A concentration gradient occurs where the concentration of something changes over a certain distance. For example, a few drops of food dye in a glass of water diffuse along the concentration gradient, from where the dye exists in its highest concentration (for instance, the brightest blue or red) to where it occurs in its lowest concentration (the water is still clear). The diffusion will continue until the concentration of the dye becomes uniform in all directions of the water. Concentration gradients are the chemical driving force behind many processes that take place near cell membranes.
In general, two types of diffusion are found in living cells: passive and active. It is, however, very rare to encounter pure passive diffusion , where molecules or ions move freely across the cell membrane, following a concentration gradient. For example, water is free to move across a membrane in either direction. But if the solutes inside the cell are barred from moving across the membrane, the resulting phenomenon is called osmosis. The water passes across the membrane into a region of higher solute concentration attempting to reach the ideal equilibrium , where for each side of the membrane the water concentration is the same. This movement leads to the buildup of osmotic pressure, however, so the flow of water stops before the membrane bursts.
Active diffusion occurs when membranes translocate or move molecules or ions from regions of low concentration to those of higher concentration. For example, many cells are able to increase the internal concentration of solutes until very high levels are reached and considerable concentration gradients are established. In this case, a process other than diffusion must be available to supply the energy. Generally, the energy comes from the hydrolysis of adenosine triphosphate (ATP) , an energy-rich molecule. Active transport is very important for tissues that are specialized, such as nerve and muscle tissues as well as secretory (or excretory) tissues like the kidneys of animals and the gills of marine life, so solutes may be absorbed against concentration gradients.
In addition, ion concentration gradients existing between two sides of a membrane produce an electrical potential difference, ranging between 50 and 100 millivolts or mV (10 −3 volt), the outside being positive with respect to the interior. This is the direct consequence of the distribution of cations, especially potassium and sodium ions. Any stimulation by electrical, mechanical, or chemical means at one point of the membrane will create a change in the potential membrane at that point. The altered potential, also called the active potential, will move as a wave over the membrane surface. This provides a means of rapid communication between different regions of a cell. In the case of an elongated nerve cell, this constitutes a nerve impulse.
It is interesting to note that this active potential is used by some fish, such as catfish and eel, to defend themselves as well as to stun their prey. The excitable membranes of the fish each develop a potential of 100 millivolts, but are stacked in such a manner that their potential differences add up to several hundred volts.
Howland, John L. (1973). Cell Physiology. New York: Macmillan.