• Other polysaccharides: Glycogen is also a storage polysaccharide. It is found as small granules and is particularly abundant in liver and muscle cells. Glycogen has a similar role and structure to starch. However, it is much more branched because it has many more 1-6 glycosidic bonds. This makes glycogen less dense and more soluble than starch. Glycogen can be also hydrolised readily by enzymes and is broken down more rapidly than starch. This is reflected in the higher metabolic rate in animals compared with plants. Cellulose is a tough structural polysaccharide. It is the major constituent of plant cell walls. Cellulose is completely permeable: it allows water and dissolved substances to enter and leave plant cells freely. The cells swell when they take in water by osmosis and the cell wall prevents the cells from bursting when this happens. Cellulose becomes impermeable when the gaps between fibres are filled with impermeable substances. Unlike starch and glycogen, cellulose cannot be hydrolysed easily. Herbivores such as cows and elephants are able to digest grass because microorganisms in their guts produce cellulase, the enzyme that digests cellulose. Humans and most other animals do not produce cellulase so they cannot obtain the nutrient content from plant cells. In wood, cellulose is further strengthened by lignin. Lignin is a highly complex non-carbohydrate polymer. It impregnates the cell walls of the water-transporting tubes (xylem) to form an impermeable lining, a process called lignification. Xylem cells die when they become completely impregnated or lignified. In addition to strengthening cells, lingin also helps prevent rot, infection and decay.
Posted on: Thu, 06 Nov 2014 13:49:00 +0000
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