Osmosis is the diffusion of water across a selectively permeable membrane. Which of the following describes osmosis?

• A. The diffusion of water molecules through a partially permeable membrane
• B. The diffusion of solutes across membranes
• C. The uptake of nutrients by ingestion
• D. The excretion of wastes from the body

Answer: (A)

Osmosis is water moving through a membrane that only lets water pass, driven by differences in water potential. This means water diffuses across a selectively permeable (partially permeable) membrane from the side with higher water potential (lower solute concentration) to the side with lower water potential (higher solute concentration). The option that describes water molecules diffusing through a partially permeable membrane matches this idea exactly. The other statements talk about diffusion of solutes, taking in nutrients, or getting rid of wastes—processes that are not osmosis, since they don’t involve water crossing a membrane.