Polarised light micrograph of crystals of the co- enzyme nicotinamide adenine dinucleotide (NAD). This is an extremely important and widespread co- enzyme found in nature. Formerly known as diphos- phopyridine nucleotide (DPN). NAD functions as a hydrogen transfer agent (it carries electrons). The hydrogen transfer is stereospecific both to the particular substrate in question and to this coenzyme. As a derivative of nicotinic acid, NAD is required in many redox reactions, as in the Krebs Cycle and the Electron Transport System in respiration. In this way, the energy carrier ATP is activated which powers most cellular activity. Magnification: x25 at 6x7cm size. x45 at 4x5ins