How is water formed in aerobic respiration?

Water is formed in aerobic respiration through the process of oxidative phosphorylation in the mitochondria.

Aerobic respiration is a biological process that takes place in the mitochondria of cells. It involves the breakdown of glucose in the presence of oxygen to produce energy, carbon dioxide, and water. The formation of water specifically occurs in the final stage of aerobic respiration, known as oxidative phosphorylation.

During oxidative phosphorylation, electrons are transferred from electron carriers to oxygen, the final electron acceptor in the electron transport chain. This process releases energy, which is used to pump protons (H+ ions) across the inner mitochondrial membrane, from the mitochondrial matrix to the intermembrane space. This creates a proton gradient, or a difference in proton concentration across the membrane.

The protons then flow back down their concentration gradient through a protein complex called ATP synthase. This flow of protons provides the energy needed for ATP synthase to convert ADP (adenosine diphosphate) into ATP (adenosine triphosphate), the main energy currency of the cell.

The oxygen that acts as the final electron acceptor becomes reduced, meaning it gains electrons. At the same time, it also picks up protons from the mitochondrial matrix to form water. The overall reaction can be summarised as follows: 1/2 O2 + 2H+ + 2e- → H2O. This means that for every molecule of oxygen that is reduced, one molecule of water is produced.

In summary, water is formed in aerobic respiration during oxidative phosphorylation, when oxygen is reduced by gaining electrons and protons. This process is crucial for life as it not only produces water, but also generates the ATP needed for cellular activities.