Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation is where most of the ATP of aerobic respiration is actually made. OCR specification module 5.2.2(e) requires you to describe the electron transport chain, the role of oxygen as the final electron acceptor, and how ATP is synthesised by chemiosmosis. This is arguably the most elegant process in biochemistry: the controlled flow of electrons down an energy gradient is used to pump protons, and the return flow of protons drives the rotation of ATP synthase.

Key Definitions:

• Oxidative phosphorylation — the synthesis of ATP driven by the oxidation of reduced NAD and reduced FAD by the electron transport chain.
• Electron transport chain (ETC) — a series of electron carriers in the inner mitochondrial membrane that transfer electrons from reduced coenzymes to oxygen.
• Chemiosmosis — ATP synthesis driven by the diffusion of H⁺ down its electrochemical gradient through ATP synthase.
• Proton motive force — the combination of the H⁺ gradient and the membrane potential across the inner mitochondrial membrane.
• Final electron acceptor — oxygen, which accepts electrons and protons to form water.

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Overview

Up to this point, respiration has produced only a small amount of ATP directly (4 ATP by substrate-level phosphorylation, 2 in glycolysis and 2 in Krebs). The bulk of energy is stored in reduced NAD (10) and reduced FAD (2) per glucose. Oxidative phosphorylation converts this stored reducing power into ATP — approximately 26–28 more ATP — making it by far the biggest ATP-producing stage.

flowchart LR
    RNAD[Reduced NAD] -->|H and 2e-| C1[Complex I]
    RFAD[Reduced FAD] -->|H and 2e-| C2[Complex II]
    C1 --> Q[Ubiquinone]
    C2 --> Q
    Q --> C3[Complex III]
    C3 --> CytC[Cytochrome c]
    CytC --> C4[Complex IV]
    C4 --> O2[O2 + 4H+ -> 2 H2O]
    C1 -. H+ pumped .-> IMS[Intermembrane space - H+]
    C3 -. H+ pumped .-> IMS
    C4 -. H+ pumped .-> IMS
    IMS -->|Flows down gradient| ATPS[ATP synthase]
    ATPS --> ATP[ATP made in matrix]

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The Electron Transport Chain

The ETC is a series of electron carriers embedded in the inner mitochondrial membrane. Electrons are passed from one carrier to the next in order of increasing electron affinity, releasing energy at each step. This energy is used to pump H⁺ ions from the matrix into the intermembrane space.

Main Components

Component	Role
NADH dehydrogenase (Complex I)	Accepts electrons from reduced NAD; pumps H⁺
Succinate dehydrogenase (Complex II)	Accepts electrons from reduced FAD (does not pump H⁺)
Ubiquinone (coenzyme Q)	Mobile carrier — shuttles electrons from I/II to III
Cytochrome b-c1 (Complex III)	Receives electrons from ubiquinone; pumps H⁺
Cytochrome c	Mobile carrier — shuttles electrons from III to IV
Cytochrome c oxidase (Complex IV)	Passes electrons to O₂; pumps H⁺
ATP synthase (Complex V)	Channel + enzyme that synthesises ATP from H⁺ flow

OCR does not require you to name every complex — but you should know that electrons pass through a series of carriers, and that some complexes act as proton pumps.

Key Features

• Each carrier is more electronegative than the previous one, so electrons flow "downhill" in energy.
• As electrons move between complexes, energy is released.
• Some of this energy is used to actively transport H⁺ from the matrix into the intermembrane space.
• This builds a steep electrochemical gradient across the inner membrane.

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Why Reduced FAD Produces Less ATP Than Reduced NAD

Reduced NAD delivers its electrons to Complex I, which pumps H⁺ and then passes the electrons on to ubiquinone. Reduced FAD, however, delivers its electrons to Complex II, which does not pump H⁺. The electrons then enter the ETC at the level of ubiquinone.

Consequence: electrons from reduced FAD bypass one proton-pumping step. This means fewer H⁺ are pumped per reduced FAD than per reduced NAD, and therefore less ATP is made per reduced FAD.

Coenzyme	Enters at	H⁺ pumps used	ATP yield (approx.)
Reduced NAD	Complex I	3 (I, III, IV)	~2.5
Reduced FAD	Complex II	2 (III, IV)	~1.5

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Oxygen: The Final Electron Acceptor

Without oxygen at the end of the chain, everything stops.

• Electrons arriving at Complex IV are passed to oxygen (O₂).
• O₂ also picks up H⁺ from the matrix.
• The final product is water (H₂O).
• For every 4 electrons received, 1 O₂ molecule and 4 H⁺ form 2 H₂O molecules.

If oxygen is not available:

• Electrons cannot be passed off the ETC.
• The whole chain "backs up" and stops.
• No H⁺ is pumped, no gradient forms, no ATP is made.
• Reduced NAD and reduced FAD accumulate, and the Krebs cycle and link reaction also stop (for lack of oxidised NAD/FAD).
• Cellular ATP levels collapse, and only glycolysis (which doesn't need oxygen) can continue — at 2 ATP per glucose.

This is why oxygen deprivation (e.g. in a heart attack, stroke or drowning) causes rapid cell death — ATP levels drop to a level insufficient to maintain ion gradients and cell integrity.

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Chemiosmosis and ATP Synthase

The pumping of H⁺ by the ETC creates a high concentration of protons in the intermembrane space and a low concentration in the matrix. This electrochemical gradient stores energy — the proton motive force.

Protons can only cross the inner membrane by flowing through ATP synthase, a huge enzyme complex with a rotating "stalk".