The movement of water across selectively permeable membranes.

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

Answer: (C)

Water moves across a selectively permeable membrane by osmosis. Osmosis is the diffusion of water across a membrane that allows water to pass but may restrict solutes. Water travels from where there is more water (lower solute concentration) to where there is less water (higher solute concentration) in order to balance solute levels on both sides. This process is passive, not requiring energy. The result is changes in cell volume depending on the surrounding solution: water enters a cell in a hypotonic environment, causing swelling; it leaves in a hypertonic environment, causing shrinking; and in an isotonic environment there is no net movement. Other terms describe different movement: diffusion of solutes, active transport that uses energy to move substances against their gradient, and facilitated diffusion that uses proteins to move certain solutes down their gradient. Osmosis specifically accounts for the movement of water itself.