A slightly simplified free energy diagram of the respiratory chain is shown opposite. Each of the three complexes is the site of a major free energy drop, which is coupled to the synthesis of one ATP molecule. The overall action of the chain is to reoxidize NADH with 0₂, and to use the released energy to produce three ATP.

Succinate makes only two ATP molecules because it comes into the chain late. The FAD, mentioned in the discussion of the citric acid cycle as being reduced by succinate to FADH₂, actually is bound to an enzyme in the form of another flavoprotein on the inner mitochondrial membrane. This flavoprotein and some phospholipid make up Complex II. The FADH₂ reduces ubiquinone directly without making any ATP, and the respiratory chain continues past ubiquinone as before, yielding only two ATP per FADH₂ oxidized.

This is the master plan by which living organisms convert organic compounds into energy. Carbohydrates are broken into glucose monomers and sent along the glycolytic pathway and citric acid cycle. Fats and proteins are chopped into two-carbon acetate units and fed directly into the cycle. The metabolite molecules are oxidized by removing hydrogens and transferring them to NAD⁺ and FAD. These molecules then carry the hydrogens to oxygen and use the released oxidation energy to synthesize ATP.