01Paying for unfavorable steps
An unfavorable reaction (ΔG > 0) can be driven by coupling it with a more favorable one sharing an intermediate. ΔG values add.
Cells couple ATP hydrolysis (ΔG ≈ −30.5 kJ/mol) to build molecules; smelting couples metal oxide reduction with carbon oxidation.
02Notebook box
ΔG_total = ΔG₁ + ΔG₂ < 0 → coupled process favorable.
Worked example: +14 kJ step + ATP (−30.5) → net −16.5 kJ, favorable.