Animals primarily use the process of cellular respiration to obtain energy. This biochemical process occurs in the cells and involves converting nutrients, primarily glucose, into adenosine triphosphate (ATP), which powers cellular activities. Cellular respiration occurs in several stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. The breakdown of glucose leads to the release of energy, which is stored in the form of ATP. By utilizing oxygen in aerobic respiration or relying on alternative pathways in anaerobic conditions, animals efficiently harness energy to sustain vital functions.