Many chemical reactions within the body require the addition of two atoms of hydrogen to a molecule in order to maintain life. These reactions are much more complex than the ones described above, because hydrogen gas is not found in the body. These kinds of reactions require "carrier" molecules, which give up hydrogen atoms to the one undergoing hydrogenation. The catalyst in biological hydrogenation is an enzyme, a complex protein that allows the reaction to take place in the blood, at a moderate temperature, and at a rate fast enough for metabolism to continue.
Hydrogenation reactions can happen to many other types of molecules as well. However, the general features for all of the reactions are the same. Hydrogen atoms add to multiple bonds in the presence of a catalyst, to product a new compound, with new characteristics. This new compound has different properties than the original molecule had.
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G. Lynn Carlson
Science EncyclopediaScience & Philosophy: Hydrazones to IncompatibilityHydrogenation - The Hydrogenation Reaction, Hydrogenation In The Research Laboratory, Hydrogenation In Industry, Biological Hydrogenation