Many fundamental biological entities carry charges. For example, from a basic biochemistry course, we know that amino acids have a charge associated with them and that the charge is pH dependent. Consequently, polymers of amino acids, namely proteins, also carry charges and the net charge carried by a protein is pH dependent. The genetically important molecule, deoxyribonucleic acid (DNA), too, is charged. The third major class of bio molecules, lipids, can also have charges associated with them. Since lipids and proteins constitute cell membranes, the surface of cells have a net charge. The charge is usually net negative. © 2014, Anamaya Publishers, New Delhi, India 2014.