Which process involves the movement of water molecules through a semi-permeable membrane down a concentration gradient?

Which process involves the movement of water molecules through a semi-permeable membrane down a concentration gradient?

• A. Diffusion
• B. Osmosis
• C. Filtration
• D. Active transport

Answer: (B)

Osmosis is the movement of water across a semipermeable membrane from a region with lower solute concentration (higher water potential) to a region with higher solute concentration (lower water potential). The membrane lets water pass but mostly blocks solutes, so water shifts to balance concentrations along the gradient. This is specifically about water and a membrane, not about solute particles diffusing or being moved by energy. Diffusion would involve other substances spreading through a medium, not necessarily crossing a membrane and not specifically the water moving to equalize concentrations. Filtration is driven by pressure differences, not by a concentration gradient. Active transport requires energy to move substances against their gradient, which is opposite to moving down the gradient.

Osmosis is the movement of water across a semipermeable membrane from a region with lower solute concentration (higher water potential) to a region with higher solute concentration (lower water potential). The membrane lets water pass but mostly blocks solutes, so water shifts to balance concentrations along the gradient. This is specifically about water and a membrane, not about solute particles diffusing or being moved by energy.

Diffusion would involve other substances spreading through a medium, not necessarily crossing a membrane and not specifically the water moving to equalize concentrations. Filtration is driven by pressure differences, not by a concentration gradient. Active transport requires energy to move substances against their gradient, which is opposite to moving down the gradient.