What Is a Diffusion Gradient?

Concentration

• A diffusion gradient, or concentration gradient, exists when an area of higher concentration is in contact with an area of lower concentration. When substances are in contact, particles may flow between them. The particles of a substance are in continuous, random motion in every direction, but there is a net flow of particles down the diffusion gradient or, in other words, from the area of higher concentration to the area of lower concentration.

Dynamic Equilibrium

• As particles flow down the diffusion gradient, their concentration at the top of the gradient decreases and their concentration at the bottom of the gradient increases, so the gradient becomes smaller, or less steep. Eventually, the concentration of particles equalizes and the diffusion gradient ceases to exist. At this point, known as dynamic equilibrium, particles continue to move, but there is no net flow of particles in any one direction and no change in concentration.

Free Energy

• The movement of a substance down a diffusion gradient liberates energy and is known as an exergonic process. The free energy liberated during the process can be calculated from the formula "G = RT ln (C2/C1), where R represents the universal gas constant, T represents temperature, ln represents natural logarithm and C2 and C1 represent the concentrations of the substance at the top and bottom of the diffusion gradient. The movement of a substance up, or against, a concentration gradient is an endergonic process, requiring an outside energy source.

Examples

• A diffusion gradient is responsible for many everyday phenomena, such as the dispersal of a vapor in air, or the dissolving of salt in water. A drop of perfume, for example, quickly evaporates to create a high concentration of perfume molecules above the drop. The perfume molecules collide with each other, spreading the molecules further and further from the initial drop, so that eventually their concentration equalizes throughout the space in which the drop was released. Similarly, a pile of table salt (sodium chloride) in water quickly dissociates into charged sodium (Na+) and chloride (Cl-) ions. The ions gradually spread out from a dense concentration above the pile to areas of lower concentration further away.