Diffusion

Diffusion is the movement or spreading out of a substance from an area of high concentration to the area of lowest concentration. Diffusion takes place at the cellular level in both living and nonliving things. Simple animals that do not have an internal circulatory system rely on diffusion to exchange gases and obtain nutrients. The root cells of green plants obtain their water from the soil by diffusion. Diffusion requires no output of energy on the part of the cell.

Diffusion is a natural phenomenon that happens under certain conditions and occurs at the molecular level. Because molecules are constantly moving, their natural tendency is for different types to mix with one another. This movement of molecules is random and depends on the amount of energy (called kinetic energy) in each molecule. As different types of molecules move about and mix together, the only pattern noticeable is their overall movement from an area of high concentration (where they are all together) to an area of lowest concentration (where there are the fewest of them). While an individual molecule may not do this exactly, the net or overall movement of the group of molecules will always move in that manner. This net movement is called diffusion.

A good example of everyday diffusion is the way tobacco smoke spreads throughout the still air of a room. Perfume does the same thing in the motionless air of a room. For the life sciences, diffusion takes place at the cellular level of both plants and animals. Since cell membranes are composed of molecules that are always in motion, there are always temporary openings in cell membranes. Living cells are always bathed in liquid. If the liquid concentration of a certain type of molecule is higher on the outside of a cell membrane than on the inside, those molecules will

diffuse through the membrane into the cell. The opposite can also occur if the liquid concentration is higher on the inside of a cell. The rate of diffusion depends on the unevenness of the distribution and the size of the molecules. When the molecules on either side of the membrane are equally distributed, no diffusion will take place.

The process of diffusion is an important method for cells to be able to exchange substances with their environment. As a result, the cells are able to refill what they have run out of and dispose what they have too much of. However, since the cell has no real control of this process, it is called passive diffusion. Diffusion also takes place in the lungs of animals. When blood enters the lungs to fill up with oxygen, it is carrying carbon dioxide, a waste product, in high concentration. Immediately, the carbon dioxide diffuses from the blood into the lungs and oxygen diffuses from the lungs into the blood.

This same exchange takes place in single-celled animals and in many simple aquatic animals as well. Animals such as sponges, flatworms, and hydras rely on diffusion to get oxygen and food from the surrounding water and to remove waste. Such simple animals can do this either because their bodies have many openings (like sponges) or because they have body walls that are only two or three cells thick (like hydras). Diffusion can work in such animals since they have simple body parts and minimal demands. However, creatures with fast metabolisms (all the chemical processes going on in a living thing) need real circulatory systems that can work fast and transport large amounts. Plants with vascular systems (internal pipelines for transporting food and water) also use diffusion to take water and oxygen into their roots. For example, if the cells of a root have only a little oxygen in them and the surrounding soil has a great deal, oxygen molecules will automatically move from the soil to the root. As with animal cells, the plant does not expend any energy to accomplish this.

[See alsoRespiratory System ]