Bonds range in their degree of strength and play a key role in the formation and function of key biological molecules. Such bonds are highly stable and help maintain the complexity of structure of large biological molecules. For example, covalent bonds form between a sugar and a phosphate group form the backbone of the DNA molecule. The atoms have partial negative and positive charges. This occurs in water molecules.
Ionic bonds are still relatively strong bonds but weaker than covalent bonds.
Such bonds form between two ions by attraction of opposite charges rather like the poles of magnets. Hydrophobic associations are not true bonds . When surrounded by hydrophilic molecules, such as water, hydrophobic molecules are repelled and associate with each other. For example, the lipid bilayer of cell membranes is pushed to the interior of the membrane and away from water. Similarly, hydrophobic regions of proteins will face away from hydrophilic molecules.