The electron transport chain is the last stage of the respiration pathway. It is the stage that produces the most ATP molecules.
The electron transport chain is a collection of carrier proteins found on the inner membrane of mitochondria. NADH release the hydrogen ions and electrons into the transport chain.
The electrons transfer their energy to the proteins in the membrane providing the energy for hydrogen ions to be pumped across the inner mitochondrial membrane.
The flow of the ions back across the membrane synthesises ATP by a protein called ATP synthase.
Oxygen is the final hydrogen ion and electron acceptor. The oxygen combines with the hydrogen ions and electrons to form water.
In total, 38 ATP molecules are produced from one molecule of glucose.
If glucose is not available for the respiration pathway, other respiratory substrates can be used via alternative metabolic pathways.
Starch, glycogen, proteins (amino acids) and fats can all be broken down into intermediates in glycolysis or the citric acid cycle. This provides alternative metabolic pathways to make ATP.