Pyruvate is produced in the last step of glycolysis, then, it is converted to the two-carbon molecule acetyl-coenzyme A (acetyl-CoA). This is carried out by a combination of three enzymes collectively known as the pyruvate dehydrogenase complex. The conversion of pyruvate to acetyl-CoA produces one . Acetyl-CoA has one less carbon than pyruvate. The third carbon of pyruvate is lost as carbon dioxide () during the conversion of pyruvate to acetyl-CoA. The citric acid cycle begins when the four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) via an aldol condensation, yielding the six-carbon molecule citrate.

The only step of the citric acid cycle listed that results in the production of as a side product is the conversion of isocitrate to alpha-ketoglutarate. In this step, the enzyme, isocitrate dehydrogenase catalyzes the conversion of isocitrate to alpha-ketoglutarate, while also converting to and as side products, and generating a molecule of in the process (i.e. reducing the carbon count from 5 in isocitrate to 4 in alpha-ketoglutarate).

The conversion of alpha-ketoglutarate to succinyl-CoA also produces a molecule of as a side product. However, this step is not listed as an answer choice.

None of the other answer choices listed produce as side products.

Isocitrate dehydrogenase activation leads to oxidative decarboxylation of isocitrate in a two step process producing alpha-ketoglutarate and . In the mitochondria, the reaction produces also a charged electron carrier molecule, , from . Isocitrate dehydrogenase, inhibited by and activated by , is a major regulator enzyme of the citric cycle.

The Krebs cycle produces the most high energy electron carriers of any process involved in cellular respiration. Per glucose molecule, the Krebs cycle produces and .

Pyruvate is converted to acetyl-CoA by the pyruvate dehydrogenase complex. Carbon dioxide is released during this reaction, and in addition to this, is reduced to .

The first step in the citric acid cycle is for acetyl-CoA to react with oxaloacetate. This forms citrate, which then continues through the cycle, ultimately reforming the oxaloacetate molecule to redo the cycle.

The pyruvate dehydrogenase complex (PDC) is preparing pyruvate for the Krebs cycle by converting it to acetyl-CoA. Because the Krebs cycle functions within the mitochondrial matrix, the PDC is also taking place there. This ensures quick and easy movement from the PDC into the Krebs cycle.

Citric acid cycle involves a series of reactions that are involved in the production of the necessary molecules for electron transport chain. The cycle starts with a two carbon molecule (acetyl-CoA) binding to a four carbon molecule (oxaloacetate). This creates a six carbon molecule (citrate) that can go through a series of reactions. Most of these reactions involve a six carbon molecule. As mentioned, acetyl-CoA has two carbons; therefore, most of the intermediates in this cycle have six carbons, or four more carbons than acetyl-CoA. One turn of citric acid cycle produces , , (carbon dioxide) and one GTP molecule(s).

The conversion of pyruvate to acetyl-CoA produces onemolecule of NADH, but remember that each glucose yields two pyruvates, so the total NADH from this first step is two. Within the citric acid cycle, there are three steps in which NADH is a byproduct, but again we must remember that each step occurs to two molecules, therefore three NADH byproducts for two molecules yields sixNADH in the cycle proper. Therefore, the total NADH produced in one turn of the citric acid cycle is eight NADH.

Flavin mononucleotide (FMN) is not produced by the citric acid cycle. This flavin coenzyme is a reactant, but not a product, since FMN will get reduced to FMNH2.

The rest of the answer choices are products of the citric acid cycle (otherwise known as the Krebs cycle).