Which term describes the movement of water across a semipermeable membrane from higher water potential to lower water potential?

• A. Diffusion
• B. Osmosis
• C. Mass flow
• D. Capillary rise

Answer: (B)

Osmosis is the movement of water across a semipermeable membrane from higher water potential to lower water potential. Water potential reflects both the tendency of water to move (pressure potential) and the effect of dissolved solutes (solute potential). Water moves toward the side with lower water potential (more negative) because that side has a greater tendency to draw in water, and the membrane allows water to pass while restricting solutes.

This differs from diffusion, which is the spread of solutes from high to low concentration and isn’t restricted to water crossing a membrane. Mass flow refers to bulk movement of fluid due to pressure differences, not the selective passage of water through a membrane. Capillary rise is driven by capillary action in narrow spaces and is not about water moving across a membrane due to a potential gradient.

In plants, osmosis explains how water moves from soil (typically higher water potential) into root cells (lower water potential), enabling uptake and transport.