Glycolysis is the process by which energy is harvested from glucose by living things. Several of the reactions of glycolysis are thermodynamically unfavorable (nonspontaneous), but proceed when they are coupled with other reactions.
Which of these reactions is (are) unfavorable?
Reaction A: Pi+glucose⟶glucose-6-phosphate+H2OΔG=13.8 kJ/mol
Reaction B: Pi+fructose-6-phosphate⟶fructose-1,6-bisphosphate+H2OΔG=16.3 kJ/mol
Reaction C: ATP+H2O⟶ADP+PiΔG=−30.5 kJ/mol
A. A
B. B
C. C
Which of these reactions can be coupled so that overall reaction is favorable?
A. A and B
B. A and C
C. B and C
What is the net change in free energy if one selection from part (b) is coupled so that the overall reaction is favorable?

1. The spontaneity of a chemical reaction depends on its standard free energy change, ∆G. If ∆G is negative, the reaction is favourable, but when it is positive, the reaction is unfavorable.

Therefore, since reaction A and B have ∆G to be positive, they are unfavorable

2. Coupling an unfavorable reaction to a favourable reaction can help the reaction to proceed in the forward direction as long as the net free energy change is negative.

Coupling reaction A and C, as well as reaction B and C will make the reactions to become favourable as net ∆G is negative in both instances.

3. A and C: net ∆G = 13.8 - 30.5 = -16.7 KJ/mol

B and C: net ∆G = 16.3- 30.5 = -14.2 KJ/mol