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 .

The fourth complex in the electron transport chain is unique in that it has the important responsibility of reducing molecular oxygen to water. Oxygen is the final electron acceptor for cellular respiration, so this is a very important role. Complex 2 is the only one that accepts electrons from  and is the smallest of the protein complexes. Complex 2 is also the one that does not have hydrogen pumping ability. Iron is a component of complexes 1, 3, and 4.

The coenzymes being referred to are  and _.  and  are used to generate the bulk of ATP at the electron transport chain. These factors are produced in both glycolysis and the Krebs cycle. In glycolysis the conversion of glyceraldehyde to 1,3-bisphosphoglycerate generates two molecules of  per molecule of glucose. The conversion of pyruvate to acetyl-CoA is the next reaction that generates . In the Krebs cycle both  and  are produced. The reactions that produce  are: isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate.

The lone reaction that produces  is the conversion of succinate to fumarate, which is carried out by an enzyme known as succinate dehydrogenase present in the electron transport chain.

Four total molecules of ATP are produced during glycolysis. Note that all of these ATP molecules are created via substrate-level phosphorylation and were made anaerobically. Recall that there are two steps in glycolysis that require ATP as a reactant, and thus, the net ATP production is two ATP per molecule of glucose.

In the citric acid cycle, succinate dehydrogenase converts succinate to fumarate with the production also of a molecule of  (flavin adenine dinucleotide) that supplies electrons to the electron transport chain. Mitochondrial glycerol-3-phosphate dehydrogenase-2 is an enzyme of the glycerol-3-phosphate shuttle that produces  and converts glycerol-3-phosphate into dihydroxyacetone phosphate. The other enzymes are part of the citric acid cycle, but do not produce  (aconitase converts citrate to isocitrate, while succinyl-CoA synthetase converts succinyl-CoA to succinate). 

The ATP yield from NADH is dependent on how the electrons from the cytoplasmic (glycolytic) NADH are transported into the mitochondria. In muscle, the glycerol-phosphate shuttle occurs, which results in 1.5 ATP per NADH. However, in the heart and liver, the malate-aspartate shuttle occurs, resulting in 2.5 ATP per NADH. This difference explains why some sources list the net ATP from glucose catabolism as 30 ATP, while others list 32 ATP.