The cell membrane, as seen under digital compound microscopes, is more than an envelope that gives mechanical strength and shape and some protection to the cell. It is also an active component of the living cell, preventing some substances from enter¬ing it and others from leaking out. It regulates the traffic in materials between the precisely ordered interior of the cell and the essentially unfavorable and potentially disruptive outer environment.

Diffusion and osmosis

Imagine a small rectangular box containing 20 marbles, all placed in a tight cluster near one end. When the box is shaken, the marbles are scattered almost evenly over the bottom of the box. Obvious as this result might seem, it is worth a closer look.

First, of all the possible directions in which a given marble might move, more lead away from the center of the cluster than toward it. Hence random movement will tend to disrupt the cluster rather than maintain it. Or, to put it another way, in the absence of any counter¬acting external influence, a dynamic system will tend to move toward the more probable disorganized state rather than toward the less probable organized state.

Another factor favors dispersion of the marbles. Movement toward the cluster has a high probability of resulting in a collision of two or more marbles, which will then be deflected. Movement away from the cluster carries much less probability of collision; a marble has a good chance of continuing on, an uninterrupted path to the outer portions of the box. On the average, then, more marbles move away from the center of concentration than toward it.

Notice that both of the above arguments are statistical. It is possible that, as a result of random motion, 20 scattered marbles will all come to form a tight cluster at one end of the box. This result has a finite possibility, but one so slight that it can justifiably be disregarded. The kind of reasoning used here is typical of most scientific reasoning. The facts and laws of science are statistical rather than absolute. They describe nature in terms of probable occurrence.

We can now make a generalization based on our example of the marbles in the box and on others like it: All other factors being equal, the net movement of the particles of a particular substance is from regions of higher to regions of lower concentration of that substance. Note that we said the net movement. There will always be some parti¬cles moving in the opposite direction, but, overall, the movement will be away from the centers of concentration. An obvious result is that the particles of a given substance tend to become distributed with relatively uniform density within any available space. When this uni¬form density is reached, the system is in equilibrium; the particles continue to move, but there is little net change in the system.

Movement of molecular-sized particles from one place to another in the manner we have been discussing is called diffusion. Diffusion is fastest by far in gases, where there is much space between the molec¬ular particles and hence relatively little chance of collision. Diffusion in a liquid is much slower; in the absence of convection currents, it takes a very long time, years in fact, for a substance to move in appre¬ciable quantity only a few feet through water.

Now consider another situation, a chamber divided into two halves by a membrane partition. Let us assume that particles of some sub¬stances can pass through the membrane, while particles of other sub¬stances cannot. Such a membrane is said to be differentially permeable (or selectively permeable).

How will the membrane affect the diffusion of materials between the two halves of the chamber? Suppose the chamber is a U-tube divided in half by a differentially permeable membrane. Suppose side A contains pure water and side B an equal quantity of sugar solution (sugar dissolved in water), both sides being subject to the same initial temperature and pressure. If the membrane is permeable to water but not to sugar, water molecules will be able to pass in both directions, from A to B and from B to A.

Such a membrane, permeable to water but not to solute, is said to be semipermeable, and movement of water through it is called osmosis. Unlike our hypothetical membrane in the U-tube, biological membranes are not, strictly speaking, semipermeable, be¬cause they are not completely impermeable to solutes; they are suffi¬ciently like semipermeable membranes, however, for the movement of water through them to be routinely termed osmosis.

But let us return to our U-tube model. Since water is already present on both sides of the membrane, as seen under digital compound microscopes, it might at first be supposed that the net effect of the movement of water molecules across the membrane would be zero, but such a supposition would be wrong. Consider the differences between the pure water and the sugar solution more care¬fully. On side A all the molecules that bump into the membrane dur¬ing a given interval are water molecules, and because the membrane is permeable to water, many of these molecules will pass through the membrane from A to B. By contrast, on side B some of the molecules bumping into the membrane during the same interval will be water molecules, which may pass through, and some will be sugar mole¬cules, which cannot pass, because the membrane is impermeable to them. At any given instant, then, part of the membrane surface on side B is in contact with sugar molecules and part is in contact with water, whereas on side A all the membrane surface is in contact with water. Hence more water molecules will move across the membrane from side A to side B per unit time than in ache opposite direction; the net osmosis will be from A to B.
The result in full accord with the earlier gen¬eralization concerning diffusion will be noted; the net movement of water mole¬cules is from the region of their greater concentration (side A) to the region of their lesser concentration (side B). But this will mean that the volume of fluid will increase on side B and decrease on side A. How long can this process continue? Will an equilibrium point be reached?

Clearly, the concentrations on the two sides of the membrane will never be equal, no matter how many water molecules move from A to B, because the fluid in B will remain a sugar solution, though an increasingly weak one, and the fluid in A will remain pure water, if the membrane is completely impermeable to sugar molecules. The net movement of water from A to B might be expected to continue indefinitely. However, this is not in fact what happens. Under normal con¬ditions, the fluid level in B will rise to a certain point and then cease to rise farther. Why? The column of fluid is, of course, being pulled downward by gravity. As the column rises, therefore, it exerts increasing downward pressure. Eventually the column of sugar solution becomes so high, and the pressure so great, that water molecules begin to move across the membrane from B to A as fast as they move into B from A. When this point is reached when water is passing through the membrane in opposite directions at the same rate-the system is in dynamic equilibrium.

The equilibrium point is also influenced by the difference in the number of solute particles per unit volume on the two sides of the membrane; the greater the difference, the higher the column will rise before equilibrium is reached. By solute particles we mean osmotically active particles (dissolved particles). If there are several kinds of so¬lutes in the same solution, then the osmotic properties of that solution are determined by the total (per unit volume) of all the particles of all kinds. If a dissolved substance separates into ions, each ion functions osmotically as a separate particle; for example, one mole of sodium chloride (NaCI) dissolved in water produces two moles of particles one of Na+ ions and one of Cl- ions. We are now in a position to make an additional generalization: If, under conditions of constant temper¬ature and pressure, two different solutions are separated by a mem¬brane permeable only to water, the net movement of the water will be from the solution with fewer osmotically active particles to the solu¬tion with more such particles.

It should now be apparent that one way to characterize the osmotic properties of a solution is to measure the number of osmotically active particles it contains per unit volume; such a measure is called the osmotic concentration. Another way, frequently used in animal physi¬ology, is to measure the pressure that must be exerted on a solution to prevent it, from taking in water when it is separated from pure water by a semipermeable membrane; the value thus obtained is called the osmotic pressure of the solution. In our U-tube example, the osmotic pressure of the sugar solution in side B is the pressure we would have to exert on the solution to prevent the fluid column in B from rising. Clearly; then, the osmotic pressure of a solution is a mea¬sure of the tendency of water to move by osmosis into it. The more dissolved particles in a solution, the greater the tendency of water to move into it, and the higher the osmotic pressure of the solution. Thus, under constant temperature and pressure, water will move from the solution with the lower osmotic pressure to the solution with the higher osmotic pressure when a semipermeable membrane separates the two solutions.

The diffusion and osmosis and the role played by semipermeable membranes at such length because cell membranes are semipermeable and the process of diffusion and osmosis are fundamental to cell life. Although the membranes of different types of cells vary widely in their permeability characteristics, as seen under digital compound microscopes, a few rough generalizations can be made: Cell mem¬branes are relatively permeable to water and to certain simple sugars, amino acids, and lipid-soluble substances. They are relatively impermeable to polysaccharides, proteins, and other very large molecules, and their permeability to small inorganic ions varies greatly, depend¬ing on the particular ion.