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Topic last updated: 30 Sep 2021
Polysaccharides are giant polymers of monosaccharides, all connected by glycosidic linkages. They have an enormous range of functions in different organisms (see above). They may have a relatively simple structure or they may form more complex molecules, depending on the monosaccharide units and the types of glycosidic bonds involved.
- Starch: the term starch covers a family of giant molecules all with relatively similar structures. They are all made up of glucose units joined by α-glycosidic bonds. The differences between them result from the ratios of α-1,4 and α-1,6 glycosidic bonds in the molecules. Starches are used as storage molecules in plants and are the staple food for most of the human population of the world, found in cereal crops (e.g. wheat, rice), root crops (e.g. sweet potato, cassava) and fruits such as bananas and plantains. Examples of starches include:
- Amylose - an unbranched polymer made in plants made up of 200 - 5000 glucose units all joined by α-1,4 glycosidic bonds. It forms spiral molecules so it is very compact – ideal as a storage molecule.
- Amylopectin - a plant polymer made up of glucose units joined mainly by α-1,4 links but with some α-1,6 linkages, so the chains branch around every 24-30 glucose units. It is not as compact as amylose, but as all the chain ends are available for enzymes to hydrolyse the bonds, it can be broken down quickly to provide glucose for cellular respiration.
- Glycogen: a complex carbohydrate used as an energy store in animal and fungal cells. In humans, glycogen is stored in the liver and the muscles, with very small amounts also stored in the brain. Structurally glycogen is very similar to amylopectin but with more α-1,6 linkages – every 8-12 glucose units. This means it is very branched and so more compact than amylopectin, and it can be broken down very rapidly to provide the glucose fuel needed for cellular respiration.
- Cellulose: a complex carbohydrate made up of β-glucose molecules joined by β -1,4 glycosidic linkages. The linking of β glucose units in this way means there are hydroxyl (-OH) groups on both sides of the polymer. Many hydrogen bonds form producing strong cross-linking between the long straight cellulose molecules. Cellulose is a very strong material but the molecules do not spiral or branch so they are not compact. Cellulose if the main structural material of plant cell walls. Most animals do not have the enzymes needed to break the β -1,4 glycosidic linkages. Cellulase enzymes are only found in some protista that live in the guts of wood eating termites, and some bacteria.
The nature of these polysaccharides depends on the isomers of glucose involved in the molecules, and the type of glycosidic bonds formed.