In chemiosmosis, the free energy from the series of redox reactions just described is used to pump hydrogen ions (protons) across the membrane.
The uneven distribution of H+ ions across the membrane establishes both concentration and electrical gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on one side of the membrane.
Many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels.
Similarly, hydrogen ions in the matrix space can only pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase.
This complex protein acts as a tiny generator, turned by the force of the hydrogen ions diffusing through it, down their electrochemical gradient.
The turning off parts of this molecular machine facilitates the addition of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion gradient.
The number of ATP molecules synthesized depends on the nature of the electron donor.
Oxidation of one molecule of NADH gives rise to 3 molecules of ATP, while that of one molecule of FADH2 produces 2 molecules of ATP.
ATP synthase (complex V) consists of two major components, F1 and F0.
The F1 headpiece is a peripheral membrane protein complex and contains the site for the synthesis of ATP from ADP and inorganic phosphate.
F0 is an integral membrane protein complex that forms the channel through which protons are across the inner membrane.
The passage of protons through the channel is coupled to the catalytic site of the F1 component for the production of ATP.
For each ATP produced, 2H+ passes through F0 from the intermembrane space to the matrix down the electrochemical proton gradient.
Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process.
Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system.
Oxygen acts as the final hydrogen acceptor.
In respiration, it is the energy of oxidation-reduction utilized for the production of proton gradient required for phosphorylation.
It is for this reason that the process is called oxidative phosphorylation.