cytochrome pathway at a point just below the entry level for NADH (Fig. 3.21). This is important because the level of FADH entry bypasses one of the sites of ATP formation, and thus each molecule of FADH that enters the electron transport chain has enough energy to form only 1.5 ATP. In con- trast, NADH entry into the electron transport chain results in the formation of 2.5 ATP (details will be mentioned later. At the end of the electron trans- port chain, oxygen accepts the electrons that are passed along and combines with hydrogen to form water. If O2 is not available to accept those elec- trons, oxidative phosphorylation is not possible and ATP formation in the cell must occur via an- aerobic metabolism. How does this ATP formation occur? The mech- anism to explain the aerobic formation of ATP is known as the chemiosmotic hypothesis. As elec- trons are transferred along the cytochrome chain the energy released is used to "pump" hydrogens protons, H released from NADH and FADH from the inside of the mitochondria across the inner mitochondrial membrane (Fig. 3.21). This results an accumulation of hydrogen ions within