Integrated Training and OPT Model Practice Test 2026 - Free Practice Questions and Study Guide

Energy currency in cells is the molecule that stores and releases energy to power cellular work. ATP, or adenosine triphosphate, fits this role because its terminal phosphate bonds hold energy that can be released quickly by hydrolysis to ADP and inorganic phosphate, providing the immediate power for muscle contraction, nerve signaling, and active transport. The body continually regenerates ATP from ADP and Pi using energy from nutrients, through processes like glycolysis and oxidative phosphorylation, making ATP a readily available energy source for both short bursts and sustained activity. The other options aren’t energy currencies: a motor unit is the functional nerve–muscle connection, not a storage of energy; work/time is a rate or measurement, and force x velocity describes power, not a biochemical energy carrier.