Eduqas Biology for A Level Book 1: 2nd Edition

Study point Both starch and glycogen are readily broken down by hydrolysis to α -glucose, which is soluble and can be transported to where energy is needed. Study point We use the word ‘cellulose’ to refer to the chains of β -glucose, i.e. the cellulose molecules and also for the bulk material that they make. Link The digestion of carbohydrates is described on p234. Link The role of the cell wall in supporting plants by keeping their cells turgid is discussed on p61. Exam tip When you write about cellulose in the cell wall, always state ‘plant cell wall’ rather than just ‘cell wall’ because the cell walls of fungi and most prokaryotes do not contain cellulose. Glycogen The main storage product in animals is glycogen. It used to be called animal starch because it is very similar to amylopectin. It also has α -1,4 and α -1,6 bonds, as shown on p19. The difference is that in glycogen, the α -1,6 bonds occur every 8–10 glucose molecules. This means glycogen has shorter α -1,4-linked chains than amylopectin and so it is more branched. Cellulose Cellulose is a structural polysaccharide and its presence in plant cell walls makes it the most abundant organic molecule on Earth. We can think of the structure of cellulose at different levels: ▪ An individual cellulose molecule consists of a long chain of β -glucose units. These glucose monomers are joined by β -1,4-glycosidic bonds to make a straight, unbranched chain. The β -link rotates adjacent glucose molecules by 180º. ▪ Hydrogen bonds form between the (OH) groups of adjacent parallel chains, contributing to cellulose’s structural stability. These parallel cellulose molecules become tightly cross- linked by hydrogen bonds to form a bundle called a microfibril. ▪ The microfibrils are, in turn, held in bundles called fibres. ▪ A cell wall has several layers of fibres, which run parallel within a layer but at an angle to the adjacent layers. This laminated structure also contributes to the strength of the cell wall. Cellulose is freely permeable, because there are spaces between the fibres. Water and its solutes can penetrate through these spaces in the cell wall, all the way to the cell membrane. Cellulose in the plant cell wall Structure of a molecule of cellulose Structure of a microfibril Fig 05 CH OH O O CH OH 1 1 4 1 4 1 4 4 O O CH OH O O CH OH O O CH OH hydrogen bonds forming cross-links O β-glucose molecules join to form a cellulose molecule cellulose molecules aggregate to form a micro bril scanning electron micrograph of a plant cell wall shows that cellulose bres are parallel within the layer but at an angle to the bres of the adjacent layer 60–70 cellulose molecules aggregate to form a micro bril cellulose micro brils aggregate into a bre up to 1700 β -glucose molecules join to form a cellulose molecule Eduqas Year 1/AS Biology 20