A level Biology exam revision resources written by A level Examiners


These are polymers, they are long chains of many monosaccharides joined together by glycosidic bonds. Polysaccharides are sometimes referred to as complex carbohydrates because although they are composed of monosaccharides they do not have sugar properties. Generally they are:

  • insoluble in water
  • not sweet tasting
  • not sugars (because they are not readily hydrolysed)

The main uses of polysaccharides in living organisms are as structural or energy storage molecules. This section will examine starch, glycogen and cellulose, which can be classified as follows:

Energy storage in plants

Energy storage in animals


Starch is formed from the condensation of many -glucose monomers forming a chain structure. Figure 6, below, shows a small portion of this structure. In living organisms starch molecules would be made up from hundreds of glucose monomers.

Figure 6

If all the glycosidic bonds present are 1-4 glycosidic bonds then amylose starch is formed. This form of starch forms long chains which tend to coil up into a helix as shown in figure 7 below.

Figure 7
glucose molecule

If some 1-6 glycosidic bonds are present in the starch molecule the form of starch produced is called amylopectin. This form is more branched than amylose and is shown in figure 8, below.

Figure 8
1-6 glycosidic bond

In most plants the starch present is not a pure substance, but is a mixture of amylose and amylopectin. Both these forms have similar properties but with one key difference.

The properties of starch make it an ideal storage molecule

  1. Starch is an insoluble molecule. This aids it's storage function as it will not affect the water potential of the cells it is present in. It also means it cannot be washed out of cells
  2. Starch is a compact molecule so a large amount can be stored within cells
  3. Starch can easily be are broken down by the enzymes to release glucose, though at different rates. This is the difference between amylose and amylopectin. Because amylopectin have more free ends it can have many enzyme molecules hydrolysing it at the same time so it releases glucose faster than amylose.

Glycogen is the main carbohydrate storage molecule in animals. Like starch it is made of -glucose chains However it is more highly branched than amylopectin starch (about 10% compared with about 5%). This allows faster enzyme hydrolysis so glycogen can release glucose very quickly.


Cellulose is formed from -glucose monomers forming a chain structure. It is found in plants and is used as a structural material in plant cell walls. Since cellulose is made from a different isomer of glucose to starch and glycogen it has a different structure which is why it has different properties. As cellulose contains -glucose it means in a chain alternate glucose molecules are inverted. This is shown in figure 9 below

Figure 9

This allows hydrogen bonds to form between the chains of glucose. Hundreds of these chains are linked together by hydrogen bonds to form cellulose microfibrils. These microfibrils have a high tensile strength. This strength is why plant cell walls can resist a lot of pressure.

The fact cellulose is a different shape to starch and glycogen also means that different enzymes are required to hydrolyse it to it's glucose monomers. The enzyme needed is called cellulase and is only present in a few organisms.

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