Oxidative Phosphorylation

This lesson is mapped to AQA 7402 Section 3.5.1 — Aerobic respiration: oxidative phosphorylation and chemiosmosis (refer to the official AQA specification document for exact wording). Oxidative phosphorylation is the final and quantitatively dominant stage of aerobic respiration: it generates approximately 26 of the ~32 ATP produced per glucose, dwarfing the output of glycolysis (2 ATP), the link reaction (0 ATP), and the Krebs cycle's substrate-level phosphorylation (2 ATP). It takes place on the inner mitochondrial membrane and couples two physically distinct events — electron flow along the electron transport chain (ETC) and proton flow through ATP synthase — through a transmembrane proton electrochemical gradient.

The conceptual breakthrough that united these events was made by Peter Mitchell in 1961, who proposed (paraphrased) that the energy released by electron transport is captured first as a transmembrane gradient of protons (a chemiosmotic gradient), and only then converted into ATP. This was a radical departure from the "high-energy chemical intermediate" model that had dominated the previous twenty years, and was initially controversial. Mitchell received the 1978 Nobel Prize in Chemistry for what is now universally accepted as the chemiosmotic hypothesis. The structural confirmation came in 1994–1997, when John Walker crystallised ATP synthase and demonstrated its rotary mechanism, sharing the 1997 Nobel Prize.

Key Definition: Oxidative phosphorylation is the synthesis of ATP from ADP and inorganic phosphate, driven by the transfer of high-energy electrons (from NADH and FADH₂) along the electron transport chain on the inner mitochondrial membrane and the resulting chemiosmotic proton gradient that powers ATP synthase.

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Mitochondrial Architecture: Why the Inner Membrane?

The mitochondrion is bounded by two membranes:

• The outer mitochondrial membrane is freely permeable to small molecules (up to ~5 kDa) because of channel proteins called porins. It does not maintain a significant ion gradient.
• The inner mitochondrial membrane is selectively impermeable to ions, and is the location of the ETC complexes and ATP synthase. It is folded extensively into cristae, which dramatically increase its surface area.

The two membranes enclose two compartments:

• The intermembrane space (between the inner and outer membranes) — a narrow compartment in which protons accumulate when pumped out of the matrix.
• The matrix (enclosed by the inner membrane) — contains the Krebs cycle enzymes, link reaction enzymes (PDH complex), mitochondrial DNA, 70S ribosomes, and substrates.

The selective impermeability of the inner mitochondrial membrane is the load-bearing structural feature: it allows a steep proton gradient to be maintained and forces protons to return to the matrix only through ATP synthase. Without this impermeability, the gradient would simply leak away and no ATP would be made.

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The Electron Transport Chain (ETC)

The ETC consists of four major protein complexes (I, II, III, IV) plus two mobile electron carriers (ubiquinone and cytochrome c), all embedded in or associated with the inner mitochondrial membrane. The complexes contain prosthetic groups capable of cyclic oxidation and reduction: iron-sulphur clusters, haem groups (in the cytochromes), and copper centres (in Complex IV). Electrons flow "downhill" through this sequence of carriers in order of increasing reduction potential (increasing affinity for electrons), releasing free energy at each step.

Key Electron Carriers

• Complex I (NADH dehydrogenase) — accepts electrons from NADH; pumps 4 H⁺ across the inner membrane per electron pair.
• Complex II (Succinate dehydrogenase) — accepts electrons from FADH₂ (this complex is also step 6 of the Krebs cycle, embedded in the membrane); does not pump protons.
• Ubiquinone (Coenzyme Q) — a small, lipid-soluble mobile carrier that transfers electrons from Complexes I and II to Complex III. Its hydrophobicity allows it to diffuse rapidly within the lipid bilayer.
• Complex III (Cytochrome bc₁ complex) — passes electrons from ubiquinone to cytochrome c; pumps 4 H⁺ per electron pair.
• Cytochrome c — a small water-soluble haem protein that shuttles electrons along the outer face of the inner membrane from Complex III to Complex IV.
• Complex IV (Cytochrome c oxidase) — the terminal electron carrier; passes electrons to oxygen; pumps 2 H⁺ per electron pair.

How the ETC Works

1. NADH donates its two electrons to Complex I. As electrons traverse Complex I, energy is released and used to pump four H⁺ ions from the mitochondrial matrix into the intermembrane space.

2. FADH₂ (bound to Complex II / succinate dehydrogenase) donates its two electrons to Complex II. Complex II does not pump protons — this is the structural reason FADH₂ generates fewer ATP than NADH.

3. Electrons from both Complex I and Complex II are passed to ubiquinone (Q), then to Complex III, which pumps four more H⁺ per electron pair via the Q-cycle mechanism.

4. Electrons pass via cytochrome c to Complex IV, which pumps two further H⁺ per electron pair.

5. At Complex IV, electrons are finally accepted by oxygen (O₂), the terminal electron acceptor. Oxygen combines with protons from the matrix to form water:

   ½O₂ + 2H⁺ + 2e⁻ → H₂O

   This is the only water produced by aerobic respiration and is sometimes called "metabolic water". In humans this generates approximately 300 mL of metabolic water per day.

graph LR
    A["NADH"] -->|"2e⁻"| B["Complex I<br/>pumps 4H⁺"]
    C["FADH₂"] -->|"2e⁻"| D["Complex II<br/>pumps 0H⁺"]
    B -->|"2e⁻"| E["Ubiquinone (Q)"]
    D -->|"2e⁻"| E
    E -->|"2e⁻"| F["Complex III<br/>pumps 4H⁺"]
    F -->|"2e⁻"| G["Cytochrome c"]
    G -->|"2e⁻"| H["Complex IV<br/>pumps 2H⁺"]
    H -->|"2e⁻ + 2H⁺ + ½O₂"| I["H₂O"]

    style B fill:#3498db,color:#fff
    style F fill:#27ae60,color:#fff
    style H fill:#e67e22,color:#fff
    style I fill:#9b59b6,color:#fff

Key Definition: Oxygen is the terminal (final) electron acceptor in the ETC. Without oxygen, the chain stops, the proton gradient collapses, and oxidative phosphorylation halts within seconds.

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Chemiosmosis and ATP Synthesis

The Proton Electrochemical Gradient (Proton Motive Force)

As electrons pass along the ETC, the energy released is used to actively pump H⁺ ions from the matrix into the intermembrane space. This creates two coupled gradients:

• A chemical gradient — higher [H⁺] in the intermembrane space than in the matrix. The pH of the intermembrane space is approximately 7.0, that of the matrix approximately 7.8 — a 0.8 pH unit difference, which corresponds to a ~6-fold concentration difference in [H⁺].
• An electrical gradient (membrane potential) — the intermembrane space becomes more positively charged (typical ΔΨ ≈ –150 to –180 mV, matrix-negative).

Together these constitute the proton motive force (Δp), a form of stored chemical and electrical energy. Mitchell's central insight (paraphrased) was that this gradient is the immediate energy source for ATP synthesis — not any phosphorylated chemical intermediate.

ATP Synthase: A Rotary Molecular Motor

The inner mitochondrial membrane is impermeable to H⁺ ions because the phospholipid bilayer presents a high-energy barrier to charged species. The only physiological route for protons to return to the matrix is through the channel of ATP synthase, a large multi-subunit enzyme spanning the inner membrane. ATP synthase has two structural sub-assemblies:

• The F₀ portion is embedded in the membrane and contains the proton channel. A ring of c-subunits rotates as protons flow through it.
• The F₁ portion projects into the matrix and contains three catalytic β-subunits surrounding a central γ-subunit. Rotation of γ (driven by F₀) cycles each β-subunit through three conformations (binding ADP + Pi, forming ATP, releasing ATP).

Approximately 4 protons flow through ATP synthase per ATP synthesised. This stoichiometry, combined with the proton-pumping ratios of the ETC complexes, gives rise to the ~2.5 ATP per NADH and ~1.5 ATP per FADH₂ yields.

Key Definition: Chemiosmosis is the movement of H⁺ ions down their electrochemical gradient through ATP synthase, driving the phosphorylation of ADP to ATP. The term was coined by Peter Mitchell to emphasise the dual nature of the gradient (chemical and electrical).

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NADH vs FADH₂: ATP Yield

Coenzyme	Entry point	Proton-pumping complexes used	Approximate ATP yield per molecule
NADH	Complex I	Complexes I, III, IV (10 H⁺ pumped)	~2.5 ATP
FADH₂	Complex II	Complexes III, IV only (6 H⁺ pumped)	~1.5 ATP

FADH₂ produces fewer ATP because it enters the chain "downstream" of Complex I — its electrons bypass the first proton-pumping step. The structural reason is thermodynamic: FAD has a higher reduction potential than NAD⁺, so FADH₂ carries lower-energy electrons that cannot drive the Complex I pump.

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Total ATP Yield per Glucose Molecule

Stage	ATP by SLP	NADH produced	FADH₂ produced
Glycolysis	2	2 (cytoplasmic)	0
Link reaction (×2)	0	2	0
Krebs cycle (×2)	2	6	2
Total	4	10	2

Calculating Total ATP (Theoretical Maximum)

• 10 NADH × 2.5 ATP = 25 ATP
• 2 FADH₂ × 1.5 ATP = 3 ATP
• 4 ATP from substrate-level phosphorylation (glycolysis + Krebs)
• Total ≈ 32 ATP per glucose (theoretical maximum)

Note: The theoretical maximum of 32 ATP is rarely achieved in practice because:

• Some energy from the proton gradient is used for transport processes (importing pyruvate and Pi into the mitochondria; exporting ATP via the ATP/ADP translocase).
• The 2 NADH from glycolysis are produced in the cytoplasm and must be shuttled into the mitochondria. The malate–aspartate shuttle (used in heart, kidney, liver) delivers them to NAD⁺ in the matrix without loss. The glycerol-phosphate shuttle (used in skeletal muscle, brain) delivers them to FAD in the inner membrane, effectively reducing their yield from ~2.5 to ~1.5 ATP each.
• Proton leakage across the inner membrane reduces the gradient slightly (basal "uncoupling").
• Older textbooks quote 36–38 ATP based on outdated stoichiometric assumptions; the modern consensus is ~30–32.

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The Role of Oxygen

Oxygen is absolutely essential for oxidative phosphorylation:

1. As the terminal electron acceptor — Oxygen accepts electrons at Complex IV and combines with protons to form water. Without oxygen, electrons cannot be removed from the chain.
2. If oxygen is absent:
   • Electrons accumulate on the upstream carriers (they become fully reduced and cannot accept further electrons from NADH or FADH₂).
   • The ETC stops.
   • H⁺ ions are no longer pumped, and the existing gradient dissipates within seconds.
   • ATP synthase cannot continue chemiosmosis without the gradient.
   • Reduced NAD and reduced FAD cannot be reoxidised, so the Krebs cycle and link reaction halt (they require oxidised NAD⁺ and FAD).
   • Only glycolysis can continue, and only if NAD⁺ is regenerated by fermentation (covered in lesson 4).

This is why mammalian tissues are exquisitely sensitive to hypoxia. The brain's reliance on aerobic respiration is so complete that ~5 minutes of anoxia causes irreversible neuronal damage.

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Inhibitors and Uncouplers

ETC Inhibitors

• Cyanide (CN⁻) and carbon monoxide (CO) bind to the haem iron and copper in Complex IV, preventing electron transfer to oxygen. The ETC backs up; the proton gradient collapses; ATP production halts. Cyanide toxicity is rapid and frequently fatal.
• Rotenone (a botanical pesticide and piscicide) inhibits Complex I, blocking electron transfer from NADH. Electrons can still enter via Complex II from FADH₂, so toxicity is partial in some tissues.
• Antimycin A inhibits Complex III.

Uncouplers

• DNP (2,4-dinitrophenol) is a small lipophilic weak acid that can carry protons across the inner mitochondrial membrane (uncoupling the proton gradient from ATP synthesis). The proton gradient is dissipated as heat rather than used to make ATP. The ETC continues to function (electrons still flow to oxygen), but no ATP is produced. DNP was briefly used as a weight-loss drug in the 1930s — and caused multiple deaths from runaway hyperthermia.
• Uncoupling protein 1 (UCP1, thermogenin) in brown adipose tissue performs the same function physiologically: it generates heat for non-shivering thermogenesis in neonates and small mammals.

Oligomycin

• Blocks the H⁺ channel in the F₀ portion of ATP synthase directly. The proton gradient builds up but cannot be used, so ATP synthesis stops. The ETC also slows because the gradient becomes too steep for further pumping (back-pressure inhibition).

graph TD
    A["Inner mitochondrial membrane"] --> B["ETC pumps H⁺ out"]
    B --> C["Steep H⁺ gradient<br/>(intermembrane space)"]
    C --> D["H⁺ returns via ATP synthase"]
    D --> E["ATP synthesised"]
    F["Cyanide / CO"] -.->|"blocks Complex IV"| B
    G["Rotenone"] -.->|"blocks Complex I"| B
    H["DNP / UCP1"] -.->|"dissipates gradient as heat"| C
    I["Oligomycin"] -.->|"blocks F₀ channel"| D

    style C fill:#3498db,color:#fff
    style E fill:#27ae60,color:#fff

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Synoptic Links

This lesson connects to:

1. AQA 7402 Section 3.6.3 — Thermoregulation: brown adipose tissue uses UCP1 to dissipate the proton gradient as heat, generating non-shivering thermogenesis in neonates. The same biochemistry that makes ATP also makes body heat.
2. AQA 7402 Section 3.1.1 — Inorganic ions: Fe²⁺/Fe³⁺ in cytochromes and Cu in Complex IV are absolute prerequisites for electron transport. Iron-deficiency anaemia limits ETC capacity.
3. AQA 7402 Section 3.5.2 — Photophosphorylation: chloroplast ATP synthesis uses an identical chemiosmotic mechanism on the thylakoid membrane — the same Mitchell logic, the same ATP synthase architecture, with the proton gradient directed lumen-to-stroma instead of intermembrane-space-to-matrix (covered comparatively in lesson 9).

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Common Errors and Mark-Loss Patterns