[Campbell Biology P.198] Cellular respiration C6H12O6 + 6O2 → 6 CO2 + 6 H2O is described as ... | Practice Question

Cellular respiration C6H12O6 + 6O2 → 6 CO2 + 6 H2O is described as having a ΔG = -686 kcal/mol. If a plant were to synthesize one mole of glucose C6H12O6 from CO2 and H2O, approximately how much free energy would be required, and would this process be spontaneous?

  • A: Approximately -686 kcal/mol; spontaneous.
  • B: Approximately +686 kcal/mol; spontaneous.
  • C: Approximately +686 kcal/mol; non-spontaneous.
  • D: Approximately -686 kcal/mol; non-spontaneous.

Explanation

The text states that if a chemical process is exergonic in one direction, then the reverse process must be endergonic, using energy. Cellular respiration has ΔG = -686 kcal/mol, meaning it is exergonic. Therefore, the reverse process, the synthesis of glucose from CO2 and H2O, must be strongly endergonic, with ΔG = +686 kcal/mol. A reaction with a positive ΔG is non-spontaneous and would not happen by itself.