Diffusion

Diffusion is the movement of molecules along a concentration gradient, from an area of high concentration to one of low concentration. Diffusion proceeds until the two concentrations are equal. Diffusion occurs in both gases and liquids.

Concentration gradients

Molecules always diffuse from areas of high concentration to areas of low concentration. The difference between the concentration of a substance in one area compared to another area is the concentration gradient. For example, placing ink on the surface of water establishes a concentration gradient in which the surface of the water has a high concentration of ink, and the rest of the water has a low concentration. As the ink diffuses, it moves from the area of high concentration to the area of low concentration, eventually resulting in a solution with equal concentrations of ink.

The importance of diffusion

Diffusion takes place not only in liquid solutions, but in gases. The odor of bread wafting through a house is an example of the diffusion of bread-smell chemicals from a high concentration to a lower concentration.

Diffusion in cells

Cells are bounded by a double membrane composed of lipids. This membrane is punctured intermittently with tiny pores. The membrane of a cell is thus selectively permeable: it keeps out certain substances but lets others pass through. The substances that pass through move in either direction, either into or out of the cell, depending on the concentration gradient. For example, very small ions pass through the lipid membrane through tiny pores in the membrane. Ions move down the concentration gradient that exists between the cytoplasm of the cell and the environment outside the cell, called the extracellular fluid. The extracellular fluid usually contains less ions than the highly concentrated cytoplasm, so ions tend to move from the cytoplasm, down the concentration gradient, into the extracellular fluid. This process is called simple diffusion.

Substances such as glucose or urea cannot pass easily into the cell because their molecules are too large, or because they are electrically charged. In these cases, the substances need assistance in getting across the membrane. Special molecules called carrier molecules, situated within the cell membrane, bind to glucose and other substances and bring about their passage into the cell. Because these substances are moving down a concentration gradient, but are assisted by carrier molecules, this type of diffusion is called carrier-facilitated diffusion.

Water diffusion: osmosis

The special case of diffusion of water into and out of cells is called osmosis . Because osmosis is the diffusion of water, it is the movement of water from an area with a high concentration of water molecules to an area with a low concentration of water molecules; that is, water diffuses from an area in which water is abundant to an area in which water is scarce. Osmosis in cells is usually defined in different terms, however. It is the movement of water from a low concentration of salts to an area with a high concentration of salts, across a semi-permeable membrane.

Resources

books

byrne, john h. an introduction to membrane transport andbioelectricity: foundations of general physiology and electrochemical signaling. 2nd ed. new york: raven press, 1994.

denny, mark. air and water: the biology and physics of life'smedia. princeton: princeton university press, 1993.

yeagle, philip. the membrane of cells. 2nd ed. san diego: academic press, 1993.

Kathleen Scogna