Which of the following best describes the main difference between passive transport and active transport across a cell membrane?

• A. Active transport only occurs in plant cells, while passive transport occurs in all cell types.
• B. Passive transport does not require energy, while active transport requires energy in the form of ATP.
• C. Both passive and active transport require the use of carrier proteins and energy.
• D. Passive transport moves substances against their concentration gradient, while active transport moves substances down their concentration gradient.

Answer: (B)

Verified step by step guidance

1. Step 1: Begin by understanding the definitions of passive transport and active transport. Passive transport is the movement of substances across a cell membrane without the use of energy, while active transport requires energy, typically in the form of ATP, to move substances.
2. Step 2: Examine the role of concentration gradients in both processes. Passive transport moves substances down their concentration gradient (from high concentration to low concentration), whereas active transport moves substances against their concentration gradient (from low concentration to high concentration).
3. Step 3: Consider the energy requirements. Passive transport does not require energy because it relies on the natural diffusion of molecules, while active transport requires energy to overcome the resistance of moving substances against their gradient.
4. Step 4: Evaluate the involvement of carrier proteins. Both passive and active transport can involve carrier proteins, but the key difference is that active transport uses these proteins in conjunction with energy to move substances against the gradient.
5. Step 5: Eliminate incorrect statements based on the above clarifications. For example, the statement that active transport only occurs in plant cells is incorrect, as active transport occurs in all cell types. Similarly, the statement that passive transport moves substances against their concentration gradient is incorrect, as this describes active transport.