The document discusses cellular respiration, which is the process that produces energy by breaking down glucose and other food molecules in the presence of oxygen. It occurs mainly in the mitochondria of cells. There are three main steps: glycolysis, the Krebs cycle, and the electron transport chain. Glycolysis produces a small amount of ATP and occurs in the cytoplasm. The Krebs cycle produces more ATP and occurs in the mitochondria. The electron transport chain produces the most ATP and also occurs in the mitochondria. Cellular respiration requires oxygen. In its absence, fermentation occurs, which still uses glycolysis but produces less ATP. Photosynthesis and cellular respiration have opposing reactions but work together to sustain life.