Hydrolysis is used during the process of digestion to break large molecules down into their smaller components. Hydrolytic reactions are exergonic reactions. In the hydrolysis of the disaccharide sucrose, the components of water are added 'across the bond' the held the two halves of the sucrose molecule together, forming two smaller monosaccharides, glucose and fructose. If water is added, withoug splitting the molecule in two, for example, if hydrogen and a hydroxyl group are added to the two sides of a double bond, the reaction is called a hydration, rather than a hydrolytic reaction. Dehydration synthesis or condensation reactions are the reverse of hydrolysis.
Dehydration synthesis plays an important role in the construction of organic polymers.
This is the reaction used to assemble monosaccharides to make complex sugars, and is used to join amino acids to make proteins, or to join nucleotides to make a nucleic acid. Several other important reactions also figure prominently in the chemistry of life. A second pair of reactions is phosphorylation and dephosphorylation, the addition and removal of a phosphate group. This reaction pair is particularly important becasue the phosphate group, due to the presence of many hydroxyls, is strongly electronegative, and bonds formed with the phosphate group have the potential to be high energy bonds. Phosphorylation is an energy storing endergonic reaction, and dephosphorylation is an energy-releasing exergonic reaction.