The electron transport chain and ATP synthesis

• The synthesis of ATP by oxidative phosphorylation involves the transfer of electrons down a series of electron carrier molecules that form the electron transport chain.
• Firstly, the hydrogen atoms produced during glycolysis and the Krebs cycle combine with coenzymes NAD and FAD.
• The reduced NAD and FAD donate the electrons of the hydrogen atoms they are carrying to the first molecule in the electron transport chain.
• The electrons then pass along a chain of electron transfer carrier molecules in a series of oxidation-reduction reactions. As the electrons move along the chain, the energy they release causes the active transport of protons across the inner mitochondrial membrane and into inter-membranal space.
• The protons accumulate in the inter-membranal space before they diffuse back into the mitochondrial matrix through ATP synthase channels embedded in the inner mitochondrial membrane.
• At the end of the electron transport chain, the electrons combine with the protons and oxygen to form water. Oxygen is therefore the final acceptor of electrons in the electron transfer chain.
• The electron transfer chain is a part of the chemiosmotic theory of oxidative phosphorylation.
• Without the role of oxygen in removing hydrogen atoms at the end of the electron transfer chain, the hydrogen ions (protons) and electrons would 'back up' along the chain, causing respiration to halt.
• Energy is released a little at a time during this process since more of it can be harvested for the benefit of the organism. This explains why the electrons carried by NAD and FAD are not transferred in one step, but instead use the electron transfer chain, where each carrier molecule is at a slightly lower energy level.
• The electrons in the electron transfer chain move gradually down an energy gradient, making their energy more useful.