3.7 Biochemical tests for molecules

Question 1

How do you test for reducing sugars?

Benedict’s test

Answer 1

1) Add Benedict’s reagent to a sample and heat it in a water bath that’s been brought to boil.
2) The colour of the precipitate changes from blue->green->yellow->orange->brick-red
3) If the test’s positive it will form a coloured precipitate.
4) The higher the concentration of reducing sugar, the further the colour change goes.

Question 2

How do you test for non-reducing sugars?

Benedict’s test

Answer 2

1) If the result of the reducing sugar is negative, there could still be a non-reducing sugar present. To test for non-reducing sugars like sucrose, first you have to break them down into monosaccharides.
2) You do this by getting a new sample of the test solution, adding dilute hydrochloric acid and carefully heating it in a water bath. You then neutralise it with sodium hydrogencarbonate. Then carry out the Benedict’s test as you would for a reducing sugar.
3) If the test’s positive it will form a coloured precipitate. If the test’s negative it will stay blue, which means it doesn’t contain any sugar.

Question 3

What do you use the test strips for?

Answer 3

• Glucose can also be tested for using test strips coated in reagent.
• The strips are dipped in a test solution and change colour if glucose is present.
• The colour change can be compared to a chart to give an indication of the concentration of glucose present.
• The strips are useful for testing a person’s urine for glucose which may indicate they have diabetes.

Question 4

What do you use the iodine test for?

Answer 4

To test starch.

• Add iodine dissolved in potassium iodide solution to the test sample.
• If starch is present, the sample changes from browny-orange to a dark, blue-black colour.
• If there’s no starch, it stays browny-orange.

Question 5

What do you use the Biuret test for?

Answer 5

To test proteins.

1) Add a few drops of sodium hydroxide solution to the test solution to make it alkaline.
2) Add some copper sulfate solution. If protein is present the solution turns purple. If protein is not present, the solution stays blue.

Question 6

What do you use the emulsion test for?

Answer 6

To test lipids.

1) Shake the test substance with 2cm^3 of ethanol for about a minute, then add 2cm^3 of distilled water.
2) If lipid is present, the solution will turn milky. If there’s no lipid the solution will stay clear.

Question 7

What is colorimetry used for?

Answer 7

• Colorimetry is used to determine the concentration of a glucose solution.
• A colorimeter is a device that measures the strength of a coloured solution by seeing how much light passes through it.
• It measures absorbance.
• The higher the glucose concentration, the lower the absorbance of the solution.

Question 8

How do you determine the concentration of a glucose solution?

Answer 8

1) Make up several glucose solutions of different known concentrations.
2) Do a Benedict’s test on each solution and use the same amount of Benedict’s solution for each.
3) Remove the precipitate- centrifuge them
4) Use a colorimeter (with a red filter) to measure the absorbance of the Benedict’s solution remaining in each tube.
5) Use the results to make the calibration curve, showing absorbance against glucose concentration.
6) Then test the unknown solution in the same way as known concentrations and use the calibration curve to find its concentration.