Why does active transport of molecules across a membrane require ATP?

• A. Because it involves passive diffusion of molecules.
• B. Because it moves molecules against their concentration gradient.
• C. Because it moves molecules along their concentration gradient.
• D. Because it relies on the natural kinetic energy of molecules.

Answer: (B)

Verified step by step guidance

1. Understand the concept of concentration gradients: Molecules naturally move from areas of high concentration to areas of low concentration, a process known as passive diffusion.
2. Recognize that active transport is different from passive diffusion: Active transport moves molecules from areas of low concentration to areas of high concentration, which is against the natural concentration gradient.
3. Identify the energy requirement: Moving molecules against their concentration gradient requires energy because it is not a spontaneous process.
4. Learn the role of ATP in active transport: ATP (adenosine triphosphate) provides the necessary energy for active transport by undergoing hydrolysis, which releases energy.
5. Connect the concepts: Since active transport requires energy to move molecules against their concentration gradient, ATP is essential to provide this energy, distinguishing it from passive processes that do not require energy input.