Qualitative tests for biological molecules

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How do we test for starch?
Add IODINE SOLUTION to a sample. If it turns from yellow-brown to BLUE-BLACK then STARCH IS PRESENT.
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Why does starch cause a iodine solution to turn blue-black?
When dissolved in potassium iodide, the iodine forms a triiodide which slips into the middle of the amylose helix.
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What are reducing sugars?
All monosaccharides and some disaccharides. They are sugars that can give electrons to other molecules.
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How do we test for reducing sugars?
HEAT a sample with BENEDICT'S SOLUTION and if there is a COLOUR CHANGE from BLUE to GREEN to YELLOW to ORANGE-RED precipitate then reducing sugars ARE PRESENT.
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What is Benedict's solution?
Alkaline copper (II) sulfate.
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Why do reducing sugars cause the colour change?
Benedict's solution contains Cu(2+) ions which the reducing sugars 'give' an electron to and therefore reduce to Cu(+) ions which form copper (I) oxide (Cu2O) which is orange-red.
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If you use Benedict's solution in excess the intensity of the red colour is proportional to ...?
The concentration of sugar. The mix will appear green if only a little precipitate is formed and fully orange-red if a lot is formed.
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What do we have to test a sample for before we prepare it to test for non-reducing sugars?
We first check there are no reducing sugars in the sample before we begin the process to hydrolyse any non-reducing sugars into reducing sugars to test for them.
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After we have tested for reducing sugars, how do we prepare a separate sample to test for non-reducing sugars?
We boil it with hydrochloric acid to hydrolyse the non-reducing sugars to reducing sugars; then we cool and then neutralise it with sodium hydrogencarbonate solution; then we test for reducing sugars.
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How would we know if a sample has BOTH reducing and non-reducing sugars?
If you test the first sample and it indicates that reducing sugars are present then you can go on and test an equal sized sample for non-reducing sugars and if there is more precipitate in the second sample, non-reducing sugars are also present.
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What is the emulsion test used to test for?
Lipids.
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What is the emulsion test?
Mix a sample thoroughly with ethanol and any lipids will dissolve into the ethanol; filter; pour into a clean test tube with water; A cloudy white emulsion indicates lipids (it is made of tiny lipid droplets).
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What does the biuret test test for?
Proteins.
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How do you do the biuret test?
You add the reagents (sometimes they are given separately as biuret A [sodium hydroxide] to be added first and then biuret B [copper sulfate] next). If the colour changes from light blue to lilac, proteins are present.
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Why does the presence of proteins cause the colour change?
It is caused when a complex is formed between the nitrogen atoms in a peptide chain and CU(2+) ions.
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Other cards in this set

Card 2

Front

Why does starch cause a iodine solution to turn blue-black?

Back

When dissolved in potassium iodide, the iodine forms a triiodide which slips into the middle of the amylose helix.

Card 3

Front

What are reducing sugars?

Back

Preview of the front of card 3

Card 4

Front

How do we test for reducing sugars?

Back

Preview of the front of card 4

Card 5

Front

What is Benedict's solution?

Back

Preview of the front of card 5
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