Figure 4.4 Complex sugars (disaccharides), such as maltose, are made from two simple sugars that have been joined together.
Polysaccharides If many simple sugars join together, a very large molecule called a polysaccharide is made. Some polysaccharide molecules contain thousands of sugar molecules joined together in a long chain. The cellulose of plant cell walls is a polysaccharide and so is starch, which is often found inside plant cells (Figure 4.5). Animal cells often contain a polysaccharide called glycogen. Most polysaccharides are insoluble, and they do not taste sweet.
Functions of carbohydrates Carbohydrates are needed for energy. One gram of carbohydrate releases 17 kJ (kilojoules) of energy. The energy is released by respiration (Chapter 11). The carbohydrate that is normally used in respiration is glucose. This is also the form in which carbohydrate is transported around an animal’s body. Human blood plasma contains dissolved glucose, being transported to all the cells. The cells then use the glucose to release the energy that they need to carry out the processes of life.
Figure 4.5 This is just a small part of a molecule of a polysaccharide, like starch.
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Cambridge IGCSE Biology
Plants also use glucose in respiration, to provide them with energy. However, they do not transport glucose around their bodies. Instead, they transport sucrose. The cells change the sucrose to glucose when they need to use it. Plants store carbohydrates as starch. It is quick and easy to change glucose into starch, or starch into glucose. Some plants store large quantities of starch in their seeds or tubers, and we use these as food. Animals do not store starch. Instead, they store carbohydrates in the form of the polysaccharide glycogen. However, only small quantities of glycogen can be stored. It is mostly stored in the cells in the liver and the muscles. The polysaccharide cellulose is used to make the criss-crossing fibres from which plant cell walls are constructed. Cellulose fibres are very strong, so the cell wall helps to maintain the shape of the plant cell.
Testing for carbohydrates We can test for the presence of sugars by adding Benedict’s solution to a food, and heating it. If the food contains reducing sugar (such as glucose or maltose), then a brick-red colour will be produced. The mixture changes gradually from blue, through green, yellow and orange, and finally brick red (Figure 4.6). If there is no reducing sugar, then the Benedict’s solution remains blue.
Figure 4.6 Positive results of the Benedict’s test. The tube on the left contained a small amount of reducing sugar, and the one on the right a larger amount.
Original material © Cambridge University Press 2014