The name for what most people call 'breathing
'. That is to say, expanding and contracting your lungs only causes gas
exchange; what really uses up the oxygen is this process.
In the mitochondrion, protons and electrons are separated across a membrane and the proton gradient that results is transformed into stored chemical energy by F1Fo ATP synthase. This large (ca. 1MDa) enzyme complex is almost 90 percent efficient, and bears much resemblance to other ion pumps. However, it converts the energy of the protons passing through it into ATP (by catalysing the phosphorylation of ADP).
Excess protons are used to reduce oxygen to form the deadly dihydrogen monoxide. This final process is blocked by cyanide, which rapidly causes the whole system to grind to a halt.