AQA AS Biology Unit 2: Polysaccharides

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  • Created on: 22-08-13 17:14
Preview of AQA AS Biology Unit 2: Polysaccharides

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Polysaccharides are substances whose molecules are polymers made of many sugars.
They are formed by combining together many monosaccharide molecules.
This process is called a condensation reaction.
The monosaccharides are joined together by glycosidic bonds.
As polysaccharides are very large molecules, they are insoluble.
This makes them very suitable for storage.
When they are hydrolysed, polysaccharides break down into disaccharides or

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Starch is a polysaccharide found in many parts of a plant in the form of small grains.
Plants store excess glucose as starch and when a plant needs more glucose for energy it
breaks down the starch down to release glucose.
Especially large amounts occur in seeds and storage organs, such as potato tubers.
It forms an important component of food and is the major energy source in most diets.…read more

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Animal cells get energy from glucose too.
But animals store excess glucose as glycogen ­ a polysaccharide of a-glucose.
Glycogen is the major carbohydrate storage product of animals.
In animals it is stored as small granules mainly in the muscles and in the liver.
Glycogen is very similar in structure to starch but has shorter chains and is more highly
More branches means that stored glucose can be released quickly, which is
important for energy release in animals.…read more

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Cellulose is made of monomers of B-glucose.
The bonds between the sugars are straight so the cellulose chains are straight.
This is because in the B-glucose units, the positions of the ­H group and the ­ OH group is
on a single carbon atom are reversed.
In B-glucose the ­ OH group is above, rather than below, the ring.
This means that to form the glycosidic links, each B-glucose molecule must be rotated 180
compared to its neighbour.…read more

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Role of Cellulose
Cellulose is a major component of plant cell walls and provides rigidity to the plant
The cellulose cell wall also prevents the cell from bursting as water enters it by
It does this by exerting an outward pressure that stops any further influx of
As a result, living plant cells are turgid and push against one another, making
herbaceous parts of the plant semi-rigid.…read more

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Starch and Glycogen Cellulose
A-glucose B-glucose
A glycosidic bonds B glycosidic bonds
H bonds within each chain H bonds between each chain
Forms helix Forms microfibrils
Can form H- bonds with water so soluble Can't form H-bonds with water so insoluble
Reacts with iodine to form purple complex Doesn't react with iodine
Easy to digest Difficult to digest
Storage role Structural role…read more


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