Q.34 The following reactions are given:

Reaction 1:
Glucose + Pi → Glucose-6-Phosphate + H₂O

can only occur when the

Reaction 2:
ATP → ADP + Pi

occurs. This is because:

1. The ΔG°′ of reaction 1 is negative.
2. The ΔG°′ of reaction 2 is positive.
3. The ΔG°′ of reaction 1 is positive and reaction 2 is negative.
4. The ΔG°′ of both reactions 1 and 2 are negative.

   The first step of glycolysis couples an unfavorable reaction (glucose phosphorylation) with a highly favorable ATP hydrolysis to make it spontaneous overall. This occurs because the positive ΔG°′ of Reaction 1 is overcome by the large negative ΔG°′ of Reaction 2.

   Correct Answer

   The ΔG°′ of reaction 1 is positive and reaction 2 is negative.

   Option Analysis

   • The ΔG°′ of reaction 1 is negative.
     Incorrect. Reaction 1 (Glucose + Pi → Glucose-6-Phosphate + H2O) has a positive ΔG°′ of about +13.8 to +16.7 kJ/mol, making it endergonic and non-spontaneous alone.

   • The ΔG°′ of reaction 2 is positive.
     Incorrect. Reaction 2 (ATP → ADP + Pi) has a large negative ΔG°′ of -30.5 kJ/mol (or ~ -7.3 kcal/mol), driving it forward exergonically.

   • The ΔG°′ of reaction 1 is positive and reaction 2 is negative.
     Correct. The coupling (net: Glucose + ATP → Glucose-6-Phosphate + ADP) yields ΔG°′ ≈ -16.7 kJ/mol, as the exergonic ATP hydrolysis pulls the endergonic phosphorylation.

   • The ΔG°′ of both reactions 1 and 2 are negative.
     Incorrect. Only Reaction 2 is negative; Reaction 1’s positive value requires coupling for feasibility in glycolysis.

   Introduction to Reaction Coupling

   In glycolysis, the phosphorylation of glucose (Glucose + Pi → Glucose-6-Phosphate + H₂O) cannot proceed alone due to its positive standard free energy change (ΔG°′ +13.8 kJ/mol). It couples with ATP hydrolysis (ATP → ADP + Pi, ΔG°′ -30.5 kJ/mol), making the net reaction exergonic. This principle is key for competitive exams like NEET and GATE Life Sciences.

   ΔG°′ Values Breakdown

   Standard free energy (ΔG°′) determines spontaneity at pH 7, 25°C:

   • Reaction 1: Endergonic (+ΔG°′), hydrolysis reverse is favorable but phosphorylation needs energy input.

   • Reaction 2: Exergonic (-ΔG°′), releases ~30 kJ/mol to drive cellular work.
     Net ΔG°′ = -16.7 kJ/mol confirms coupling’s role in the first glycolytic step.

   Reaction	Equation	ΔG°′ (kJ/mol)	Spontaneity
   1	Glucose + Pi → G6P + H₂O	+13.8 to +16.7	Non-spontaneous
   2	ATP → ADP + Pi	-30.5	Spontaneous
   Coupled	Glucose + ATP → G6P + ADP	-16.7	Spontaneous

   Biological Significance

   Hexokinase catalyzes the coupled reaction, trapping glucose in cells for metabolism. This prevents reversal, ensuring glycolytic flux. In plants and microbes (relevant to your botany focus), similar coupling sustains energy homeostasis.

   Exam Preparation Tips

   • Memorize: ATP hydrolysis ΔG°′ ≈ -30 kJ/mol; glucose phosphorylation +14 kJ/mol.

   • Practice: Identify coupling in pathways like gluconeogenesis reverse.

   • Common trap: Confusing ΔG°′ (standard) with actual ΔG (cellular concentrations).