Unit 2 - Testing for biological molecules

Question 1

Explain the iodine test.

Answer 1

• Tests for the presence of starch
• Add iodine dissolved in potassium iodide solution to test sample
• If starch is present the colour will change from brown/orange to dark blue/black
• If no starch is present, the colour stays brown/orange

Question 2

Explain the Biuret test.

Answer 2

• Test for the presence of protein
• The test solution needs to be alkaline so first step is to add few drops of sodium hydroxide solution
• Next add the copper (ii) sulphate solution
• If protein is present (positive result) the solution turns pale purple
• If no protein is present (negative result) then the solution stays pale blue

Question 3

What is used to test for starch?

Answer 3

Iodine

Question 4

Why does iodine turn starch blue?

Answer 4

When dissolved in potassium iodide the iodine forms a tri-iodide ion, which slips into the middle of the amylose helix.

Question 5

What does the biuret test really discover?

Answer 5

Peptide bonds in proteins

Question 6

How is the colour formed in the biuret test?

Answer 6

Colour is formed by a complex between the nitrogen atoms in a peptide chain and Cu²⁺ ions

Question 7

Explain the Emulsion test.

Answer 7

• Tests for the presence of lipids
• Shake test sample with ethanol for about 1 minute then pour solution into water
• If lipid is present, solution turns milky
• The more lipid present, the more noticeable the milky colour
• If there is no lipid present the solution stays clear

Question 8

What is used to test for reducing sugars?

Answer 8

Benedict’s solution

Question 9

Explain the steps in the Benedict’s test.

Answer 9

• Add Benedicts reagent (blue) to sample
• Heat it in water bath that’s been brought to boil
• Watch for colour change
• Colour of precipitate changes from: blue - green - yellow - orange - brick red
• If positive result, forms coloured precipitate
• The higher the concentration of reducing sugar, further the colour change goes (used to compare amount of reducing sugars in different solutions)

Question 10

Define reducing sugar

Answer 10

• A sugar which can give electrons (reduce) to other molecules
• Reducing sugars include all monosaccharides and some disaccharides

Question 11

What causes the change in colour of the Benedict’s solution?

Answer 11

The Cu²⁺ in benidict solution reduces into Cu⁺ making copper oxide giving that orange colour

Question 12

What else can test for reducing sugars?

Answer 12

Commercially manufactured test strips
Glucose strips like for testing for diabetes etc

Question 13

What are biosensors?

Answer 13

• Biosensors use biological components to determine the presence and concentration of molecules
• Glucose blood strips are an example
• Blood is the alayte which is placed onto the test strip,
• Molecular recognition uses glucose oxidase immobilised upon the surface of the strip
• Glucose in the blood reacts with a glucose oxidase producing gluconic acid causing a change in the tranducer (a change in the current between the terminals of a glucose monitor)
• Data is processed by the glucose monitor to give a reading dependent upon the current

Question 14

Why are glucose test strips useful and how do glucose test strips work?

Answer 14

• Useful to test a person’s urine for reducing sugars which could indicate if person has diabete
• Also can help a diabetic person to manage their bloods
• Glucose tested using test strips coated in reagent (Benedict’s solution)
• Strips dipped in test solution and change colour if glucose (reducing sugar) is present
• Colour change is compared to chart to give indication of the concentration of glucose present

N.B this test only shows presence of a reducing sugar, which could be glucose, this is why a blood test is then done to determine if this is glucose

Question 15

What is a more accurate method than the Benedict’s test to compare the amount of reducing sugars in different solutions?

Answer 15

Filter the solution and weigh the precipitate

Question 16

What is the test for non-reducing sugars?

Answer 16

Benedict’s test

Question 17

What must first happen to the sugar solution before it can tested for non-reducing sugars?

Answer 17

Boiled with hydrochloric acid to hydrolyse the bonds and free up the reducing sugar group e.g. Sucrose split into glucose and fructose

Question 18

After adding hydrochloric acid to free up the reducing sugar group, what must be added to the testing solution to neutralise the acid?

Answer 18

Sodium hydrogen carbonate

Question 19

What are the non-reducing sugars Benedict’s test results?

Answer 19

• Positive result, forms coloured precipitate.
• Colour changes:
  
  • Blue - Green - Yellow - Orange - Red
• Negative result, stays blue so no sugar present

Question 20

How can colorimetry be used to calculate the concentraction of reducing sugar present?

Answer 20

• A colorimeter is a piece of equipement that quantitatively measures the absorbance, or transmission, of light be a coloured solution.
• The more concentrated a solution is the more light it will absorb and the less light it will transmit.