any substance that is composed of two molecules of simple sugars (sugar) (monosaccharides) linked to each other. Sucrose, which is formed following photosynthesis in green plants, consists of one molecule of glucose and one of fructose; lactose (milk sugar), found in the milk of all mammals, consists of glucose and galactose; and maltose, a product of the breakdown of starches during digestion, consists of two molecules of glucose. Another important disaccharide, trehalose, which is found in the circulating fluid of many insects, also consists of two molecules of glucose, but they are linked in a way such that trehalose differs from maltose.