Bioenergetics — Approximately 7.3 kcal per mole are released under standard biochemical conditions when which event occurs?

Introduction / Context:
ATP is the primary energy currency in cells. The standard free energy change (ΔG°′) for hydrolysis of the terminal phosphoanhydride bond in ATP to ADP + Pi is often quoted as about −7.3 kcal per mole (−30.5 kJ per mole). Understanding this value helps explain how ATP hydrolysis drives endergonic processes through energetic coupling.

Given Data / Assumptions:

• We are referring to biochemical standard conditions (pH ~7, 1 M activities).
• “Terminal phosphate bond” denotes the γ-phosphate of ATP.
• Other options involve processes with different energetics.

Concept / Approach:
Hydrolysis of ATP → ADP + Pi liberates free energy due to relief of electrostatic repulsion, resonance stabilization of products, and better solvation. This energy can be harnessed by enzymes to drive unfavorable reactions (e.g., biosyntheses, transport, mechanical work). While complete oxidation of glucose releases far more energy, and NAD+ reduction to NADH stores energy (not releases it), the specific ~7.3 kcal/mole value corresponds to ATP γ-bond hydrolysis.

Step-by-Step Solution:

Verification / Alternative check:
Biochemistry references consistently list ATP → ADP + Pi near −7.3 kcal/mol at standard conditions; in vivo ΔG can be more negative depending on concentrations.

Why Other Options Are Wrong:

• Glucose → CO2 + H2O: much larger energy release than 7.3 kcal/mol.
• NAD+ → NADH: reduction captures energy; hydrolysis value does not apply.
• Coupled sucrose synthesis: net ΔG depends on both reactions; the 7.3 figure is for ATP hydrolysis alone.

Common Pitfalls:
Assuming the energy is stored in the phosphate bond itself; the free energy change reflects system-level transformations, not a single “high-energy bond.”

Final Answer:
the terminal phosphate bond of ATP is broken by hydrolysis