Starch and glycogen are energy storage molecules, starch in plants and glycogen in animals. Starch and glycogen are both large, insoluble molecules and therefore do not affect the osmotic properties of cells, but they can be rapidly broken down to form glucose, which is used as an energy source.
The two forms of glucose, α-glucose and β-glucose, form different polysaccharides. Starch is a polymer of α-glucose and exists in two forms, known as amylose and amylopectin. Amylose consists of many thousands of α-glucose monomers, joined by 1,4 glycosidic bonds only. The numbers 1 and 4 refer to the positions of the carbon atoms in the glucose molecules. The long chain coils to form a helix (Figure 2.03).
Cellulose is a polymer of β-glucose, joined by 1,4 glycosidic bonds. The properties of cellulose are different from those of starch and cellulose is a structural polysaccharide, found in plant cell walls. Long, straight chains of cellulose molecules form bundles known as microfibrils, which in turn form cellulose fibres (Figure 2.05). The chains of cellulose fibres are held together by hydrogen bonding between projecting –OH groups. Cellulose fibres have a high tensile strength and the way in which they are arranged imparts considerable strength to plant cell walls.
Amylopectin also consists of many thousands of α-glucose monomers, but it is a branched molecule. Branching occurs as a result of the formation of 1,6 glycosidic bonds (i.e. between carbon atoms 1 and 6), as illustrated in Figure 2.03. Amylose and amylopectin molecules form starch grains in many plant cells. CH2OH OH
cellulose microfibrils in a plant cell wall microfibril cell walls
CH2OH
O
OH
O OH
O 1,6 glycosidic bond, forming a branch
HO
0.5 mm
O CH2OH OH
CH2OH
O O OH
OH
CH2
O O OH
2 Biological molecules
Glycogen is similar in structure to amylopectin, but is more highly branched because the 1,6 glycosidic bonds between the α-glucose monomers form more frequently. Glycogen molecules clump together to form glycogen granules in many animal cells, including liver cells.
The glycosidic bonds in disaccharides and in polysaccharides can be broken by the process of hydrolysis. In living organisms, disaccharides and polysaccharides are broken down to monosaccharides in the process of digestion. The chemical test for non-reducing sugars involves acid hydrolysis of glycosidic bonds.
OH
plant cells
CH2OH O O
OH
OH
O O
cellulose molecules
OH
1,4 glycosidic bonds
Figure 2.03 Glycosidic bonds in the structure of amylopectin amylose
Figure 2.05 The structure of cellulose molecules, microfibrils and fibres.
amylopectin
Figure 2.04 The structure of amylose and amylopectin.
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