Diffusion

Diffusion is the net movement of particles from a region of higher concentration to one of lower concentration, driven by random thermal motion.

Brownian motion: The irregular motion of particles suspended in a fluid, caused by collisions with its molecules. Its random molecular collisions provide a visible example of the motion underlying diffusion.
Concentration: The amount of a substance present in a given volume or other measure of space. A spatial difference in concentration identifies the gradient associated with net diffusion.
Osmosis: The net movement of solvent across a selectively permeable membrane toward higher solute concentration. It is a membrane-mediated case of diffusion involving solvent movement.
Convection: Transport of heat or matter by the bulk motion of a fluid. Unlike diffusion, convection carries particles through coordinated fluid movement.
Diffusion limitation: A constraint that arises when diffusion cannot supply or remove substances rapidly enough. Slow transport can limit reaction rates and the size of biological structures.
Fick's laws of diffusion: Equations relating diffusive flux to concentration gradients and their change over time. They quantify how concentration gradients drive particle flux.
Random walk: A mathematical model in which successive steps are chosen randomly. Many independent random steps produce the large-scale spreading seen in diffusion.
Gas diffusion: The spreading of gas molecules through random motion and collisions. It explains how gases mix and how molecules move through porous materials.
Active transport: The energy-dependent movement of substances across membranes, often against concentration gradients. It can move substances uphill in concentration, unlike passive diffusion.
Diffusion time: The characteristic time required for diffusion across a specified distance. The time grows roughly with the square of distance, making long-range diffusion slow.

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