Which energy currency fuels active transport?

• 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, and the direct energy currency used is ATP. Membrane pumps are ATPases that hydrolyze ATP to ADP and inorganic phosphate, and the released energy drives conformational changes in the transporter so substrates are moved from one side of the membrane to the other. A classic example is the Na+/K+ ATPase, which uses ATP to push three Na+ ions out and two K+ ions in, creating essential ionic gradients. AMP is just the hydrolysis product and cannot supply energy in that cycle; NADH is an electron carrier tied to generating ATP indirectly, not to powering a pump directly; GTP is used in some specific cellular processes but is not the general energy source for active transport. So ATP is the energy currency fueling active transport.