Chemiosmosis & oxidative phosphorylation (theory-heavy)

Mitchell's chemiosmotic hypothesis, in one sentence

The energy of electron transfer is conserved as a transmembrane proton-motive force, which then drives ATP synthesis — the two are coupled only by the proton gradient

The two components of the proton-motive force

Chemical (ΔpH) + electrical (membrane potential Δψ). Δψ dominates in mitochondria.

Why FADH₂ yields less ATP than NADH

It enters at Complex II, which pumps no protons, bypassing Complex I. Fewer protons pumped → ~1.5 vs ~2.5 ATP

Which complexes pump protons

I, III, IV. Not II.

What "coupling" means, and how to prove a system is coupled

Electron transport and ATP synthesis are obligately linked via the gradient. Proof: block ATP synthase (oligomycin) and electron transport also stops (respiratory control)

Respiratory (acceptor) control

Flux through the ETC is governed by ADP availability. No ADP → gradient maxes out → pumping stops even with O₂ and fuel present

Rotational catalysis (Boyer) at ATP synthase

Proton flow through F₀ spins the c-ring and γ-subunit; this rotation forces the three β-subunits through L → T → O conformations, releasing ATP from the T (tight) site

Roughly how many protons per ATP

~4 (≈3 through the synthase + ~1 for phosphate/ADP transport). Reflects the c-ring having 8–17 subunits per revolution

Uncoupler: mechanism and net effect

A proton ionophore (DNP, FCCP) carries H⁺ back across, collapsing the gradient. Electron transport speeds up, ATP synthesis falls, energy is lost as heat

Physiological uncoupler and its role

Thermogenin (UCP1) in brown adipose tissue — deliberate heat generation (non-shivering thermogenesis)

Inhibitor vs uncoupler — the diagnostic

Inhibitor: O₂ use ↓ AND ATP ↓ (both stop). Uncoupler: O₂ use ↑ but ATP ↓. This exact contrast is a favourite question

Why oxygen is essential even though it appears only at Complex IV

It is the terminal electron acceptor. Without it, the chain backs up, all carriers stay reduced, NADH can't be reoxidised, and TCA + β-oxidation halt

The two NADH shuttles and their ATP consequence

Malate–aspartate (delivers NADH → ~2.5 ATP → total 32); glycerol-3-phosphate (delivers FADH₂ → ~1.5 ATP → total 30

Why cytosolic NADH needs a shuttle at all

The inner mitochondrial membrane is impermeable to NADH; only its electrons are transferred in