In biological systems, covalent bonds are called strong bonds. This means that they are not normally broken under biological conditions unless by enzymic catalysis. This is in opposition to weak bonds like hydrogen and ionic bonds which are easily broken under normal biological conditions of temperature and pressure. There is a type of covalent bond called a polar covalent bond. In Biological systems, polar covalent bonds are important because these kinds of bonds allow the formation of another kind of weak bond called a hydrogen bond .
Water is an example of a molecule that has polar covalent bonds and engages in hydrogen bonding.
In addition to polar covalent bonds, there are nonpolar covalent bonds . In biological systems, if a molecules has a predominance of nonpolar covalent bonds, that substance is hydrophobic.