Practical Biochemistry - Qualitative Tests

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  • Created by: shyde7
  • Created on: 02-05-16 00:33
Why are they called qualitative tests?
Because they simply determine the presence or absence of a particular biological molecule.
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What type of molecule is starch?
A carbohydrate
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What is the procedure for testing for starch?
Add iodine solution (in potassium iodide) to a sample.
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What would happen if a sample tested positive for starch?
There would be a colour change in the liquid from yellow or brown to blue or black.
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What happens chemically when iodine is dissolved in potassium iodide?
The iodine forms a triiodide ion (I3-).
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What causes the colour change?
The triiodide 'slipping' into the middle of the amylose helix.
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How is the iodine test used in brewing?
To check that any chains of glucose in the fermentation mix are relatively short.
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How is 'relatively short' defined?
Less than 9 molecules short in straight chains, less than 60 in branched chains.
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Why is it important for the chains to be relatively short in brewing?
Because longer chains can make the beer cloudy which looks unappetizing.
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What type of molecule is a reducing sugar?
A carbohydrate.
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What does the term 'reducing sugar' include?
All monosaccharides and some disaccharides.
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Why are they known as reducing sugars?
Because they can reduce (give electrons to) other molecules.
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What is the procedure for testing for a reducing sugar?
Heat a reducing sugar with Benedict's solution.
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What is Benedict's solution?
Alkaline copper (II) sulfate
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How can you tell if the sample is or contains a reducing sugar?
There is a colour change from blue to green to yellow to orange or red.
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What is the chemical process that causes this colour change?
Benedict's solution contains Cu^2+ ions, which reduce to Cu+ ions. This forms a red-orange copper (I) oxide (Cu2O) precipitate.
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What is a precipitate?
An insoluble solid that emerges from a solution.
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What happens if you use Benedict's solution in excess?
The intensity of the red colour is proportional to the concentration of sugar.
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What colour will the reaction mix turn if a little precipitate is formed?
Green.
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What colour will the reaction mix turn if a lot of precipitate is formed?
Red.
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Which other, simpler method could you use to test for reducing sugars?
You can use commercially manufactured test strips. Dip the strip into the test solution and compare with the calibration card supplied.
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How is this simpler method often used in daily life?
To test for glucose in the urine of diabetic patients.
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In testing for non-reducing sugars, why do we have to hydrolyse the bond first?
To 'free up' the 'reducing groups'.
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What is the four-step method when testing for non-reducing sugars?
1. Test the sample to make sure there are no reducing sugars to begin with. 2. Take another sample and boil it with hydrochloric acid. 3. Cool the solution and neutralise with sodium hydrogencarbonate solution. 4. Test for reducing sugars again.
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Why do we boil the sample with hydrochloric acid?
To hydrolyse the sucrose into glucose and fructose.
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How would you know if you has a positive result?
The solution would change colour (to green, yellow, orange or red).
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What would a positive result indicate?
That a non-reducing sugar was present in your first sample.
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What would you do if there were both reducing and non-reducing sugars present in a sample?
You test for non-reducing sugars with an equal sized sample. If there are non-reducing sugars present, the precipitate from the second sample would have more mass than precipitate from the first.
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How would you extract the precipitate from the mixture?
Filtration.
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Which test is used to test for the presence of lipids?
An emulsion test.
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What is the procedure for an emulsion test?
Mix a sample thoroughly with ethanol, filter ad pour the solution into water.
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What would indicate the presence of lipids?
A cloudy white emulsion.
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What is the emulsion made of?
Tiny lipid droplets that come out of solution when mixed with water.
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What would you use to test for proteins?
A buriet test.
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What reagents would you use in this test and in which order?
Biuret A is sodium hydroxide, and goes in first. Biuret B is copper sulfate, which goes in second.
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How would you know that a protein is present?
The colour changes from light blue to lilac.
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How is this colour formed?
A complex forms between the nitrogen atoms in a peptide chain and Cu^2+ ions.
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This complex means that the test really detects the presence of?
Peptide bonds, not proteins themselves.
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Card 2

Front

What type of molecule is starch?

Back

A carbohydrate

Card 3

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What is the procedure for testing for starch?

Back

Preview of the front of card 3

Card 4

Front

What would happen if a sample tested positive for starch?

Back

Preview of the front of card 4

Card 5

Front

What happens chemically when iodine is dissolved in potassium iodide?

Back

Preview of the front of card 5
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