Cellular respiration is the process by which cells turn food into usable energy in the form of ATP. It takes place in the mitochondria through aerobic or anaerobic respiration. Aerobic respiration breaks down glucose in the presence of oxygen to produce carbon dioxide, water, and ATP through three stages: glycolysis, the Krebs cycle, and oxidative phosphorylation. Glycolysis is the first stage where glucose is broken down to form pyruvic acid, releasing energy and forming NADH.