module 2 - testing for biological molecules

Question 1

how do you test for the presence of starch?

Answer 1

add iodine dissolved in potassium iodide solution to test sample (the iodine test)

Question 2

describe the iodine test for starch

Answer 2

Tests for the presence of starch
1. Add iodine dissolved in potassium iodide solution to test sample
2. If starch is present the colour will change from brown/orange to dark blue/black
3. If no starch is present, the colour stays brown/orange

Question 3

how do you test for the presence of a protein?

Answer 3

use the Biuret test

Question 4

explain the Biuret test for proteins?

Answer 4

Test for the presence of protein
1. The test solution needs to be alkaline so first step is to add few drops of sodium hydroxide solution
2. Next add the copper (ii) sulphate solution
3. If protein is present (positive result) the solution turns pale purple
4. If no protein is present (negative result) then the solution stays pale blue

Question 5

What does the biuret test really discover?

Answer 5

Peptide bonds in proteins

Question 6

How is the colour formed in the biuret test?

Answer 6

Colour is formed by a complex between the nitrogen atoms in a peptide chain and Cu2+ ions

Question 7

what test is used to test for lipids?

Answer 7

the emulsion test

Question 8

Explain the Emulsion test.

Answer 8

Tests for the presence of lipids
1. Shake test sample with ethanol for about 1 minute then pour solution into water
2. If lipid is present, solution turns milky
The more lipid present, the more noticeable the milky colour
3. If there is no lipid present the solution stays clear

Question 9

What is used to test for reducing sugars?

Answer 9

Benedict’s reagent

Question 10

Explain the steps in the Benedict’s test.

Answer 10

1. Add Benedicts reagent (blue) to sample
2. Heat it in water bath that’s been brought to boil
3. Watch for colour change
4. Colour of precipitate changes from: blue - green - yellow - orange - brick red
5. If positive result, forms coloured precipitate
6. The higher the concentration of reducing sugar, the further the colour change goes (used to compare amount of reducing sugars in different solutions)

Question 11

Define reducing sugar

Answer 11

A sugar which can give electrons (reduce) to other molecules
Reducing sugars include all monosaccharides and some disaccharides

Question 12

What causes the change in colour of the Benedict’s solution?

Answer 12

The Cu2+ in benidict solution reduces into Cu+ making copper oxide giving that orange colour

Question 13

What is a more accurate method than the Benedict’s test to compare the amount of reducing sugars in different solutions?

Answer 13

Filter the solution and weigh the precipitate

Question 14

What is the test for non-reducing sugars?

Answer 14

Boiled with hydrochloric acid to hydrolyse the bonds and free up the reducing sugar group e.g. Sucrose split into glucose and fructose

Question 15

How can colorimetry be used to calculate the concentraction of reducing sugar present?

Answer 15

A colorimeter is a piece of equipement that quantitatively measures the absorbance, or transmission, of light be a coloured solution.
The more concentrated a solution is the more light it will absorb and the less light it will transmit.