Match the inhibitors in Group 1 with their respective targets in Group 2.Group 1Group 2(P) Oligomycin(i) Cytochrome bc1 complex(Q) Antimycin A(ii) Photosystem II(R) 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)(iii) K+ ionophore(S) Valinomycin(iv) FO ATP synthase

The correct match is P-iv, Q-i, R-ii, S-iii.Oligomycin blocks the proton channel of the FO part of ATP synthase.Antimycin A blocks the cytochrome bc1 complex (complex III) at the Qi site.DCMU (diuron) binds the QB site of photosystem II and stops electron flow from it.Valinomycin is a K+ ionophore that carries potassium across membranes and dissipates the membrane potential.The other options interchange these targets, for example giving oligomycin to complex III.

Oxidative Phosphorylation Questions and Answers

Q: Which one of the following compounds does NOT block electron transport?

The compound that does not block electron transport is Oligomycin.Oligomycin inhibits ATP synthase by binding to its Fo proton channel. It blocks phosphorylation, not the electron transport chain itself; in tightly coupled mitochondria electron flow slows only indirectly, because the proton gradient builds up, and it resumes at once when an uncoupler is added.The other three are true electron transport inhibitors: rotenone blocks Complex I (NADH to ubiquinone), antimycin A blocks Complex III at the Qi site, and cyanide binds the haem a3-CuB centre of cytochrome c oxidase (Complex IV).

Q: Rotenone is a chemical often used to kill insect pests on crop plants and fishes in lakes. Rotenone acts by inhibiting electron transport from the NADH dehydrogenase enzyme in Complex I to ubiquinone in the mitochondrial electron transport chain. Which one of the following explains why plants can tolerate rotenone application?

Plants tolerate rotenone because they have additional NAD(P)H dehydrogenases that are resistant to rotenone.Plant mitochondria possess alternative, type II NAD(P)H dehydrogenases on both faces of the inner membrane. They pass electrons from NADH or NADPH directly to ubiquinone without going through Complex I and without pumping protons, and they are insensitive to rotenone. When rotenone blocks Complex I, electrons still reach the rest of the chain through these bypass enzymes, so respiration continues.Plant Complex I itself is rotenone-sensitive, and tolerance is not attributed to degradation of the compound or to specific efflux channels. Insects and fish lack the bypass, which is why rotenone kills them.

Q: Electrons from cytoplasmic NADH are brought into mitochondria by malate-aspartate shuttle pathway. Select the correct option from below for which this shuttle specifically operate.

The malate-aspartate shuttle operates specifically in heart and liver.NADH made by glycolysis in the cytosol cannot cross the inner mitochondrial membrane. In the malate-aspartate shuttle cytosolic oxaloacetate is reduced to malate, which enters the mitochondrion and is reoxidised, regenerating NADH inside. The electrons enter the respiratory chain at Complex I, giving about 2.5 ATP per NADH. Heart, liver and kidney rely on this efficient shuttle.Skeletal muscle and brain mainly use the glycerol 3-phosphate shuttle, which passes the electrons to FAD and so into the chain at ubiquinone, giving only about 1.5 ATP. The pancreas is not the tissue associated with this shuttle.

Q: Oxysomes for F0-F1 particles are present on :

Oxysomes are present on the inner mitochondrial membrane.Oxysomes, or elementary particles, are the F0-F1 particles of ATP synthase. The F0 part is embedded in the inner mitochondrial membrane and forms a proton channel; the F1 head projects into the matrix and makes ATP. They line the cristae, where oxidative phosphorylation takes place.The outer mitochondrial and outer chloroplast membranes carry no ATP synthase. Thylakoids do hold an ATP synthase (CF0-CF1), but the term oxysome refers to the mitochondrial particles.

Q: ................................ is/are the only nonprotein component/s of the electron transport system.

The only nonprotein component is Coenzyme-Q.Coenzyme Q (ubiquinone) is a small lipid-soluble quinone that moves freely in the inner mitochondrial membrane, carrying electrons from complexes I and II to complex III. It is the one carrier of the chain that is not a protein or a group bound inside a protein.Cytochromes are haem proteins and Fe-S centres are iron-sulphur clusters held in proteins. NAD is a soluble coenzyme that feeds electrons into complex I rather than a component of the chain, and FAD (with FMN) is a prosthetic group bound within flavoproteins.

Q: During respiration correct sequence of electron acceptors in ATP synthesis is :

The correct sequence is Cyt b, c, a, a3.Electrons pass along the chain in order of increasing reduction potential. Cytochrome b is part of complex III, which passes electrons through cytochrome c1 to the mobile cytochrome c. Cytochrome c then delivers them to complex IV, where they pass from cytochrome a to a3 and finally to oxygen.The other orders put a3 before a, or c before b, or reverse the chain entirely.

Q: Cyanide or azide used in poisoning, irreversibly bind to ................... that acts as a critical link between aerobic respiration and oxygen.

Cyanide and azide bind to Cytochrome C Oxidase.Cytochrome c oxidase (complex IV) is the last enzyme of the respiratory chain; it passes electrons to molecular oxygen, reducing it to water. Cyanide and azide bind to the iron of its haem a3, the site where oxygen binds, and so stop electron flow and aerobic ATP production.Succinate dehydrogenase (complex II) and NADH dehydrogenase (complex I) feed electrons into the chain, and coenzyme Q carries them to complex III; none of them reacts with oxygen. Their inhibitors are different, such as malonate for complex II and rotenone for complex I.

Which one of the following compounds does NOT block electron transport?

A.Cyanide
B.Rotenone
C.Oligomycin
D.Antimycin A

Exam Year(s)

GATE Life Sciences - 2012,

Answer: C

The compound that does not block electron transport is Oligomycin.

Oligomycin inhibits ATP synthase by binding to its F_o proton channel. It blocks phosphorylation, not the electron transport chain itself; in tightly coupled mitochondria electron flow slows only indirectly, because the proton gradient builds up, and it resumes at once when an uncoupler is added.

The other three are true electron transport inhibitors: rotenone blocks Complex I (NADH to ubiquinone), antimycin A blocks Complex III at the Q_i site, and cyanide binds the haem a₃-Cu_B centre of cytochrome c oxidase (Complex IV).

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Rotenone is a chemical often used to kill insect pests on crop plants and fishes in lakes. Rotenone acts by inhibiting electron transport from the NADH dehydrogenase enzyme in Complex I to ubiquinone in the mitochondrial electron transport chain. Which one of the following explains why plants can tolerate rotenone application?

A.The Complex I in plants is resistant to rotenone.
B.Plants inactivate rotenone by enzymatic degradation.
C.Plants have specific channels that efflux rotenone out of the cell.
D.Plants have additional NAD(P)H dehydrogenases that are resistant to rotenone.

Exam Year(s)

GATE Life Sciences - 2023,

Answer: D

Plants tolerate rotenone because they have additional NAD(P)H dehydrogenases that are resistant to rotenone.

Plant mitochondria possess alternative, type II NAD(P)H dehydrogenases on both faces of the inner membrane. They pass electrons from NADH or NADPH directly to ubiquinone without going through Complex I and without pumping protons, and they are insensitive to rotenone. When rotenone blocks Complex I, electrons still reach the rest of the chain through these bypass enzymes, so respiration continues.

Plant Complex I itself is rotenone-sensitive, and tolerance is not attributed to degradation of the compound or to specific efflux channels. Insects and fish lack the bypass, which is why rotenone kills them.

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Match the inhibitors in Group 1 with their respective targets in Group 2.

Group 1	Group 2
(P) Oligomycin	(i) Cytochrome bc₁ complex
(Q) Antimycin A	(ii) Photosystem II
(R) 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)	(iii) K⁺ ionophore
(S) Valinomycin	(iv) F_O ATP synthase

A.P-i, Q-ii, R-iii, S-iv
B.P-iv, Q-i, R-ii, S-iii
C.P-iii, Q-i, R-iv, S-ii
D.P-iv, Q-ii, R-i, S-iii

Exam Year(s)

GATE Life Sciences - 2024,

Answer: B

The correct match is P-iv, Q-i, R-ii, S-iii.

Oligomycin blocks the proton channel of the F_O part of ATP synthase.
Antimycin A blocks the cytochrome bc₁ complex (complex III) at the Q_i site.
DCMU (diuron) binds the Q_B site of photosystem II and stops electron flow from it.
Valinomycin is a K⁺ ionophore that carries potassium across membranes and dissipates the membrane potential.

The other options interchange these targets, for example giving oligomycin to complex III.

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Electrons from cytoplasmic NADH are brought into mitochondria by malate-aspartate shuttle pathway. Select the correct option from below for which this shuttle specifically operate.

A.Skeletal muscle
B.Pancreas
C.Brain
D.Heart and Liver

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: D

The malate-aspartate shuttle operates specifically in heart and liver.

NADH made by glycolysis in the cytosol cannot cross the inner mitochondrial membrane. In the malate-aspartate shuttle cytosolic oxaloacetate is reduced to malate, which enters the mitochondrion and is reoxidised, regenerating NADH inside. The electrons enter the respiratory chain at Complex I, giving about 2.5 ATP per NADH. Heart, liver and kidney rely on this efficient shuttle.

Skeletal muscle and brain mainly use the glycerol 3-phosphate shuttle, which passes the electrons to FAD and so into the chain at ubiquinone, giving only about 1.5 ATP. The pancreas is not the tissue associated with this shuttle.

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Oxysomes for F₀-F₁ particles are present on :

A.Thylakoids
B.Outer mitochondrial membrane
C.Outer chloroplast membrane
D.Inner mitochondrial membrane

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: D

Oxysomes are present on the inner mitochondrial membrane.

Oxysomes, or elementary particles, are the F₀-F₁ particles of ATP synthase. The F₀ part is embedded in the inner mitochondrial membrane and forms a proton channel; the F₁ head projects into the matrix and makes ATP. They line the cristae, where oxidative phosphorylation takes place.

The outer mitochondrial and outer chloroplast membranes carry no ATP synthase. Thylakoids do hold an ATP synthase (CF₀-CF₁), but the term oxysome refers to the mitochondrial particles.

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................................ is/are the only nonprotein component/s of the electron transport system.

A.Cytochromes
B.NAD and FAD
C.Fe-S center
D.Coenzyme-Q

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: D

The only nonprotein component is Coenzyme-Q.

Coenzyme Q (ubiquinone) is a small lipid-soluble quinone that moves freely in the inner mitochondrial membrane, carrying electrons from complexes I and II to complex III. It is the one carrier of the chain that is not a protein or a group bound inside a protein.

Cytochromes are haem proteins and Fe-S centres are iron-sulphur clusters held in proteins. NAD is a soluble coenzyme that feeds electrons into complex I rather than a component of the chain, and FAD (with FMN) is a prosthetic group bound within flavoproteins.

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During respiration correct sequence of electron acceptors in ATP synthesis is :

A.Cyt b, c, a3, a
B.Cyt c, b, a, a3
C.Cyt a3, a, b, c
D.Cyt b, c, a, a3

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: D

The correct sequence is Cyt b, c, a, a3.

Electrons pass along the chain in order of increasing reduction potential. Cytochrome b is part of complex III, which passes electrons through cytochrome c₁ to the mobile cytochrome c. Cytochrome c then delivers them to complex IV, where they pass from cytochrome a to a₃ and finally to oxygen.

The other orders put a₃ before a, or c before b, or reverse the chain entirely.

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Cyanide or azide used in poisoning, irreversibly bind to ................... that acts as a critical link between aerobic respiration and oxygen.

A.Succinate dehydrogenase
B.Coenzyme Q
C.NADH dehydrogenase
D.Cytochrome C Oxidase

Exam Year(s)

MH SET - 2020, SET Exam - 2020,

Answer: D

Cyanide and azide bind to Cytochrome C Oxidase.

Cytochrome c oxidase (complex IV) is the last enzyme of the respiratory chain; it passes electrons to molecular oxygen, reducing it to water. Cyanide and azide bind to the iron of its haem a₃, the site where oxygen binds, and so stop electron flow and aerobic ATP production.

Succinate dehydrogenase (complex II) and NADH dehydrogenase (complex I) feed electrons into the chain, and coenzyme Q carries them to complex III; none of them reacts with oxygen. Their inhibitors are different, such as malonate for complex II and rotenone for complex I.

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