At equilibrium, is the number of molecules on each side of a membrane guaranteed to be equal?

• A. Yes
• B. No, the rate of movement on both sides is equal, but the numbers may differ
• C. No, numbers on both sides are always equal
• D. Depends on temperature

Answer: (B)

In diffusion across a membrane, equilibrium means no net transfer of molecules across the membrane. Molecules keep moving back and forth, but the forward and backward fluxes are equal, so the net flow is zero. The actual number of molecules on each side doesn’t have to be the same; it depends on the volumes of the compartments and how many started there. If the two sides have different volumes, you can have different counts while the concentrations (molecules per unit volume) can balance so that there’s no net movement. Temperature changes how fast molecules move (the rate) but doesn’t by itself force equal numbers on both sides. So the correct idea is that rates are balanced at equilibrium, but the numbers on each side may differ.