Osmosis is defined as what?

• A. Osmosis is the diffusion of water across a selectively permeable membrane.
• B. Osmosis is the diffusion of water across any membrane.
• C. Osmosis is active transport of water requiring ATP.
• D. Osmosis is diffusion of solutes across a membrane.

Answer: (B)

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves from the side with higher water potential (lower solute concentration) to the side with lower water potential (higher solute concentration) to balance solute levels. The membrane must be selectively permeable to water, otherwise osmosis wouldn’t occur; many solutes can’t cross freely, so water shifts to even out concentrations. This movement is passive and does not require energy. Diffusing water across any membrane isn’t precise, because if the membrane isn’t water-permeable, osmosis can’t happen, and diffusion of solutes is a different process.