Читать книгу Cell Biology - Stephen R. Bolsover - Страница 36

It is difficult to overstate the importance of membranes to living cells; without them life as we know it could not exist. The plasma membrane, also known as the cell membrane or plasmalemma, defines the boundary of the cell. It regulates the movement of materials into and out of the cell and facilitates electrical and chemical signaling between cells. Other membranes define the boundaries of organelles and provide a matrix upon which complex chemical reactions can occur. In the following section, the basic structure of the cell membrane will be outlined.

The basic structure of a biological membrane is shown in Figure 2.1. Approximately half the mass is phospholipid, molecules comprising an electrically charged head group and long hydrocarbon tails. The head groups readily associate with water, a property called being hydrophilic. In contrast the tails are hydrophobic and avoid contact with water. Phospholipid molecules spontaneously organize to form a bilayer about 4 nm thick with hydrophilic surfaces formed of their heads and a hydrophobic interior formed of their tails. All the membranes of the cell, including the plasma membrane, also contain proteins. These may be embedded within the membrane and extracted from it only with great difficulty, in which case they are integral proteins (e.g. connexin, Figure 2.6); or they may be associated with the inner or outer surface and thus separated with relative ease, in which case they are peripheral proteins (e.g. Ras, Medical Relevance 7.1 on page 108). Membrane proteins are free to move laterally, within the plane of the membrane. Integral plasma membrane proteins are often glycosylated: they have sugar residues attached on the side facing the extracellular medium. In addition to phospholipids and proteins, eukaryotic cell membranes also contain cholesterol. Cholesterol makes the membrane more fluid. Cholesterol is essential for life but excess cholesterol in the bloodstream is strongly implicated in the development of atherosclerosis and heart disease.

Molecules of oxygen are uncharged. Although they dissolve readily in water, they are also able to dissolve in the hydrophobic interior of lipid bilayers. Oxygen molecules can therefore pass from the extracellular medium into the interior of the plasma membrane, and from there on into the cytoplasm, in a simple diffusion process (Figure 2.2). Water itself also passes across the plasma membrane in a process known as osmosis (In Depth 2.1), as do the uncharged hormones of the steroid family. Indeed, any foreign chemical that is not strongly hydrophilic will readily pass into and out of the cell. In contrast, simple ions and charged molecules are strongly hydrophilic. They cannot dissolve in the hydrophobic interior of the membrane and therefore cannot cross membranes by simple diffusion (Figure 2.2). In this case, specialized proteins are present in our cells to facilitate their entry and exit. We will cover these in detail in Chapter 9.

Figure 2.1. Membranes comprise a lipid bilayer plus integral and peripheral proteins.

Figure 2.2. Small uncharged molecules can pass through membranes by simple diffusion, but hydrophilic solutes cannot.