So, why of is oxygen essential for the aerobic pro ductic of ATP ? Remember that the purpose of the electron transport chain is to move electrons down a series of cytochromes to provide energy to drive ATP production in the mitochondria. This process, il lustrated in Fig. 3.2 requires each element in the electron transport chain to undergo a series of oxida tion-reduction reactions. If the last cytochrome (ie cytochrome a) remains in a reduced state, it would be unable to accept more electrons, and the electron transport chain would stop. However, when oxygen is present, the last chrome in the chain can be oxidized by oxygen. That is, oxygen, derived from the air we breathe, allows electron transport to continue by functioning as the final electron acceptor of the electron transport chain. This oxidizes cytochrome a3 and allows electron transport and oxidative phos phorylation to continue. At the last step in the elec tron transport chain, oxygen accepts two electrons that were passed along the electron transport c from either NADH or FADH. This reduced oxygen molecule now binds with two protons (H+) to form water (Fig. 3.21)
As mentioned earlier. NADH and FADH dif fer in the amount of ATP that can be formed from each of these molecules. Each NADH formed in the mitochondria donates two electrons to the elec tron transport system at the first proton pump (Fig. 3.21). These electrons are then passed to the second and third proton pumps until they are fi nally passed along to oxygen. The first and sec ond electron pumps transport four protons each whereas the third electron pump transports two protons, for a total of ten. Because four protons are required to produce and transport one ATP from the mitochondria to the cytoplasm, the total ATP production from one NADH molecule is 2.5 ATP (10 protons 4 protons per ATP 2.5 ATP). Note that ATP molecules do not exist in halves and that the decimal fraction of ATP indicates an average number of ATP molecules that are produced per NADH.
Compared to NADH, each FADH molecule pro duces less ATP because the electrons from FADH are donated later in the electron transport chain than those by NADH (Fig. 3.21). Therefore, the electrons from FADH activate only the second and third proton pumps. Because the first proton pump is bypassed, the electrons from FADH result in the pumping of six protons (four by the second and two by the third pump). Because four protons are required to produce and transport one ATP from the mitochondria to the cytoplasm, the total ATP production from one FAD molecule is 1.5 ATP (6 protons 4 protons per ATP 1.5 ATP). See A Closer Look 3.5 for more details on the quantity of ATP produced in cells