Osmosis is the movement of which substance across a selectively permeable membrane?

• A. Movement of solute particles across a membrane
• B. Movement of water across a selectively permeable membrane
• C. Active transport of ions
• D. Bulk transport

Answer: (B)

Osmosis centers on the diffusion of water across a selectively permeable membrane. Water flows from where there is more free water (higher water potential / lower solute concentration) to where there is less free water (lower water potential / higher solute concentration) until the system tends toward balance. The membrane allows water to pass while restricting many solutes, so the net movement is water rather than solute particles. This is a passive process, not requiring energy, and it can be aided by water channels called aquaporins. The other options describe moving solutes, energy-dependent transport, or moving large amounts of material in vesicles, which are not osmosis.