The electron transport system.

Feed NADH at complex I or FADH₂ at complex II and watch electrons pass via ubiquinone, complex III, cytochrome c and complex IV to O₂, while protons pile up in the intermembrane space and flow back through F₀ to make ATP.

Show that one FADH₂ yields one ATP fewer than one NADH, then stall the whole chain using only the O₂ slider.

O₂ is used throughout respiration; FADH₂ gives 3 ATP.

• The energy held in NADH + H⁺ and FADH₂ is released when they are oxidised through the electron transport system (ETS), a chain of carriers in the inner mitochondrial membrane, with the electrons finally passed to O₂ to form H₂O.
• Complex I (NADH dehydrogenase) oxidises NADH made in the matrix and passes the electrons to ubiquinone in the inner membrane.
• Ubiquinone also receives reducing equivalents from FADH₂ through complex II, which is generated when succinate is oxidised in the citric acid cycle.
• Reduced ubiquinone (ubiquinol) passes its electrons to cytochrome c through the cytochrome bc₁ complex (complex III).
• Cytochrome c, a small protein sitting on the outer face of the inner membrane, shuttles electrons as a mobile carrier between complexes III and IV.
• Complex IV is cytochrome c oxidase, containing cytochromes a and a₃ and two copper centres.
• Electron flow through complexes I to IV is coupled to ATP synthase (complex V). The yield depends on the donor: one NADH gives 3 ATP, one FADH₂ gives 2 ATP.
• O₂ acts only at the very end, but it is vital: as the final hydrogen acceptor it pulls hydrogen out of the system and so drives the whole process.
• In photophosphorylation, light energy builds the proton gradient; in respiration the energy of oxidation-reduction builds it, so ATP synthesis here is called oxidative phosphorylation.
• ATP synthase has two parts: F₁, a peripheral headpiece with the site where ATP is made from ADP and Pi, and F₀, an integral membrane complex forming the proton channel. For each ATP made, 4 H⁺ pass through F₀ from the intermembrane space into the matrix, down the electrochemical proton gradient.

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