Biochemistry : Citric Acid Cycle Carbohydrate Intermediates

Study concepts, example questions & explanations for Biochemistry

varsity tutors app store varsity tutors android store

Example Questions

Example Question #209 : Catabolic Pathways And Metabolism

Which molecule is not a citric acid cycle intermediate?

Possible Answers:

Phosphoenolpyruvate

Isocitrate

Succinate

Citrate

Correct answer:

Phosphoenolpyruvate

Explanation:

Phosphoenolpyruvate (PEP) is an intermediate in glycolysis, not the citric acid cycle. PEP is the product of the ninth reaction in glycolysis, which involves the enolase-catalyzed conversion of 2-phosphoglycerate into PEP. All other molecules are indeed intermediates in the citric acid cycle.

Example Question #210 : Catabolic Pathways And Metabolism

Pyruvate enters the citric acid cycle after being converted to a molecule with how many carbons?

Possible Answers:

Correct answer:

Explanation:

The three-carbon molecule pyruvate produced from glycolysis 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; this third carbon from pyruvate was lost as carbon dioxide during its conversion to acetyl-CoA via the pyruvate dehydrogenase complex.

Example Question #1 : Citric Acid Cycle Carbohydrate Intermediates

What is the intermediate between citrate and isocitrate?

Possible Answers:

Succinate

Cis-aconitate

Oxaloacetate

Fumarate

Correct answer:

Cis-aconitate

Explanation:

The citric acid cycle begins when a four-carbon molecule, oxaloacetate combines with acetyl-CoA (a two carbon molecule) to produce the six-carbon molecule citrate. The enzyme citrate synthase carries out this reaction. Citrate then becomes the six-carbon molecule cis-aconitate via catalysis by aconitase. The same enzyme then converts cis-aconitate to isocitrate, which is an isomer of citrate.

Example Question #2 : Citric Acid Cycle Carbohydrate Intermediates

What vitamin does pyruvate dehydrogenase need in order to make pyruvate into acetyl-CoA for the citric acid cycle? 

Possible Answers:

Thiamine (B1)

Pyridoxine (B6)

Niacin (B3)

Methylcobalamin (B12)

Correct answer:

Thiamine (B1)

Explanation:

Thiamine (B1) acts as a cofactor to enable pyruvate dehydrogenase to convert pyruvate from glycolysis into acetyl-CoA so it can enter the citric acid cycle. 

Example Question #3 : Citric Acid Cycle Carbohydrate Intermediates

The citric acid cycle begins when the two-carbon acetyl group from acetyl-CoA combines with the four-carbon molecule __________ to form the six-carbon molecule citrate. 

Possible Answers:

Oxaloacetate

Aspartate

Aspartate

Pyruvate

Malate

Correct answer:

Oxaloacetate

Explanation:

Oxaloacetate combines with acetyl-CoA to form citrate. This is the first stage of the citric acid cycle. Eventually, citrate will lose two molecules of  to regenerate the four-carbon molecule oxaloacetate.

Example Question #4 : Citric Acid Cycle Carbohydrate Intermediates

There are at least four types of glucose transporter in the body. GLUT1 and GLUT3 are located in most tissues including the brain and the red blood cells. These glucose transporters rapidly take up glucose from the blood but have the lowest  value. GLUT2 is commonly found in the liver and the pancreas. GLUT2 has a lower affinity for glucose but has the highest  value. GLUT4 is common in skeletal tissues and in adipose tissues. This transporter is normally not active for uptake unless stimulated by insulin or during exercise.  

During strenuous exercise, GLUT4 will be highly active. Which of the following intermediates will also increase?

I. Pyruvate

II. 

III. ADP

Possible Answers:

II only

II and III

III only

I only

I and II

Correct answer:

I and II

Explanation:

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.  

Example Question #5 : Citric Acid Cycle Carbohydrate Intermediates

Which of the following Krebs cycle intermediate molecules can be used directly in another pathway to make fatty acids?

Possible Answers:

Oxaloacetate

Alpha-ketoglutarate

Citrate

Aspartate

Succinyl-CoA

Correct answer:

Citrate

Explanation:

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.

Example Question #142 : Carbohydrate Metabolism

How many atoms of carbon are present in the citric acid cycle intermediate, fumarate?

Possible Answers:

6

2

4

3

1

Correct answer:

4

Explanation:

The citric acid cycle intermediate, fumarate, contains four atoms of carbon.

As a frame of reference, one molecule of glucose, the starting material for glycolysis, contains six atoms of carbon. The carbohydrate products of glycolysis are two molecules of pyruvate, with one molecule of pyruvate containing three atoms of carbon.

In preparation for entering the citric acid cycle, pyruvate loses one molecule of carbon dioxide, and therefore one molecule of carbon, to form acetyl-CoA, which contains two atoms of carbon. Acetyl-CoA is then combined with a molecule of oxaloacetate, which contains four atoms of carbon, to produce a molecule of citrate, which contains six atoms 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 to four atoms. 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 fumarate, and therefore, fumarate contains four atoms of carbon. 

Example Question #6 : Citric Acid Cycle Carbohydrate Intermediates

How many molecules of carbon are present in the citric acid cycle intermediate, malate?

Possible Answers:

4

1

6

2

3

Correct answer:

4

Explanation:

The citric acid cycle intermediate, malate, contains four atoms of carbon.

A single glucose molecule, which is the starting material for glycolysis, contains six carbon atoms. Glycolysis produces two pyruvate molecules, and one pyruvate molecule contains three carbon atoms.

Prior to entering the citric acid cycle, pyruvate loses one carbon dioxide molecule (e.g. one carbon atom), forming acetyl-CoA, which contains two carbon atoms. Acetyl-CoA then combines with one oxaloacetate molecule, a four-carbon molecule, to produce a molecule of citrate, which contains six carbon 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 to four atoms. 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 contains four atoms of carbon. 

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: