Testing for biological molecules 1

Question 1

How is food sample prepared for the food tests

Answer 1

Grind food with a mortar and pestle with distilled water until it turns into a paste and stir the mixture

Then add more distilled water and filter the solid food particles out to get a food solution (filtrate) ready for use

Question 2

What solution is used to test for starch

Answer 2

Iodine solution

Question 3

Describe the procedure used to test starch

Answer 3

• Place 3cm³ of food solution to test tube
• Add 1cm³ of a solution containing iodine and potassium iodide (iodine solution) to it

Question 4

If starch is present what colour would the iodine solution turn

Answer 4

A blue-black colour

Question 5

If starch is absent what colour would the iodine solution turn

Answer 5

It will remain orange

Question 6

What colour is the iodine solution

Answer 6

Orange

Question 7

What test is used to test for proteins

Answer 7

The biuret test

Question 8

Describe the procedure used to test proteins

Answer 8

• • Add 3cm³ of the food sample to test tube
  • Add 3cm³ of dilute sodium hydroxide solution to the food sample
  • Then add a few drops of copper(II) sulphate and
    mix

OR

• Add biuret solution (a premix of dilute sodium hydroxide and copper(II) sulphate) to the prepared food sample and mix

Question 9

If protein is present in the biuret test what colour would the solution turn

Answer 9

Purple or lilac

Question 10

If protein is absent in the biuret test what colour would the solution turn

Answer 10

It will remain blue

Question 11

What bonds do the biuret test look for

Answer 11

Peptide bonds

Question 12

Why would a solution of amino acids not give a positive result for the biuret test

Answer 12

They do not contain peptide bonds

Question 13

Which food sample does not need to be filtered to test. Why

Answer 13

Lipids - It can stick to the filter paper

Question 14

What test is used to test lipids

Answer 14

Emulsion test

Question 15

Describe the method used to test lipids

Answer 15

• Add 3 cm³ of ethanol and water to 3cm³ food solution
• Shake the solution

Question 16

What will form if lipids are present in the emulsion test

Answer 16

White cloudy emulsion

Question 17

What would happen if lipids were absent whilst conducting the emulsion test

Answer 17

The solution stays clear

Question 18

What are reducing sugars

Answer 18

Sugars that can donate an electron to another molecule

Question 19

Are all monosaccharides reducing sugars

Answer 19

Yes

Question 20

Are all disaccharides reducing sugars

Answer 20

No - only some are

Question 21

State examples of disaccharides that are reducing sugars

Answer 21

• Maltose
• Lactose

Question 22

State an example of a disaccharide that is a non-reducing sugar

Answer 22

Sucrose

Question 23

Describe the method used to test for reducing sugars

Answer 23

• Place 3cm³ of food solution in a boiling tube
• Add 3cm³ of Benedict’s solution to the solution
• Place boiling tube into a water bath

Question 24

State what would happen if reducing sugars were absent whilst conducting the Benedict’s test

Answer 24

Colour in the solution would stay blue

Question 25

State the various precipitates that will form if lipids are present in the Benedict’s test

Answer 25

• Green precipitate - Some traces of reducing sugar
• Yellow precipitate - Small amount of reducing sugar
• Orange precipitate - Moderate amount of reducing sugar
• Brick red precipitate - Large amount of reducing sugar

Question 26

Describe the method used to test for non-reducing sugars in an unknow solution

Answer 26

• Carry out the Benedict’s test first and record any changes - To see if it also has any reducing sugars
• • Take a fresh boiling tube adding 3cm³ of unknown solution to it
• Add 3cm³ of dilute hydrochloric acid to it
• Gently boil solution in water bath for few mins
• Then add 3cm³ of dilute sodium hydroxide
• Add 3cm³ of Benedict’s solution to boiling tube and heat in water bath for a few mins
• Note colour change

Question 27

If a first Benedict’s test (testing reducing sugars) for an unknown solution was negative BUT the second Benedict’s test (testing non-reducing) was orange what does that mean

Answer 27

• Reducing sugar is not present
  BUT
• Moderate amount of non-reducing sugar present

Question 28

If a first Benedict’s test (testing reducing sugars) for an unknown solution was green BUT the second Benedict’s test (testing non-reducing) was red what does that mean

Answer 28

• Very small reducing sugar is present
  BUT
• Large amount of non-reducing sugar present

Question 29

If a first Benedict’s test (testing reducing sugars) for an unknown solution was resulted in a red precipitate, why can’t a test for non-reducing sugars be conducted

Answer 29

If a non-reducing sugar was present we would not be able to see a colour change beyond red

Question 30

Why does the test for non-reducing sugars require the sodium hydroxide

Answer 30

It makes the solution alkaline because Benedict’s solution cant work under acidic conditions

Question 31

State when non-reducing sugars can be tested

Answer 31

If there is little to no reducing sugars in the initial Benedict’s test