hydrolysis

Meaning: Hydrolysis is the cleavage of a bond by water: H goes onto one fragment and OH onto the other, so one molecule becomes two. It is the reverse of condensation (dehydration synthesis). Examples include peptide bonds to amino acids, glycosidic bonds to monosaccharides, esters to acid plus alcohol, and the phosphoanhydride bonds of ATP. Many hydrolyses are favorable but slow, so cells use enzymes (hydrolases such as proteases and lipases); in the lab, acid or base and heat are used.

Equation: A–B + H₂O → A–OH + B–H. A–B is a molecule with a cleavable bond and H₂O is water; these are chemical species, so no SI unit applies. For ATP: ATP + H₂O → ADP + Pi, ΔG°′ ≈ −30 kJ/mol (SI unit: J/mol).

On the MCAT: Expect questions that ask for the products of breaking a bond, that name an enzyme as a hydrolase, or that ask you to recognize that a reaction consuming water is hydrolysis while one releasing water is condensation.

Common confusion: Hydrolysis is not hydration. Hydration adds water across a double bond without splitting the molecule. Also, ATP hydrolysis releases energy because its products are more stabilized than ATP, not because breaking a bond releases energy. Breaking a bond costs energy.