Question 1

The compound 2,4-dinitrophenol (DNP),an uncoupler,was briefly used as a weight-loss drug.Some of its effects in people who took the drug included weight loss and higher than normal body temperature.Some people even died.Explain the first two effects of the compound in biochemical terms.

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Question 2

Explain briefly the current model for how the proton motive force that is generated by electron transport is used to drive the ATP synthesis reaction.

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Question 3

Show the path of electrons from ubiquinone (Q or coenzyme Q)to oxygen in the mitochondrial respiratory chain.One of the two compounds (Q and O₂)has a standard reduction potential (E'°)of 0.82 V,and the other,0.045 V.Which value belongs to each compound? How did you deduce this?

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Question 4

Compound X is an inhibitor of mitochondrial ATP synthesis.It was observed that when compound X was added to cells,the NAD⁺/NADH ratio decreased.Would you expect X to be an uncoupling agent or an inhibitor of respiratory electron transfer? Explain.

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Question 5

When the $\Delta$G'° of the ATP synthesis reaction is measured on the surface of the ATP synthase enzyme,it was found to be close to zero.Describe briefly why this is so.

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Question 6

Diagram the path of electron flow from NADH to the final electron acceptor during electron transport in mitochondria.For each electron carrier,indicate whether only electrons,or both electrons and protons,are accepted/donated by that carrier.Also,indicate where electrons from succinate oxidation enter the chain of carriers.

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Question 7

The standard reduction potential for ubiquinone (Q or coenzyme Q)is 0.045 V,and the standard reduction potential (E'°)for FAD is -0.219 V.Using these values,show that the oxidation of FADH₂ by ubiquinone theoretically liberates enough energy to drive the synthesis of ATP.The Faraday constant,,is 96.48 kJ/V·mol.$\Delta$G'° for ATP synthesis is +30.5 kJ/mol.

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Question 8

During electron transfer through the mitochondrial respiratory chain,the overall reaction is: NADH + 1/2 O₂ + H⁺ $\rightarrow$ NAD⁺ + H₂O.The difference in reduction potentials for the two half-reactions ($\Delta$E'°)is +1.14 V.Show how you would calculate the standard free-energy change,$\Delta$G'°,for the reaction as written above.(The Faraday constant,,is 96.48 kJ/V·mol. )

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Question 9

The skunk cabbage (Symphocarpus foetidus)can maintain a temperature of 10-25 °C higher than the temperature of the surrounding air.Suggest a mechanism for this.

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Question 10

When the F₁ portion of the ATP synthetase complex is removed from the mitochondrial membrane and studied in solution,it functions as an ATPase.Why does it not function as an ATP synthetase?

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Question 11

As you read and answer this question,you are (presumably)consuming oxygen.What single reaction accounts for most of your oxygen consumption?

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Question 12

Consider a liver cell carrying out the oxidation of glucose under aerobic conditions.Suppose that we added a very potent and specific inhibitor of the mitochondrial ATP synthase,completely inhibiting this enzyme.Indicate whether each of the following statements about the effect of this inhibitor is true or false;if false,explain in a sentence or two why it is false.
____ (a)ATP production in the cell will quickly drop to zero.
____ (b)The rate of glucose consumption by this cell will decrease sharply.
____ (c)The rate of oxygen consumption will increase.
____ (d)The citric acid cycle will speed up to compensate.
____ (e)The cell will switch to fatty acid oxidation as an alternative to glucose oxidation,and the inhibitor will therefore have no effect on ATP production.

Accepted Answer

(a) False. Mitochondrial ATP synthesis will cease, but to compensate, cells will accelerate the production of ATP by glycolysis, preventing ATP levels from dropping to zero. (b) False. The acceleration of glycolysis noted above will actually increase the rate of glucose consumption. (c) False. Because electron transfer through the respiratory chain is tightly coupled to ATP synthesis, blocking ATP synthase blocks electron flow and oxygen consumption. (d) False. The citric acid cycle is an oxidative pathway, producing NADH. When electron flow from NADH to O₂ is blocked, NADH accumulates, NAD+ is depleted, and the citric acid cycle slows for lack of an electron acceptor (NAD+). (e) False. Oxidation of fats produces NADH, FADH₂, and acetyl-CoA, which is further oxidized via the citric acid cycle. For the reasons noted above, blocking electron flow through the respiratory chain prevents ATP synthesis with energy from fatty acid oxidation.

Question 13

Describe,in simple diagrams and a few words,the chemiosmotic theory for coupling oxidation to phosphorylation in mitochondria.

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Question 14

Describe and explain the two different ratios that affect the rate of respiration in mitochondria.What is accomplished by these control mechanisms?

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Question 15

Give an example of (a)an uncoupler of oxidative phosphorylation,and (b)an inhibitor of respiration.(c)Describe the difference in the effects of such uncouplers and inhibitors on mitochondrial function.

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Question 16

Mitochondria carrying out oxidative phosphorylation consume oxygen.Explain what happens to this oxygen,and describe the effect of an uncoupling agent such as 2,4-dinitrophenol on the rate of oxygen consumption.Assume there is a sufficient supply of oxidizable substrate,ADP,and P_i.

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Question 17

Although molecular oxygen (O₂)does not participate directly in any of the reactions of the citric acid cycle,the cycle operates only when O₂ is present.Explain this observation.

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Question 18

Cyanide ion (CN^-)blocks electron transfer in mitochondria at the level of cytochrome a + a₃.2,4-Dinitrophenol (DNP)is an uncoupler of mitochondrial oxidative phosphorylation.Draw graphs (ATP synthesized vs.time and O₂ consumed vs.time)with labeled curves to show the effects of adding each of these compounds separately to a suspension of mitochondria supplied with O₂,succinate,ADP,and P_i.Use an arrow to indicate the time of drug addition.

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Question 19

In his studies of alcoholic fermentation by yeast,Louis Pasteur noted that the sudden addition of oxygen (O₂)to a previously anaerobic culture of fermenting grape juice resulted in a dramatic decrease in the rate of glucose consumption.This "Pasteur effect" can be counteracted by the addition of 2,4-dinitrophenol (DNP),an uncoupler of oxidative phosphorylation. (a)Why would the yeast cells consume less glucose in the presence of oxygen? Can you estimate how much less glucose they would use? (b)Why would DNP counteract or prevent the Pasteur effect?

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Question 20

For each of the following explain whether they are transported into or out of the mitochondrion and comment briefly on the mechanism for each:
(1)NADH
(2)Inorganic phosphate
(3)ADP

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Quiz 19: Oxidative Phosphorylation and Photophosphorylation

The compound 2,4-dinitrophenol (DNP),an uncoupler,was briefly used as a weight-loss drug.Some of its effects in people who took the drug included weight loss and higher than normal body temperature.Some people even died.Explain the first two effects of the compound in biochemical terms.

Explain briefly the current model for how the proton motive force that is generated by electron transport is used to drive the ATP synthesis reaction.

Show the path of electrons from ubiquinone (Q or coenzyme Q)to oxygen in the mitochondrial respiratory chain.One of the two compounds (Q and O₂)has a standard reduction potential (E'°)of 0.82 V,and the other,0.045 V.Which value belongs to each compound? How did you deduce this?

Compound X is an inhibitor of mitochondrial ATP synthesis.It was observed that when compound X was added to cells,the NAD⁺/NADH ratio decreased.Would you expect X to be an uncoupling agent or an inhibitor of respiratory electron transfer? Explain.

When the $\Delta$ G'° of the ATP synthesis reaction is measured on the surface of the ATP synthase enzyme,it was found to be close to zero.Describe briefly why this is so.

The skunk cabbage (Symphocarpus foetidus)can maintain a temperature of 10-25 °C higher than the temperature of the surrounding air.Suggest a mechanism for this.

When the F₁ portion of the ATP synthetase complex is removed from the mitochondrial membrane and studied in solution,it functions as an ATPase.Why does it not function as an ATP synthetase?

As you read and answer this question,you are (presumably)consuming oxygen.What single reaction accounts for most of your oxygen consumption?

Describe,in simple diagrams and a few words,the chemiosmotic theory for coupling oxidation to phosphorylation in mitochondria.

Describe and explain the two different ratios that affect the rate of respiration in mitochondria.What is accomplished by these control mechanisms?

Give an example of (a)an uncoupler of oxidative phosphorylation,and (b)an inhibitor of respiration.(c)Describe the difference in the effects of such uncouplers and inhibitors on mitochondrial function.

Mitochondria carrying out oxidative phosphorylation consume oxygen.Explain what happens to this oxygen,and describe the effect of an uncoupling agent such as 2,4-dinitrophenol on the rate of oxygen consumption.Assume there is a sufficient supply of oxidizable substrate,ADP,and P_i.

Although molecular oxygen (O₂)does not participate directly in any of the reactions of the citric acid cycle,the cycle operates only when O₂ is present.Explain this observation.

For each of the following explain whether they are transported into or out of the mitochondrion and comment briefly on the mechanism for each: (1)NADH (2)Inorganic phosphate (3)ADP

Quiz 19: Oxidative Phosphorylation and Photophosphorylation

The compound 2,4-dinitrophenol (DNP),an uncoupler,was briefly used as a weight-loss drug.Some of its effects in people who took the drug included weight loss and higher than normal body temperature.Some people even died.Explain the first two effects of the compound in biochemical terms.

Explain briefly the current model for how the proton motive force that is generated by electron transport is used to drive the ATP synthesis reaction.

Show the path of electrons from ubiquinone (Q or coenzyme Q)to oxygen in the mitochondrial respiratory chain.One of the two compounds (Q and O2)has a standard reduction potential (E'°)of 0.82 V,and the other,0.045 V.Which value belongs to each compound? How did you deduce this?

Compound X is an inhibitor of mitochondrial ATP synthesis.It was observed that when compound X was added to cells,the NAD+/NADH ratio decreased.Would you expect X to be an uncoupling agent or an inhibitor of respiratory electron transfer? Explain.

When the Δ\DeltaΔ G'° of the ATP synthesis reaction is measured on the surface of the ATP synthase enzyme,it was found to be close to zero.Describe briefly why this is so.

The skunk cabbage (Symphocarpus foetidus)can maintain a temperature of 10-25 °C higher than the temperature of the surrounding air.Suggest a mechanism for this.

When the F1 portion of the ATP synthetase complex is removed from the mitochondrial membrane and studied in solution,it functions as an ATPase.Why does it not function as an ATP synthetase?

As you read and answer this question,you are (presumably)consuming oxygen.What single reaction accounts for most of your oxygen consumption?

Describe,in simple diagrams and a few words,the chemiosmotic theory for coupling oxidation to phosphorylation in mitochondria.

Describe and explain the two different ratios that affect the rate of respiration in mitochondria.What is accomplished by these control mechanisms?

Give an example of (a)an uncoupler of oxidative phosphorylation,and (b)an inhibitor of respiration.(c)Describe the difference in the effects of such uncouplers and inhibitors on mitochondrial function.

Mitochondria carrying out oxidative phosphorylation consume oxygen.Explain what happens to this oxygen,and describe the effect of an uncoupling agent such as 2,4-dinitrophenol on the rate of oxygen consumption.Assume there is a sufficient supply of oxidizable substrate,ADP,and Pi.

Although molecular oxygen (O2)does not participate directly in any of the reactions of the citric acid cycle,the cycle operates only when O2 is present.Explain this observation.

For each of the following explain whether they are transported into or out of the mitochondrion and comment briefly on the mechanism for each: (1)NADH (2)Inorganic phosphate (3)ADP

Show the path of electrons from ubiquinone (Q or coenzyme Q)to oxygen in the mitochondrial respiratory chain.One of the two compounds (Q and O₂)has a standard reduction potential (E'°)of 0.82 V,and the other,0.045 V.Which value belongs to each compound? How did you deduce this?

Compound X is an inhibitor of mitochondrial ATP synthesis.It was observed that when compound X was added to cells,the NAD⁺/NADH ratio decreased.Would you expect X to be an uncoupling agent or an inhibitor of respiratory electron transfer? Explain.

When the $\Delta$ G'° of the ATP synthesis reaction is measured on the surface of the ATP synthase enzyme,it was found to be close to zero.Describe briefly why this is so.

When the F₁ portion of the ATP synthetase complex is removed from the mitochondrial membrane and studied in solution,it functions as an ATPase.Why does it not function as an ATP synthetase?

Mitochondria carrying out oxidative phosphorylation consume oxygen.Explain what happens to this oxygen,and describe the effect of an uncoupling agent such as 2,4-dinitrophenol on the rate of oxygen consumption.Assume there is a sufficient supply of oxidizable substrate,ADP,and P_i.

Although molecular oxygen (O₂)does not participate directly in any of the reactions of the citric acid cycle,the cycle operates only when O₂ is present.Explain this observation.