TOPIC 6: MITOCHONDRIAL vs PHOTOSYNTHETIC ETC (theory)

The single most important shared principle

Both use chemiosmosis: an electron transport chain builds a proton gradient across a closed membrane, and an F-type ATP synthase uses it to make ATP by rotational catalysis

Mitochondrial vs photosynthetic — energy source

vMito: chemical oxidation of NADH/FADH₂. Photo: light (photons)

Electron donor and terminal acceptor — mito

Donor NADH/FADH₂; terminal acceptor O₂ → H₂O

Electron donor and terminal acceptor — photosynthesis

Donor H₂O (split at PSII); terminal acceptor NADP⁺ → NADPH

Direction of electron flow — the key contrast

Mito: downhill (high → low energy), releasing energy. Photo: uphill, driven twice by light (Z scheme), storing energy

O₂ — consumed or produced

Mito consumes O₂; photosynthesis produces it (from water splitting at the Mn₄Ca cluster of PSII)

Mobile carriers — the analogues

Mito: ubiquinone + cytochrome c. Photo: plastoquinone + plastocyanin

The proton-pumping cytochrome complex in each

Mito: Complex III (cyt bc₁). Photo: cytochrome b₆f

Where protons accumulate

Mito: intermembrane space. Photo: thylakoid lumen

ATP synthase orientation

Mito: F₁ head into matrix. Photo: CF₁ head into stroma

Linear vs cyclic photophosphorylation

Linear: makes ATP + NADPH + O₂. Cyclic (around PSI only): makes ATP only — no NADPH, no O₂; adjusts the ATP:NADPH ratio

The synthesis sentence for top marks

The two systems are mechanistically identical and thermodynamically opposite: photosynthesis reduces CO₂ to carbohydrate storing light energy; respiration oxidises carbohydrate releasing it. Same chemistry, opposite direction.