The movement of solutes and water across a semipermeable membrane due to a force such as gravity or blood pressure is called?

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

Answer: (A)

Filtration is the movement of water and small solutes across a semipermeable membrane driven by a pressure difference, such as hydrostatic pressure from gravity or blood pressure. That pressure pushes fluid and dissolved substances through membrane pores, so both water and solutes can pass while larger molecules stay behind. This distinguishes filtration from diffusion, which is the random spread of particles down a concentration gradient without a bulk pressure force; and from osmosis, which is specifically water moving across a membrane in response to solute concentration differences. Active transport, on the other hand, requires cellular energy to move substances against their gradient. A common real-world example is fluid moving from blood into surrounding tissues or into the filtrate in the kidneys due to hydrostatic pressure.