Oxidative phosphorylation is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. It uses the energy released by the electron transport chain to drive the synthesis of ATP, producing the vast majority of cellular energy.
How Oxidative Phosphorylation Works
The Electron Transport Chain
NADH and FADH2 from glycolysis and the citric acid cycle donate electrons to protein complexes embedded in the inner mitochondrial membrane. These complexes (Complex I through IV) pass electrons through a series of redox reactions, each with a progressively higher reduction potential.
Proton Pumping
As electrons move through the chain, the energy released is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space. Complexes I, III, and IV each contribute to building this proton gradient. The result is a high concentration of protons in the intermembrane space and a low concentration in the matrix.
The Proton Motive Force
The electrochemical gradient created by the proton concentration difference and the membrane potential is called the proton motive force. This force stores potential energy, much like water behind a dam.
ATP Synthesis
Protons flow back into the matrix through ATP synthase (Complex V), a molecular turbine. As protons pass through the enzyme, it rotates, driving the phosphorylation of ADP to ATP. Approximately three to four ATP molecules are produced per ten protons that flow through.
Oxygen as Terminal Electron Acceptor
Oxygen is the final electron acceptor at Complex IV. It accepts electrons and combines with protons to form water. Without oxygen, the electron transport chain backs up, and oxidative phosphorylation stops.
ATP Yield
Complete oxidation of one glucose molecule yields approximately 30–32 ATP molecules through oxidative phosphorylation, far exceeding the 2 ATP produced by glycolysis alone.
Practical Measurement of Oxygen Consumption Rate
Measure oxygen consumption rate (OCR) using a Seahorse XF Analyzer or a Clark-type oxygen electrode. For Seahorse assays, seed cells at 1–4 × 10⁴ cells/well in an XF96 plate, culture overnight, then replace medium with XF DMEM containing 10 mM glucose and 2 mM glutamine. Equilibrate at 37°C for 1 hour. Measure basal OCR for three cycles (3 minutes mix, 2 minutes wait, 3 minutes measure). Inject 1 µM oligomycin (Complex V inhibitor) through port A — the drop in OCR reveals the fraction coupled to ATP synthesis (ATP-linked respiration). Inject 0.5 µM FCCP (uncoupler) through port B — OCR reaches its maximum (maximal respiratory capacity), and the spare respiratory capacity is calculated as (maximal OCR − basal OCR). Inject 0.5 µM rotenone (Complex I inhibitor) plus 1 µM antimycin A (Complex III inhibitor) through port C — OCR drops to non-mitochondrial levels, confirming that the measured signal originates from mitochondrial respiration. For isolated mitochondria, use a Clark electrode: add 0.5 mg of mitochondria to 1 mL of respiration buffer (225 mM mannitol, 75 mM sucrose, 10 mM KCl, 10 mM Tris-HCl pH 7.2, 5 mM KH2PO4, 0.1% BSA) with 5 mM succinate or 5 mM pyruvate plus 2 mM malate as substrates. State 3 respiration (ADP-stimulated) is initiated by adding 0.5 mM ADP; state 4 (resting) is measured after ADP depletion.
Real-World Application
In a study of mitochondrial dysfunction in Parkinson’s disease, fibroblasts from patients with PINK1 mutations show reduced basal OCR (40 pmol/min vs. 80 pmol/min in controls) and minimal spare respiratory capacity. Rotenone injection confirms that Complex I activity is specifically impaired. These findings support the hypothesis that mitochondrial dysfunction contributes to dopaminergic neuron degeneration in Parkinson’s disease.
