Testing For Biological Molecules

Question 1

What can a reducing sugar do?

Answer 1

It can donate electrons or reduce another chemical

Question 2

Benedict’s test for reducing sugars

Answer 2

1) Place the sample in a test tube if liquid (if solid, crush and add to water)
2) Add an equal volume of Benedict’s reagent
3) Heat the mixture gently in a water bath for 5 minutes

Question 3

What happens in Benedict’s test?

Answer 3

The reducing sugars will react with Copper ions in Benedict’s reagent
Electrons will be added to the blue Cu2+ ions
They are reduced to brick red Cu+ ions

Question 4

Colours for Benedict’s test

Answer 4

Green - low concentration of reducing sugar
Yellow/Amber/Orange - medium concentration of reducing sugar
Red - high concentration of reducing sugar

Question 5

Example of a non reducing sugar

Answer 5

Sucrose

Question 6

How to test for non reducing sugars

Answer 6

1) Place sample in a test tube
2) Add Hydrochloric acid and heat in a water bath
3) Neutralise the solution with Sodium Hydrogencarbonate
4) Carry out Benedict’s test as usual

Question 7

Why does Benedict’s reagent work for non reducing sugars after adding HCl?

Answer 7

HCl will hydrolyse (break down) glycosidic bonds to form monosaccharides
The resulting monosaccharides will have an aldehyde or ketone group that can donate electrons to Copper (II) Sulfate, forming a precipitate

Question 8

What is Benedict’s reagent?

Answer 8

An alkaline solution of Cooper (II) Sulfate

Question 9

How does the iodine test for starch work?

Answer 9

A few drops of iodine dissolved in potassium iodide solution is added to the sample
If the colour changes from orange to blue/black, starch is present

Question 10

Why is iodine used in a potassium iodide solution?

Answer 10

Because iodine is insoluble in water

Question 11

How do reagent strips work?

Answer 11

The strips contain chemicals in them and when the sample reacts with these chemicals, there is a colour change
The result is compared to a chart

Question 12

Advantage of reagent strips

Answer 12

The concentration can be determined using a colour coded chart

Question 13

How to use colorimetry to determine concentration of sugars

Answer 13

1) Filter was placed in the colorimeter
2) The colorimeter was calibrated using distilled water
3) Benedict’s test was carried out on samples
4) Solutions were filtered to remove precipitate
5) The % transmission of each glucose solution was measured using the colorimeter
6) These values were used to plot a calibration curve to determine unknown concentrations

Question 14

How does a colorimeter work?

Answer 14

Measures transmission of light by a coloured solution

More concentrated solution = more light absorbed = less light transmitted

Question 15

How do biosensors work?

Answer 15

Molecular recognition - a protein enzyme or antibody or single strand of DNA is immobilised to a surface, such as a glucose strip. This will bind to the specific molecule being investigated

Transduction - this interaction causes a change in a transducer, which will produce a response, such as a dye being released on a test strip

Display - this produces a visible, qualitative or quantitative result, like a colour on a test strip or reading on a test machine

Question 16

Test to identify lipids

Answer 16

Sample is mixed with ethanol
Solution is mixed with water and shaken
If white emulsion is formed, a lipid is present

Question 17

Separating amino acids using thin layer chromatography

Answer 17

Stationary phase - silica gel
Mobile phase - organic solvent
1) Draw a line on the plate to place the amino acids
2) Dot the amino acids onto the line using a capillary tube
3) The plate was placed in a jar with the solvent and closed
4) Plate was removed from the solvent and a pencil line was drawn along the solvent front
5) Plate was sprayed with ninhydrin spray so the colours would show up

Question 18

Identifying proteins

Answer 18

1) Add Biuret’s reagent to the sample (this makes the solution blue)
2) If colour changes from blue to mauve/purple, then proteins are present
If colour remains the same, no proteins are present

Question 19

Conditions for Biuret’s test to work

Answer 19

There must be at least 2 peptide bonds in any protein molecule, so with amino acids and dipeptides, the test will come up negative

Question 20

What is Buiret’s reagent?

Answer 20

Copper (II) Sulfate solution and an alkali

Question 21

Limitations of the Biuret’s test

Answer 21

It’s qualitative and does not show how much protein is in a sample
If the sample contains amino acids or dipeptides, the result will be negative