Covalent bonds are the forces that hold atoms together as molecules. For example, the two O-H bonds in water molecules are covalent bonds. The covalent bonds most important in biology (C-C and C-H) have bond energies in the range of 300-400 kJ/mol.

Non-covalent interactions (also called noncovalent forces or noncovalent bonds) are weak interactions between ions, molecules, and parts of molecules. They help shape individual molecules and groups of molecules and ions, but are weak enough to be continually broken and re-formed in the dynamic molecular interplay that is life. In fact, Figure 2.1 shows that biologically important noncovalent interactions are 10-100 times weaker than covalent bonds.

The different types of noncovalent interactions are summarized in Figure 2.2. All are fundamentally electrostatic in nature; that is, all depend on the forces that electrical charges exert on one another. Note that all but hydrogen bonds decrease with distance.

For cells, both covalent bonds and noncovalent interactions are important. DNA, for example, is composed of two intertwined chains of polynucleotides. The forces that hold together the atoms of the nucleotides in each individual chain are covalent. The forces that hold the two chains together, however, are noncovalent hydrogen bonds. The weaker hydrogen bond forces are strong enough to keep the two chains together, but weak enough to enable the cell to pull the chains apart when necessary to perform DNA replication (see here) or other processes that require the chains to be separated.