Biochemical Tests for Molecules

Question 1

Starch

Iodine test

Answer 1

• add iodine dissolved in potassium iodide solution to test sample
• if starch is present, brown/orange to dark blue/black
  no starch, stays brown/orange

Question 2

Protein

Biuret test

Answer 2

• test solution needs to be alkaline so add few drops of sodium hydroxide solution
• add some copper (ii) sulphate solution
• if protein present (positive result), turns pale purple
• no protein (negative result), stays pale blue

Question 3

Lipid

Emulsion test

Answer 3

• shake test sample with ethanol for about 1 minute then pour solution into water
• if lipid is present, solution turns milky
• more lipid, more noticeable the milky colour is
• no lipid, solution stays clear

Question 4

Glucose

Test strips

Answer 4

• glucose tested using test strips coated in a reagent
• strips dipped in test solution and change colour if glucose is present
• colour change compared to chart to give indication of the concentration of glucose present

Question 5

why are glucose test strips useful?

Answer 5

• useful to test a person’s urine for glucose to indicate if person has diabetes

Question 6

reducing sugar

Benedicts test

Answer 6

• reducing sugars include all monosaccharides and some disaccharides
• add Benedicts reagent (blue) to sample and heat it in water bath that’s been brought to boil
• colour of precipitate changes from:
  blue - green - yellow - orange - brick red
• if positive result, forms coloured precipitate
• higher the concentration of reducing sugar, further the colour change goes (used to compare amount of reducing sugars in different solutions)

Question 7

what is a more accurate method to compare the amount of reducing sugars in different solutions?

Answer 7

filter the solution and weigh the precipitate

Question 8

Non-reducing sugars

Benedicts test

Answer 8

• if negative result for reducing sugar then non-reducing sugar may be present
• firstly, have to break non-reducing sugars to monosaccharides
• add dilute HCL to new test sample and heat in water bath that’s been brought to boil
• neutralise with sodium hydrogencarbonate and carry out benedicts test
• positive result, forms coloured precipitate
• negative result, stays blue so no sugar present