What process describes the diffusion of free water molecules across a partially permeable membrane?

The diffusion of free water molecules across a partially permeable membrane is specifically described by osmosis. This process involves the movement of water from an area of lower solute concentration (where there are more free water molecules) to an area of higher solute concentration (where there are fewer free water molecules) through a membrane that allows water to pass but restricts solute molecules. Osmosis is vital for maintaining cell turgor in plant cells, regulating the internal environment of cells, and ensuring proper physiological function.

While diffusion generally refers to the movement of particles from an area of higher concentration to one of lower concentration, osmosis is a specialized case focusing exclusively on water molecules and occurs in relation to solute concentrations. This distinction is crucial for understanding fluid balance in biological systems. In contrast, filtration refers to the physical separation of particles based on size through a membrane, and active transport involves the movement of substances against their concentration gradient, requiring energy input.