NAD (Nicotinamide Adenine Dinucleotide) is a hydrogen acceptor, used as a coenzyme in many reactions especially in respiration to transfer (pairs of) hydrogen atoms to other molecules. Dehydrogenation - the removal of hydrogen - is effectively a form of oxidation. Accompanying this is the acceptance/gain and donation/loss of electrons.
NAD consists of two nucleotides, joined by their phosphate groups . On either side of the central
phosphate groups are 5-membered
ribose rings, attached to other rings (bases) containing nitrogen atoms (blue). On one end of the molecule is a double ring structure - the base
adenine - and at the other end is a slightly simpler single ring structure -
nicotinamide - which is a derivative of vitamin B3.
The
oxidised form of the molecule carries a positive charge (so it may be called NAD
+) due to the arrangement of bonds on one of the
nitrogen atoms in the nicotinamide section.
The
reduced form of NAD (sometimes called NADH, NADH
2 or NADH/H
+) has accepted an electron [cancelling the positive charge], and a hydrogen atom which can be seen on the nicotinamide section. The other hydrogen remains as an H
+ ion.
In photosynthesis (and also in some reactions in animal cells), hydrogen is transferred using
NADP - which has an additional phosphate group attached to carbon 2 of the ribose of adenosine.