GRE Subject Test: Biology : Understanding the Krebs Cycle

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding The Krebs Cycle

What are the net products of the Krebs cycle from one glucose molecule?

Possible Answers:

4 NADH, 2 FADH2, 2 GTP, 6 CO2

2 pyruvate, 2 NADH, 2 ATP

6 NADH, 2 FADH2, 2 GTP, 4 CO2

3 NADH, 1 FADH2, 1 GTP, 2 CO2

Correct answer:

6 NADH, 2 FADH2, 2 GTP, 4 CO2

Explanation:

The Krebs cycle is very important for the production of the electron carriers NADH and FADH2. NADH is produced in a higher quantity, partially due to the fact that it is more energetically favorable during the electron transport chain. During one turn of the Krebs cycle, one acetyl-CoA is used to generate a CoA residue, three NADH, one GTP, one FADH2, and two carbon dioxide. Each glucose molecule produces two acetyl-CoA, fueling two turns of the Krebs cycle and doubling these amounts.

The net products of glycolysis are 2 pyruvate, 2 ATP, and 2 NADH.

Example Question #1 : Understanding The Krebs Cycle

Which of the following products is produced in the highest quantity during the Krebs cycle?

Possible Answers:

Correct answer:

Explanation:

For every two molecules of pyruvate that enter the Krebs cycle, six molecules of NADH are generated. Only four molecules of carbon dioxide, two molecules of FADH2, and two molecules of ATP (GTP) are produced. The reason this has significance is because NADH is an important electron carrier that will help produce large amounts of ATP during the electron transport chain. The primary purpose of the Krebs cycle is to generate large amounts of this electron carrier in order to fuel the electron transport chain and oxidative phosphorylation, which is eventually responsible for mass production of ATP.

Example Question #3 : Understanding The Krebs Cycle

What molecule joins with acetyl CoA in order to produce citrate?

Possible Answers:

Ketoglutarate

Oxaloacetate

Succinate

Pyruvate

Correct answer:

Oxaloacetate

Explanation:

Acetyl CoA (a two-carbon molecule) enters the Krebs cycle by joining with oxaloacetate (a four-carbon molecule) in order to create the six-carbon molecule citrate.

Example Question #4 : Understanding The Krebs Cycle

__________ is the loss of electrons caused by __________ agents.

Possible Answers:

None of these

Oxidation . . . oxidizing 

Reduction . . . reducing

Oxidation . . . reducing

Reduction . . . oxidizing

Correct answer:

Oxidation . . . oxidizing 

Explanation:

Oxidation is the process by which a molecule loses an electron, which is caused by an oxidizing agent. Reduction is the process by which a molecule gains an electron, which is caused by a reducing agent. Remember: OIL RIG (Oxidation Is Loss of electrons, Reduction Is Gain of electrons)

Example Question #9 : Cell Metabolism

Which of the following are high energy intermediates produced during the citric acid cycle?

I.

II. 

III. 

IV. 

Possible Answers:

I, II, III, and IV

I and III

II and III

I, III, and IV

I, II, and III

Correct answer:

I and III

Explanation:

 and  are the only high energy intermediates produced in cellular respiration.  is produced instead in the light reactions of photosynthesis or in the pentose phosphate pathway. However, all three of these can pass on their high energy electrons to reduce other substrates. Even though  is produced in cellular respiration, it is not a high energy intermediate.  is the most oxidized form of carbon and cannot be tapped into for further energy by standard metabolic processes.

Example Question #2 : Understanding The Krebs Cycle

Which molecule reacts with oxaloacetate (four-carbons) to form citrate (six-carbons) in the beginning of the citric acid cycle?

Possible Answers:

Fumarate

Alpha-ketoglutarate

Malate

Acetyl CoA

Succinate

Correct answer:

Acetyl CoA

Explanation:

The acetyl group from the molecule acetyl CoA (two-carbons) is added to oxaloacetate to form citrate in the beginning of the citric acid cycle. Alpha-ketoglutarate (five-carbons), succinate (four-carbons), fumarate (four-carbons), and malate (four-carbons) are all intermediates of the citric acid cycle. Acetyl-CoA can come from various metabolic pathways including glycolysis (and subsequent pyruvate dehydrogenation) and beta-oxidation of fatty acids.

Example Question #6 : Understanding The Krebs Cycle

Which high energy intermediate is produced when pyruvate and coenzyme A react to form acetyl-CoA and carbon dioxide?

Possible Answers:

Correct answer:

Explanation:

The decarboxylation of pyruvate results in the production of NADH. NADH can then pass its high energy electrons through the electron transport chain, which leads to ATP production. The other high energy intermediates, except for NADPH, are produced in other stages of cellular respiration (glycolysis and Krebs cycle). Cyclic AMP is an intracellular second messenger that is involved in signal transduction and regulation of many cellular processes.

Example Question #8 : Understanding The Krebs Cycle

The hydrolysis of ATP to ADP and an inorganic phosphate is a(n) __________ reaction.

I. exergonic

II. endergonic

III. spontaneous

IV. nonspontaneous

Possible Answers:

I and III

II only

I only

II and IV

I and IV

Correct answer:

I and III

Explanation:

The hydrolysis of ATP releases energy, making it an exergonic reaction. Endergonic reactions require energy from a source such as the hydrolysis of ATP. ATP will spontaneously hydrolyze to form ADP and Pi, this is because the products have greater entropy, lower free energy, and thus a negative . ATP is unstable because it has three negatively-charged phosphate groups that repel one another, and because upon hydrolysis, the phosphate group(s) exhibit resonance.

Example Question #3 : Understanding The Krebs Cycle

Glucose is not the only molecule from which ATP can be derived. Which molecule type produces the most amount of ATP per gram?

Possible Answers:

Lipids

Carbohydrates

Alcohol

Nucleic acids 

Proteins

Correct answer:

Lipids

Explanation:

Lipids yield the most amount of ATP per gram because they contain the most reduced form of carbon. Alcohols, such as ethanol, are a close second because their carbons are more reduced than they are in carbohydrates. A general guide is that lipids produce about 9kcal per gram, alcohols produce about 7kcal per gram, carbohydrates and proteins produce about 4kcal per gram and nucleic acids produce about 2kcal per gram, although they are rarely used for energy.

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