In what ways is active transport different from passive transport across a cell membrane?

• A. Passive transport can move molecules against their concentration gradient.
• B. Active transport requires energy input, while passive transport does not.
• C. Active transport only moves substances down their concentration gradient.
• D. Passive transport involves carrier proteins that use ATP.

Answer: (B)

Verified step by step guidance

1. Step 1: Define passive transport. Passive transport is the movement of molecules across a cell membrane without the use of energy, typically down their concentration gradient (from high concentration to low concentration). Examples include diffusion, osmosis, and facilitated diffusion.
2. Step 2: Define active transport. Active transport is the movement of molecules across a cell membrane that requires energy input, usually in the form of ATP, to move substances against their concentration gradient (from low concentration to high concentration).
3. Step 3: Highlight the key difference. The main distinction is that active transport requires energy input, while passive transport does not. Passive transport relies on the natural kinetic energy of molecules and their concentration gradient.
4. Step 4: Clarify misconceptions. Passive transport cannot move molecules against their concentration gradient; this is a characteristic of active transport. Additionally, passive transport does not involve carrier proteins that use ATP—this is specific to active transport.
5. Step 5: Provide examples for clarity. Examples of passive transport include simple diffusion and facilitated diffusion (which uses carrier proteins but does not require ATP). Examples of active transport include the sodium-potassium pump and proton pumps, which use ATP to move ions against their concentration gradient.