What is the diffusion of water across a selectively permeable membrane called?

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

Answer: (B)

Water moves across a selectively permeable membrane by osmosis, a diffusion process specific to water. It occurs because the two sides have different solute concentrations, creating a difference in water potential. Water flows from the side with higher water potential (lower solute concentration) to the side with lower water potential (higher solute concentration) until equilibrium is reached. The membrane lets water through but restricts many solutes, so water shifts to balance solute levels, often aided by aquaporin channels. Osmosis is a special case of diffusion focused on water movement; other transport types involve solute diffusion, facilitated diffusion with channels or carriers, or active transport that uses energy to move substances against their gradient.