Hydrophilic
 
Most biological molecules contain electronegative atoms such as O, N, or S. These molecules or groups are polar and can act as hydrogen bond acceptors or donors, allowing them to interact more or less strongly with other such polar groups, with groups possessing charges of opposite sign, or with solvent water, which is equipped with partial positive and negative charges. Molecules or groups that bear full electrostatic charges, such as the NH₃ ⁺  and COO^– groups of amino acids in neutral solution, are extremely polar and especially difficult to remove from water. Hydrophilicity, or hydrophilic character, provides a quantitative index of a molecule's polarity, by indicating the difficulty of removing it from solvent water.