High-energy phosphates in cells: Considering ATP, ADP, AMP, and water, which molecule typically offers the greatest amount of readily available free energy per molecule for cellular processes?

Introduction / Context:
Cells require an immediately usable energy currency to drive endergonic processes. Adenosine triphosphate (ATP) fulfills this role through its terminal phosphoanhydride bonds, which liberate significant free energy upon hydrolysis under cellular conditions.

Given Data / Assumptions:

• ATP has two high-energy phosphoanhydride bonds; ADP has one; AMP has none.
• Hydrolysis free energy depends on intracellular concentrations of ATP, ADP, Pi, Mg2+, and pH.
• Water itself is not an energy currency; it participates as a reactant or product.

Concept / Approach:
ATP → ADP + Pi and ATP → AMP + PPi are both strongly exergonic, with the latter often coupled to pyrophosphatase to make the reaction effectively irreversible. The high phosphoryl-transfer potential of ATP enables it to donate phosphate groups and drive otherwise unfavorable steps in metabolism, transport, and mechanical work.

Step-by-Step Solution:
Compare energy content: ATP > ADP > AMP with respect to useful phosphoryl transfer. Note that ATP hydrolysis provides the largest immediate energy release per molecule among the choices. Exclude water: not a chemical energy store for work. Select ATP as the principal high-energy currency.

Verification / Alternative check:
Standard biochemistry texts show that cellular Delta G for ATP hydrolysis is more negative than standard-state values because of actual metabolite ratios (high ATP/ADP). This reinforces ATP as the prime energy donor.

Why Other Options Are Wrong:

• ADP/AMP: Lower phosphoryl-transfer potential; ADP is a product of ATP hydrolysis, AMP even lower.
• H2O / Pi: Not energy currencies; they are participants in hydrolysis and phosphorylation reactions.

Common Pitfalls:
Confusing high-energy bonds with instability; ATP is kinetically stable in the absence of enzymes but thermodynamically capable of driving reactions when catalysis is present.

Final Answer:
ATP provides the greatest readily available free energy per molecule.