Cellular respiration is the biochemical process by which cells convert nutrients, primarily glucose, into energy. This process involves breaking down food molecules in the presence of oxygen to produce adenosine triphosphate (ATP), the energy currency of the cell. It occurs in several stages, including glycolysis, the Krebs cycle, and oxidative phosphorylation. Unlike photosynthesis, which uses sunlight to create food, cellular respiration focuses on extracting and utilizing the energy stored in food molecules to fuel cellular activities.