The movement of water across a selectively permeable membrane, from low to high concentration, is called what?

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

Answer: (A)

Water moving across a selectively permeable membrane toward higher solute concentration is osmosis. This is a passive transport process, so it doesn’t require energy. The membrane lets water pass but restricts many solutes, so water flows from the side with lower solute concentration (higher water potential) to the side with higher solute concentration (lower water potential) in an effort to balance concentrations on both sides. Osmosis continues until equilibrium is reached or other forces stop it. It’s different from diffusion, which involves solute particles moving from high to low concentration; from active transport, which uses energy to move substances against a gradient; and from filtration, which is driven by pressure.