Question

Is the fermentation of sugar by yeast an exergonic process or an endergonic reaction?
Justify your answer based on the energy changes involved.

Answer 1

The fermentation of sugar by yeast is an exergonic process (ΔG < 0) — it proceeds spontaneously and releases free energy.

Justification:
- During fermentation (e.g. glucose → 2 ethanol + 2 CO2 in yeast) the reduced, high‑energy bonds in glucose are partly oxidized to lower‑energy products. The free energy of the products is lower than that of the reactants, so the overall Gibbs free energy change is negative.
- Cells capture part of that released free energy as ATP by substrate‑level phosphorylation (glycolysis yields a net ∼2 ATP/glucose); the remainder is lost as heat. Thus the pathway releases usable energy even though it yields far less ATP than complete aerobic respiration.
- In thermodynamic terms ΔG = ΔH − TΔS is negative for the overall fermentation under physiological conditions (although if products build up strongly the reaction can be driven toward equilibrium and become less favorable).

So fermentation is exergonic overall, providing the energy needed to make ATP and sustain yeast metabolism.