(Use the outline to review the material in this section. With it you can jump to specific areas. Before you jump around, however, follow the text in the order in which it is presented.)
The movement of materials into and out of cells influences the internal concentrations of the molecules. So, you can see that what goes in or out will have a profound effect on the ability of cells to function properly. The ability of cells to move materials in and out efficiently can determine whether they will live or die. How do cells control these movements? They do it by both passive and active transport. We will examine these processes in this section. Before we begin, however, I want to stress the importance of the plasma membrane that surrounds cells. To get in or out of a cell, material must pass through this membrane. It is the membrane that is the gatekeeper of cells.
Many materials move into cells without requiring the expenditure of cellular energy. The transport mechanism in such cases is considered passive or inactive because the force moving these molecules or ions is not external to them. It comes from the kinetic energy (energy of movement) the particles already have. They have this motion because they have heat. All molecules are in constant motion above a very cold temperature called absolute zero (-273.16oC or -459.69o F). The amount of molecular movement depends upon the temperature and physical properties of a substance. At room temperature a chair is solid (very little shifting of the molecules in the chair), water is liquid (more relative motion of the molecules) and air is a gas (far more motion of the molecules). But if we cool its molecules down enough, the air can be converted to a liquid and at even colder temperatures it will become a solid. We think of oxygen as a gas because that is its state at normal temperatures, but we have heard about the liquid oxygen that is used to fuel rockets. The difference is one of temperature. The less heat, the less kinetic energy and the less motion of molecules.
Well, so what! Why concern ourselves with molecular motion? We must understand molecular motion in order to understand diffusion. Without understanding diffusion we cannot understand passive transport.
Because of molecular motion, molecules will tend to move around until they are randomly distributed in their space. In the diagram, the blue dots are originally congregated on the left side of the container, where they originally fell. In time the dots have moved so that they are equally distributed in the solution (the diagram on the right). They have diffused. Diffusion is the movement of a substance from an area of its greater concentration to an area of its lesser concentration. This happens because the random motion of the substance (the blue dots in the diagram) will cause them to bump into each other and the walls of the container until they have spread out from their source. Eventually as many dots will move to the left as move to the right and an equilibrium is achieved. That is, there is no net movement of the dots in any given direction. Notice that even though the movement of individual dots is totally random, the changes in concentration that occur are not random. The concentration in denser areas goes down and the concentration in less dense areas goes up. When we have a difference in concentration between two regions, we have a concentration gradient. Substances always move in the direction of the concentration gradient, i.e. from where they are more concentrated to where they are less concentrated.
You can see this happen if you release a few drops of food color into a glass of water. At first the water will become very dark where the drops of color land. After a while, however, the individual molecules of dye will diffuse away from the region where they were first concentrated and the water will become uniformly colored. Try this! What do you think will speed up the process? Think of two things that will speed the diffusion and then try them to see if they really work. Now try to explain the speeding effect in terms of molecular motion. Please email me the results of this experiment and your explanations.
Cell membranes allow some materials to pass through them and do not allow others to do so. They are, therefore, selectively permeable. The diagram shows three containers, each with a selectively permeable membrane separating two solutions. Blue dots are dissolved in water in all three. The membrane is selectively permeable. It is permeable to water, but not to blue dots. That is, it will allow water to pass through, but not the blue dots. Now we have to be careful. We know that substances will diffuse from areas of their greater concentration to areas of their lesser concentration. It is obvious (I hope) that were there no membrane in the tubes, the blue dots would diffuse until they were randomly distributed in the tubes. But they cannot reach the right side of any of the tubes because the membrane is impermeable to them. They always remain in the left compartment of the tubes. But what about the water? It is free to diffuse through the membrane and will do so, from an area of greater concentration of water to an area of lesser concentration of water. What will happen to the water in these tubes? There are three possibilities. 1. The net movement will be the same on both sides of the membrane and the situation will remain as it is in tube A. 2. water will flow from left to right (tube B). 3. Water will flow from right to left (tube C). Before you read further, decide what will happen to the water. Don't guess, try to reason logically using what you have learned about diffusion, concentration gradients and selectively permeable membranes.
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© copyright June B. Steinberg, 2000