A Level Biology Practice Exam

Which process is described as diffusion that occurs without energy input?

1. Osmosis

2. Exocytosis

3. Facilitated diffusion

4. Active transport

The correct answer is: Osmosis

The correct selection is osmosis, which is a specialized form of diffusion specifically referring to the movement of water molecules across a selectively permeable membrane. This process occurs in response to differences in solute concentration, allowing water to move from areas of lower solute concentration to areas of higher concentration until equilibrium is reached. Importantly, osmosis does not require any energy input from the cell, aligning perfectly with the description provided in the question. Facilitated diffusion, while also a passive transport process, specifically involves the movement of solutes across a membrane through protein channels or carriers. While it does not require energy, it does not accurately represent the specific concept of water movement that osmosis describes. Active transport, on the other hand, is a process that does require energy to move substances against their concentration gradient, making it incompatible with the concept of energy-independent diffusion. Exocytosis refers to the process of vesicle transport out of the cell, which also requires energy.