In the Krebs cycle we oxydized the carbon molecules of our glucose. Some of this energy produced ATP (from ADP, of course;) most of the energy is in the electron carriers NAD+ and FAD. In the final stage of respiration, high-level electrons are passed step-by-step to the low energy level of oxygen. The energy they yield in the course of this passage is ultimately used to regenerate ATP from ADP. This step-by-step passage is made possible by a series of electron carriers, each of which holds the electrons at a slightly lower level.
These carriers make up what is known as the electron transport chain. At the top of the energy hill the electrons are held by NADH and FADH2. Most of the potential energy of the glucose molecule now resides in these electron acceptors.
The Krebs cycle yielded two molecules of FADH2 and six molecules of NADH for each molecule of glucose. The oxidation of pyruvic acid to acetyl CoA yielded two molecules of NADH. ALso, two molecules of NADH were produced in glycolysis. In the presence of oxygen, the electrons held by these two NADH molecules are also transported into the mitochondrion whre they are fed into the electron transport chain. In the process, NAD+ is regenerated in the cytoplasm, allowing glycolysis to continue.
Among the principal components of the electron transport chain are molecules known as cytochromes. These molecules consist of a protein and a heme group, analogous to that of hemoglobin, in which an atom of iron is enclosed in a pophyrin ring. Although similar, the protein structures of the individual cytochromes differ enough to enable them to hold electrons at different energy levels. The iron atom of each cytochrome alternately accepts and releases and electron, passing it along to the next cytochrome at a slightly lower energy level until the electrons, their energy spent, are accepted by oxygen. The energy released in this downhill passage of electrons is harnessed, as we shall see, to form ATP molecules from ADP. Such ATP formation is known as oxidative phosphorylation. At the end of the chain, the electrons are accepted by oxygen, which combines with protons (H+ ions) from the solution to produce water.
Quantitative measurements show that for every two electrons that pass from NADH to oxygen, three molecules of ATP are formed from ADP and phosphate. For every pair of electrons that passes from FADH2, which holds them at a slightly lower energy level than NADH, two molecules of ATP are formed. In oxydative physphorylation, the electron transfer potential of NADH and FADH2 is converted to the phosphate transfer potential of ATP.
-._
`,
Krebs
Cycle
_,' ___
` ->(FMN) ___
->(CoQ) ______
->(cyt. b)______
->(cyt. c)_____
->(cyt. a)____
->(cyt. a3)
The principal eletron-carrier molecules of the electron transport chain. At least nine other carrier molecules function as intermediates between the carriers shown here.
Flavin Mononucleide (FMN) and Coenzyme Q (CoQ) transfer electrons and protons. The cytochromes transfer only electrons. The electrons carried by NADH enter the chain when they are transferred to FMN; those carried by FADH2 either the chain farther down the line at CoQ. The electons are ultimately accepted by oxygen, which combines with protons (hydrogen ions) in the solution to form water.
Source: Curtis Barnes, Biology, Fifth Edition. | 
