AS Biology- Unit 1 - Tests

The various tests for substances- Protein, Lipids etc.

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  • Created on: 07-05-12 13:07

Tests in Unit 1 - Biology.

Protein tests detect peptide links.

Reducing sugars include all monosaccharides and some disaccharides (maltose).   Reduction is a chemical reaction involving the gain of electrons. A reducing sugar is therefore a sugar that can donate electrons to (or reduce)  another chemical (in this case Benedict's reagent). 

Non-reducing sugars do not change the colour of Benedicts regeant when they are heated with it. To detect a non-reducing sugar it must first be broken down into monosaccharide components by hydrolysis.

Starch is easily detected by it's ability to change the colour of the iodine in potassium iodide solution from yellow to blue-black. The test is carried out at room temperature.

Lipids- the cloudy colour is due to any lipid in the sample being finely dispersed in the water to form an emulsion. Light passing through this emulsion is refracted as it passes from oil droplets to water droplets, making it appear cloudy.

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Test for Protein.

-The test solution needs to be alkaline so you must first add a few drops of Sodium Hydroxide solution at room temperature to the sample.

-Then you add some Copper (II) Sulphate solution and mix gently.

-A purple colouration indicates the prescence of peptide bonds and hence a protein. If no protein is present, the solution remains blue. 

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Tests for Carbohydrates- Reducing Sugars.

-If the sample is not already in liquid form, grind it up in water.

- Add a few drops of Benedict's Reagent (which is blue).

-Heat the solution in a gently boiling water bath for 5 minutes.

-If the sample contains reducing sugars then it will gradually turn a brick red colour due to the formation of a red precipitate.

-The results of Benedict's test according to the concentration of reducing sugar present:

(None Present) Light Blue--Green--Yellow--Orange--Red (Highest Concentration)

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Tests for Carbohydrates- Non-Reducing Sugars.

-Add Benedict's Reagent to the sample and heat for 5 minutes. If the Benedict's Reagent does not change colour (the solution reamins blue) then a reducing sugar is not present.

-Add a few drops of dilute hydrochloric acid in another test tube with another component of the sample in it and heat the solution. The dilute hydrochloric acid will hydrolyse any disaccharide present into it's monosaccharide components.

-Add some Sodium Hydrocarbonate Solution to the solution in order to neutralise the Hydrochloric Acid (Benedict's Reagent doesn't work in acidic conditions.

-Test the solution with pH paper to check that the solution is alkaline.

-Re-test the resulting solution by adding Benedict's Reagent and heating it.

-If a Non-Reducing Sugar was present in the original sample, the Benedict's reagent will now turn orange-brown. This is due to the Reducing Sugars that were produced from the hydrolysis of the Non-Reducing Sugar.

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Tests for Carbohydrates- Starch.

-Add a few drops of Iodine Solution to the Sample.

-Shake or Stir the Solution.

-The prescence of Starch is indicated by a change of colour from browny-orange to a dark, black-blue colour. 

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Test for Lipids.

-Take a completely dry and grease-free test tube.

-Add Ethonal to the sample being tested.

-Shake the solution thoroughly to dissolve any lipids in the sample.

-Add water to the solution and shake gently.

-A cloudy-white colour indicates the prescence of a lipid.

- As a control, repeat the procedures using water instead of the sample; the final solution should remain clear.

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The audio is terrible sorry :( 1/5

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