Final answer:
Without oxygen, the electron transport chain backs up and shuts down, halting oxidative phosphorylation and greatly reducing ATP production. Cells switch to anaerobic methods like glycolysis followed by fermentation, which yield far less ATP, potentially leading to cell death.
Step-by-step explanation:
If oxygen became unavailable, the electron transport chain (ETC) and the rate of cellular respiration would be significantly impacted. Oxygen serves as the final electron acceptor in the ETC, so without it, electrons cannot be efficiently passed down the chain, leading to a backup. This halt in electron flow prevents the formation of a proton gradient across the mitochondrial membrane, which is necessary for the production of ATP through oxidative phosphorylation. Consequently, the cells would need to switch to anaerobic pathways to produce ATP, such as glycolysis followed by lactic acid fermentation or alcohol fermentation to regenerate NAD+, but these processes are much less efficient than aerobic respiration.
Without aerobic respiration, cells are unable to produce the large amounts of ATP that are typically generated. For each glucose molecule, aerobic conditions can yield 36-38 ATP, but without oxygen, only about two ATP are produced via glycolysis alone. This drastic reduction in ATP production can lead to cell dysfunction and ultimately cell death if oxygen deprivation continues.