In essence, electron bifurcation amounts to a short-term energy loan, made on the promise of prompt repayment. As we’ve noted, the reaction of H2 with CO2 is exergonic overall (releasing energy) but the first few steps are endergonic (requiring an energy input). Electron bifurcation contrives to use some of the energy that is released in the later, exergonic, steps of CO2 reduction to pay for the difficult first steps.7 As more energy is released in the last few steps than needs to be spent in the first few steps, some energy can be conserved as a proton gradient across a membrane (Figure 18).
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