Which of the following statements is true only for active transport and not for facilitated diffusion?

• A. It requires energy input in the form of ATP.
• B. It utilizes membrane proteins to transport substances.
• C. It involves movement of molecules down their concentration gradient.
• D. It occurs without the need for cellular energy.

Answer: (A)

Verified step by step guidance

1. Step 1: Begin by understanding the key difference between active transport and facilitated diffusion. Active transport requires energy input, typically in the form of ATP, while facilitated diffusion does not require energy and relies on the concentration gradient.
2. Step 2: Analyze the options provided in the problem. Identify which statements describe characteristics of active transport and which describe facilitated diffusion.
3. Step 3: Evaluate the first option: 'It requires energy input in the form of ATP.' This is a defining feature of active transport, as it moves molecules against their concentration gradient using energy.
4. Step 4: Evaluate the second option: 'It utilizes membrane proteins to transport substances.' Both active transport and facilitated diffusion use membrane proteins, so this is not exclusive to active transport.
5. Step 5: Evaluate the remaining options: 'It involves movement of molecules down their concentration gradient' and 'It occurs without the need for cellular energy.' Both of these are characteristics of facilitated diffusion, not active transport. Therefore, the correct answer is the first option.