During strenuous exercise, GLUT4 will be active to bring glucose into the cell. Glucose is then pushed through glycolysis to generate pyruvate. Pyruvate is then pushed through pyruvate dehydrogenase complex, where it is converted into acetyl-CoA, which feeds into the Krebs cycle (assuming aerobic conditions) to generate ATP, NADH,, and carbon dioxide.

Most of the intermediate molecules in the Krebs cycle can, rather than continuing through the cycle itself, go through other pathways to form macromolecules. Citrate can be used to create fatty acids and sterols. Alph-ketoglutarate can be used to make some of the amino acids. Succinyl-CoA can be used to make porphyrins, heme, and chlorophyll. Aspartate can be used to make some of the amino acids. Oxaloacetate can be used in gluconeogenesis to create glucose.

The citric acid cycle intermediate,fumarate, containsfouratoms of carbon.

As a frame of reference, one molecule ofglucose, the starting material for glycolysis, containssixatoms of carbon. The carbohydrate products of glycolysis are two molecules of pyruvate, with one molecule ofpyruvatecontainingthreeatoms of carbon.

In preparation for entering the citric acid cycle, pyruvate loses one molecule of carbon dioxide, and therefore one molecule of carbon, to formacetyl-CoA, which containstwoatoms of carbon. Acetyl-CoA is then combined with a molecule ofoxaloacetate, which containsfouratoms of carbon, to produce a molecule ofcitrate, which containssixatoms of carbon, and is the starting point for the citric acid cycle.

Citrate undergoes a number of a reactions, via the citric acid cycle, most notably two reactions in which a single molecule of carbon dioxide, and therefore carbon, is lost, thereby decreasing the total number of carbons tofour atoms．两个反应去除碳是有限公司nversion of isocitrate to alpha-ketoglutarate and the conversion of alpha-ketoglutarate to succinyl-CoA. No additional carbons are removed prior to the production of fumarate, and therefore, fumarate containsfouratoms of carbon.

The citric acid cycle intermediate,malate, containsfouratoms of carbon.

A singleglucosemolecule, which is the starting material for glycolysis, containssixcarbon atoms. Glycolysis produces two pyruvate molecules, and onepyruvatemolecule containsthreecarbon atoms.

Prior to entering the citric acid cycle, pyruvate loses one carbon dioxide molecule (e.g. one carbon atom), formingacetyl-CoA, which containstwocarbon atoms. Acetyl-CoA then combines with oneoxaloacetatemolecule, afour-carbon molecule, to produce a molecule ofcitrate, which containssixcarbon atoms, and is the starting material for the citric acid cycle.

Citrate undergoes a number of a reactions in the citric acid cycle, including two reactions where one atom of carbon dioxide (e.g. carbon) is lost, which decreases the total number of carbons tofouratoms. The two reactions that remove carbons are the conversion of isocitrate to alpha-ketoglutarate and the conversion of alpha-ketoglutarate to succinyl-CoA. No additional carbons are removed prior to the production of malate. Therefore, malate containsfouratoms of carbon.

Acetyl-CoA is not part of the cycle but is oxidized by it. There are two decarboxylations in the cycle, from isocitrate to alpha-ketoglutarate, and from alpha-ketoglutarate to succinyl-CoA. In total, three equivalents ofare produced in the cycle. Isocitrate is a compound in the cycle, produced from citrate.

Even though anis generated when malate is dehydrogenated to oxaloacetate, this oxidation is very unfavorable because of the addition of a reactive ketone in place of an alcohol on the 2nd carbon. In fact, the only way this reaction can proceed is if oxaloacetate concentration is very low. All of the other reactions have large negativevalues.

The formation of alpha-ketoglutarate from isocitrate using the enzyme alpha-ketoglutarate dehydrogenase is an irreversible reaction due to its largely negativevalue.

This question is providing us with a scenario in whichions enter a cell. We're further told that it will take a single molecule of ATP to move three of these ions out of the cell. Finally, we are being asked to determine the total number of acetyl-CoA molecules that must pass through the Krebs cycle in order to provide the energy necessary for the export of theseions.

First, we'll need to determine the total number of ATP molecules generated from the passage of a single molecule of acetyl-CoA through the Krebs cycle. It's important to remember that the passage of acetyl-CoA through the Krebs cycle generates one molecule of ATP directly by substrate-level phosphorylation, but it also produces other intermediate energy carriers in the form ofand．

For each acetyl-CoA ran through the cycle, one molecule ofand three molecules ofare produced. Furthermore, each molecule ofwill go on to donate its electrons to the electron transport chain to generatemolecules of ATP per molecule ofoxidized. Likewise, eachwill also produce ATP via oxidative phosphorylation, but at a rate ofmolecules of ATP per molecule ofoxidized.

Adding these up, we obtain:

ATP via substrate-level phosphorylation

Adding these values up, we have a total ofmolecules of ATP produced for every molecule of acetyl-CoA oxidized. Now that we know how much ATP is produced from one acetyl-CoA, we can calculate the number needed to move theions out of the cell.

The only step of the citric acid cycle (also known as the Krebs cycle, or the TCA cycle) thatdirectlyproduces ATP or GTP is the conversion of succinyl-CoA to succinate.

In this reaction, succinyl-CoA is converted to succinate with the assistance of the enzyme, succinyl-CoA synthetase. During this reaction, ADP + Pi (or GDP + Pi) is also converted to ATP (or GTP) using the energy from the breaking of the bond between CoA and succinate. Thus, the overall reaction appears as:

While side products of some of the other reactions listed produce intermediaries that may be used to produce ATP in the future, these reactions do notdirectlyproduce ATP.

The only citric acid cycle (also known as the Krebs cycle or TCA cycle) step listed that does not result in the production ofas a side product is the conversion of fumarate to malate.

In the conversion of fumarate to malate, fumarate is chemically combined with water in the presence of the enzyme fumarase to produce malate. In this conversion, there is no concomitant production of．

In each of the other reactions listed,is converted toandas side products.