A Level Biology Practice Exam

What is osmosis?

1. Diffusion of nutrients across a cell membrane

2. Active transport of water against a gradient

3. Diffusion of water through a partially permeable membrane down a water potential gradient

4. Movement of solutes through a membrane using energy

The correct answer is: Diffusion of water through a partially permeable membrane down a water potential gradient

Osmosis is defined as the diffusion of water through a selectively permeable membrane in response to a water potential gradient. This process occurs when water molecules move from an area of higher water potential (where there is a higher concentration of water molecules, and a lower concentration of solute) to an area of lower water potential (where there is a lower concentration of water molecules, and a higher concentration of solute). This movement continues until equilibrium is reached or until other factors such as pressure prevent further movement. Importantly, osmosis does not require energy input, as it is a passive transport process driven by the concentration gradient of water molecules rather than the active transport of solutes. In contrast to other options, diffusion of nutrients across a cell membrane refers to the movement of solutes, not water, while active transport involves the movement against a gradient using energy, which does not apply to osmosis. Finally, the movement of solutes using energy describes a different process entirely, primarily linked to solute transport rather than water. Thus, the correct definition of osmosis aligns precisely with the concept of water moving through a membrane driven by the difference in water potential.