Why is oxygen essential for life?

Oxygen is essential for life primarily because it is required for cell metabolism, particularly in the process of cellular respiration. In aerobic organisms, oxygen serves as the final electron acceptor in the electron transport chain, a key component of cellular respiration. This biochemical process takes place in the mitochondria of cells and is vital for producing adenosine triphosphate (ATP), the energy currency of the cell. ATP is used to power various cellular functions and processes that sustain life, such as muscle contraction, nerve impulse propagation, and biosynthesis of macromolecules.

While the other choices mention important aspects of body functions, they do not accurately reflect the primary role of oxygen in sustaining life. For instance, oxygen does not provide energy directly in the same way that nutrients like glucose do; rather, it supports the metabolic processes that generate energy. Oxygen does not aid in digestion or serve as a replacement for carbon dioxide in a direct sense, as carbon dioxide is a waste product of metabolism that is expelled from the body. Therefore, acknowledging oxygen's critical role in cell metabolism helps to clarify its vital function for sustaining life.