Which type of diffusion involves water moving across a membrane from high water concentration to lower water concentration?

• A. Osmosis
• B. Facilitated diffusion
• C. Active transport
• D. Simple diffusion

Answer: (A)

Osmosis is water moving across a selectively permeable membrane from higher water concentration to lower water concentration. Water flows toward the side with more solutes (less water) to balance concentrations, and this happens without energy input. The membrane lets water pass (often through aquaporin channels) while many solutes cannot cross as easily, so the driving force is the difference in water concentration across the membrane. This is why osmosis is the correct concept here. The other processes involve moving solutes or require energy: facilitated diffusion moves substances down their gradient with help from transport proteins; simple diffusion moves small, nonpolar molecules across; active transport pushes substances against their gradient using energy.