Dehydration synthesis is the chemical process in which two molecules are covalently linked together with the release of a water molecule. Many physiologically important compounds are formed by dehydration synthesis, for example, complex carbohydrates, proteins, DNA, and RNA.
Sugar molecules can be covalently linked together by dehydration synthesis, also called condensation reaction. The resulting stable bond is called a glycosidic bond. To form the bond, a hydroxyl (-OH) group from one reactant and a hydrogen atom from the other form water, while the remaining oxygen links the two compounds. For each additional bond that is formed, another molecule of water is released, literally dehydrating the reactants. For example, individual glucose molecules (monomers) can undergo repeated dehydration synthesis to create a long chain or branched compound. Such a compound, with repeating identical or similar subunits, is called a polymer. Given the diverse set of sugar monomers, and variation in the location of the linkage, a virtually unlimited number of sugar polymers can be built.
Plants produce simple carbohydrates from carbon dioxide and water in a process called photosynthesis. Plants store the resulting sugars (i.e., energy) as starch, a polysaccharide that is created from glucose molecules by dehydration synthesis. Cellulose is likewise built from glucose monomers and is the building block of the cell wall in plants.
Animals consume complex carbohydrates and break them down. The monosaccharides are then used for energy production or stored in the form of glycogen. Glycogen is a branched polysaccharide made from glucose monomers by dehydration synthesis. Furthermore, monosaccharides are used as raw material for small organic building blocks like nucleic acids, amino acids, and fatty acids.
Most animals cannot digest cellulose that is synthesized by plants. Instead, the insoluble fiber passes through the digestive system with very beneficial side effects: it helps pass food along and increases the amount of water that is retained in the intestine. Some animals, such as cows, have bacteria in their gut that produce enzymes to break down cellulose, thereby making glucose available to the cow.
How can amylose (the linear part of starch), glycogen, and cellulose all be made of the same base component but differ in their properties? The difference lies in the type of linkage between individual glucose molecules. Cellulose has β-1,4 linkages of glucose, meaning that a glucose monomer with carbon number one in β-form (i.e., the hydroxyl group at carbon number one is pointing up) is linked to carbon number 4 in the neighboring glucose monomer. The glucose monomers in amylose are connected with α-1,4 linkages. Glycogen also has α-1,4 linkages, but additional side chains with α-1,6 linkage.
