Energy source for active transport mechanisms.

• A. Adenosine Triphosphate (ATP)
• B. Adenosine Monophosphate (AMP)
• C. Guanosine Triphosphate (GTP)
• D. Nicotinamide Adenine Dinucleotide (NADH)

Answer: (A)

Active transport needs energy to move substances against their gradient. The energy currency used for most primary active transport is ATP, which powers pumps by hydrolyzing to ADP and inorganic phosphate. This energy drives conformational changes in transport proteins (like Na+/K+-ATPase and Ca2+-ATPase), enabling ions to be moved across the membrane against their gradients and creating essential cellular ion and nutrient balances. The other options don’t serve as the general energy source for these pumps: AMP is a lower-energy form, NADH is an electron carrier used in redox reactions rather than directly powering transport, and GTP is used in some specific cellular processes but not the typical energy source for membrane pumps. In secondary active transport, the gradient created by ATP-powered pumps provides the energy for moving other substances. Therefore ATP is the best answer.