Cellular respiration is a catabolic process in which organic molecules are broken down to create usable energy via the electron transport chain. This process requires oxygen in humans and most other organisms, and produces carbon dioxide, water, heat, and usable energy in the form of ATP. Although different organic molecules, such as sugars, amino acids, and lipids, can be used as substrates in cellular respiration, glucose acts as the main substrate. For glucose, the equation for cellular respiration is C6H12O6 plus six O2, leads to six CO2 plus six H2O plus energy, the reverse of photosynthesis. This reaction occurs in multiple steps, each at a different location within a cell. While glycolysis takes place in the cytoplasm, pyruvate oxidation and the citric acid cycle occur in the mitochondrial matrix, and oxidative phosphorylation happens in the inner mitochondrial membrane. Together, these processes power cellular activities such as flagellar movement, and muscle contraction, through the breakdown of organic molecules to produce ATP.