Which process describes the movement of water across a membrane?

• A. Osmosis
• B. Diffusion
• C. Active Transport
• D. Transpiration

Answer: (A)

Water moving across a membrane is osmosis, a specific kind of diffusion that involves water crossing a selectively permeable membrane to balance solute concentrations on both sides. Water tends to move from area with higher water potential (fewer dissolved substances) to area with lower water potential (more dissolved substances) until the sides are balanced. Diffusion describes general movement of molecules from high to low concentration, but osmosis specifically refers to water crossing a membrane. Active transport uses energy to move substances against their gradient, not simply water flow. Transpiration is the plant process of water loss from leaves, not the movement of water across a membrane inside cells. So osmosis is the process at work.