Compare dehydration synthesis and hydrolysis reaction.
Dehydration synthesis and hydrolysis refer to two opposite organic reactions. The former results in the combination of monomers to form polymers and the latter takes place when polymers are split into the monomers that they are created from.
When two organic molecules combine to form a larger molecule, for example in the case of monosaccharides combining to form a disaccharide, there is the loss of a water molecule in the reaction. This is referred to dehydration synthesis as a molecule of water is being eliminated. For a reverse reaction to take place, i.e. for the disaccharide formed to split into the constituent monosaccharides an extra molecule of water is required in the reactants. This is the reason why hydrolysis reactions take place only in the presence of water.
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