Figure 2.7: A glucose molecule is made up of carbon, hydrogen and oxygen.Structural composition
Carbohydrates are made up of monomers known asmonosaccharides. The monosaccharide
that makes up most carbohydrates is glucose. Other monosaccharides include fructose,
galactose and deoxyribose (discussed later). These monomers can be joined together by
glycosidic bonds. When two monosaccharides are chemically bonded together, they form
disaccharides. An example of a disaccharide is sucrose (table sugar), which is made up of
glucose and fructose. Other dissacharides include lactose, made up of glucose and galactose,
and maltose, made up of two glucose molecules. Monosaccharides and dissachardies are
often referred to as sugars, or simple carbohydrates. Several monosaccharides join together
to formpolysaccharides. Examples of polysaccharides you will encounter include glycogen,
starch and cellulose. Polysaccharides are usually referred to as complex carbohydrates as
they take longer to break down.
Figure 2.8: Examples of food sources of various monosaccharides, disaccharides and polysaccharides.
Role in animals and plants
The main function of carbohydrates is as energy storage molecules and as substrates (starting
material) for energy production. Carbohydrates are broken down by living organisms to re-
lease energy. Each gram of carbohydrate supplies about 17 kilojoules (kJ) of energy.Starch
andglycogenare both storage polysaccharides (polymers made up of glucose monomers)
and thus act as a store for energy in living organisms. Starch is a storage polysaccharide in
plants and glycogen is the storage polysaccharide for animals. Cellulose is found in plant cell
walls and helps gives plants strength. All polysaccharides are made up of glucose monomers,
Chapter 2. The chemistry of life 31