Enzymes catalyse both the breaking down and the building up of large molecules. Food contains large molecules that are too large to be absorbed from the gut, so digestive enzymes break them into small molecules.
Carbohydrases such as amylase break down carbohydrates like starch into sugars. Proteases break down proteins into amino acids. Lipases break down lipids (fats and oils) into fatty acids and glycerol.
Cells also use enzymes to build molecules. Sugars are joined together to make carbohydrates, such as starch. Amino acids are joined to make proteins. Fatty acids and glycerol are joined to make lipids. Each of these reactions is catalysed by a different enzyme, because enzymes are specific.