Final answer:
Hydrides from NADH and FADH2 are not directly transferred to oxygen because the electron transport chain uses these electrons in a multi-step process to create an electrochemical gradient for ATP synthesis, with oxygen acting as the final electron acceptor. In the absence of oxygen, alternative pathways oxidize NADH, but less ATP is produced.
Step-by-step explanation:
Hydrides from NADH and FADH2 are not directly transferred to oxygen in cellular respiration because the electrons they carry are used in a multi-step process known as the electron transport chain (ETC). Instead of a direct transfer, electrons from NADH and FADH2 are passed down a series of protein complexes within the ETC. As these electrons travel through the chain, they lose energy, which is harnessed to pump hydrogen ions across the mitochondrial membrane, creating an electrochemical gradient that drives the synthesis of ATP. The final electron acceptor in the chain is oxygen; it combines with electrons and hydrogen ions to form water. This indirect transfer is essential for the generation of ATP and the avoidance of electron backup that would halt cellular respiration.
In situations where there is no oxygen, such as in anaerobic glycolysis, NADH is oxidized by other means, such as by lactate dehydrogenase, resulting in the production of less ATP. Additionally, anaerobic respirers have distinct complexes for electron transfer to their final electron acceptors that generate smaller electrochemical gradients and thus less ATP.