Overview of Cellular Respiration Organic compounds such as glucose store energy in their arrangements of atoms. These molecules are broken down and their energy extracted in cellular respiration. The first stage of cellular respiration occurs in the cytosol, while the second and third stages occur in mitochondria. In cellular respiration, electrons are transferred from glucose to electron carriers such as NAD+ and finally to oxygen. The energy released by this transfer of electrons is used to make ATP. Carbon dioxide and water are given off as by-products. Let’s learn about the stages of cellular respiration. Glycolysis is a series of steps in which a glucose molecule is broken down into two molecules of pyruvate. As the chemical bonds in glucose are broken, electrons and hydrogen ions are picked up by NAD+, forming NADH. Glucose is oxidized and NAD+ is reduced. A small amount of ATP is also produced in glycolysis by substrate-level phosphorylation. But most of the energy released by the breakdown of glucose is carried by the electrons attached to NADH. The pyruvate molecules are oxidized to acetyl CoA as they enter the mitochondrion, releasing carbon dioxide. The acetyl CoA molecules enter a series of reactions called the citric acid cycle. More carbon dioxide is released as the citric acid cycle completes the oxidation of glucose. In the citric acid cycle, a small amount of ATP is formed by substrate-level phosphorylation, but most of the energy released by the oxidation of glucose is carried by NADH and FADH2. In oxidative phosphorylation, the final stage of cellular respiration, the NADH and FADH2 molecules produced in glycolysis and the citric acid cycle donate their electrons to the electron transport chain. Electrons move down the chain to the end, where oxygen exerts a strong pull on the electrons. Oxygen, electrons, and hydrogen ions combine, forming water. The electron transport chain converts the chemical energy from moving electrons to a form of energy that can be used to drive oxidative phosphorylation, which produces most of the ATP generated by cellular respiration.