Biochemical Tests For Molecules

Question 1

How do you carry out a test for determining a reducing sugar? What is the positive result of the test?

Answer 1

• pour 2cm cubed of sample solution into a test tube
• add 5 cm cubes of BENEDICTS reagent
• heat gently in a water bath (85 degrees) for 2 minutes
• if the test is positive it will form a coloured precipitate, with the higher the concentration of reducing sugar, the further the colour change
• blue = green, yellow, orange, brick red

Question 2

How do you carry out a test for determining a non- reducing sugar? What is the positive result of the test?

Answer 2

-if the result of the reducing sugars test is negative, then you have to test for a non reducing sugar

• pour 2cm cubed of sample solution into a test tube
• add 1cm cubed of 1m dilute hcl
• boil in a water bath for 1 minute
• neutralise solution with sodium hydrogencarbonate powder
• check pH with indicator paper
• then carry out benedicts test as you would for a reducing sugar

-if tests positive it’ll form a coloured precipitate, but if negative solution will remain blue, meaning it contains no sugar at all.

Question 3

How do you carry out the test for determining starch? What is the positive result of the test?

Answer 3

• pour 1cm cubed of sample into test tube
• add a few drops of iodine solution

-positive result is change from brown/orange to blue black

Question 4

How do you carry out the test for determining proteins? What is the positive result of the test?

Answer 4

• pour 2cm cubed of sample solution into test tube
• add an equal amount of biuret solution

-positive result is colour change from blue to purple

Question 5

What is the test for determining a lipid? What is the positive result of the test?

Answer 5

• pour 1cm cubed of ethanol into a test tube
• add 1cm cubed of sample solution
• shake vigorously for a minute to dissolve
• add an equal volume of water to test tube

-positive result solution will turn milky and form a cloudy emulsion