Carbohydrates are biomolecules that are composed of carbon, hydrogen and oxygen atoms in the ratio of 1:2:1. We can represent the proportion of these elements within carbohydrate molecules with the formula CH2O. Most carbohydrates are characterized as either monosaccharides, disaccharides or polysaccharides. The term “saccharide” is just another word for sugar. The prefixes mono, di and poly refer to the number of sugars in the molecule. “Mono” means one, so a monosaccharide is a carbohydrate made of one unit of sugar. The prefix “di” means two, so a disaccharide is a carbohydrate made of two units of sugar. And “poly” means many, so a polysaccharide is made of many sugar units bonded together. Let’s talk about monosaccharides first. Monosaccharides are the building blocks, or monomers, of all carbohydrates. Common monosaccharides include glucose, fructose, and galactose. Glucose is by far the most abundant monosaccharide. It is water soluble, easily transported through an organism, and is the energy source for cellular respiration and the production of ATP. Fructose is the primary monosaccharide found in fruits and plants, and galactose is the primary monosaccharide found in milk. All of these monosaccharides are six carbon sugars with the chemical formula C6H12O6. They can be depicted chemically as either straight chains or rings. Disaccharides are formed when monosaccharides are joined together through dehydration reactions forming glycosidic linkages. Common disaccharides include maltose, which is made up of two glucose molecules; sucrose (also known as table sugar), which is made up of glucose and fructose; and lactose (or milk sugar) which contains glucose and galactose. Polysaccharides are formed when glucose monomers link together to form long chains. These long chains of glucose units are ideal for storing energy. The chains can be straight or branched. Plants store energy in the form of amylose, which has straight chains, or amylopectin, which is branched. Animals differ from plants in that they store energy in the form of glycogen, which is a highly branched polysaccharide that can be broken down quickly to supply energy to tissues. Other polysaccharides such as cellulose, chitin and peptidoglycan serve as structural molecules in organisms. The most abundant polysaccharide is cellulose. Cellulose is a straight chain polymer of glucose like amylose, but it differs in the configuration of the bonds between the glucose units. Most organisms are unable to break these bonds and cannot use cellulose as a source of energy. Instead cellulose is used to add strength to plant cell walls. Chitin is a structural polysaccharide found in animals and fungi. It makes up the exoskeleton of insects and crustaceans. Its unique properties are a result of chitin having amino groups attached to its sugar monomers. Peptidoglycans are complex polysaccharides found in the cell walls of bacteria. The macromolecule is both flexible and rugged due to its structure. Each monomer of the polysaccharide has a peptide chain attached to it. Often, we refer to carbohydrates as being either simple sugars or complex carbohydrates. Monosaccharides and disaccharides are commonly referred to as simple sugars. The term complex carbohydrates refers to the polysaccharides.