Qualitative tests for biological molecules

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.
1 of 15
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.
2 of 15
What are reducing sugars?
All monosaccharides and some disaccharides. They are sugars that can give electrons to other molecules.
3 of 15
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.
4 of 15
What is Benedict's solution?
Alkaline copper (II) sulfate.
5 of 15
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.
6 of 15
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.
7 of 15
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.
8 of 15
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.
9 of 15
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.
10 of 15
What is the emulsion test used to test for?
Lipids.
11 of 15
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).
12 of 15
What does the biuret test test for?
Proteins.
13 of 15
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.
14 of 15
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.
15 of 15

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
View more cards

Comments

No comments have yet been made

Similar Biology resources:

See all Biology resources »See all Biological molecules, organic chemistry and biochemistry resources »