What is the main energy source that cells use for most of their work?

• A. ADP
• B. ATP
• C. NADH
• D. Glucose

Answer: (B)

Energy in cells is stored in a form that can be released quickly to power a wide range of activities, and that form is ATP. The terminal phosphate bonds in ATP are high-energy, so when the bond is broken, a burst of energy is released that can drive endergonic processes like muscle contraction, active transport across membranes, and the synthesis of complex molecules. After ATP donates its energy, it becomes ADP and inorganic phosphate, and the cell can quickly rebuild ATP from ADP and Pi using energy captured from nutrients through cellular respiration. This setup lets cells rapidly respond to changing needs. Other molecules have important roles but aren’t the immediate energy source for work: NADH stores energy as electrons for redox reactions, not as the direct fuel for ongoing tasks, and glucose is a fuel that must be processed to generate ATP. ATP serves as the universal, readily usable energy currency for most cellular work.