Osmosis refers to the diffusion of water across a membrane.

• A. Diffusion of water across a membrane
• B. Diffusion of solutes across a membrane
• C. Active transport of water
• D. Bulk flow of water and solutes

Answer: (A)

Osmosis is water moving across a selectively permeable membrane in response to differences in solute concentration. Water travels from where there is more water (lower solute concentration) to where there is less water (higher solute concentration) to help balance concentrations, and it does so passively without using energy. This movement can occur directly through the lipid bilayer or through aquaporin channels. The other options describe diffusion of solutes, active transport requiring energy, or bulk flow driven by pressure—none of which define osmosis. So the statement that best captures osmosis is the diffusion of water across a membrane.