Biochemistry : Reactants and Products of the Citric Acid Cycle

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #1 : Reactants And Products Of The Citric Acid Cycle

Which of these molecules is not a product of the citric acid cycle?

Possible Answers:

Ubiquinol (QH2)

CO2

Flavin mononucleotide (FMN)

Pyruvate

NADH

Correct answer:

Flavin mononucleotide (FMN)

Explanation:

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?

Possible Answers:

Glucose

Acetyl-CoA

Acetate

Correct answer:

Acetyl-CoA

Explanation:

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 #1 : Reactants And Products Of The Citric Acid Cycle

The first reaction of the citric acid cycle is an aldol condensation involving which two molecules?

Possible Answers:

 and acetyl-CoA

 and pyruvate

Acetyl-CoA and oxaloacetate

Oxaloacetate and malate

Correct answer:

Acetyl-CoA and oxaloacetate

Explanation:

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 #3 : Reactants And Products Of The Citric Acid Cycle

At what step is  produced during the citric acid cycle? 

Possible Answers:

During conversion of succinate into fumarate by succinate dehydrogenase

During conversion of succinate into fumarate by succinate dehydrogenase

During conversion of oxaloacetate and acetyl-CoA to form citric acid by citrate synthase

During conversion of citric acid into isocitrate by acontinase

Correct answer:

During conversion of succinate into fumarate by succinate dehydrogenase

Explanation:

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 #4 : Reactants And Products Of The Citric Acid Cycle

In the Krebs cycle, which step involves the formation of GTP?

Possible Answers:

Succinate  fumarate 

Alpha-ketoglutarate  succinyl-CoA

None of these, GTP is not a product of Krebs cycle

Isocitrate  alpha-ketoglutarate

Succinyl-CoA  succinate 

Correct answer:

Succinyl-CoA  succinate 

Explanation:

Succinyl-CoA synthetase performs substrate level phosphorylation at this step in the Krebs cycle, such that .

Example Question #1 : Reactants And Products Of The 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.

Possible Answers:

Correct answer:

Explanation:

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 #1 : Reactants And Products Of The Citric Acid Cycle

Which of the following molecules are initial reactants of the citric acid cycle?

Possible Answers:

Pyruvate

None of these are initial reactants of citric acid cycle

NADH

Correct answer:

None of these are initial reactants of citric acid cycle

Explanation:

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 #1 : Reactants And Products Of The Citric Acid Cycle

A glucose molecule entering the cell can indirectly produce __________  and __________  in the citric acid cycle.

Possible Answers:

three . . . one

six . . . two

one . . . three

two . . . six

Correct answer:

six . . . two

Explanation:

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 #8 : Reactants And Products Of The Citric Acid Cycle

Which of the following is true regarding the citric acid cycle?

Possible Answers:

The citric acid cycle produces GTP

The citric acid cycle produces carbon dioxide

Most of the intermediates of this cycle have four more carbons than acetyl-CoA

More than one of these are true

Correct answer:

More than one of these are true

Explanation:

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).

Example Question #10 : Reactants And Products Of The Citric Acid Cycle

Which of the following molecules involved in the citric acid cycle initiates the cycle by combining with acetyl-CoA?

Possible Answers:

Citrate

Malate

Fumarate

Oxaloacetate

Succinate

Correct answer:

Oxaloacetate

Explanation:

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.

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