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Oxidative Phosphorylation Notes
1. Overall idea
Oxidative phosphorylation has two linked parts: A. Electron transport chain Electrons move from NADH and FADH₂ to oxygen B. Chemiosmosis / ATP synthesis The energy released is used to pump H⁺ ions , and their return through ATP synthase drives ATP formation
2. Main names you need to know
Reduced electron carriers
- NADH = reduced nicotinamide adenine dinucleotide
- FADH₂ = reduced flavin adenine dinucleotide These carry high-energy electrons from glycolysis, the link reaction, and the TCA cycle.
Electron transport chain complexes
Complex I
NADH dehydrogenase Also called NADH:ubiquinone oxidoreductase
Complex II
Succinate dehydrogenase
Coenzyme Q
Also called ubiquinone Reduced form = ubiquinol
Complex III
Cytochrome bc₁ complex Also called ubiquinol:cytochrome c oxidoreductase
Cytochrome c
A small mobile electron carrier protein
Complex IV
Cytochrome c oxidase
ATP synthase
Also called Complex V Full name: F₀F₁-ATP synthase
3. Electron flow, with equations
From NADH
At Complex I : NADH → NAD⁺ + H⁺ + 2e⁻ This means NADH is oxidised. It loses electrons. Those 2 electrons enter Complex I and are eventually passed to coenzyme Q. A simplified equation for Complex I is: NADH + H⁺ + Q + 4H⁺(matrix) → NAD⁺ + QH₂ + 4H⁺(intermembrane space) Where:
- Q = ubiquinone
- QH₂ = ubiquinol So Complex I:
- accepts electrons from NADH
- reduces ubiquinone to ubiquinol
- pumps protons across the membrane
From FADH₂
4 cyt c(Fe²⁺) + O₂ + 8H⁺(matrix) → 4 cyt c(Fe³⁺) + 2H₂O + 4H⁺(intermembrane space) So Complex IV:
- takes electrons from cytochrome c
- transfers them to oxygen
- forms water
- pumps protons
4. Overall electron transport summary
equations
For NADH oxidation linked to oxygen reduction
NADH + H⁺ + 1/2 O₂ → NAD⁺ + H₂O
This is the overall redox result for one NADH.
For FADH₂ oxidation
FADH₂ + 1/2 O₂ → FAD + H₂O
These overall equations show the electron donor and final electron acceptor, but they do not show proton pumping directly.
5. Proton pumping
The complexes that pump protons are:
- Complex I
- Complex III
- Complex IV The complex that does not pump protons is:
- Complex II So the electron flow creates a proton gradient by moving H⁺ from the matrix to the intermembrane space.
6. ATP synthesis equation
Once the proton gradient is made, ATP synthase uses it to drive: ADP + Pi → ATP + H₂O Where:
- ADP = adenosine diphosphate
- Pi = inorganic phosphate Sometimes it is also written simply as: ADP + Pi + energy → ATP The energy comes from the proton motive force.
7. The part that spins: ATP synthase
This is the part you asked about, and yes, it is very important.
ATP synthase has two main parts
F₀ portion
- found in the inner mitochondrial membrane
- forms a channel for protons
- this is the membrane-embedded part
F₁ portion
- projects into the matrix
- contains the catalytic sites where ATP is made So:
- F₀ = proton channel and rotor part
- F₁ = catalytic headpiece
8. Which part actually spins?
The rotating parts are mainly:
10. Binding change mechanism
This is the classic explanation for how ATP is formed. Each β subunit can exist in 3 conformations:
- L state = loose
- T state = tight
- O state = open
What each state does
L (Loose)
Binds ADP and Pi
T (Tight)
Forces ADP and Pi together to form ATP
O (Open)
Releases ATP As the γ subunit rotates , it forces each β subunit to change shape: L → T → O That is how rotation drives ATP synthesis.
11. Simple sequence of the spinning
mechanism
You can think of it like this: H⁺ flows through F₀ → c-ring rotates → γ subunit rotates → β subunits change shape → ADP + Pi become ATP → ATP is released
12. Very important distinction
Do protons spin the whole enzyme?
No. They do not spin the whole enzyme. They rotate the rotor parts :
- c-ring
- γ subunit The catalytic head stays mostly fixed.
13. Full pathway in one chain
Here is the whole thing in order: NADH → Complex I → ubiquinone (CoQ) → Complex III → cytochrome c → Complex IV → O₂ or FADH₂ → Complex II → ubiquinone (CoQ) → Complex III → cytochrome c → Complex IV → O₂ At the same time: Complexes I, III, and IV pump H⁺ into the intermembrane space Then: H⁺ returns through ATP synthase (Complex V) Then: ADP + Pi → ATP
14. Short exam version with equations
In oxidative phosphorylation, electrons from NADH and FADH₂ pass through the electron transport chain in the inner mitochondrial membrane. NADH is oxidised according to the
