MCAT Biology : Krebs Cycle

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

Example Question #1 : Krebs Cycle

Within the Krebs cycle, L-malate and NAD+ come together to form oxaloacetate, NADH, and H+. What type of chemical reaction is responsible for this step in the cycle?

Possible Answers:

Hydration

Dehydration

Decarboxylation

Oxidation

Correct answer:

Oxidation

Explanation:

In order for oxaloacetate to be formed, malate must lose electrons, which is the definition of an oxidation reaction. Alternately, NAD+ is reduced (gains electrons) to form NADH and H+.

Example Question #1 : Krebs Cycle

During cellular respiration, where is NADH produced?

Possible Answers:

The cytosol and mitochondrial matrix

The cytosol

The mitochondrial intermembrane space

The nucleus

The endoplasmic reticulum

Correct answer:

The cytosol and mitochondrial matrix

Explanation:

NADH is produced during glycolysis, which occurs in the cytoplasm. NADH is also produced during the Krebs cycle, which occurs in the mitochondrial matrix. The protons generated in the production of NADH are later used in the intermembrane space to power ATP synthase during oxidative phosphorylation.

Example Question #2 : Krebs Cycle

If the Krebs cycle is overstimulated, the body will produce too much of which of the following molecules?

Possible Answers:

Pyruvate

Acetyl CoA

Carbon dioxide

Glucose

Oxygen

Correct answer:

Carbon dioxide

Explanation:

Of the answer choices, only carbon dioxide is a product of the Krebs cycle. If the cycle is overstimulated, too much of the products will be formed and the body will have too much carbon dioxide.

Glucose is the reactant that fuels glycolysis to produce pyruvate, which is then converted to acetyl CoA for the Krebs cycle. As such, each of these would be depleted as reactants fueling an overstimulation of the Krebs cycle.

Example Question #3 : Krebs Cycle

Where is the Krebs cycle carried out in eukaryotic cells?

Possible Answers:

Cytosol

Inner membrane of the mitochondria

Mitochondrial matrix

Nucleus

Correct answer:

Mitochondrial matrix

Explanation:

During the Krebs cycle, or citric acid cycle, acetyl CoA is oxidized to CO2 and NAD+ and FADH are reduced to NADH and FADH2, respectively. This process is carried out in the mitochondrial matrix of eukaryotic cells.

The electron transport chain is carried out in the inner membrane of the mitochondria, while glycolysis is carried out in the cytosol. 

Example Question #4 : Krebs Cycle

Cellular respiration is the set of metabolic reactions that occur in cells to produce energy in the form of ATP. During cellular respiration, high energy intermediates are created that can then be oxidized to make ATP. During what stage are these intermediates produced?

Possible Answers:

The citric acid cycle

Glycolysis

The citric acid cycle and glycolysis

Oxidative phosphorylation and the citric acid cycle

Oxidative phosphorylation

Correct answer:

The citric acid cycle and glycolysis

Explanation:

The citric acid (Krebs) cycle and glycolysis yield high energy intermediates that can then be used to make ATP. Each turn of the citric acid cycle generates NADH and FADH2, and each cycle of glycolysis generates NADH. These intermediates can then donate their electrons and become oxidized in the electron transport chain. Production of these electron donors is essential to the function of the electron transport chain.

Example Question #5 : Krebs Cycle

Acetyl-CoA is a react in the citric acid cycle, while and  are products. If twelve molecules of  are produced over a period of time, how many  molecules are produced during this period?

Possible Answers:

Twenty-four

Twelve

Two

Four

Correct answer:

Four

Explanation:

Each turn of the citric acid cycle is powered by one molecule of acetyl-CoA, resulting in three and one . The net reaction is:

Since twelve are produced, there must have been an input of four acetyl-CoA molecules and four total turns in the cycle. As a result, four  molecules were produced.

Example Question #6 : Krebs Cycle

Which statement is false regarding the citric acid cycle?

Possible Answers:

All of these

Another name for it is the Krebs cycle 

Oxygen is directly required for the citric acid cycle to occur 

It occurs in the mitochondrial matrix for eukaryotes

6 GTP molecules would be produced starting with 3 glucose molecules

Correct answer:

Oxygen is directly required for the citric acid cycle to occur 

Explanation:

Oxygen is needed for the electron transport chain to occur which oxidizes  and . If there is no oxygen available then Krebs cycle would not occur since there would be no oxidized electron carriers. Therefore oxygen is only indirectly required for the Krebs cycle to occur, not directly.

Example Question #16 : Biochemistry And Metabolism

James took a neural sample and separated the cell body from the axon. He noticed that when he placed both parts on a glucose plate, the cell body began releasing carbon dioxide. What could explain the result? 

Possible Answers:

The carbon dioxide came from the plate 

The carbon dioxide is used as a messenger to communicate with the axon

The cell body is degrading 

The cell body contains mitochondria 

None of these

Correct answer:

The cell body contains mitochondria 

Explanation:

The cell body of a neuron is where the mitochondria and all other organelles are located. Recall from the Krebs cycle that carbon dioxide is produced as a byproduct. Anaerobic respiration, which occurs in the cytoplasm does not release carbon dioxide (in humans) and produces lactic acid instead. Note that in certain organisms like yeast, fermentation produces ethanol (two-carbons) and carbon dioxide since pyruvate, the product of glycolysis is a three-carbon molecule. 

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