The mitochondria is the powerhouse of the cell! In order to produce energy, cells need to break carbohydrates like glucose down into usable energy molecules called ATP (adenosine triphosphate). ATP is a high-energy molecule because its three phosphate groups are weakly bound, and removing one releases large amounts of energy for cell use. The mitochondria is the organelle responsible for generating ATP.
First, glucose enters the cytoplasm of the cell, where it enters the first stage of cellular respiration, called glycolysis. Glycolysis is a ten-reaction biochemical pathway that ultimately produces two molecules of a chemical called pyruvate from glucose, plus two molecules of ATP. Pyruvate then enters the matrix of the mitochondria and undergoes pyruvate oxidation, wherein it is further broken down into two molecules of acetyl-CoA. Finally, acetyl-CoA is further oxidized in a process called the Krebs Cycle, where it binds with a molecule of oxaloacetate to form citric acid. This citric acid then undergoes a series of biochemical modifications to reproduce oxaloacetate, two more ATP molecules, NADH, and FADH₂. The oxaloacetate is then recycled, so the cycle can begin anew.
All of the sequential steps of metabolism—glycolysis, pyruvate oxidation, and the Krebs Cycle—produce a total of ten NADH and two FADH2 molecules. These molecules, which are formed by the oxidation of glucose byproducts during the various stages of aerobic respiration, are high-energy electron carriers (the “H” indicates the presence of an electron). The final stage of respiration—the electron transport chain (oxidative phosphorylation)—occurs in the cristae of the mitochondria (the space between the inner and outer mitochondrial membranes). NADH and FADH₂ deposit their electrons to a series of electron acceptors, which increase in affinity for electron binding the further down the chain the electron travels. Oxygen is the final, and strongest, electron acceptor, leading to the formation of CO₂.
The movement of these electrons to successively stronger electron-binding proteins releases a large amount of energy (corresponding to the increased stability of the successive electron-protein complexes), which is eventually used to activate an intermembrane protein called ATP-synthase. ATP-synthase pumps H+ ions out of the intermembrane space (where they had been aggregating due to the oxidation of NADH and FADH₂) back into the mitochondrial matrix. This corresponds to moving H+ from an area of very high to very low concentration, which produces enough energy to generate the phosphorylation of ADP to ATP. This process is called chemiosmosis, because a chemical (H+), not water, moves to a more favorable concentration gradient to produce energy.