What process involves the movement of water across a semi-permeable membrane?

The process being described is osmosis, which specifically refers to the movement of water molecules through a semi-permeable membrane. In osmosis, water moves from an area of lower solute concentration (where there is a higher concentration of water) to an area of higher solute concentration (where there is a lower concentration of water) in order to balance solute concentrations on both sides of the membrane.

This process is passive, meaning it does not require energy input; it relies instead on the concentration gradient of water and solutes. Semi-permeable membranes allow certain molecules, like water, to pass through while blocking others, which is key to maintaining homeostasis in cells and regulating their internal environment.

The other options involve different mechanisms: diffusion refers to the movement of solutes, active transport requires energy to move molecules against their concentration gradient, and endocytosis involves the engulfing of substances into a cell rather than the movement of water across membranes. This makes osmosis a unique and vital process for cellular function and hydration.