[B1] Tests for Biological Molecules

Question 1

What does Benedict’s Test identify?

Answer 1

The presence of Reducing Sugars (Carbohydrates)

Question 2

What reagent/chemicals is used to test for Reducing Sugars (Carbohydrates)?

Answer 2

Benedict’s reagent, Sodium Carbonate, Copper(II) Suphate

Question 3

What are the steps to test for a Reducing Sugar?

Answer 3

[1] Add solution to the test tube.

[2] Add benedict’s reagent (blue).

[3] Heat for 3-5 minutes in a water bath at 75°.

[4] Identify any colour changes.

[Info] The concentration of reducing sugars determines the colour change.

• Blue/Green = None or less
• Yellow/Orange = Moderate
• Red/Brick red = High

[Understanding] Copper(II) ions in benedict’s solution is reduced to Copper(I), which is insoluble, by the reducing sugars present which causes the colour change.

[Understanding] Sodium carbonate in the Benedicts solution makes the solution alkaline to allow the redution to take place.

Question 4

What are the extra steps are taken to test for Non-Reducing Sugars rather than Reducing Sugars?

Answer 4

[1] Using a new sample of the test solution, add dilute HCl (to hydrolyse).

[2] Heat for 3-5 minutes in water bath at 75°.

[3] Add sodium hydrogen-carbonate (which neutralises the solution).

[4] Carry out Benedict’s Test (same as reducing sugars).

[Info] Colour changes apply for non-reducing sugars also.

Question 5

What molecules does Iodine test for?

Answer 5

Starch

Question 6

What is used to test for Starch?

Answer 6

Iodine (Potassium Iodide)

Question 7

What are the steps to test for Starch?

Answer 7

[1] Place solution in a test tube.

[2] Add iodine to the solution and gently shake.

[3] Check for any colour changes.

[Info] If starch is present, the solution changes to blue-black, otherwise it remains brown-brown.

[Understanding] Iodine slips into amylose coil (present in starch) to produce a blue-black colour.

Question 8

What test/substance is used to test for Lipids?

Answer 8

Emulsion Test, ethanol

Question 9

What is identified using the Emulsion test?

Answer 9

Lipids

Question 10

What are the steps used to test for Lipids?

Answer 10

[1] Place sample in test tube. If solid, crush it with a pestle and mortar first.

[2] Add ethanol and mix thoroughly (for lipids to dissolve within ethanol).

[3] Pour the solution into water and mix.

[4] Identify any changes in appearance.

[Info] If lipids are present, a cloudy emulsion will appear (because lipids are insoluable). The “cloudiness” reflects the abundance of lipids.

Question 11

What is the Biuret test used to identify?

Answer 11

Proteins

Question 12

What test/substance is used to identify proteins?

Answer 12

Biuret test, Sodium Hydroxide and Copper(II) Sulphate Solution

Question 13

What is the steps used to test for Proteins?

Answer 13

[1] Place the sample in a test tube.

[2] Add a few drops of sodium hydroxide solution (to make the solution alkaline).

[3] Add some copper(II) sulphate solution (bright blue) and mix.

[4] Identify any colour changes.

[Info] If proteins are present, the solution will turn purple, otherwise it will remain blue.

[Info] The colours are pale, so you need to look carefully.

[Understanding] Copper(II) is able to form a complex with peptide bonds (which are the bonds which hold amino acids together; a chain or amino acids is a protein) in an alkali solution, which is made with the sodium hydroxide.

Question 14

What device can be used to accuratly measure a colour change?

Answer 14

Spectrophotometer

Question 15

Examples of reducing sugars?

Answer 15

Glucose, Maltose, Lactose, Fructose

Question 16

Example of a non-Reducing sugar?

Answer 16

Sucrose