Krebs cycle

Question 1

The conversion of oxaloacetate to citrate in the Krebs cycle requires the addition of how many carbon atoms?

A 1
B 2
C 3
D 4

Answer 1

B 2

Acetyl-CoA reacts with oxaloacetate in the Krebs cycle to add two carbon atoms (acetyl has a methyl carbon and a carbonyl carbon). This step finish the loop of the Krebs cycle before it begins again with citrate. Although pyruvate has 3 carbon atoms, a decarboxylation steps precedes its conversion to Acetyl-CoA, so only 2 carbon atoms remain to add to oxaloacetate

Question 2

Where does the Krebs Cycle occur in human beings?

A Mitochondrial matrix
B Outer mitochondrial membrane
C Inner mitochondrial membrane
D Intermembrane space of the mitochondria

Answer 2

A) Mitochondrial matrix

The inner and outer mitochondrial membranes are permeable to the products of glycolysis, such as NADH and pyruvate. The Krebs Cycle utilizes these two molecules in the mitochondrial matrix in order to produce NADH, ATP, and FADH2.

Question 3

In the absence of oxygen, pyruvate from glycolysis cannot be further utilized for energy synthesis through the Krebs cycle because?

A Oxygen is needed to react with pyruvate and convert it to acetyl CoA
B Glycolysis becomes more energetically favorable than the Krebs cycle, thus pyruvate circulates through glycolysis for energy synthesis
C There is not NAD+ to react with pyruvate and convert it to acetyl CoA
D Oxygen drives the entrance of pyruvate into the Krebs cycle

Answer 3

C ) There is not NAD+ to react with pyruvate and convert it to acetyl CoA

Recall that in the electron transport chain, NADH donates electrons, and oxygen is the final electron acceptor. In the absence of oxygen, NADH cannot donate electrons, and thus cannot become NAD+. NAD+ is a critical reactant for the conversion of pyruvate into acetyl CoA, the compound that enters the Krebs cycle, thus without NAD+, the reaction does not occur and neither does the Krebs cycle.

Question 4

One of the steps of the citric acid cycle is the conversion of succinate to fumarate, which results in the reduction of FAD. This process is shown by the diagram below. Which of the following statements is (are) true regarding the reaction that occurs?

I. Fumarate will rotate polarized light differently than succinate does.
II. FAD becomes reduced to FADH2
III. The chemical potential energy of fumarate is lower than succinate.

A III only
B I and III only
C II and III only
D I, II, and III

Answer 4

II and III only

Question 5

In the Krebs cycle, malate and NAD+ are converted to NADH and oxaloacetate. The malate converts to oxaloacetate by having one of its alcohol groups converted into a carbonyl. From the perspective of malate, this type of reaction is an example of which of the following?

A reduction
B oxidation
C dehydration
D condensation

Answer 5

oxidation

giveaway is the loss of hydrogen. In organic chemistry, this suggests an oxidation reaction. Additionally, the increase in the number of bonds to the oxygen atom (from C-O of the alcohol to C=O of the carbonyl) is also suggestive of oxidation. NAD+ is reduced to NADH. Therefore our answer is (B).

Question 6

Carnitine is a compound required for the transport of fatty acids from the cytosol into the mitochondria. A carnitine deficiency would most directly impact which of the following metabolic reactions?

A Gluconeogenesis
B Krebs Cycle
C Electron Transport Chain
D Glycolysis

Answer 6

Krebs Cycle

Fatty acids are used by the mitochondria for energy production by being degraded into acetyl groups for Acetyl-CoA which lead directly into the Krebs Cycle. Carnitine deficiency would have downstream effects on the Electron Transport Chain but only because of the Krebs Cycle. Glycolysis does not involve fatty acids and does not occur in the mitochondria, therefore this process would be unaffected. Gluconeogeneis is roughly a reversal of glycolysis and so would be unaffected as well.

Question 7

Which of the following is NOT considered a proton pump?

A Complex IV
B Complex III
C Complex II
D Complex I

Answer 7

Complex III

The mitochondrial electron transport chain involves multiple mitochondrial redox carriers, and energy obtained through the transfer of electrons down this chain is used to pump protons from the mitochondrial matrix into the intermembrane space. This creates an electrochemical proton gradient across the mitochondrial inner membrane and allows ATP synthase to generate ATP from ADP and inorganic phosphate. There are four membrane-bound complexes (Complex I, II, III, and IV) in the mitochondria, and each is embedded in the inner membrane. All of them involve the translocation of protons, except Complex II, which is a parallel electron transport pathway to Complex I (accepts electrons from NADH, incoming from the Kreb’s Cycle). Therefore, Complex II does not transport protons to the intermembrane space and subsequently contributes less energy to the overall electron transport chain.

Question 8

Which of the following is NOT common to the degradation of all amino acids?

A The use of the amino groups for synthesis of new amino acids
B The passage of the carbon skeletons to the gluconeogenic pathway
C The separation of the amino group(s) from the carbon skeleton
D The process occurs mainly in the liver in mammals

Answer 8

B The passage of the carbon skeletons to the gluconeogenic pathway

For amino acids to enter the blood, they must first pass out of organelles via amino acid transporters. Subsequent degradation occurs mainly in the liver (and kidneys too), and it involves deamination (removal of an amine group) that moves the amino group to alpha-ketoglutarate, forming glutamate. In most cases, the amino group is then removed through the urea cycle and excreted as urea, but amino acid degradation can also produce uric acid or ammonia. Moreover, after removal of one or more amino groups, the remainder of the molecule (the carbon skeleton) can be used to synthesize new amino acids or for energy, by entering glycolysis or the Kreb’s Cycle.

Question 9

Which of the following is NOT correct in regard to fatty acid transport into the mitochondrial matrix?

A Fatty acid transport into the mitochondrial matrix is the rate-limiting step in beta oxidation.
B The inner mitochondrial membrane is impermeable to fatty acyl CoA.
C A specialized carrier system transports activated fatty acids from the cytosol to mitochondria.
D Fatty acyl groups that enter the matrix are not committed to oxidation to acetyl-CoA

Answer 9

D Fatty acyl groups that enter the matrix are not committed to oxidation to acetyl-CoA.

Prior to fatty acid oxidation (beta-oxidation) occurring in the mitochondria, fatty acids must be activated before they can be carried into the mitochondria. However, the inner mitochondrial membrane is impermeable to fatty acids, so a specialized carrier system (carnitine) transports activated fatty acids from the cytosol to the mitochondria. Then, once activated, the acyl CoA is transported into the mitochondrial matrix. The eventual acetyl-CoA (product of beta-oxidation) will then enter the Citric Acid Cycle.

Question 10

The relase of CO2 during the Krebs cycle is a result of which type of reaction?

A decarboxylation
B carbonyl reduction
C carbonic acid neutralization
D beta-oxidation

Answer 10

A) decarboxylation

Beta-oxidation is used in fatty acid metabolism and is not relevant for the Krebs cycle.
Carbonic acid neutralization would involve the reaction of carbonic acid with some base, evolving CO2.

However, this would require the presence of CO2 already (as carbonic acid is simply a form of dissolved CO2).

The question is asking how the CO2 is produced by the Krebs cycle. Reduction of a carbonyl group would result in its conversion to an alcohol or an alkane, not CO2. Decarboxylation reactions involve the loss of a carboxyl group (-COO-) which, with the rearrangement of electrons, is CO2.

Question 11

In animals, which of the following does NOT occur in the mitochondria?

A Glycolysis
B Electron transport
C Citric acid cycle
D Krebs Cycle

Answer 11

A Glycolysis

Question 12

Cellular respiration has essentially three metabolic processes. What are they?

Answer 12

1) glycolysis,
2) the Citric acid cycle (also known as the Krebs Cycle),
3) oxidative phosphorylation, and each of these three take place in specific regions in animal cells

Question 13

Where does glycolysis take place in the cell?

Answer 13

Glycolysis occurs in the cytosol

Question 14

The Citric acid cycle (also known as the Krebs Cycle) take place in the?

Answer 14

in the mitochondrial matrix

Question 15

dative phosphorylation (via the electron transport chain) occurs in the?

Answer 15

inner mitochondrial membrane.

Question 16

Without O2 what would cellular respiration consists of and where would it happend?

Answer 16

glycolysis and fermentation
in the cytosol.

Question 17

After Glycolysis and the Citric Acid Cycle (Kreb’s Cycle), most of the energy from the original glucose molecule is now in the form of what?

A Pyruvate
B ATP
C H2O
D High-energy electrons associated with electron carriers

Answer 17

D) High-energy electrons associated with electron carriers

Glycolysis, taking place in the cytoplasm, is designed to break down glucose into two pyruvate molecules, which yields a net amount of 2 ATP and 2 NADH. The Citric Acid Cycle (aka the Kreb’s Cycle), occurring in the mitochondria, takes pyruvate (which eventually becomes Acetyl CoA) and produces ATP, NADH, and FADH2. By the end of these two processes, most of the energy is in the form of electron carriers NADH (10 molecules to be exact) and FADH2 (2 molecules).

Question 18

The release of CO2 during the Krebs cycle is a result of which type of reaction?

A. Decarboxylation
B. Carbonyl reduction
C. Carbonic acid neutralization
D. Beta-oxidation

Answer 18

A. Decarboxylation

Question 19

Mature red blood cells have hemoglobin and no organelles. These cells metabolize glucose, which generates lactate, and can be used by the liver in gluconeogenesis. Which of the following does NOT explain why mature red blood cells metabolize glucose to lactate in order to generate energy?

A) Anaerobic oxidation of pyruvate regenerates the NAD+ required for glycolysis to continue
B) Oxygen is not available for the aerobic oxidation of glucose
C) Mature red blood cells have no citric acid cycle
D ) Mature red blood cells have no mitochondria

Answer 19

Oxygen is not available for the aerobic oxidation of glucose

Question 20

in humans, proline can be synthesized from glutamate which in turn can be derived from α-ketoglutarate. It can therefore be concluded that:

A proline is necessary for the Krebs cycle.
B glutamate and proline play similar roles in proteins.
C proline production is an irreversible reaction in the body.
D proline is not an essential amino acid in humans.

Answer 20

D) proline is not an essential amino acid in humans.

Question 21

The conversion of oxaloacetate to citrate in the Krebs cycle requires the addition of how many carbon atoms?

A 1
B 2
C 3
D 4

Answer 21

B) 2

Acetyl-CoA reacts with oxaloacetate in the Krebs cycle to add two carbon atoms (acetyl has a methyl carbon and a carbonyl carbon). This step finish the loop of the Krebs cycle before it begins again with citrate. Although pyruvate has 3 carbon atoms, a decarboxylation steps precedes its conversion to Acetyl-CoA, so only 2 carbon atoms remain to add to oxaloacetate.

Question 22

In the citric acid cycle, how many molecules of ATP are produced per molecule of acetyl CoA that enters the cycle?

A 1
B 2
C 3
D 32

Answer 22

A) 1

Question 23

What does Citric acid cycle produces the over all?

Answer 23

The citric acid cycle produces
1 ATP, 3 NADH, and 1 FAHD2

Question 24

Which of the following molecules can be converted into acetyl-CoA and used for energy within the citric acid cycle?

I. fats
II. carbohydrates
III. proteins

A II only
B II and III only
C I and II only
D I, II, and III

Answer 24

D I, II, and III

Question 25

Within the citric acid cycle, malate dehydrogenase converts malate to oxaloacetate. The reaction utilizes NAD+ from which an NADH molecule is produced. This reaction serves as an example of:

A elimination.
B combustion.
C oxidation reduction.
D decarboxylation.

Answer 25

C oxidation reduction

In this reaction NAD+ is reduced to become NADH and malate is oxidized (as it loses electrons and donates them to the NAD+ molecule). Thus (C) is the correct answer.

Question 26

How many molecules of NADH are produced from 2 glucose molecules through the citric acid cycle?

A 6
B 8
C 10
D 12

Answer 26

D) 12

The citric acid cycle produces 1 ATP, 3 NADH, and 2 CO2 per acetyl-CoA molecule. Each glucose provides two acetyl-CoA molecules, thus 4*3 = 12 NADH molecules. Thus (D) is the correct answer.

Question 27

Which of the following is true regarding the electron carrying ability of NADH and FADH2?

A FADH2 produces double the ATP molecules than that produced by NADH
B FADH2 produces half the ATP molecules than that produced by NADH
C FADH2 produces the same number of ATP molecules produced by NADH
D FADH2 produces less ATP molecules than that produced by NADH but greater than half

Answer 27

D) FADH2 produces less ATP molecules than that produced by NADH but greater than half.

FADH2 produces 2 molecules of ATP in the Krebs cycle and NADH produces 3 molecules of ATP, thus (D) is the correct answer.

Question 28

How are electrons extracted from the citric acid cycle for use in the electron transport chain?

A. Reduction of ATP and GTP
B. Reduction of NAD+ and FAD+
C. Oxidation of NAD+ and FAD
D. Oxidation of ATP and GTP

Answer 28

B. Reduction of NAD+ and FAD+

Question 29

Where does the Krebs Cycle occur in human beings?

A. Outer mitochondrial membrane
B. Inner mitochondrial membrane
C. Intermembrane space of the mitochondria
D. Mitochondrial matrix

Answer 29

D. Mitochondrial matrix

Question 30

High cellular concentrations of what molecule would inhibit the entry of pyruvate into the citric
acid cycle?

A. Coenzyme A
B. Pyruvate
C. AMP
D. NADH

Answer 30

D. NADH