ATP hydrolysis
Summary
ATP hydrolysis is the reaction ATP + H₂O → ADP + Pi, which is strongly exergonic (ΔG°′ about −30 kJ/mol). Cells couple it to endergonic processes, and the energy release comes from the products being more stable, not from the bond breaking itself.
Detail
Meaning: ATP hydrolysis is the cleavage of ATP's terminal phosphoanhydride bond by water, giving ADP and inorganic phosphate (Pi). It is strongly exergonic because the products are more stable: charge repulsion among ATP's phosphates is relieved, Pi is stabilized by resonance, and the products are better solvated. Cells harness it by coupling, often by transferring the phosphate to a substrate or protein.
Equation: ATP + H₂O → ADP + Pi; ΔG = ΔG°′ + RT ln Q. ΔG is the free energy change (J/mol), ΔG°′ is the standard value at pH 7 (about −30 kJ/mol), R is the gas constant (J/(mol·K)), T is temperature (K), and Q is the reaction quotient (unitless).
On the MCAT: Passages feature a pump, motor protein or kinase. You explain why ATP is needed or add ΔG values for coupled reactions (+15 kJ/mol plus about −30 kJ/mol is net negative). Cells keep ATP high relative to ADP, so the actual ΔG is more negative than ΔG°′.
Common confusion: Students say energy is released by breaking the bond, but breaking a bond costs energy. The release comes from the products being more stable. Also, hydrolysis alone only gives off heat; useful work requires coupling to another process.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
- Biochemistry (Voet and Voet)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.