Which process moves water across a semipermeable membrane from higher water potential to lower water potential?

Which process moves water across a semipermeable membrane from higher water potential to lower water potential?

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

Answer: (D)

Water moves across a semipermeable membrane when there’s a difference in water potential on the two sides. Water potential is essentially the tendency of water to move; adding solutes lowers this potential, so the side with more solutes has lower water potential. Water travels from the side with higher water potential (more pure water) to the side with lower water potential (more solutes) through the membrane, until the potentials balance. The membrane being semipermeable allows water to pass but blocks many solutes, creating this directional flow. This is what osmosis is. In contrast, diffusion and facilitated diffusion involve solutes moving down their concentration gradient, and active transport uses energy to move substances against their gradient, which is why they aren’t the process described here.

Water moves across a semipermeable membrane when there’s a difference in water potential on the two sides. Water potential is essentially the tendency of water to move; adding solutes lowers this potential, so the side with more solutes has lower water potential. Water travels from the side with higher water potential (more pure water) to the side with lower water potential (more solutes) through the membrane, until the potentials balance. The membrane being semipermeable allows water to pass but blocks many solutes, creating this directional flow. This is what osmosis is. In contrast, diffusion and facilitated diffusion involve solutes moving down their concentration gradient, and active transport uses energy to move substances against their gradient, which is why they aren’t the process described here.