The citric acid cycle, also known as the Kreb's cycle, occurs within the mitochondria of eukaryotic cells. In prokaryotic cells, it occurs in the cytosol.

The citric acid cycle takes place in the mitochondrial matrix.

Glycolysis takes place in the cytosol, and the electron transport chain involves both the intermembrane space and the inner mitochondrial membrane.

Pyruvate from glycolysis is transported into the mitochondrial matrix for the citric acid cycle. Energy from the citric acid cycle allows protons to be pumped to the intermembrane space. The electron transport chain involves proteins along the inner mitochondrial membrane, eventually resulting in the activation of ATP synthase due to the influx of protons along their gradient.

In cellular respiration, glucose first undergoes glycolysis and is broken down into two pyruvate molecules. As the pyruvate passes through the citric acid cycle, three molecules of  are produced. The radioactive oxygen molecules would be found in the .

 is formed when electrons removed from glucose are used to reduce .  is produced by the phosphorylation of . The oxygen in  enters the mitochondrion as gaseous molecular oxygen from the atmosphere, not from glucose. Finally,  is reduced to water in cellular respiration and serves as a reactant, rather than a product, in cell metabolism.

The Krebs cycle takes place in the mitochondrial matrix. The products of glycolysis, which takes place in the cytosol, are brought to the mitochondria for the Krebs cycle and electron transport chain. The electron carriers generated during the Krebs cycle (NADH and FADH₂) are then used in the electron transport chain, which takes place on the inner membrane of the mitochondria.

A turn of the Krebs cycle produces one ATP, three NADH, one FADH₂, and two CO₂.

Acetyl-CoA is not produced during Krebs cycle. It is produced from the decarboxylation of a pyruvate molecule, which occurs before the Krebs cycle can begin. Each turn of Krebs cycle is initiated by one acetyl-CoA molecule. Remember that there are two acetyl-CoA produced from the two pyruvate molecules (end product of glycolysis). For every glucose molecule, the Krebs cycle produces two cycles: two ATP, six NADH, two FADH₂, and four CO₂.

Acetyl-CoA is the molecule that enters as the primary reactant in the Krebs cycle.

During glycolysis glucose is the primary reactant. Glucose contains six carbons. The process of glycolysis converts one molecule of glucose into two molecules of pyruvate with three carbons each. Pyruvate then undergoes a decarboxylation reaction before entering the Krebs cycle. Each pyruvate loses one carbon to create carbon dioxide during this reaction, with the end product of acetyl-CoA. Acetyl-CoA is, thus, a two-carbon chain.

The ratio of carbon in acetyl-CoA to carbon in glucose is two-to-six, or 1:3.

The role of the Krebs cycle is to produce the intermediates NADH and FADH₂, which will serve as electron donors in the electron transport chain (ETC). At the same time, the ETC creates NAD⁺ and FADH⁺ as byproducts. The products can then be turned around to continue fueling the Krebs cycle. Since the ETC will not function in an anaerobic environment, neither will the Krebs cycle. The reactants will not be replenished, and the cycle will be unable to continue.

Oxygen is not directly involved as a reactant or product of the Krebs cycle. Oxygen is only directly used as an electron receptor in the electron transport chain.

NAD⁺ and FADH are used as reactants in the citric acid cycle to make NADH and FADH₂, which are used in the electron transport chain to convert additional ADP into ATP. All of the other selections are products in the citric acid cyclce. Protons (H⁺) are a byproduct when NAD⁺ is converted to NADH. Carbon dioxide (CO₂) is produced during carbohydrate conversions in the cycle. One GTP molecule is produced by the cycle, and contains almost equivalent energy to ATP.

An anabolic reaction is one in which larger molecules are made from combining smaller molecules. Even without knowing the exact mechanics of the reactions given in the answer choices, we know that we are looking for a reaction in which multiple molecules combine to form a single molecule.

Out of the options, there is only one time where a larger molecule is made by the combination of two smaller ones: when acetyl CoA (2 carbons) and oxaloacetate (4 carbons) come together in order to create citrate (6 carbons).

The generation of pyruvate from glucose results in two smaller molecules from one larger molecule; this is a catalysis reaction. The conversion of glucose-6-phosphate to fructose-6-phosphate is an isomerization reaction. The transition from citrate to ketoglutarate is processed through an intermediate, but is ultimately a catalysis reaction.

Fermentation is a catabolic process which does not require oxygen. In contrast, Krebs cycle (citric acid cycle) and oxidative phosphorylation (chemiosmosis and electron transport) do use oxygen. Aerobic respiration is much more efficient than anaerobic respiration in producing ATP.