What process describes the movement of molecules across a membrane?

The movement of molecules across a membrane is best described by diffusion, which is a passive transport process. In diffusion, molecules move from an area of higher concentration to an area of lower concentration until equilibrium is reached. This process does not require energy and occurs naturally due to the random motion of particles.

Diffusion is a fundamental concept in biology, as it explains how substances such as oxygen and carbon dioxide move across cell membranes in living organisms. It is the mechanism behind many physiological processes, including gas exchange in the lungs and nutrient uptake in cells.

While other processes like osmosis, active transport, and facilitated transport also describe movement across membranes, each has specific conditions and requirements. Osmosis, for instance, refers specifically to the movement of water molecules through a selectively permeable membrane, primarily driven by differences in solute concentration. Active transport, on the other hand, requires energy to move molecules against their concentration gradient. Facilitated transport involves carrier proteins that help certain molecules cross the membrane but does not require energy, as it also moves molecules down their concentration gradient.

Thus, diffusion is a key process for understanding how substances naturally move and distribute in biological systems.