What is diffusion in a biological context?

• A. The passive movement of molecules from high concentration to low concentration until equilibrium.
• B. The active movement of molecules against their concentration gradient.
• C. The diffusion of water that requires energy.
• D. The diffusion of solutes across a membrane.

Answer: (A)

Diffusion in biology is the passive movement of molecules from regions of higher concentration to regions of lower concentration, driven by the random thermal motion of particles. This process continues until concentrations become equal, at which point there is no net movement. Because it relies on natural molecular motion, no cellular energy is needed. This idea explains why substances spread through a solution and, for many small or nonpolar molecules, can cross membranes without pumps. It also helps distinguish diffusion from active transport, which moves substances against their gradient using energy, and from osmosis, which is the diffusion of water across a selectively permeable membrane.