describe the role of NAD and FAD in transferring hydrogen to carriers in the inner mitochondrial membrane

Respiration – Role of NAD+ and FAD in Transferring Hydrogen to Carriers in the Inner Mitochondrial Membrane

Learning Objectives

  • Describe the redox chemistry of NAD+ and FAD and how they become the reduced carriers NADH and FADH2.

  • Identify where NADH and FADH2 are produced in the three‑stage aerobic respiration pathway.

  • Explain how the reduced carriers enter the electron transport chain (ETC) and name all ETC components (Complex I–V, ubiquinone, cytochrome c).

  • Link electron flow to chemiosmosis, the proton‑motive force, and the P/O ratios for ATP synthesis.

  • Understand why fermentation is required when O₂ is absent and how it limits ATP yield.

  • Define the respiratory quotient (RQ), give substrate‑specific values and show how RQ relates to the amount of NADH/FADH2 produced.

  • Carry out a simple calculation of total ATP yield from one molecule of glucose.

1. Where Are the Stages of Respiration Located?

StageCellular LocationKey Enzyme(s) that Produce Reduced Co‑enzymesReduced Co‑enzyme(s) Formed (per glucose)
Glycolysis (Embden‑Meyerhof pathway)CytosolGlyceraldehyde‑3‑phosphate dehydrogenase2 NADH
Link reaction (pyruvate → acetyl‑CoA)Mitochondrial matrixPyruvate dehydrogenase complex2 NADH
Krebs (citric acid) cycleMitochondrial matrixIsocitrate dehydrogenase, α‑ketoglutarate dehydrogenase, succinate dehydrogenase6 NADH + 2 FADH2
Oxidative phosphorylation (ETC & ATP synthase)Inner mitochondrial membraneComplex I–V, ubiquinone, cytochrome cEnergy from NADH/FADH2 used to make ATP

2. Redox Chemistry of the Electron Carriers

The two co‑enzymes act as reversible electron carriers:

NAD+ + 2e⁻ + H⁺ ⇌ NADH  (accepts two electrons and one proton)

FAD + 2e⁻ + 2H⁺ ⇌ FADH2  (accepts two electrons and two protons)

In glycolysis, the link reaction and the Krebs cycle these reductions store the energy of the electrons for transport to the inner mitochondrial membrane.

3. Entry of Electrons into the Electron Transport Chain

The ETC consists of four large protein complexes plus a fifth complex that synthesises ATP. Mobile carriers shuttle electrons between the complexes.

Complex / CarrierSource of ElectronsPrimary FunctionProtons Pumped (per pair of e⁻)
Complex I – NADH‑ubiquinone oxidoreductaseNADH (matrix)Transfers electrons to ubiquinone (Q) → QH24
Complex II – Succinate‑ubiquinone oxidoreductaseFADH2 (produced by succinate dehydrogenase)Reduces Q to QH20 (no proton pumping)
Ubiquinone (Q)Mobile lipid‑soluble carrierShuttles electrons from Complex I or II to Complex III–
Complex III – Cytochrome bc1 complexQH2Transfers electrons to cytochrome c; pumps protons4
Cytochrome cMobile water‑soluble carrierCarries electrons from Complex III to Complex IV–
Complex IV – Cytochrome c oxidaseCytochrome c (reduced)Final electron acceptor: O2 + 4e⁻ + 4H⁺ → 2H2O; pumps protons2
Complex V – ATP synthaseUses the proton‑motive forceADP + Pi + 4H⁺out → ATP + H₂O + 3H⁺in– (synthesises ATP)

Chemiosmosis – Coupling Electron Flow to ATP Synthesis

Electrons travelling from NADH (Complex I) or FADH2 (Complex II) to O₂ release free energy that is used to pump protons from the matrix into the inter‑membrane space. The resulting electrochemical gradient (the proton‑motive force) drives Complex V, producing ATP from ADP and inorganic phosphate.

4. P/O Ratios and Total ATP Yield

Electron DonorComplex of EntryTotal Protons Pumped (per pair of e⁻)P/O Ratio* (ATP per donor)
NADHComplex I4 (I) + 4 (III) + 2 (IV) = 10≈ 2.5 ATP
FADH2Complex II0 (II) + 4 (III) + 2 (IV) = 6≈ 1.5 ATP

*The Cambridge syllabus expects the rounded figures 2.5 ATP per NADH and 1.5 ATP per FADH2.

Sample calculation – ATP from one molecule of glucose (aerobic)

  1. Reduced co‑enzymes produced: 10 NADH + 2 FADH2

  2. ATP from oxidative phosphorylation:

    • 10 NADH × 2.5 ATP = 25 ATP

    • 2 FADH2 × 1.5 ATP = 3 ATP

  3. Substrate‑level phosphorylation:

    • Glycolysis – 2 ATP (net)

    • Krebs cycle – 2 ATP (or GTP) per glucose

  4. Total ATP ≈ 25 + 3 + 2 + 2 = 32 ATP (rounded to 30 ATP in many textbooks; the Cambridge exam marks the 30 ATP figure).

5. Anaerobic Pathways – Why Fermentation Is Needed

When O₂ is unavailable the ETC cannot re‑oxidise NADH. Glycolysis would soon stop because NAD⁺ is depleted. Fermentation provides an alternative electron acceptor, regenerating NAD⁺ and allowing glycolysis to continue.

  • Lactate fermentation (muscle)

    Pyruvate + NADH → Lactate + NAD⁺ (catalysed by lactate dehydrogenase). Net ATP from glycolysis = 2 ATP per glucose.

  • Ethanol fermentation (yeast)

    Pyruvate → Acetaldehyde + CO₂ (pyruvate decarboxylase);

    Acetaldehyde + NADH → Ethanol + NAD⁺ (alcohol dehydrogenase). Net ATP = 2 ATP per glucose.

Because oxidative phosphorylation does not occur, the P/O contribution is zero, reducing the total ATP yield from ~30 ATP (aerobic) to only the 2 ATP obtained by substrate‑level phosphorylation.

6. Respiratory Quotient (RQ)

RQ = CO₂ produced ÷ O₂ consumed. The value depends on the type of substrate oxidised because different substrates generate different numbers of NADH and FADH2.

SubstrateTypical RQReason (relative NADH/FADH2 production)
Glucose (C₆H₁₂O₆)≈ 1.0Complete oxidation yields equal moles of CO₂ and O₂ consumption (6 CO₂, 6 O₂). NADH dominates, giving a high O₂ demand.
Palmitate (C₁₆H₃₂O₂)≈ 0.7β‑oxidation produces relatively more FADH₂ than NADH, requiring less O₂ per CO₂ released.

RQ calculation example – Glucose

Complete aerobic oxidation: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

RQ = 6 CO₂ ÷ 6 O₂ = 1.0

7. Summary – Key Points to Remember

  1. NAD+ and FAD are reversible electron carriers; they become NADH and FADH2 during glycolysis, the link reaction and the Krebs cycle.

  2. NADH donates electrons to Complex I; FADH2 donates to Complex II.

  3. Ubiquinone (Q) shuttles electrons to Complex III; cytochrome c carries them to Complex IV, where O₂ is reduced to H₂O.

  4. Complex V (ATP synthase) uses the proton‑motive force generated by Complexes I, III and IV to synthesise ATP (chemiosmosis).

  5. Proton pumping yields ≈ 10 H⁺ per NADH and ≈ 6 H⁺ per FADH2, giving P/O ratios of ≈ 2.5 ATP/NADH and ≈ 1.5 ATP/FADH2.

  6. Overall aerobic ATP yield from one glucose ≈ 30 ATP (including 4 ATP from substrate‑level phosphorylation).

  7. In the absence of O₂, fermentation regenerates NAD⁺, allowing glycolysis to continue but limiting ATP production to 2 ATP per glucose.

  8. RQ values (glucose ≈ 1.0, fatty acids ≈ 0.7) reflect the differing amounts of NADH and FADH2 generated by each substrate.

Suggested diagram: Schematic of the inner mitochondrial membrane showing Complex I–V, ubiquinone (Q), cytochrome c, the flow of electrons from NADH (entering at I) and FADH₂ (entering at II), proton pumping, and the final reduction of O₂ to H₂O at Complex IV.