Cell Membrane

All cells are bounded by cell membranes, which are similar in all cells. In prokaryotic cells, the membranes appear to be much-folded, the convolutions extending to the interior of the cell and having the effect of increasing surface area. Plant and animal cells are alike in this respect; however, a significant difference between the two is found in the cell wall. In plants, cell walls are secreted by plant cells and lie outside of the cell membrane. Animal cells for the most part do not exhibit this characteristic. (Cell walls are further described in an upcoming section of this chapter.)

The cell membrane, or plasmalemma, has three layers, as seen through an electron microscope. There is a light line in the center bounded by dark lines on each side. The center portion is made of phospholipids, and the dark lines are made of protein. Such a membrane is called a unit membrane. It is perforated by many holes.

While the cell wall is freely permeable to both water and dissolved materials, the cell membrane exercises selectivity; that is, it allows some materials to pass through and restricts others. This selective permeability may be thought to relate to pore size, with molecules and ions smaller than the openings being able to pass through, and molecules larger than the openings being restricted. This reasonable postulate, which depends on the constant motion of molecules and their tendency to diffuse, accords with some observations; but other factors come into play. Dependency on diffusion alone would be too slow a process. A cell membrane is a living structure and can exercise selectivity entirely separate from the presence of apertures. The movement of materials through a membrane involves the expenditure of energy and is called active transport. Enzymes are involved. The permeability of the membrane constantly changes. A substance that is allowed to pass through at one time may be disallowed at another time. Materials become dissolved in the membrane, migrate across it, and emerge on the other side. Certain molecules are moved across the cell membrane by carriers. They become attached to carrier molecules, are transported through the membrane, and are released on the other side. These migrations do not involve movement through holes. The capacity of a cell membrane to allow or disallow the passage of solutes depends on several factors. Ions having a charge of plus one (+1) tend to increase permeability. Ions having a charge of plus two (+2) tend to decrease permeability. Nitrates and phosphates tend to increase cellular metabolism and, hence, accelerate the movement of dissolved materials through the membrane. When calcium ions are deficient, the membrane tends to be damaged and develops leaks.