1.4 Starch, glycogen and cellulose

What is starch?

• Polysaccharide that is found in many parts of a plant in the form of small grains

• Especially large amounts occur in seeds and storage organs, such as potato tubers

• Major energy source in most diets

• Made up of chains of alpha glucose monosaccharides linked by glycosidic bonds that are formed by condensation reactions

• The chains may be branched or unbranched. The unbranched chain is wound into a tight coil that makes the molecule very compact

What is the structure of starch like for its main role to be energy storage?

• Insoluble and therefore doesn’t affect water potential, so water is not drawn into the cells by osmosis

• Being large and insoluble, it does not diffuse out of cells

• Compact, so a lot of it can be stored in a small place

• When hydrolysed it forms alpha glucose, which is both easily transported and readily used in respiration

• The branched form has many ends, each of which can be acted on by enzymes simultaneously meaning that glucose monomers are released very rapidly

What is glycogen?

• Found in animals and bacteria but never in plant cells

• Very similar structure to starch but has shorter chains and is more highly branched

• Major carbohydrate storage product of animals

• In animals it is stored as small granules mainly in the muscles and liver

• The mass of carbohydrate that is stored is relatively small because fat is the main storage molecule in animals

How does the structure of glycogen suit it for storage?

• Insoluble and therefore does not tend to draw water into the cells by osmosis

• Being insoluble, it does not diffuse out of cells

• Compact, so a lot of it can be stored in a small space

• It is more highly branched than starch and so has more ends that can be acted on simultaneously by enzymes. It is therefore more rapidly broken down to form glucose monomers, which are used in respiration. This is important to animals which have a higher metabolic rate and therefore respiratory rate than plants because they are more active

What is cellulose? 

• Made of monomers of beta glucose

• Straight, unbranched chains

• The chains run parallel to one another, allowing hydrogen bonds to form cross-linkages between adjacent chains

• While each individual hydrogen bond adds very little to the strength of the molecule, the sheer overall number of them makes a considerable contribution to strengthening cellulose, making it the valuable structural material that it is

  • Cellulose molecules are grouped together to form microfibrils which, in turn, are arranged in parallel groups called fibres

What is the role of cellulose in plant cells?

• Major component of plant cell walls and provides rigidity to the plant cell

• The cellulose cell wall also prevents the cell from bursting as water enters it by osmosis. It dooes this by exerting an inward pressure that stops any further influx of water. 

• As a result, living plant cells are turgid and push against one another, making non-woody parts of the plant semi-rigid. This is especially important in maintaining stems and leaves in a turgid state so that they can provide the maximum surface area for photosynthesis