The Electron Transport Chain and Chemiosmosis is where the vast majority of the ATP is synthesized. This is what makes Aerobic Respiration far more efficient per glucose than Anaerobic Respiration.
So far in Glycolysis, Acetyl CoA step, and Krebs we have made 4 ATP, 10 NADH + H+ , and 2 FADH2. Note that the latter 2 molecules are reduced coenzymes. If you want more information on the previous stages of Cellular Respiration, visit my other posts. Now onto the subject at hand!
In the Electron Transport Chain, the main thing that goes on is that the energy stored in the reduced coenzymes sets up a proton gradient through redox reactions. The gradient will be high concentration on one side and low on the other. The protons cannot just flow across the inner membrane (cristae folds) of the mitochondria into the matrix so they must go through an ATP Synthase. This gives the energy to phosphorylate ADP and P into ATP. This is called Oxydative Phosphorylation.
Lets now go into detail:
The NADH + H+ and the FADH2 are now in the Matrix of the mitochondria. Remember that these are reduced coenzymes and electron carriers. They will now oxidize (lose electrons). The electrons they donate will go through a series of carrier proteins embedded in the cristae inner mitochondrial membrane to a final electron acceptor – oxygen. The electrons, as they are going through from protein to protein, will allow H+ to go through the cristae into the inter membrane space to accumulate.
Note: Cristae dramatically increase the surface area, allowing mitochondria to be crazy good at their jobs.
There are four multi protein complexes the electron goes through.
The electrons from the NADH (which carried them from Glycolysis and Krebs) will go to the first protein, Flavin Mononucleotide (FMN) in Complex I. This electron then will travel to an Iron Sulfur protein called Ubiquinone through redox reaction. Ubiquinone is not a part of complex I or complex III because it flows freely. Its not directly attached to other proteins.
The remaining three protein complexes are called cytochromes which have a heme (iron) group that allows them to easily accept and donate electrons. Note that as the electrons are going down, they are decreasing in energy state as a lot is lost to the environment as heat.
Now lets step away from this electron for a bit. What happened with FADH2? Well, FADH2 gave its electrons to protein complex II which consists of peripheral proteins attached to the inside facing part of the Cristae folds. This will occur at a far lower energy than the electrons from NADH + H. Thus NADH + H is responsible for more ATP production.
All the electrons then pass through protein complex III, cytochrome c, and protein complex IV. The electrons will go to the final electron acceptor which is oxygen to make water, a waste product of cellular respiration.
Remember as all of this is going on protons are being pumped into the inter membrane space. This is called the proton motive force as the protons want to flow down the concentration gradient.
During Chemiosmosis, protons flow down an enzyme called ATP Synthase which uses the energy from the protons. This energy is applied to phosphorylate ADP and P into ATP, the main energy molecule of the cell. ATP is extremely difficult to produce with no energy input so Chemiosmosis is a way to couple that reaction with energy released from the exited protons going back into the matrix.
Chemiosmosis and Electron Transport chain result in the production of ATP in a process called Oxidative Phosphorylation. In Oxidative Phosphorylation, there is a net gain of 26-28 ATP.
Throughout cellular respiration the products are water, 32-34 ATP maximum, and carbon dioxide per one glucose molecule.
-Post by Sai Rachakonda