Complex II: The other way the proton motive force begins is through complex II, where FADH feeds e– and H+ from the oxidation of a product of the citric acid cycle, to the quinones. Complex II is less efficient than complex I.
Complex I: One way the proton motive force begins is with the donation of H+ from NADH to flavin mononucleotide (FMN) to make FMNH2. 4H+ move to the outside of the cell when FMNH2 donates 2e– to the Fe/S proteins in complex I.
16), protons are transported from the matrix of the mitochondria across the inner mitochondrial membrane to the intermembrane space located between the inner and outer mitochondrial membranes
Imagine you’re standing on the roof of a tall building, holding an orange. If you let the orange drop over the side of the building, by the time it reaches the ground it will have gained so much speed that it will hit the ground with great force and smash. Because of the large difference in height from where it was dropped and where it landed, there’s a great amount of energy.
How this happens is that during electron transport, H+ are pushed to the outside of the membrane. The H+ come from both NADH and the dissociation of H2O into H+ and OH–. As you’ll see in a minute, it’s a bit more complicated, but the overall result is accumulation of a positive charge outside the cell and a negative charge within.
The diffusible electron carriers NADH and FADH2 carry hydrogen atoms (protons and electrons) from substrates in exergonic catabolic pathways such as glycolysis and the citric acid cycle to other electron carriers that are embedded in membranes.
Complex III: Quinones are reduced in the (a series of oxidation and reduction reactions of the coenzyme Q that result in the release of additional H+ to the outside of the membrane). Then electrons are passed one at a time from the Q-cycle to complex III, which contains the heme-containing proteins (specifically, cytochrome bc1) and an FeS protein.
Chemiosmosis involves the pumping of protons through channels in the mitochondrial membrane from the inner to outer compartments which then results in a proton gradient.
Creating the H+ gradient is the function of the electron transport chain which acts as an energy converter that uses the exergonic flow of electrons to pump H+ across the membrane, from the mitochondrial matrix into the intermembrane space.