After the Krebs cycle is completed, oxygen enters the respiration pathway as the electron acceptor at the end of the electron transport chain.
The oxidation takes place in a series of steps, like the electron chain of photosynthesis, but with different transport molecules. Many of the latter are cytochromes (proteins with an iron‐containing porphyrin ring attached) where the electron exchanges take place on the iron atoms. Others are iron‐sulfur proteins with iron again the exchange site. Three complexes of carriers are embedded together with proteins in the inner mitochondrial membrane where they assist in the chemiosmotic production of ATP (see below). The most abundant electron carrier, coenzyme Q (CoQ), carries electrons and hydrogen atoms between the others.
The transport chain often is likened to a series of magnets, each stronger than the last, which pull electrons from one weaker carrier and release it to the next stronger one. The last acceptor in the line is oxygen, an atom of which accepts two energy‐depleted electrons and two hydrogen ions (protons) and forms a molecule of water.
Energy from the transport chain establishes a proton gradient across the inner membrane of the mitochondrion and supplies the energy for the embedded protein complexes—which also are proton pumps and sites of the chemiosmotic process. As electrons are pulled from NADH and FADH 2, protons (H +) also are released, and the protein complexes pump them into the intermembrane space. Since the membrane is impermeable to protons, they accumulate there, and thus both a H + gradient and an electrochemical gradient are established between the inner membrane space and the matrix. Embedded in the membrane, however, are complexes of the enzyme ATP synthase with inner channels through which protons can pass. As the protons move down the gradient, their energy binds a phosphate group to ADP, an oxidative phosphorylation, making ATP.
The importance of the Krebs cycle and oxidative phosphorylation is evident when the net yield of ATP molecules produced from each molecule of glucose is calculated. Each turn of the Krebs cycle produces one ATP, three molecules of NADH, and one of FADH 2. (Remember that it takes two turns of the cycle to release the six carbons of glucose as CO 2 so this number is doubled for the final count.) The retrieval of energy from the oxidative phosphorylations and the chemiosmotic pumping are an impressive 34 ATPs (four from the two NADH molecules produced in glycolysis and added to the transport and phosphorylation chain; six from the NADH molecule produced in the conversion of pyruvate to acetyl CoA; and 18 from the six molecules of NADH, four from the two FADH molecules, and two directly produced in two turns of the Krebs cycle.) The net yield from glycolysis is only two ATP molecules.
The number of enzymes and the precise mechanisms of the respiratory pathways may seem to be an unnecessarily complex way for cells to obtain energy for metabolic work. But, if electrons were added directly to oxygen, the reaction probably would produce enough heat to damage the cells and result in too small an amount of captured energy to be a significant source for future energy needs.