Q.38 When freshly isolated intact mitochondria were incubated with ADP and inorganic phosphate
neither the oxygen consumption nor the ATP synthesis could be detected. Addition of succinate resulted
in increased oxygen consumption as well as ATP synthesis with time. Subsequent addition of cyanide to
this system will result in which one of the following?
(A) Both oxygen consumption and ATP synthesis are inhibited
(B) Oxygen consumption continues but ATP synthesis is inhibited
(C) Oxygen consumption is inhibited but ATP synthesis continues
(D) Both oxygen consumption and ATP synthesis continue

You’re dealing with a classic oxidative phosphorylation and ETC inhibition question. Let’s turn this into a clear, SEO-friendly explanation article.

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Introduction

In mitochondrial bioenergetics, understanding how inhibitors affect the electron transport chain (ETC) and oxidative phosphorylation is crucial for exams like CSIR NET, GATE, university entrances, and medical biochemistry.

This question explores what happens when cyanide is added to a mitochondrial preparation that is actively consuming oxygen and synthesizing ATP using succinate as a substrate.

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Question Restatement

Freshly isolated intact mitochondria are incubated with ADP and inorganic phosphate (Pi). Initially:

• No oxygen consumption

• No ATP synthesis

When succinate is added:

• Oxygen consumption increases

• ATP synthesis increases

Then, cyanide is added. What happens next?

Options:
(A) Both oxygen consumption and ATP synthesis are inhibited
(B) Oxygen consumption continues but ATP synthesis is inhibited
(C) Oxygen consumption is inhibited but ATP synthesis continues
(D) Both oxygen consumption and ATP synthesis continue

Correct Answer: (A) Both oxygen consumption and ATP synthesis are inhibited

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Concept Overview: Mitochondria, Succinate, and Cyanide

To understand the correct option, you need to recall:

• Succinate is a substrate of Complex II of the electron transport chain. It donates electrons via FADH22 into the ETC.

• Electrons flow:
  Succinate → Complex II → CoQ → Complex III → Cytochrome c → Complex IV → O22

• Oxygen consumption reflects electron flow to oxygen (final electron acceptor).

• ATP synthesis depends on the proton gradient generated by complexes I, III, and IV.

Cyanide (CN⁻) is a potent inhibitor of Complex IV (cytochrome c oxidase). It blocks the transfer of electrons from cytochrome c to oxygen.

When Complex IV is blocked:

• Electron flow stops.

• Oxygen is no longer reduced to water → O22 consumption stops.

• Proton pumping stops → proton motive force collapses → ATP synthesis stops.

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Step-by-Step Interpretation of the Experimental Setup

1. Initial state: Mitochondria + ADP + Pi (no substrate)

   • No fuel (like succinate, malate, or NADH-linked substrate) → no electrons enter the ETC.

   • Result:

     • No electron transport

     • No oxygen consumption

     • No ATP synthesis

2. After adding succinate

   • Succinate donates electrons via Complex II.

   • Electron transport through Complex II → CoQ → III → IV.

   • Complexes III and IV pump protons → proton gradient builds.

   • ATP synthase uses this gradient → ATP synthesis increases.

   • Continuous electron flow to O22 → oxygen consumption increases.

3. Addition of cyanide (CN⁻)

   • Cyanide binds to Complex IV (cytochrome c oxidase).

   • Electron transfer from cytochrome c to O22 is blocked.

   • Entire electron flow upstream backs up because the terminal step is blocked.

   Consequences:

   • No electrons reach O22 → O22 consumption stops.

   • Proton pumping via Complex IV stops; electron flow through III also halts → no new proton gradient formed.

   • Without sufficient proton motive force → ATP synthesis stops.

Therefore, both oxygen consumption and ATP synthesis are inhibited.

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Explanation of Each Option

(A) Both oxygen consumption and ATP synthesis are inhibited – Correct

• Cyanide inhibits Complex IV.

• Result:

  • Electron transport stops.

  • Oxygen is no longer reduced to water → O22 usage stops.

  • Proton gradient is no longer maintained → no ATP synthesis via oxidative phosphorylation.

This matches the known biochemical effect of cyanide on mitochondria.

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(B) Oxygen consumption continues but ATP synthesis is inhibited – Incorrect

This pattern would suggest:

• Electron transport continues.

• Proton gradient is uncoupled from ATP synthesis (e.g., due to uncouplers like DNP or CCCP).

In an uncoupled state:

• Oxygen consumption is high (ETC runs fast).

• ATP synthesis is low or absent.

However, cyanide is not an uncoupler. It is a respiratory chain inhibitor, specifically at Complex IV. Thus, it blocks electron flow itself, not just ATP synthesis. So both O22 consumption and ATP production stop, not just ATP.

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(C) Oxygen consumption is inhibited but ATP synthesis continues – Incorrect

This scenario is not possible for oxidative phosphorylation because:

• ATP synthesis by ATP synthase depends on the proton gradient generated by ETC activity.

• If oxygen consumption (and therefore electron transport) is inhibited, the proton gradient cannot be maintained.

• Without this gradient, ATP synthase cannot keep producing ATP via oxidative phosphorylation.

So, if oxygen consumption stops, ATP synthesis by mitochondria also stops. This option contradicts the coupling mechanism of oxidative phosphorylation.

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(D) Both oxygen consumption and ATP synthesis continue – Incorrect

This would imply that cyanide:

• Has no effect on the ETC or ATP synthesis.

But cyanide is a well-known poison that blocks mitochondrial respiration by inhibiting Complex IV. It causes rapid cessation of:

• Oxygen consumption

• ATP synthesis

Therefore, this option is incompatible with basic mitochondrial bioenergetics.

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Key Takeaways

• Cyanide inhibits Complex IV (cytochrome c oxidase).

• It blocks electron flow to oxygen, so oxygen consumption stops.

• The proton gradient collapses, and ATP synthesis stops.

• Thus, when cyanide is added after succinate in intact mitochondria, both oxygen consumption and ATP synthesis are inhibited → Option (A).