Carbohydrates are made from sugars. A single sugar molecule, such as glucose, is a small unit called a monomer. When many sugar monomers join together in a long chain, they make a large molecule called a polymer. Starch, glycogen and cellulose are all polymers of sugar.
Joining monomers to make a polymer is called synthesis. Plants make starch to store energy, and animals make glycogen. Polymers are large and do not dissolve easily, so they make a good store.
The reverse is breakdown. Carbohydrase enzymes, such as amylase, break down starch into sugars. The small sugars can then be moved around the body and used in respiration. Iodine solution turns blue-black with starch, and Benedict's reagent turns orange-red when heated with a sugar such as glucose.