Carbohydrates

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Monosaccharides

Carbohydrates can be broken down in extrememly smaller molecules. 

Each individual unit is called a monomer. These monomers join to make longer chains such as polymers.

In carbohydrates, the smallest indivdual monomer unit is called a monosaccharide.

Common monosaccharides include:-

- Glucose

- Fructose

- Lactose

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The Benedicts Test

Using the Benedicts solution test is a good way to test for reducing sugars (e.g. glucose).

The steps of this test are:-

1) Put your monosaccharide solution into a testube.

2) Put a small amount of Benedict's solution into the testube.

3) Heat in a water bath for 3-4 minutes.

4) If a monosaccharide is present, it should turn a brick-red colour.

For any other disaccharide or no sugar, it should remain blue. 

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Disaccharides

There are three main disaccharides:-

1) Maltose (glucose + glucose).

2) Sucrose (glucose + fructose).

3) Lactose (glucose + galactose)

Disaccharides have a glycosidic bond.

With the then exception of Maltose, they cannot be tested via Benedict's reagant.

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Testing for Disaccharides

There is a method for the testing of disaccharides.

Method:

1) Using 2cm3 of the food solution, carry out a normal Benedict's test.

2) If there is no change, a reducing sugar is not present.

3) Therefore, add 2cm3 of a different sample  of food solution to 2cm3 of hydrochloric acid and gently heat in a water bath for 5 minutes. The HCl will hydrolyse the disaccharide.

4) Then add some sodium hydrogencarbonate to the mixture in order to neutralize the acid (as Benedict's solution won't work unless neutral).

5) Re-test the solution by carrying out the Benedict's test.

6) If a non-reducing sugar was present in the original sample, the solution will now turn brick-red.

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Polysaccharides

Formed by many monosaccharide bonds joined by glycosidic bonds.

They are insoluble. (large molecules.)

- Hydrolysing polysaccharides breaks them down into mono- and disaccharides.

Polysaccharides can be used for:-

1) Storage (e.g. Starch)

2) Structure. (support for plants)

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Testing for Starch

You can test for starch using the iodine test:-

1) Place 2cm3 into test tube.

2) Add two drops of iodine and stir.

3) If starch is present, the solution will turn blue-black.

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