Q. Until the Krebs cycle, aerobic respiration can be described without mentioning oxygen, the chemical element after which the reaction gets its name. Where in the process does this chemical element take part? What is its significance?

Oxygen enters the aerobic respiration in its final phase the respiratory chain. It is of fundamental importance because it is accountable for the maintenance of the hydrogen concentration gradient between the spaces separated by the inner mitochondrial membrane. This gradient promotes the functioning of the ATP synthetase and thus the phosphorylation of ATP to form ADP. In the space inside the inner membrane oxygen binds to free hydrogens to form water and this hydrogen consumption keeps the hydrogen gradient and the proton traffic through the ATP synthetase.

The entire aerobic respiration process has the intent to make the ATP synthetase work. Aerobic beings, for instance, we humans need to breathe oxygen to maintain that hydrogen concentration gradient and keep the ATP synthetase working.