayer membrane is made up of hydrophobic lipids, the solution on one side of the membrane cannot leak across into the solution on the other side. In this way, the plasma membrane forms a boundary around the cell’s contents.