What must occur to stop the process of diffusion or osmosis?

• A. Equilibrium; the solutes or water level are the same on both sides of a membrane
• B. A higher external temperature will stop diffusion
• C. A membrane becomes completely impermeable
• D. Active transport must occur for diffusion to stop

Answer: (A)

Diffusion and osmosis stop when equilibrium is reached across the membrane. When the solute concentrations (or osmolarity) are equal on both sides, there is no net movement of particles, so the processes cease. These are passive processes driven by gradients, so movement continues only as long as a gradient exists. In osmosis, water moves toward the side with higher solute concentration until hydrostatic pressure balances osmotic pressure, and then net flow stops.

Temperature changes alter the rate of diffusion but do not inherently stop it. A membrane being completely impermeable would prevent diffusion entirely, but that describes a barrier, not the universal stopping condition. Active transport uses energy to move substances against a gradient and does not cause diffusion to stop.