Which statement about osmosis is true?

• A. Water diffuses through a selectively permeable membrane
• B. Water and solute molecules diffuse equally across a membrane
• C. Energy is required for water movement
• D. Solutes diffuse across the membrane down their concentration gradient

Answer: (A)

Osmosis is the diffusion of water across a selectively permeable membrane, moving from where water concentration is higher (or solute concentration is lower) to where water concentration is lower (or solute concentration is higher). The statement that water diffuses through a selectively permeable membrane reflects this idea: water can pass through the membrane, while many solutes cannot, so water movement occurs to balance solute concentrations.

The other ideas don’t fit osmosis as defining features. Water and solute molecules do not diffuse equally across the membrane because the membrane is selectively permeable and often restricts solute movement. Osmosis is a passive process and does not require energy. Solutes may diffuse across the membrane if the membrane is permeable to them, but that diffusion is separate from osmosis and isn’t the defining feature.