Biological Molecules - Carbohydrates

Topics covered include:
- monosaccharides
- hydrolysis
- condensation reactions
- disaccharides
- polysaccharides

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Carbohydrates - Monosaccharides

The formula shared by glucose and other simple sugars (e.g. fructose) is C6H12O6.

These simple sugars are made up of a single sugar unit and are known as monosaccharides.


The molecular formula C6H12O6 does not indicate how the atoms bond together. Different positions of the -H and -OH groups on the molecule are responsible for the different properties of the sugars.

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Carbohydrates - Disaccharides (1)

Each single sugar unit is a monomer and capable of combining with another to form a disaccharide.
When two monomers join together, a unit of water is given off. This is a condensation reaction.
The bond formed between the two sugar units is called a glycosidic bond.

A glycosidic bond can also be broken to release separate monomer units, when this happens water is taken in. This is a hydrolysis reaction.


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Carbohydrates - Disaccharides (2)

Different disaccharides are made by joining together different monosaccharides.

Disaccharide Component monosaccharides
lactose <- glucose and galactose
maltose <- glucose and glucose
sucrose <- glucose and fructose

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Carbohydrates - Polysaccharides (1)

Like disaccharides, polysaccharides consist of monomer units linked together by glycosidic bonds, but there are chains of these sugar units instead of just two monomer units, these are known as polymers.

(part of a branched section of a starch molecule. Starch is a polymer of the sugar α glucose.)

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Carbohydrates - Polysaccharides (2)

Uses of polysaccharides
- Starch is stored in organisms as a future energy source.
- Glycogen is stored in the liver, which releases glucose for energy when the blood sugar is low
- Cellulose has long molecules which help form a tough protective layer around plant cells. Cellulose fibres are very strong.
- Pectins are used alongside cellulose in the cell wall.

(Together cellulose and pectins give exceptional mechanical strength while still allowing permeation to a wide range of substances)

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