What is osmosis?

• A. The diffusion of solutes across a membrane.
• B. The diffusion of water molecules across a selectively permeable membrane from an area of lower solute concentration to higher.
• C. The diffusion of water across a selectively permeable membrane from higher water potential to lower water potential.
• D. The active transport of ions across a membrane.

Answer: (C)

Osmosis is the movement of water across a selectively permeable membrane driven by a gradient in water potential. Water potential is like the “availability” of water; it’s higher in areas with few solutes and lower where solute concentration is higher. Water moves from the region of higher water potential to the region of lower water potential to balance conditions on both sides of the membrane, and this movement happens passively—no energy required—so the membrane must be permeable to water but not necessarily to solutes.

The best description matches this exact idea: water diffuses across a selectively permeable membrane from higher water potential to lower water potential. This captures both the substance moving (water) and the driving force (water potential gradient), which is the core feature of osmosis.