MCQs on Biological Oxidation

Dr Ashish Anjankar

High Energy Compounds

Q1. Which of the following is considered the primary high-energy compound in cellular metabolism?
A. AMP
B. ADP
C. ATP
D. cAMP

Q2. High-energy compounds are characterized by:
A. High thermal energy
B. High entropy
C. High free energy of hydrolysis
D. Low pH

Q3. Which of the following has the highest standard free energy of hydrolysis?
A. ATP
B. Creatine phosphate
C. Glucose-6-phosphate
D. Phosphoenolpyruvate (PEP)

Q4. Phosphocreatine acts as an energy reservoir primarily in:
A. Liver
B. Brain
C. Skeletal muscle
D. Adipose tissue

Q5. The high energy of ATP is due to:
A. High pH of the cell
B. Weak phosphate bonds
C. Repulsion between negatively charged phosphate groups
D. Presence of adenosine

Q6. Which of the following is not a high-energy phosphate compound?
A. 1,3-Bisphosphoglycerate
B. Phosphoenolpyruvate
C. Glucose-6-phosphate
D. Creatine phosphate

Q7. Hydrolysis of ATP to ADP releases approximately:
A. 3.4 kcal/mol
B. 5.6 kcal/mol
C. 7.3 kcal/mol
D. 9.8 kcal/mol

Q8. Which of the following is a low-energy phosphate compound?
A. ATP
B. AMP
C. Phosphoenolpyruvate
D. Creatine phosphate

Q9. Which molecule transfers a phosphate group to ADP during substrate-level phosphorylation in glycolysis?
A. Glucose-6-phosphate
B. 3-Phosphoglycerate
C. 1,3-Bisphosphoglycerate
D. Pyruvate

Q10. ATP is structurally composed of:
A. Adenine + glucose + 2 phosphate groups
B. Adenine + ribose + 3 phosphate groups
C. Ribose + deoxyribose + 1 phosphate group
D. Guanine + ribose + 3 phosphate groups

Answer Key:

  1. C. ATP

  2. C. High free energy of hydrolysis

  3. D. Phosphoenolpyruvate (PEP)

  4. C. Skeletal muscle

  5. C. Repulsion between negatively charged phosphate groups

  6. C. Glucose-6-phosphate

  7. C. 7.3 kcal/mol

  8. B. AMP

  9. C. 1,3-Bisphosphoglycerate

  10. B. Adenine + ribose + 3 phosphate groups

Electron Transport Chain and Components

Q1. The electron transport chain is located in which part of the mitochondrion?
A. Outer mitochondrial membrane
B. Inner mitochondrial membrane
C. Mitochondrial matrix
D. Intermembrane space

Q2. The final electron acceptor in the electron transport chain is:
A. NAD⁺
B. FAD
C. Oxygen
D. Water

Q3. Complex I of the ETC is also known as:
A. Cytochrome c oxidase
B. NADH dehydrogenase
C. Succinate dehydrogenase
D. ATP synthase

Q4. Which complex of the ETC does not pump protons?
A. Complex I
B. Complex II
C. Complex III
D. Complex IV

Q5. Which of the following is a mobile electron carrier in the ETC?
A. Complex II
B. Cytochrome c
C. Complex IV
D. ATP synthase

Q6. The function of Complex IV (cytochrome c oxidase) is to:
A. Transfer electrons from NADH to ubiquinone
B. Oxidize succinate
C. Transfer electrons to molecular oxygen
D. Synthesize ATP

Q7. How many protons are pumped into the intermembrane space by Complex I per pair of electrons?
A. 1
B. 2
C. 4
D. 6

Q8. Which of the following complexes accepts electrons directly from FADH₂?
A. Complex I
B. Complex II
C. Complex III
D. Complex IV

Q9. Ubiquinone (coenzyme Q) transfers electrons between:
A. Complex IV and oxygen
B. Complex I/II and Complex III
C. Complex III and Complex IV
D. NADH and oxygen

Q10. The proton gradient generated by the ETC is used to:
A. Generate heat
B. Produce glucose
C. Drive ATP synthesis via ATP synthase
D. Break down fatty acids

Answer Key:

  1. B. Inner mitochondrial membrane
  2. C. Oxygen
  3. B. NADH dehydrogenase
  4. B. Complex II
  5. B. Cytochrome c
  6. C. Transfer electrons to molecular oxygen
  7. C. 4
  8. B. Complex II
  9. B. Complex I/II and Complex III
  10. C. Drive ATP synthesis via ATP synthase

 

Inhibitors of the Electron Transport Chain (ETC)

Q1. Which of the following inhibits Complex I of the electron transport chain?
A. Rotenone
B. Antimycin A
C. Cyanide
D. Oligomycin

Q2. Antimycin A inhibits which complex of the ETC?
A. Complex I
B. Complex II
C. Complex III
D. Complex IV

Q3. Cyanide and carbon monoxide inhibit:
A. Complex I
B. Complex II
C. Complex III
D. Complex IV

Q4. Oligomycin directly inhibits:
A. Cytochrome oxidase
B. Proton pumping
C. ATP synthase
D. Ubiquinone

Q5. Which inhibitor blocks the transfer of electrons from cytochrome b to cytochrome c₁ in Complex III?
A. Rotenone
B. Antimycin A
C. Cyanide
D. Malonate

Q6. Malonate inhibits which enzyme in the ETC pathway?
A. NADH dehydrogenase
B. Cytochrome oxidase
C. ATP synthase
D. Succinate dehydrogenase

Q7. Which of the following is an uncoupler rather than an inhibitor?
A. Rotenone
B. Antimycin A
C. Dinitrophenol (DNP)
D. Cyanide

Q8. Inhibitors of ETC generally result in:
A. Increased ATP production
B. Enhanced oxygen consumption
C. Blocked electron flow and reduced ATP synthesis
D. Activation of glycolysis

Q9. Which inhibitor blocks the binding of oxygen in the electron transport chain?
A. Malonate
B. Dinitrophenol
C. Cyanide
D. Rotenone

Q10. Which of the following statements is TRUE about uncouplers like DNP?
A. They inhibit Complex IV
B. They stop oxygen consumption
C. They prevent proton gradient formation
D. They allow electron transport but block ATP synthesis

Answer Key:

  1. A. Rotenone

  2. C. Complex III

  3. D. Complex IV

  4. C. ATP synthase

  5. B. Antimycin A

  6. D. Succinate dehydrogenase

  7. C. Dinitrophenol (DNP)

  8. C. Blocked electron flow and reduced ATP synthesis

  9. C. Cyanide

  10. D. They allow electron transport but block ATP synthesis

Oxidative Phosphorylation: Sites & Mechanism

Q1. Oxidative phosphorylation occurs in which part of the mitochondrion?
A. Outer membrane
B. Inner membrane
C. Matrix
D. Intermembrane space

Q2. The enzyme complex responsible for ATP synthesis during oxidative phosphorylation is:
A. Complex I
B. Cytochrome c oxidase
C. ATP synthase (Complex V)
D. Succinate dehydrogenase

Q3. What drives the synthesis of ATP in oxidative phosphorylation?
A. Breakdown of glucose
B. Flow of electrons
C. Proton gradient across the inner mitochondrial membrane
D. Hydrolysis of GTP

Q4. The movement of protons back into the mitochondrial matrix through ATP synthase is called:
A. Electron transport
B. Chemiosmosis
C. Substrate-level phosphorylation
D. Oxidation

Q5. Which of the following is not directly involved in oxidative phosphorylation?
A. NADH
B. FADH₂
C. Glucose
D. Oxygen

Q6. The number of ATP molecules produced from oxidation of one NADH via oxidative phosphorylation is approximately:
A. 1
B. 2
C. 2.5
D. 3.5

Q7. Which of the following statements about oxidative phosphorylation is TRUE?
A. It occurs in the cytosol
B. It does not require oxygen
C. It uses a proton motive force to produce ATP
D. It involves substrate-level phosphorylation

Q8. The chemiosmotic theory was proposed by:
A. Urey
B. Peter Mitchell
C. Krebs
D. Pasteur

Q9. During oxidative phosphorylation, oxygen functions as:
A. A proton donor
B. A substrate for ATP synthase
C. The final electron acceptor
D. A coenzyme

Q10. Which of the following is produced at the end of oxidative phosphorylation?
A. CO₂
B. Pyruvate
C. Water and ATP
D. NADH

Answer Key

  1. B. Inner membrane

  2. C. ATP synthase (Complex V)

  3. C. Proton gradient across the inner mitochondrial membrane

  4. B. Chemiosmosis

  5. C. Glucose

  6. C. 2.5

  7. C. It uses a proton motive force to produce ATP

  8. B. Peter Mitchell

  9. C. The final electron acceptor

  10. C. Water and ATP

 

Uncouplers and Inhibitors of Oxidative Phosphorylation

Q1. Which of the following is a classical uncoupler of oxidative phosphorylation?
A. Oligomycin
B. Rotenone
C. Dinitrophenol (DNP)
D. Antimycin A

Q2. Uncouplers of oxidative phosphorylation:
A. Inhibit electron transport
B. Prevent formation of proton gradient
C. Allow ATP synthesis without electron transport
D. Allow electron transport but prevent ATP synthesis

Q3. Oligomycin inhibits oxidative phosphorylation by:
A. Inhibiting Complex I
B. Uncoupling ATP synthesis from electron transport
C. Blocking ATP synthase
D. Inhibiting cytochrome oxidase

Q4. Which of the following is not an uncoupler?
A. Dinitrophenol
B. Thermogenin
C. Valinomycin
D. Cyanide

Q5. Which of the following is true about uncouplers?
A. They block electron flow in the ETC
B. They prevent oxygen consumption
C. They dissipate the proton gradient
D. They increase ATP production

Q6. Cyanide poisoning inhibits oxidative phosphorylation by:
A. Blocking Complex II
B. Blocking ATP synthase
C. Inhibiting cytochrome c oxidase (Complex IV)
D. Uncoupling electron transport

Q7. Thermogenin (UCP-1) is a physiological uncoupler found in:
A. Liver
B. Skeletal muscle
C. Brown adipose tissue
D. White adipose tissue

Q8. What happens to oxygen consumption in the presence of an uncoupler like DNP?
A. Decreases
B. Stops
C. Remains unchanged
D. Increases

Q9. Which of the following is an ATP synthase inhibitor?
A. Thermogenin
B. Valinomycin
C. Oligomycin
D. DNP

Q10. Uncouplers generate heat because:
A. They accelerate glycolysis
B. They inhibit ATP synthase
C. They allow protons to leak back into the matrix, bypassing ATP synthase
D. They stimulate fatty acid oxidation

Answer Key

  1. C. Dinitrophenol (DNP)

  2. D. Allow electron transport but prevent ATP synthesis

  3. C. Blocking ATP synthase

  4. D. Cyanide

  5. C. They dissipate the proton gradient

  6. C. Inhibiting cytochrome c oxidase (Complex IV)

  7. C. Brown adipose tissue

  8. D. Increases

  9. C. Oligomycin

  10. C. They allow protons to leak back into the matrix, bypassing ATP synthase

 

Substrate-Level Phosphorylation

Q1. Substrate-level phosphorylation involves the direct transfer of:
A. ADP to NADH
B. A phosphate group to ADP to form ATP
C. Electrons to oxygen
D. Phosphate to glucose

Q2. Which of the following is a site of substrate-level phosphorylation in glycolysis?
A. Hexokinase reaction
B. Phosphofructokinase-1 reaction
C. 1,3-Bisphosphoglycerate to 3-Phosphoglycerate
D. Glucose to glucose-6-phosphate

Q3. Substrate-level phosphorylation occurs in which of the following cellular locations?
A. Only in mitochondria
B. Only in cytoplasm
C. In both cytoplasm and mitochondria
D. Only in the endoplasmic reticulum

Q4. Which of the following reactions in the TCA cycle is an example of substrate-level phosphorylation?
A. Isocitrate → α-ketoglutarate
B. Succinyl-CoA → Succinate
C. Malate → Oxaloacetate
D. Citrate → Isocitrate

Q5. Which enzyme is responsible for ATP production from 1,3-bisphosphoglycerate in glycolysis?
A. Hexokinase
B. Pyruvate kinase
C. Phosphoglycerate kinase
D. Enolase

Q6. Substrate-level phosphorylation differs from oxidative phosphorylation in that it:
A. Requires oxygen
B. Occurs only in mitochondria
C. Involves direct phosphate transfer from a high-energy intermediate
D. Requires electron transport chain

Q7. In glycolysis, how many ATP molecules are generated by substrate-level phosphorylation per glucose molecule?
A. 2
B. 4
C. 6
D. 8

Q8. Which of the following compounds can directly donate a phosphate group to ADP in substrate-level phosphorylation?
A. Glucose-6-phosphate
B. Pyruvate
C. 1,3-Bisphosphoglycerate
D. Lactate

Q9. Which of the following enzymes catalyzes substrate-level phosphorylation in the final step of glycolysis?
A. Aldolase
B. Pyruvate kinase
C. Enolase
D. Phosphoglucomutase

Q10. Substrate-level phosphorylation in the TCA cycle produces:
A. NADH
B. GTP or ATP
C. FADH₂
D. CO₂

Answer Key

  1. B. A phosphate group to ADP to form ATP

  2. C. 1,3-Bisphosphoglycerate to 3-Phosphoglycerate

  3. C. In both cytoplasm and mitochondria

  4. B. Succinyl-CoA → Succinate

  5. C. Phosphoglycerate kinase

  6. C. Involves direct phosphate transfer from a high-energy intermediate

  7. B. 4

  8. C. 1,3-Bisphosphoglycerate

  9. B. Pyruvate kinase

  10. B. GTP or ATP

Revision MCQs

  1. Which complex of the mitochondrial electron transport chain transfers electrons from NADH to ubiquinone?
    A. Complex I (NADH dehydrogenase)
    B. Complex II (Succinate dehydrogenase)
    C. Complex III (Cytochrome bc1)
    D. Complex IV (Cytochrome c oxidase)

  2. In oxidative phosphorylation, the proton gradient across the inner mitochondrial membrane is used by which enzyme to synthesise ATP?
    A. ATP synthase (FoF1 ATPase)
    B. Cytochrome c oxidase
    C. Succinate dehydrogenase
    D. NADH dehydrogenase

  3. Which of the following is not a mobile electron carrier in the mitochondrial ETC?
    A. Ubiquinone (Coenzyme Q)
    B. Cytochrome c
    C. Iron‑sulfur protein
    D. Cytochrome P450

  4. Where in mitochondria does the TCA cycle occur?
    A. Outer membrane
    B. Intermembrane space
    C. Mitochondrial matrix
    D. Inner membrane

  5. Which molecule serves both as a substrate in TCA cycle and feeds electrons into ETC via Complex II?
    A. Fumarate
    B. Succinate
    C. α‑Ketoglutarate
    D. Oxaloacetate

  6. The respiratory control (rate of oxygen consumption) is primarily regulated by:
    A. NADH concentration alone
    B. Availability of ADP (ADP phosphorylation)
    C. Concentration of oxygen in the cytosol
    D. Availability of FAD

  7. Which inhibitor blocks electron flow at Complex IV (cytochrome c oxidase)?
    A. Rotenone
    B. Antimycin A
    C. Cyanide (CN⁻)
    D. Malonate

  8. What is the P/O ratio (ATP molecules / O atom reduced to water) approximately for NADH oxidation via the mitochondrial ETC?
    A. ~1.5
    B. ~2.5
    C. ~3.0
    D. ~4.0

  9. Which enzyme catalyses the conversion of pyruvate to acetyl CoA and is inhibited by arsenic compounds?
    A. Pyruvate dehydrogenase complex
    B. Pyruvate carboxylase
    C. Lactate dehydrogenase
    D. Pyruvate kinase

  10. Which of the following statements about its coenzyme is correct for the enzyme in Q9?
    A. It requires thiamine pyrophosphate, lipoic acid, FAD, NAD⁺ and CoA.
    B. It requires NADPH instead of NAD⁺.
    C. It uses biotin as essential cofactor.
    D. It uses pyridoxal phosphate as cofactor.

  11. In the electron transport chain, which complex does not pump protons across the inner mitochondrial membrane?
    A. Complex I
    B. Complex II
    C. Complex III
    D. Complex IV

  12. Which of the following molecules has the highest standard reduction potential (E°)?
    A. NAD⁺/NADH
    B. FAD/FADH₂
    C. Ubiquinone/ubiquinol (Q/QH₂)
    D. O₂/H₂O

  13. The main site of reactive oxygen species (ROS) generation in mitochondria is:
    A. Complex I and III leak of electrons
    B. Complex II overload
    C. ATP synthase malfunction
    D. Pyruvate dehydrogenase complex

  14. Which compound is a competitive inhibitor of succinate dehydrogenase (Complex II)?
    A. Cyanide
    B. Malonate
    C. Antimycin A
    D. Rotenone

  15. Which of the following is true about uncouplers of oxidative phosphorylation (like 2,4‑dinitrophenol)?
    A. They inhibit electron transfer in Complex III.
    B. They increase the proton gradient.
    C. They dissipate proton gradient, increasing respiration but reducing ATP yield.
    D. They block ATP synthase’s catalytic subunit.

  16. In the TCA cycle, which step produces GTP (or ATP in some organisms)?
    A. Conversion of isocitrate to α‑ketoglutarate
    B. Oxidation of succinate to fumarate
    C. Conversion of succinyl CoA to succinate
    D. Oxidation of malate to oxaloacetate

  17. What happens to the respiratory control ratio (RCR) in mitochondria when ADP is added (assuming oxygen and substrate are not limiting)?
    A. RCR decreases
    B. RCR increases
    C. RCR remains same
    D. Cannot predict

  18. Which coenzyme donates electrons into the ETC at a point downstream of Complex I?
    A. NADH
    B. FADH₂
    C. Cytochrome c
    D. Cytochrome a₃

  19. In a patient with cyanide poisoning, which among the following remains reduced?
    A. Cytochrome c oxidase (Complex IV)
    B. Cytochrome c
    C. Ubiquinone
    D. NAD⁺

  20. Which enzyme of TCA cycle is inhibited by fluoroacetate (via conversion to fluorocitrate)?
    A. Aconitase
    B. Isocitrate dehydrogenase
    C. α‑Ketoglutarate dehydrogenase
    D. Succinate dehydrogenase

Answer Key

  1. A

  2. A

  3. D

  4. C

  5. B

  6. B

  7. C

  8. B

  9. A

  10. A

  11. B

  12. D

  13. A

  14. B

  15. C

  16. C

  17. B

  18. B

  19. A

  20. A

 

Case Based and Analytical MCQs

Q1.

A 22-year-old male experiences muscle cramps and weakness after intense exercise. Laboratory findings reveal elevated lactate levels. Which mitochondrial complex is most likely under-functioning, impairing oxidative phosphorylation?

A. Complex I (NADH dehydrogenase)
B. Complex II (Succinate dehydrogenase)
C. Complex IV (Cytochrome c oxidase)
D. ATP Synthase

Q2.

A patient was administered rotenone during a mitochondrial experiment. Which of the following statements best describes the effect of this drug?

A. It blocks the transfer of electrons from FADH₂ to CoQ
B. It uncouples oxidative phosphorylation by increasing membrane permeability
C. It inhibits Complex I, preventing NADH electron transfer to CoQ
D. It directly inhibits ATP synthase

Q3.

A 3-month-old infant presents with hypotonia, lactic acidosis, and poor feeding. Biochemical tests suggest a mitochondrial disorder involving Complex IV. Which of the following enzymes is primarily affected?

A. NADH dehydrogenase
B. Succinate dehydrogenase
C. Cytochrome c oxidase
D. ATP synthase

Q4.

Which of the following changes would most likely result from inhibition of ATP synthase by oligomycin?

A. Increased oxygen consumption
B. Accumulation of protons in the mitochondrial matrix
C. Decreased proton gradient across inner mitochondrial membrane
D. Decreased ATP production despite normal electron flow

Q5.

An experimental drug acts as an uncoupler of oxidative phosphorylation. Which of the following best explains its mechanism?

A. It inhibits Complex III, blocking electron flow
B. It allows protons to bypass ATP synthase, dissipating the proton gradient
C. It increases the affinity of cytochrome oxidase for oxygen
D. It enhances NADH production from the TCA cycle

Q6.

During a research study, a compound is found to specifically inhibit the transfer of electrons from Complex III to Complex IV. Which molecule is most likely targeted?

A. Coenzyme Q
B. Cytochrome b
C. Cytochrome c
D. FAD

Q7.

A patient suffers from a rare deficiency of cytochrome c. Which of the following processes would be directly impaired?

A. Oxidation of succinate to fumarate
B. Transport of electrons from Complex III to Complex IV
C. Formation of NADH in glycolysis
D. Beta-oxidation of fatty acids

Q8.

A mitochondrion is exposed to dinitrophenol (DNP). What is the most likely immediate biochemical outcome?

A. Increased proton gradient
B. Enhanced ATP synthesis
C. Decreased oxygen consumption
D. Dissipation of proton motive force

Q9.

Which of the following correctly pairs an ETC complex with its electron donor?

A. Complex I – FADH₂
B. Complex II – NADH
C. Complex III – CoQ
D. Complex IV – Succinate

Q10.

A 45-year-old man presents with cyanide poisoning. Cyanide acts by inhibiting which of the following enzymes in the ETC?

A. NADH dehydrogenase
B. Cytochrome c oxidase
C. Succinate dehydrogenase
D. ATP synthase

✅ Answer Key:

  1. C. Complex IV (Cytochrome c oxidase)

  2. C. It inhibits Complex I, preventing NADH electron transfer to CoQ

  3. C. Cytochrome c oxidase

  4. D. Decreased ATP production despite normal electron flow

  5. B. It allows protons to bypass ATP synthase, dissipating the proton gradient

  6. C. Cytochrome c

  7. B. Transport of electrons from Complex III to Complex IV

  8. D. Dissipation of proton motive force

  9. C. Complex III – CoQ

  10. B. Cytochrome c oxidase

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