An organic compound is a compound whose molecules contain C, and usually at least one C-C or C-H bond. Very small carbon-containing molecules that do not follow the above rules, such as CO2 and simple carbonates, are considered inorganic. Life on earth would not be possible without carbon. Other than water, most molecules of living cells are carbon-based, and hence are referred to as organic compounds. The main classes of organic compounds we will investigate in this section include carbohydrates, lipids, proteins and nucleic acids.
Each of these classes of compounds consists of large molecules built from small subunits. The smallest of these subunits is called a monomer. Several monomers bond together to form polymers. Each of these polymers is characterised by a specific structure owing to the chemical bonds formed. These structures are related to the function of the compound in living organisms. We will therefore study each class of compounds under the following headings:
- molecular make-up: the main elements that make up the class of compounds
- structural composition: how the monomers join up together to form polymers
- biological role: importance of these molecules to animals and plants
- chemical test: how to detect the presence of each class of compounds
Carbohydrates consist of carbon (C), hydrogen (H) and oxygen (O).
Carbohydrates are made up of monomers known as monosaccharides. 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 disaccharides include lactose, made up of glucose and galactose, and maltose, made up of two glucose molecules. Monosaccharides and disaccharides are often referred to as sugars, or simple carbohydrates. Several monosaccharides join together to form polysaccharides. 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.
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 release energy. Each gram of carbohydrate supplies about 17 kilojoules (kJ) of energy. Starch and glycogen are 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, but the difference in the properties of these substances can be attributed to the way in which the glucose molecules join together to form different structures. Below are images of glycogen and starch.
Chemical tests to identify presence of carbohydrates
Substances containing starch turn a blue-black colour in the presence of iodine. An observable colour change is therefore the basis of a chemical test for the compound.