Which complex in the ETC reduces O2 into water?

Complex IV, also known as cytochrome c oxidase, is responsible for the reduction of molecular oxygen (O2) to water (H2O) in the electron transport chain (ETC). This complex plays a crucial role in cellular respiration, particularly during the final stage of oxidative phosphorylation.

In Complex IV, electrons are transferred from cytochrome c to the enzyme, and through a series of redox reactions, O2 is reduced. This reduction involves the addition of protons (H+) from the surrounding medium and the electrons, leading to the formation of water. This process not only helps to expel the final product of respiration—water—but also contributes to the establishment of a proton gradient across the inner mitochondrial membrane, which is essential for ATP synthesis through ATP synthase.

The activity of Complex IV is vital for maintaining the flow of electrons through the ETC and ensuring that the respiratory pathway continues efficiently. It is also where the "breathing" aspect of cellular respiration occurs, as it directly handles the oxygen we inhale and converts it into a usable byproduct, water, while also facilitating the continuation of the ETC.