'AQA AS Biology Practice Exam 2025 – The Complete All-in-One Guide for Exam Success!'

The diffusion of water across a selectively permeable membrane is defined as osmosis. This process occurs when water molecules move from an area of higher concentration (where there are more water molecules relative to solute) to an area of lower concentration (where there are fewer water molecules relative to solute), through a membrane that allows the passage of water but restricts solute movement.

Osmosis is crucial for maintaining cell turgor, hydration, and overall fluid balance within biological systems. During osmosis, water will continue to move until there is an equal concentration of solute on both sides of the membrane, achieving equilibrium.

In contrast, active transport involves the movement of substances against their concentration gradient and requires energy, while facilitated diffusion is the process of passive transport of molecules across a membrane via specific transmembrane proteins, also not involving water. Filtration refers to the movement of water and solutes through a membrane due to pressure differences, not concentration gradients. Thus, osmosis specifically denotes the movement of water and is the correct definition in this context.