Osmosis is the movement of water across a membrane.

• A. Through protein channels
• B. From high concentration to low concentration
• C. From solutes to water
• D. Across a membrane

Answer: (D)

Osmosis is water movement in response to a solute gradient across a selectively permeable membrane. The essential idea is that water flows across a membrane to balance solute concentrations on both sides, often through water channels called aquaporins, but the membrane is the key requirement. Without a membrane, you’re just describing diffusion, not osmosis. Saying water moves through channels is sometimes true, but it’s not what defines osmosis itself; likewise, solutes moving or diffusion from high to low concentration don’t capture the membrane-bound, water-specific transport that osmosis describes. Across a membrane best captures the defining condition of osmosis.