Which of the following best describes whether diffusion can occur against a concentration gradient?

• A. No, diffusion requires ATP to move substances against the gradient.
• B. No, diffusion only occurs from areas of high concentration to low concentration.
• C. Yes, but only if carrier proteins are present.
• D. Yes, diffusion can move substances from low to high concentration without energy input.

Answer: (B)

Verified step by step guidance

1. Understand the concept of diffusion: Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration, driven by the concentration gradient, without requiring energy input.
2. Clarify the term 'against a concentration gradient': Moving against a concentration gradient means moving substances from an area of low concentration to an area of high concentration, which typically requires energy input.
3. Distinguish diffusion from active transport: Active transport is a process that moves substances against a concentration gradient using energy, often in the form of ATP, and may involve carrier proteins.
4. Evaluate the options provided: Diffusion does not require ATP, does not involve carrier proteins, and cannot move substances from low to high concentration. It only occurs from areas of high concentration to low concentration.
5. Conclude that the correct description is: 'No, diffusion only occurs from areas of high concentration to low concentration.' This aligns with the definition of diffusion as a passive process driven by the concentration gradient.