Final answer:
Without oxygen, cells resort to anaerobic respiration or fermentation to generate ATP through substrate-level phosphorylation. This process regenerates the necessary NAD+ for glycolysis but results in less ATP and a change in pH that can affect organic molecules like enzymes.
Step-by-step explanation:
When cells lack oxygen, they cannot rely on oxidative phosphorylation to generate ATP because it requires oxygen as the final electron acceptor in the electron transport chain (ETC). In the absence of oxygen, cell metabolism shifts to anaerobic respiration or fermentation. These pathways allow for the continued production of ATP through substrate-level phosphorylation during glycolysis by regenerating the electron carrier NAD+ from NADH, essential for glycolysis to proceed. During fermentation, pyruvate, the end product of glycolysis, is converted into different end products such as lactic acid in lactic acid fermentation or ethanol in alcoholic fermentation. This process does not produce as much ATP as oxidative phosphorylation, but it allows cells to survive temporarily during anoxic conditions. The change in pH resulting from the accumulation of acidic fermentation products can affect various organic molecules, especially enzymes, which can lose functionality or change shape due to the altered pH environment. Reduced ATP production can affect protein synthesis, cellular processes, and muscle function, particularly in the heart where sustained aerobic metabolism is crucial.