What is osmosis and direction of water movement in a hypertonic solution?

• A. Osmosis is the diffusion of water across a selectively permeable membrane; water moves toward higher solute concentration (from hypotonic to hypertonic).
• B. Osmosis is the diffusion of solutes across a membrane; water moves from hypertonic to hypotonic solutions.
• C. Osmosis is active transport of water using ATP; water moves randomly.
• D. Osmosis is the diffusion of water through any membrane regardless of solute concentration.

Answer: (A)

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves toward higher solute concentration, from the side with lower solute concentration (hypotonic) to the side with higher solute concentration (hypertonic). In a hypertonic solution, the outside environment has more solute, so water flows out of the cell into that surrounding solution, causing the cell to shrink. Osmosis is a passive process, requiring no energy, and it specifically involves water moving through a membrane that is permeable to water. The other statements either describe diffusion of solutes instead of water, claim active transport, or imply water moves freely through any membrane regardless of solute conditions, which isn’t accurate.