Macromolecules I: Carbohydrates and Lipids

Most of the large molecules that build living things are polymers -- long chains built from repeating smaller units called monomers, linked together by a reaction called dehydration synthesis, which removes a water molecule each time it forms a new bond. Breaking a polymer back down into its monomers runs the reaction in reverse, called hydrolysis, which adds a water molecule back in at each bond broken -- your digestive system runs hydrolysis constantly to break the food you eat into absorbable pieces.

Carbohydrates are built from sugar monomers called monosaccharides, such as glucose. Two linked sugars form a disaccharide (table sugar, sucrose, is one), and long chains form polysaccharides: starch stores energy in plants, glycogen stores energy in animals, and cellulose builds rigid plant cell walls -- three different polysaccharides built from the same basic sugar monomer, arranged differently for three different jobs. Lipids, by contrast, are not true polymers, since they are not built from one repeating monomer; a typical fat (a triglyceride) is one glycerol backbone attached to three fatty acid chains. Saturated fats have no double bonds between carbons in their fatty acid chains, pack tightly, and are usually solid at room temperature (most animal fats); unsaturated fats have at least one double bond, which puts a kink in the chain that keeps them loose and liquid at room temperature (most plant oils). Lipids store roughly twice the energy per gram that carbohydrates do, and also provide insulation, cushioning, and, as phospholipids, the cell membrane itself -- a structure returned to in Unit 3.