Cellular Respiration is the production of ATP, or energy, within the cell. ATP is made by the joining of phosphate groups to ADP. The energy from the ATP is released when the three phosphate bonds are broken. Glycolysis, which occurs in the cytosol, is the first step in cellular respiration.

During glycolysis, glucose, a six-carbon sugar goes through oxidation, or is broken down, into two pyruvates, or two three-carbon sugars. Here, two ATP and two NADH molecules are gained. There are ten steps in glycolysis, the first five are known as the energy investment phase, and the last five are known as the energy payoff phase. During the investment phase, ATP is spent to phosphorylate the fuel molecules; during the payoff phase, ATP is produced by substrate-level phosphorylation, along with NADH. The next step in cellular respiration is the Krebs cycle.

If oxygen is present, the pyruvates will enter the mitochondria. Here, oxidation is completed. Carbon dioxide is released from the pyruvates, and electrons are transferred by enzymes to NAD to form NADH. The enzymes of the Krebs cycle are located in the mitochondrial matrix. NADH and FADH that are produced during this cycle pass on the electrons that have been removed to the election transport chain; this is the process of oxidative phosphorylation. Located in the inner membrane of the mitochondria are folds that form cristae; this provides are larger surface area with more room for copies of the chain. During the electron transport chain, electrons are able to pass through the chain by carriers. One type of carrier would be the proteins known as cytochromes; another carrier would be a ubiquinone. The electron transport chain does not make ATP, but the enzyme ATP synthase does from the ADP and inorganic phosphate. The electron transport chain pumps hydrogen across the membrane from the matrix into the intermembrane. From the hydrogen ion concentration, a protein gradient is formed that connects the redox reactions, or electron reduction, to ATP synthesis. This is known as chemiosmosis. Because of the electron transport chain and oxidative phosphorylation, 38 ATP are able to be produced. These ATP are constantly regenerated by the cell.