Cellular respiration occurs in three distinct, yet inter-connected, series of reactions: glycolysis, citric acid cycle, and the electron transport chain (oxidative phosphorylation). The products of these reactions include CO2, water, and energy. Although most of the energy is lost as heat, almost half is captured in a form that the cell can use through the synthesis of ATP adenosine triphosphate), an energy-rich molecule.
Cellular respiration includes aerobic reactions (require oxygen) and anaerobic reactions (do not require oxygen). For each glucose molecule that is decomposed completely by cellular respiration, up to thirty-eight molecules of ATP can be produced. All but two ATP molecules are formed by the aerobic reactions.
An ATP molecule that loses its terminal phosphate becomes an ADP (adenosine diphosphate) molecule, which has only two phosphates. However, ATP can be resynthesized from an ADP by using energy released from cellular respiration to reattach a phosphate, a process known as phosphorylation. Thus, ATP and ADP molecules shuttle back and forth between the energy-releasing reactions of cellular respiration and the energy-utilizing reactions of the cell.
ATP is not the only kind of energy-carrying molecule within a cell, but it is the primary one. Without enough ATP, cells quickly die.
Aerobic respiration: is a sequence of reactions that begins with pyruvic acid produced by glycolysis moving from the cytosol into the mitochondrion. From each pyruvic acid, enzymes inside the mitochondria remove two hydrogen atoms, a carbon atom, and two oxygen atoms generating NADH and a CO2 and leaving a 2-carbon acetic acid. The acetic acid then combines with a molecule of coenzyme A derived from the vitamin pantothenic acid) to form acetyl CoA. CoA "carries" the acetic acid into the citric acid cycle.
The carbon dioxide produced by the formation of acetyl CoA and in the citric acid cycle dissolves in the cytoplasm, diffuses from the cell, and enters the bloodstream. Eventually, the respiratory system excretes the carbon dioxide.
Neither glycolysis not the citric acid cycle uses oxygen directly although they are part of the aerobic metabolism of glucose. Instead, the final enzyme of the electron transport chain gives up a pair of electrons that combine with two hydrogen ions (provided by the hydrogen carriers) and an atom of oxygen to form a water molecule:
2e- + 2H+ + 1/2 O2 = H2O
Thus, oxygen is the final electron "carrier". In the absence of oxygen, electrons cannot continue to pass through the electron transport chain, and aerobic respiration grinds to a halt.