The electron transport chain is the last stage of the respiration pathway and is the stage that produces the most ATP molecules. The electron transport chain is a collection of proteins found on the inner membrane of mitochondria. NADH and FADH2 release the 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 membrane. The flow of the ions across the membrane synthesises ATP by a protein called ATP synthase.
Three ATP are produced from each NADH, and two ATP are produced from each FADH2, which transfers high energy electrons to the electron transport chain. This results in a total gain of 34 ATP molecules in the electron transport chain.
Oxygen is the final electron acceptor. The oxygen combines with the hydrogen to form water. If oxygen is not present then hydrogen cannot pass through the electron transport chain resulting in a reduction of ATP molecules produced.
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.
Membranes are able to localise the site of metabolic pathways. The pathways can occur on the membrane surfaces and the inner compartments the membranes create.
Membranes create a large surface area and small compartments allowing for high reaction rates to occur.
All three domains of life (archaea, bacteria and eukaryotes) need to produce energy so they need to have respiration pathways within each cell.
ATP (adenosine triphosphate) is the energy-carrying molecule used in cells because it can release energy very quickly.
Energy is released from ATP when the end phosphate is removed. Once ATP has released energy, it becomes ADP (adenosine diphosphate), which is a low energy molecule.
ADP can be recharged back into ATP by adding a phosphate. This requires energy.
These molecules can be recycled so that a constant stream of energy rich ATP is available for all metabolic pathways in the cell.
Almost all cellular processes need ATP to give a reaction its required energy.
ATP can transfer energy and phosphorylate (add a phosphate) to other molecules in cellular processes such as DNA replication, active transport, synthetic pathways and muscle contraction.