Which molecule serves as the energy currency used by cells in most processes?

• A. NADH
• B. FADH2
• C. ATP
• D. GTP

Answer: (C)

ATP serves as the energy currency used by cells in most processes. Its three phosphate groups store energy in the bonds between them, and when ATP is hydrolyzed to ADP and inorganic phosphate, a usable amount of energy is released to power a wide range of cellular work—from active transport and muscle contraction to biosynthesis and enzyme regulation. Because ATP can be rapidly regenerated from ADP and Pi through glycolysis, the citric acid cycle, and oxidative phosphorylation, it provides a versatile and readily available energy source for nearly all cellular activities. Other carriers like NADH and FADH2 transport high-energy electrons to drive ATP production, and GTP serves in specific roles, but ATP is the primary, universal energy currency across most processes.