All living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases (usually CO₂ and O₂), taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell. This membrane is a complex structure that is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell, and for interacting with the environment surrounding the cell.

There are two ways that the molecules move through the membrane: passive transport and active transport. Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. Passive transport does not require such an energy expenditure, and occurs spontaneously.

The principle means of passive transport is diffusion. Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout the system.

An important concept in understanding diffusion is the concept of equilibrium. There are two types of equilibrium. Static equilibrium occurs when there is no action taking place. Dynamic equilibrium occurs when two opposing actions occur at the same rate. For example, consider a bucket full of water. It is in a state of static equilibrium because the water level stays the same. The water is not moving. If you were to poke a hole in the bottom of the bucket, water would leak out. This system would not be at equilibrium because there is action taking place - water is leaking out - and the water level in the bucket would drop.

However, if you were to begin pouring water into the bucket at the same rate that it was leaking out, the water level in the bucket would stay the same because the rate at which the water is entering the bucket is equal to the rate at which it is leaking out. This is an example of dynamic equilibrium, and it applies to nearly everything that happens in the natural world.

Diffusion occurs when a system is not at equilibrium. As an example, suppose you drop one drop of ink into a glass of water. At first, all of the ink molecules are in a small space and they are moving around in a random way. They move in straight lines and change direction only when they collide with each other or the surrounding water molecules. Some of the ink molecules near the edge of the drop move away from the center of the drop. As a matter of fact, most of the molecules move away from the center of the drop.

Most of the molecules continue to move away from the original center of the drop. They move in all different directions, and some may even move back toward the center. Still, more are moving away from the drop than toward it until they find the wall of the glass. Then they start moving back toward the center again. More and more molecules bounce off of the glass until they start moving toward the center, then they pass the center and move toward the other side. Eventually the number of molecules moving away from the center equals the number moving toward the center, and equilibrium is established. At this point the molecules are evenly spread throughout the water, and diffusion stops. Have the molecules stopped moving? Is this a static or dynamic equilibrium?

Several factors affect how fast a molecule will diffuse. The first of these is the kinetic energy of the molecule, which is most frequently measured as the temperature of the system. Molecules in a system at a higher temperature will have more energy and will move faster, and hence diffuse faster, than molecules of the same type in a low-temperature system. The size of the molecule also affects how rapidly it will diffuse. At the same temperature, smaller molecules will move more rapidly than larger molecules because it takes more energy to get the larger molecule moving. Other factors include any charges on the molecule (positive or negative) and the nature of the material that the molecules are moving through.

Diffusion can occur through a cell membrane. The membrane allows small molecules like water (H₂O), oxygen (O₂), carbon dioxide (CO₂), and others to pass through easily. It is said to be permeable to these molecules. If a cell is floating in a water solution (like the ocean) that has some oxygen dissolved in it, the oxygen molecules will move into the cell. They will also move out of the cell at the same rate, and a dynamic equilibrium will exist. However, if the cell uses some of the oxygen as it comes into the cell, more oxygen will move into the cell than out of the cell.

So the oxygen effectively moves from a region of high concentration (the seawater) to a region of low concentration (the cell), and diffusion occurs. Likewise, as the chemical reactions in the cell use up oxygen they produce carbon dioxide. The concentration of carbon dioxide inside the cell increases so that more CO₂ molecules strike the inside of the cell and move out than strike the outside of the cell and move in. So the overall effect is that the CO₂ moves out of the cell.

Osmosis is a special case of diffusion. In this case, a large molecule like starch is dissolved in water. The starch molecule is too large to pass through the pores in the cell membrane, so it cannot diffuse from one side of the membrane to the other. The water molecules can, and do, pass through the membrane. Hence the membrane is said to be semipermeable, since it allows some molecules to pass through but not others. However, on the side of the membrane with the starch, the starch molecules interfere with the movement of the water molecules, preventing them from leaving as rapidly as they enter. Thus, more water flows into the side with the starch than flows out, and the starch gets diluted.

If the starch (or some other large molecule like a protein) is in a cell, the water moves into the cell faster than it leaves, and the cell swells. The cell membrane acts somewhat like a balloon, and if too much water enters the cell, the cell can burst, which kills the cell. So cells usually have some kind of mechanism for preventing too much water from entering or pumping the water out or simply making a tough outer coat that will not rupture.

Things are more difficult when the starch or other large molecule is on the outside of the cell. Then the cell loses water faster than it comes in, and the cell shrinks, which might not be too bad except that the cell needs the water for the chemical reactions that take place inside that keep it alive. In fact, this principle is used in food preservation. Foods that are packed in salt or sugar prevent bacterial growth by essentially sucking the water out of the bacterial cells (or, more properly, preventing water from entering the cells) and preventing their growth.

There are other ways that cells use to move materials across the cell membrane, most of which involve active transport, requiring the use of energy. The cell membrane also has other functions besides controlling the movement of materials into and out of the cell, and the membranes of specialized cells have very complex functions. So we see that the cell membrane is a very intricate and important component of the cell.

Teacher Materials:

Teaching Tips

Evaluation Keys

Student Materials:

	Background Reading
Osmosis in Elodea	Dynamic Equilibrium	Factors Affecting Diffusion
Osmosis	Osmosis and Blood Cells	Quiz

Science Education Connection Department of Biochemistry The University of Arizona Wednesday, February 12, 1997 johnmcc@dakotacom.net http://biology.arizona.edu/sciconn/lessons/mccandless/ All contents copyright © 1997; 1998. All rights reserved.