Oxidative phosphorylation, incorporating two interdependent processes – the flow of electrons through electron transport chain down to the oxygen and chemiosmotic coupling -, is the final stage of cellular respiration.
Highly energetic electrons that are extracted during the decomposition of food molecules by cellular metabolic pathways are stored in electron carriers – NADH and FADH2. Energy stored in these molecules is converted into cellular energy currency (i.e. ATP) only via oxidative phosphorylation. Actually, most of the ATP production during cellular respiration happens here. Take a look at the diagram below that shows the interconnectedness between cellular energy pathways.
In the mitochondrial matrix, the electron carriers NADH and FADH2 deposit the electrons they gained from glycolysis and the citric acid cycle in the electron transport chain – a series of proteins embedded in the inner mitochondrial membrane. In the electron transport chain, electrons are passed from one molecule to another as a series of redox reactions. The energetically “downhill” movement of electrons through the chain causes pumping of protons into the intermembrane space by the first, third, and fourth complexes. As a result, the electrochemical gradient (i.e. proton gradient) forms across the inner membrane of mitochondria.
This gradient is used by chemiosmotic coupling to make ATP through phosphorylation of ADP. In this process, protons flow down their concentration gradient into the matrix through the membrane protein ATP synthase, causing it to spin (like a water wheel) and catalyze conversion of ADP to ATP. Take a look at the diagram below which summarizes oxidative phosphorylation.
You may wonder where the oxygen takes role in cellular “respiration”? As electrons complete their flow through electron transport chain elements, oxygen accepts them. Also taking up protons from the environment, oxygen forms water in this process. If oxygen isn’t there to accept electrons (for instance, because a person is not breathing in enough oxygen), the electron transport chain will stop running, and ATP will no longer be produced by chemiosmosis. Without enough ATP, cells can’t carry out the reactions they need to function, and, after a long enough period of time, may even die.
