Explain the significance of oxygen as the final electron acceptor in the Electron Transport Chain. What happens if oxygen is not available?

Oxygen plays a crucial role in the process of cellular respiration, particularly in the Electron Transport Chain (ETC), which is the final stage of aerobic respiration. Here's a step-by-step explanation:

1. The ETC is located in the inner mitochondrial membrane and is composed of a series of proteins and organic molecules. It receives electrons from NADH and FADH2, two electron carrier molecules produced in earlier stages of cellular respiration.

2. As electrons are passed along the chain, they move from a higher energy level to a lower one. This energy is used to pump protons (H+) across the inner mitochondrial membrane, from the matrix into the intermembrane space, creating a proton gradient.

3. Oxygen acts as the final electron acceptor in the ETC. It accepts the electrons that have been passed along the chain and picks up protons to form water (H2O). This removal of electrons ensures that the ETC does not become saturated with electrons and can continue to function.

4. The significance of oxygen is that without it, the ETC would not be able to function. The chain would become backed up with electrons, the proton gradient would not be established, and ATP (the cell's energy currency) would not be produced.

5. If oxygen is not available (an anaerobic condition), the cell must rely on anaerobic respiration or fermentation to produce ATP. In these processes, other molecules serve as the final electron acceptors, but they are less efficient and produce less ATP than aerobic respiration. In human cells, the lack of oxygen leads to lactic acid fermentation, which can cause muscle fatigue.