Water moves across a membrane from an area of lower solute concentration to an area of higher solute concentration by which principle?

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

Answer: (A)

Osmosis is at work here: water moves across a semipermeable membrane from the side with lower solute concentration to the side with higher solute concentration. This happens because the higher-solute area exerts osmotic pressure that pulls water through the membrane to help equalize solute levels. The membrane is more permeable to water than to the solute, so water can cross while many solutes cannot. This movement is driven by the osmotic gradient, not by energy use. It’s different from diffusion, which involves solutes moving down their own concentration gradients; it’s different from active transport, which requires cellular energy to push substances against their gradients; and it’s different from filtration, which is driven by hydrostatic pressure. In dialysis contexts, creating or leveraging an osmotic gradient with the dialysate can draw water across the membrane, facilitating fluid removal.