AP Biology : Understanding the Citric Acid Cycle

Study concepts, example questions & explanations for AP Biology

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Example Questions

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Example Question #1 : Understanding The Citric Acid Cycle

Where does the citric acid cycle take place in eukaryotic cells?

Possible Answers:

The nucleus

The cytosol

Endoplasmic reticulum

Mitochondria

Golgi body

Correct answer:

Mitochondria

Explanation:

The citric acid cycle, also known as the Kreb's cycle, occurs within the mitochondria of eukaryotic cells. In prokaryotic cells, it occurs in the cytosol.

Example Question #51 : Cellular Respiration

The citric acid cycle takes place in the __________.

Possible Answers:

cytosol

mitochondrial matrix

intermembrane space

inner mitochondrial membrane

Correct answer:

mitochondrial matrix

Explanation:

The citric acid cycle takes place in the mitochondrial matrix.

Glycolysis takes place in the cytosol, and the electron transport chain involves both the intermembrane space and the inner mitochondrial membrane.

Pyruvate from glycolysis is transported into the mitochondrial matrix for the citric acid cycle. Energy from the citric acid cycle allows protons to be pumped to the intermembrane space. The electron transport chain involves proteins along the inner mitochondrial membrane, eventually resulting in the activation of ATP synthase due to the influx of protons along their gradient.

Example Question #51 : Cellular Respiration

A sample of rats were fed glucose  containing radioactive oxygen. After a few minutes, where would the radioactive oxygen be found?

Possible Answers:

Correct answer:

Explanation:

In cellular respiration, glucose first undergoes glycolysis and is broken down into two pyruvate molecules. As the pyruvate passes through the citric acid cycle, three molecules of  are produced. The radioactive oxygen molecules would be found in the .

 is formed when electrons removed from glucose are used to reduce  is produced by the phosphorylation of . The oxygen in  enters the mitochondrion as gaseous molecular oxygen from the atmosphere, not from glucose. Finally,  is reduced to water in cellular respiration and serves as a reactant, rather than a product, in cell metabolism.

Example Question #52 : Cellular Respiration

Where does the Krebs cycle take place?

Possible Answers:

Mitochondrial matrix

On the inner membrane of the mitochondria

Intermembrane space of the mitochondria

In the cytosol

Correct answer:

Mitochondrial matrix

Explanation:

The Krebs cycle takes place in the mitochondrial matrix. The products of glycolysis, which takes place in the cytosol, are brought to the mitochondria for the Krebs cycle and electron transport chain. The electron carriers generated during the Krebs cycle (NADH and FADH2) are then used in the electron transport chain, which takes place on the inner membrane of the mitochondria.

Example Question #53 : Cellular Respiration

Which of the following molecules is produced during the Krebs cycle?

I. FADH2

II. ATP

III. Acetyl-CoA

Possible Answers:

II and III

I only

II only

I and II

Correct answer:

I and II

Explanation:

A turn of the Krebs cycle produces one ATP, three NADH, one FADH2, and two CO2.

Acetyl-CoA is not produced during Krebs cycle. It is produced from the decarboxylation of a pyruvate molecule, which occurs before the Krebs cycle can begin. Each turn of Krebs cycle is initiated by one acetyl-CoA molecule. Remember that there are two acetyl-CoA produced from the two pyruvate molecules (end product of glycolysis). For every glucose molecule, the Krebs cycle produces two cycles: two ATP, six NADH, two FADH2, and four CO2.

Example Question #55 : Cellular Respiration

The ratio of carbons in one acetyl-CoA molecule to one glucose molecule is __________.

Possible Answers:

Correct answer:

Explanation:

Acetyl-CoA is the molecule that enters as the primary reactant in the Krebs cycle.

During glycolysis glucose is the primary reactant. Glucose contains six carbons. The process of glycolysis converts one molecule of glucose into two molecules of pyruvate with three carbons each. Pyruvate then undergoes a decarboxylation reaction before entering the Krebs cycle. Each pyruvate loses one carbon to create carbon dioxide during this reaction, with the end product of acetyl-CoA. Acetyl-CoA is, thus, a two-carbon chain.

The ratio of carbon in acetyl-CoA to carbon in glucose is two-to-six, or 1:3.

Example Question #56 : Cellular Respiration

Under anaerobic conditions, a eukaryotic cell will not undergo the Krebs cycle. Why is this?

Possible Answers:

Lack of NADH and FADH2

Lack of NAD+ and FADH+

The reactant for the Krebs cycle is oxygen

The end product for the Krebs cycle is oxygen

None of the other answers

Correct answer:

Lack of NAD+ and FADH+

Explanation:

The role of the Krebs cycle is to produce the intermediates NADH and FADH2, which will serve as electron donors in the electron transport chain (ETC). At the same time, the ETC creates NAD+ and FADH+ as byproducts. The products can then be turned around to continue fueling the Krebs cycle. Since the ETC will not function in an anaerobic environment, neither will the Krebs cycle. The reactants will not be replenished, and the cycle will be unable to continue.

Oxygen is not directly involved as a reactant or product of the Krebs cycle. Oxygen is only directly used as an electron receptor in the electron transport chain.

Example Question #54 : Cellular Respiration

Which of the following is not a product formed during the citric acid cycle?

Possible Answers:

GTP

NAD+

H+

FADH2

CO2

Correct answer:

NAD+

Explanation:

NAD+ and FADH are used as reactants in the citric acid cycle to make NADH and FADH2, which are used in the electron transport chain to convert additional ADP into ATP. All of the other selections are products in the citric acid cyclce. Protons (H+) are a byproduct when NAD+ is converted to NADH. Carbon dioxide (CO2) is produced during carbohydrate conversions in the cycle. One GTP molecule is produced by the cycle, and contains almost equivalent energy to ATP.

Example Question #8 : Understanding The Citric Acid Cycle

Which of the following is an example of an anabolic reaction?

Possible Answers:

Citrate being converted to ketoglutarate

Glucose-6-phosphate being isomerized to form fructose-6-phosphate

Acetyl CoA combining with oxaloacetate to form citrate

Glucose being converted to two pyruvate molecules

Correct answer:

Acetyl CoA combining with oxaloacetate to form citrate

Explanation:

An anabolic reaction is one in which larger molecules are made from combining smaller molecules. Even without knowing the exact mechanics of the reactions given in the answer choices, we know that we are looking for a reaction in which multiple molecules combine to form a single molecule.

Out of the options, there is only one time where a larger molecule is made by the combination of two smaller ones: when acetyl CoA (2 carbons) and oxaloacetate (4 carbons) come together in order to create citrate (6 carbons).

The generation of pyruvate from glucose results in two smaller molecules from one larger molecule; this is a catalysis reaction. The conversion of glucose-6-phosphate to fructose-6-phosphate is an isomerization reaction. The transition from citrate to ketoglutarate is processed through an intermediate, but is ultimately a catalysis reaction.

Example Question #54 : Cellular Respiration

Which of the following processes occurs without oxygen?

Possible Answers:

Electron transport chain

Fermentation

Krebs cycle 

Citric acid cycle

Correct answer:

Fermentation

Explanation:

Fermentation is a catabolic process which does not require oxygen. In contrast, Krebs cycle (citric acid cycle) and oxidative phosphorylation (chemiosmosis and electron transport) do use oxygen. Aerobic respiration is much more efficient than anaerobic respiration in producing ATP.  

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