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Example Questions
Example Question #948 : Biochemistry
Which of the following steps of the citric acid cycle results in the production of as a side product?
None of the answers listed result in the production of as a side product.
The conversion of alpha-ketoglutarate to succinyl-CoA
The conversion of succinyl-CoA to succinate
The conversion of malate to oxaloacetate
The conversion of isocitrate to alpha-ketoglutarate
None of the answers listed result in the production of as a side product.
The correct answer is that none of the citric acid cycle steps listed result in the production of . The only step of the citric acid cycle that results in the production of is the conversion of succinate to fumarate (catalyzed by succinate dehydrogenase). In this reaction, is concomitantly converted to using the hydrogen molecules removed from succinate by succinate dehydrogenase. This reaction was not listed in the answer choices though, and therefore none of the reactions listed produced .
Each of the reactions listed did produce other side products. The conversions of isocitrate to alpha-ketoglutarate, alpha-ketoglutarate to succinyl-CoA, and malate to oxaloacetate all result in the production of as a side product, but not . The conversion of succinyl-CoA to succinate results in the production of ATP or GTP and CoA-SH as side products, but not .
Example Question #1 : Reactants And Products Of The Citric Acid Cycle
Which of these molecules is not a product of the citric acid cycle?
Flavin mononucleotide (FMN)
NADH
Ubiquinol (QH2)
Pyruvate
CO2
Flavin mononucleotide (FMN)
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).
Example Question #1 : Reactants And Products Of The Citric Acid Cycle
What is pyruvate converted to before it enters the citric acid cycle?
Glucose
Acetate
Acetyl-CoA
Acetyl-CoA
Pyruvate is the end product of glycolysis. After glycolysis, the three-carbon molecule pyruvate is converted into 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 also produces one molecule of . Acetyl-CoA has one less carbon than pyruvate. The third carbon from pyruvate is lost as carbon dioxide () during the conversion of pyruvate to acetyl-CoA. Recall that since glucose is a six-carbon molecule, two molecules of pyruvate (three carbons each) are formed via glycolysis.
Example Question #2 : Reactants And Products Of The Citric Acid Cycle
The first reaction of the citric acid cycle is an aldol condensation involving which two molecules?
and acetyl-CoA
and pyruvate
Oxaloacetate and malate
Acetyl-CoA and oxaloacetate
Acetyl-CoA and oxaloacetate
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.
Example Question #2 : Reactants And Products Of The Citric Acid Cycle
At what step is produced during the citric acid cycle?
During conversion of oxaloacetate and acetyl-CoA to form citric acid by citrate synthase
During conversion of succinate into fumarate by succinate dehydrogenase
During conversion of succinate into fumarate by succinate dehydrogenase
During conversion of citric acid into isocitrate by acontinase
During conversion of succinate into fumarate by succinate dehydrogenase
During the conversion of succinate into fumarate by succinate dehydrogenase, a single molecule of is reduced to as it accepts the hydrogens from succinate. then feeds its electrons into the electron transport chain in the inner mitochondrial membrane.
Example Question #1 : Reactants And Products Of The Citric Acid Cycle
In the Krebs cycle, which step involves the formation of GTP?
Alpha-ketoglutarate succinyl-CoA
None of these, GTP is not a product of Krebs cycle
Succinate fumarate
Isocitrate alpha-ketoglutarate
Succinyl-CoA succinate
Succinyl-CoA succinate
Succinyl-CoA synthetase performs substrate level phosphorylation at this step in the Krebs cycle, such that .
Example Question #34 : Citric Acid Cycle
From a single molecule of glucose, two molecules of pyruvate are formed which can be converted into two acetyl-CoA molecules to enter the citric acid cycle.
Starting from a single molecule of glucose, how many molecules of NADH are formed in a single turn of the citric acid cycle including the conversion of pyruvate into acetyl-CoA? Do not include NADH formed during glycolysis, which is not part of the citric acid cycle.
The conversion of pyruvate to acetyl-CoA produces one molecule 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 six NADH in the cycle proper. Therefore, the total NADH produced in one turn of the citric acid cycle is eight NADH.
Example Question #2 : Reactants And Products Of The Citric Acid Cycle
Which of the following molecules are initial reactants of the citric acid cycle?
NADH
None of these are initial reactants of citric acid cycle
Pyruvate
None of these are initial reactants of citric acid cycle
Citric acid cycle inputs are derived from glycolysis outputs. Glycolysis produces pyruvate molecules, , and ATP. The pyruvate molecules undergo reactions that convert the three carbon pyruvate to a two carbon acetyl CoA and an one carbon carbon dioxide. The acetyl-CoA molecules are then used as the initial inputs for the citric acid cycle, as they are combined with oxaloacetate. Note that pyruvate itself does not enter the citric acid cycle. and are electron carriers that are produced in the citric acid cycle and are used in electron transport chain to generate ATP.
Example Question #33 : Citric Acid Cycle
A glucose molecule entering the cell can indirectly produce __________ and __________ in the citric acid cycle.
six . . . two
three . . . one
two . . . six
one . . . three
six . . . two
A glucose (six carbons) molecule enters glycolysis and produces two three carbon molecules (pyruvate). Each pyruvate is broken down into a two carbon acetyl-CoA molecule that enters the citric acid cycle. Each acetyl-CoA molecule produces three and one in the citric acid cycle. This means that two acetyl-CoA (derived from one glucose molecule) produces six and two molecules in the citric acid cycle.
Example Question #2 : Reactants And Products Of The Citric Acid Cycle
Which of the following is true regarding the citric acid cycle?
More than one of these are true
Most of the intermediates of this cycle have four more carbons than acetyl-CoA
The citric acid cycle produces GTP
The citric acid cycle produces carbon dioxide
More than one of these are true
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).
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