The primary fuel source for cellular respiration is glucose. During this metabolic process, glucose is broken down in the presence of oxygen to produce ATP (adenosine triphosphate), which cells use for energy. Cellular respiration occurs in several stages: glycolysis, the citric acid cycle, and oxidative phosphorylation. Each stage effectively extracts energy from glucose, supporting vital cellular functions and maintaining energy balance within the organism. In the absence of oxygen, some cells can perform anaerobic respiration with alternative pathways.