Many biological reactions lead to an increase in order, and thus a decrease in entropy (Δ S < 0). An obvious example is the reaction that links amino acids together to form a protein . A solution of protein molecules has a lower entropy than does a solution of the same amino acids unlinked, because the free movement of any amino acid in a protein is restricted when it is bound in a long chain. For the linking reaction to proceed, a compensatory decrease in free energy must occur elsewhere in the system, as is discussed in Chapter 4.
When two monosaccharides undergo dehydration synthesis whereby a molecule of water is released, as two hydrogen atoms and one oxygen atom are lost from the two monosaccharides, the new molecule, consisting of two monosaccharides, is called a disaccharide and is conjoined together by a glycosidic or ether bond . The reverse reaction can also occur, using a molecule of water to split up a disaccharide and break the glycosidic bond; this is termed hydrolysis . The most well-known disaccharide is sucrose , ordinary sugar (in scientific contexts, called table sugar or cane sugar to differentiate it from other sugars). Sucrose consists of a glucose molecule and a fructose molecule joined together. Another important disaccharide is lactose , consisting of a glucose molecule and a galactose molecule. As most humans age, the production of lactase , the enzyme that hydrolyzes lactose back into glucose and galactose , typically decreases. This results in lactase deficiency , also called lactose intolerance .