Determine the direction that each of the reactions will progress. Assume that the reactants and products are present in equimolar amounts. The standard free energy of hydrolysis of ATP is – 30.5 kJ/mol. fructose + ATP fructose 6-phosphate + ADP The standard free energy of hydrolysis for fructose 6-phosphate is -15.9 kJ/mol. 3-phosphoglycerate + ATP 1,3-bisphosphoglycerate + ADP The standard free energy of hydrolysis for 1,3-bisphosphoglycerate is -49.3 kJ/mol. creatine + ATP creatine phosphate + ADP The standard free energy of hydrolysis for creatine phosphate is -43.0 kJ/mol.
The correct answer and explanation is:
To determine the direction of each reaction, we compare the net standard free energy change (ΔG°’) of the reaction. If ΔG°’ is negative, the reaction proceeds forward; if positive, it proceeds in reverse. Since reactants and products are present in equimolar amounts, we use standard free energy values directly.
Reaction 1:
fructose + ATP → fructose 6-phosphate + ADP
- ΔG°’ for ATP hydrolysis = –30.5 kJ/mol
- ΔG°’ for fructose 6-phosphate hydrolysis = –15.9 kJ/mol
So, the reverse of this reaction would be:
fructose 6-phosphate + ADP → fructose + ATP
ΔG°’ = (–15.9) – (–30.5) = +14.6 kJ/mol
That means the forward reaction (fructose + ATP → fructose 6-phosphate + ADP) has
ΔG°’ = –14.6 kJ/mol, so it proceeds forward.
Reaction 2:
3-phosphoglycerate + ATP → 1,3-bisphosphoglycerate + ADP
- ΔG°’ for ATP hydrolysis = –30.5 kJ/mol
- ΔG°’ for 1,3-bisphosphoglycerate hydrolysis = –49.3 kJ/mol
Reverse reaction:
1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP
ΔG°’ = (–49.3) – (–30.5) = –18.8 kJ/mol
Thus, the forward reaction has ΔG°’ = +18.8 kJ/mol, so it proceeds in reverse.
Reaction 3:
creatine + ATP → creatine phosphate + ADP
- ΔG°’ for ATP hydrolysis = –30.5 kJ/mol
- ΔG°’ for creatine phosphate hydrolysis = –43.0 kJ/mol
Reverse:
creatine phosphate + ADP → creatine + ATP
ΔG°’ = (–43.0) – (–30.5) = –12.5 kJ/mol
So, forward reaction has ΔG°’ = +12.5 kJ/mol, hence the reaction proceeds in reverse.
✅ Summary:
| Reaction | Direction |
|---|---|
| Fructose + ATP → fructose 6-phosphate + ADP | Forward |
| 3-phosphoglycerate + ATP → 1,3-bisphosphoglycerate + ADP | Reverse |
| Creatine + ATP → creatine phosphate + ADP | Reverse |
Explanation
The direction a biochemical reaction proceeds depends largely on its standard Gibbs free energy change (ΔG°’). If ΔG°’ is negative, the reaction is thermodynamically favorable and will proceed forward. If positive, the reverse reaction is favored.
ATP hydrolysis releases a significant amount of energy (–30.5 kJ/mol), making it a common driving force in coupled biochemical reactions. To evaluate the direction of each reaction, we analyze the free energy change of the overall process by comparing the ΔG°’ of ATP hydrolysis with that of the product being formed.
In the first reaction, fructose is phosphorylated to fructose 6-phosphate using ATP. The hydrolysis of fructose 6-phosphate is less exergonic (–15.9 kJ/mol) than ATP, so the overall ΔG°’ is negative, making the forward reaction spontaneous.
In the second and third reactions, ATP is used to form high-energy compounds—1,3-bisphosphoglycerate and creatine phosphate. However, both of these compounds release more energy upon hydrolysis (–49.3 and –43.0 kJ/mol respectively) than ATP. Therefore, synthesizing them from ATP is unfavorable, meaning the reverse reactions are favored thermodynamically.
This analysis is crucial in metabolism, where ATP is used to drive otherwise unfavorable reactions by coupling them with its highly exergonic hydrolysis.
