Molecules in fluids undergo continual random thermal motion. When a substance is distributed unevenly, that random motion produces a net spreading from regions where the substance is more concentrated toward regions where it is less concentrated. This process is diffusion.

A concentration gradient means that concentration changes from one position to another. Diffusion tends to reduce such gradients.

The important point is that individual molecules do not sense where concentration is lower and deliberately move there. Molecules move randomly in all directions. There are simply more molecules available to cross from the high-concentration region toward the low-concentration region than in the reverse direction, creating a net flux down the gradient.

Example

Suppose a dye molecule is initially concentrated in one corner of a beaker of still water. Dye molecules immediately move in many directions because of thermal motion. At first, many more molecules leave the highly concentrated region than return to it. Over time the dye spreads throughout the water.

At uniform concentration, molecular motion does not stop. Molecules continue crossing imaginary boundaries in both directions, but the average rates are equal, so there is no net diffusive flux.

What changes the rate?

Diffusion is generally faster when:

• the concentration difference is larger;
• the distance over which molecules must spread is smaller;
• the diffusing species moves more readily through the medium;
• temperature increases molecular motion, although the quantitative effect depends on the system.

Large distances matter greatly. Diffusion is very effective over cellular micrometer scales but becomes slow over macroscopic distances, which is one reason multicellular organisms use bulk flow and circulatory systems for long-range transport.

Diffusion is a passive process: a net flux down a concentration gradient does not require a molecular motor or direct ATP consumption. Mathematical laws such as Fick's law quantify this same physical tendency.