This document discusses how ATP (adenosine triphosphate) is used and regenerated in cells to provide energy for cellular processes. ATP provides energy by releasing an energized phosphate group, leaving ADP (adenosine diphosphate) and free energy. Enzymes facilitate the breakdown of complex organic molecules like glucose through cellular respiration, releasing free energy that is used to re-energize phosphate groups and reform ATP molecules. In this way, ATP acts as a carrier of chemical energy within cells, transferring energy from the breakdown of nutrients to power energy-requiring cellular work like building molecules.