Active transport differs from diffusion in that active transport

1. Analyze the question The question asks for a feature that sets active transport apart from diffusion. The answer must be true of active transport and NOT true of diffusion.

2. Explain the concept

• Diffusion is the movement of molecules from a region of higher concentration to a region of lower concentration (down the concentration gradient). It is passive and needs no energy from the cell.
• Active transport is the movement of substances across a cell membrane from a region of lower concentration to a region of higher concentration (against the concentration gradient). It needs energy in the form of ATP and uses carrier proteins.

3. Evaluate each option

• A. Is a very fast process: Not a defining feature. Diffusion can be fast too, and speed does not distinguish the two processes.
• B. Allows the movement of substances against the concentration gradient: True for active transport only. Diffusion can never do this. This is the key difference.
• C. Takes place in both light and dark reactions of photosynthesis: Incorrect. Light and dark reactions are about energy capture and carbon fixation, not about distinguishing transport types.
• D. Occurs in both plant and animal tissues: True, but diffusion also occurs in both, so it does not set them apart.

4. Answer

5. Exam tip Remember: Diffusion = downhill, no energy. Active transport = uphill, needs energy (ATP). Whenever you see "against the concentration gradient" in a transport question, it points to active transport.