The speed at which diffusion occurs, measured by the number of diffusing molecules passing through a membrane per unit time, depends on (1) the magnitude of the concentration difference across the membrane (the "steepness" of the concentration gradient), (2) the permeability of the membrane to the diffusing substances, (3) the temperature of the solution, and (4) the surface area of the membrane through which the substances are diffusing.
The magnitude of the concentration difference across a membrane serves as the driving force for diffusion. Regardless of this concentration difference, however, the diffusion of a substance across a membrane will not occur if the membrane is not permeable to that substance. With a given concentration difference, the speed at which a substance diffuses through a membrane will depend on how permeable the membrane is to it. In a resting neuron, for example, the plasma (cell) membrane is about twenty times more permeable to potassium (K+) than to sodium (Na+); consequently, K+ diffuses much more rapidly than does Na+. Changes in the protein structure of the membrane channels, however, can change the permeability of the membrane. This occurs during the production of a nerve impulse (see chapter 7), when specific stimulation opens Na+ channels temporarily and allows a faster diffusion rate for Na+ than for K+.
In areas of the body that are specialized for rapid diffusion, the surface area of the cell membranes may be increased by numerous folds. The rapid passage of the products of digestion across the epithelial membranes in the small intestine, for example, is aided by tiny fingerlike projections called microvilli (discussed in chapter 3). Similar microvilli are found in the kidney tubule epithelium, which must reabsorb various molecules that are filtered out of the blood.
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