AQA AS Biology Unit 1: Carbohydrates

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  • Created on: 22-08-13 16:10
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Carbohydrates
Carbohydrates contain only three elements:
Carbon
Hydrogen
Oxygen
They can be found in one of three forms:
Monosaccharides
Disaccharides
Polysaccharides
In carbohydrates, the basic monomer unit is a sugar, otherwise known as a saccharide.
A single monomer is therefore called a monosaccharide.
A pair of monosaccharides can be combined to form a disaccharide.
Monosaccharides can also be combined in much larger numbers to form polysaccharides.

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Monosaccharides
Monosaccharides are sweet-tasting, soluble substances.
They have the general formula (CH2O) n where n can be any number from 3-7.
The most important monosaccharide is glucose.
This molecule is a hexose ­ 6 carbon sugar and has the formula C6H12O6
Glucose forms a six-sided ring.
There are many isomers of glucose, with the same chemical formula but different structural
formula.
These isomers include galactose and fructose.…read more

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Disaccharides
Disaccharides are formed when two monosaccharides are joined together.
When the monosaccharides join, a molecule of water is removed and the reaction is called a
condensation reaction.
The bond that is formed is called a glycosidic bond.
When water is added to a disaccharide, it breaks the glycosidic bond forming
monosaccharides.
This is called hydrolysis.…read more

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Polysaccharides
Polysaccharides are polymers.
They are formed by combining together many monosaccharide molecules.
The monosaccharides are joined together by glycosidic bonds that were formed by
condensation reactions.
As polysaccharides are very large molecules, they are insoluble.
This feature makes them suitable for storage.
When they are hydrolysed, polysaccharides break down into disaccharides or
monosaccharides.
Some polysaccharides, such as cellulose are not used for storage but give structural support
to plant cells.
Starch is a polysaccharide made up of amylose and amylopectin.…read more

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