Diffusion

Cells are bounded by a double membrane composed of lipids. This membrane is punctured intermittently Simple diffusion (top) and carrier-facilitated diffusion (bottom) in a red blood cell. Illustration by Hans & Cassidy. Courtesy of Gale Group. 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.