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. 

Reduction potential refers to the spontaneity of the reduction half reaction. Remember that reduction refers to a gain of electrons. Thus, reduction potential is similar to the property of electronegativity. It can also be thought of a molecule's tendency to gain electrons or as a measure of its unwillingness to give up electrons.

Since oxygen is the final electron acceptor in the electron transport chain, we know that the reduction of oxygen is highly spontaneous (highly positive E, and highly negative G). It is this reason that the electrons from NADH and FADH₂ must be passed step-wise to oxygen. Otherwise, there is such a large release of energy that too much would be lost to heat and become unavailable to do work for the cell.

In this question, we're asked to determine which scenario would cause a reduction in the amount of  produced by mitochondria.

First, let's start with  and . Both of these cofactors serve as high-energy electron carriers, which donate their electrons into the mitochondrial electron transport chain to ultimately produce . Therefore, high levels of these cofactors would not be expected to reduce  production.

Next, let's consider the effect of a higher pH in the matrix than in the intermembrane space. When the above mentioned cofactors donate their electrons into the electron transport chain, protons are actively pumped from the matrix into the intermembrane space. The result of this is that the intermembrane space becomes significantly more acidic than the matrix. This is needed, because the protons are then able to spontaneously flow down their proton gradient to produce . Therefore, we would expect that a higher pH (more basic) in the matrix is the equivalent to saying that the intermembrane space has a lower pH (more acidic). Consequently, this lower pH in the intermembrane space would be expected to produce  rather than inhibit its production.

Finally, lets consider how the concentration of  affects  production. In order to produce  via the electron transport chain,  needs to be phosphorylated. Therefore, if there is not much  around to phosphorylate, then we would expect that most of the cell's adenosine is already in the form of . Thus, we would expect low  concentrations to reduce  production.

The combustion of glucose to yield carbon dioxide and water refers to aerobic metabolism (oxidative phosphorylation). This process releases energy, so Gibbs free energy is negative. A negative Gibbs free energy indicates that the products are at a lower energy than the reactants, meaning that the reaction is thermodynamically favorable (spontaneous). Lastly, aerobic metabolism is called so because it requires oxygen. Thus, all of the answers are correct.

The complexes that function as a part of the electron transport chain accept electrons from  and .  When they accept the high energy electrons, they also pump hydrogens across the mitochondrial inner membrane to ultimately be used at the ATP synthase.  Creation of ATP is the end goal, but it is not made directly by the electron transport chain complexes.

Oxygen is required for the electron transport chain to function because it is the final electron acceptor for oxidative respiration. Once the high energy electron carriers,  and , have delivered electrons to the chain, the electrons run through the protein complexes. When they finish moving through all of the complexes, something must be available for them to attach to. Oxygen is the molecule responsible for this. It accepts the electrons and in turn is converted into .