The movement of water molecules from an area of high concentration to an area of low concentration across a semi-permeable membrane is called what?

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

Answer: (B)

Osmosis is the movement of water across a selectively permeable membrane from an area of higher water potential (lower solute concentration) to an area of lower water potential (higher solute concentration). The membrane lets water pass but restricts most solutes, so water flows toward the side with more dissolved particles to balance concentrations on both sides. This transport is passive, meaning no energy is required. Diffusion is a broader term for the movement of any substance from high to low concentration, not restricted to water or membranes. Facilitated diffusion uses specific transport proteins for certain molecules, and active transport requires energy to move substances against their gradient. For example, if a cell is placed in a salty solution, water moves out by osmosis, because the outside has a lower water potential due to the high solute concentration.