Plants make carbohydrates by a process called photosynthesis by combining the gas, carbon dioxide (CO2) with the liquid, water, with the help of energy in the form of sunlight in this endothermic reaction. The important gas, oxygen is released, and this helps to maintain the composition of the air. The reaction occurs in green leaves because the chloropyll traps this energy. Animals obtain their energy by a process called respiration.
Carbohydrates burn to give the gas carbon dioxide and water, and this proves that carbon and hydrogen are present in a carbohydrate. Two chemical tests are used to identify carbohydrates. If starch is present, the iodine solution turns blue-black, while benedicts solution turns red-brown if heated with glucose or maltose. Starch does not dissolve well in water and forms a colloid Light rays are scattered as they pass through it.
Starch is a large molecule or polymer, made from many molecules of glucose in a reaction called polymerisation. As water molecules are lost in this process, the reaction is called condensation. When food containing starch is eaten it is broken down by digestion, helped by biological catalysts called enzymes and stomach acid. Water molecules are involved in the breaking up of starch, and the reaction is described as hydrolysis. Large molecules are broken down to allow them to be absorbed into the blood. The breakdown of sucrose gives two products, one of which is found in starch, while the other is fructose.