What molecule is composed of adenosine and three phosphate groups and acts as the energy currency of the cell?

• A. ATP
• B. NADH
• C. ADP
• D. AMP

Answer: (A)

ATP is the molecule that acts as the energy currency of the cell. It consists of adenosine attached to three phosphate groups. The energy stored in the bonds between these phosphate groups, especially the bond to the outermost (gamma) phosphate, is released when ATP is hydrolyzed to ADP and inorganic phosphate. That energy powers many cellular tasks, such as muscle contraction, active transport across membranes, and driving biosynthetic reactions. Cells continually regenerate ATP from ADP and phosphate through processes like glycolysis, the citric acid cycle, oxidative phosphorylation, and, in plants, photosynthesis.

NADH, by contrast, is an electron carrier used in redox reactions. ADP and AMP are lower-energy forms with fewer phosphate groups and must be phosphorylated to become ATP; they are not the immediate energy currency themselves.