H2O
Summary
H₂O is water, a bent polar molecule that hydrogen bonds, dissolves ions and polar molecules, and acts as a reactant in hydrolysis or a product in condensation. Most testable: nonpolar groups cluster away from water (hydrophobic effect), largely because that raises water's entropy.
Detail
Meaning: Water is a bent, polar molecule. Oxygen carries a partial negative charge and each hydrogen a partial positive charge, so water hydrogen bonds with other water molecules and with polar or charged solutes. It is therefore a good solvent for ions and polar molecules and a poor one for nonpolar molecules. Water is also a reactant or product: hydrolysis uses water to break a bond (peptide, glycosidic, ester, phosphoanhydride), and condensation (dehydration synthesis) joins monomers and releases water. Water is amphoteric and autoionizes slightly.
Equation: Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C, where Kw is the ion-product constant of water (unitless by convention) and [H⁺] and [OH⁻] are molar concentrations in mol/L.
On the MCAT: Expect to label a reaction as hydrolysis or condensation from a structure, or to explain protein folding. Nonpolar side chains bury in the interior away from water, while polar and charged side chains face the solvent. Acid-base questions use pH + pOH = 14 at 25 °C.
Common confusion: Students say nonpolar groups cluster because they attract each other strongly. The main driver is water's entropy: ordered water around exposed nonpolar surfaces is released when they cluster. Also, neutral means [H⁺] = [OH⁻]. That is pH 7 only at 25 °C, because Kw changes with temperature.
Sources
- Lehninger Principles of Biochemistry
- Biochemistry (Berg, Tymoczko, Stryer)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.