The conversion of succinate to fumarate is the only reaction that occurs outside of the normal Krebs cycle. Complex II of the electron transport chain has an enzyme known as succinate dehydrogenase. This enzyme is responsible for the conversion of succinate to fumarate. Fumarate is return to the cycle where it is then oxidized to malate continuing the cycle. Each of the other reactions of the Krebs cycle listed all occur in the inner mitochondrial matrix; whereas the conversion of succinate to fumarate occurs at the inner mitochondrial membrane.

ATP synthase uses the proton gradient across the inner membrane to generate ATP. The ATP synthase is essentially like a rotary motor. The proton gradient serves as the priming of the ATP synthase. As proton are moved from the outer mitochondrial matrix back into the mitochondrial matrix they are providing mechanical energy to turn the pump. As the pump is being turned ATP synthase utilizes a unit of ADP and inorganic phosphate to generate one molecule of ATP. This is done for every three turns of the ATP synthase.

Complex IV is also known as cytochrome c oxidase because it accepts the electrons from cytochrome c and directs them towards the four electron reduction of oxygen to form two molecules of water. ATP synthase is directly responsible for the generation of ATP by utilizing one unit of ADP and one unit of inorganic phosphate along with the proton motive force (PMF). Complex II is also known as succinate dehydrogenase which is responsible for one of the reaction of the Krebs cycle: succinate to fumarate. This reaction generates one molecule of . Complex I is also known as  dehydrogenase in that it oxidizes the coenzyme .

Complex I is also called NADH-Coenzyme Q (CoQ) reductase because it transfers 2 electrons from NADH to CoQ. Complex I was formerly known as NADH dehydrogenase. This complex binds NADH and takes up two electrons.The last step of this complex is the transfer of two electrons one at a time to CoQ. The process of transferring electrons from NADH to CoQ by complex I results in the overall transport of protons from the matrix side of the inner mitochondrial membrane to the inter membrane space where the hydrogen ion concentration increases generating a proton motive force which is utilized by ATP synthase.

Complex II of the electron transport chain is generally apart of both the electron transport chain as well as the Krebs cycle. It is the the succinate dehydrogenase that carried out the conversion of succinate to fumarate in the Krebs cycle. The only enzyme of the citric acid cycle that is an integral membrane protein. The conversion of succinate to fumarate generates an .  then transfers its electrons one at a time through complex II. The final step of this complex is the transfer of two electrons one at a time to coenzyme Q.

Complex II of the electron transport chain is generally apart of both the electron transport chain as well as the Krebs cycle. It is the the succinate dehydrogenase that carried out the conversion of succinate to fumarate in the Krebs cycle. The only enzyme of the citric acid cycle that is an integral membrane protein. The conversion of succinate to fumarate generates an .  then transfers its electrons one at a time through complex II. The final step of this complex is the transfer of two electrons one at a time to coenzyme Q.

Ubiquinone functions to carry electrons in oxidative phosphorylation from the first enzyme complex to the second enzyme complex.  It does not receive electrons from  nor  directly. 

Oxidative phosphorylation takes place in the mitochondria in a eukaryote.  The process is made possible by the double membrane within the mitochondria.

Complex I pumps 4 protons, complex IV pumps 4 protons, and the interaction between complex III and complex II is more complicated.

Complex II pumps no electrons in itself, but releases the fully reduced quinone species, , which interacts with complex III through the Q cycle. Simplified, the net result of the Q cycle is that 4 protons are pumped out into the intermembrane space. complex III pumps 2 protons from the mitochondrial matrix and 2 protons from .

This is a simplification of the 4 complexes, providing only the information necessary to complete the question. But a full understanding of the 4 complexes, and the flow of electrons is nonetheless essential for understanding why each complex pumps the number of protons it does.

 first delivers its electrons to complex 2 of the electron transport chain. Subsequently, the electrons are delivered to ubiquinone, and then they move through complex 3, cytochrome C, and complex 4. Complex 2, therefore, is the only protein complex that directly accepts electrons from .