Testing for Carbohydrates

Question 1

Reducing sugar method

Answer 1

• Reducing sugars can donate electrons or reduce another molecule or chemical.
• The chemical test uses Benedict’s reagent which is an alkaline solution of copper sulfate.

Method:

1. Place sample in boiling tube; if it’s not liquid, grind it up or blend with water.
2. Add equal volume of Benedict’s reagent.
3. Heat mixture gently in boiling water bath for five mins.

Question 2

Reducing sugar results

Answer 2

• Reducing sugars with react with copper ions in Benedict’s reagent.
• This results in the addition of electrons to Cu2+ ions, reducing them to brick red Cu+ ions.
• brick red precipitate formed = positive for reducing.
• The amount of Cu2+ left in solutions depends on the concentration of reducing sugars present.
• It is a qualitative test unless a colorimeter is used.

Question 3

Non reducing sugar method

Answer 3

• Sucrose is the most common non reducing sugar.
• To test for non reducing using Benedict’s reagent, you need to:
  1. Heat a new sample with dilute hydrochloric acid.
  2. Neutralise it with sodium hydrogencarbonate.
  3. Heat sample with Ben’s reagent.
• If there is a colour change towards brick red then reducing sugars are present.

Question 4

Iodine test for Starch

Answer 4

• Add iodine dissolved in potassium iodide solution to the sample.
• If starch is present, it turns from brown orange to blue black.

Question 5

Reagent strips

Answer 5

• This tests for glucose quickly and easily.
• Using the colour coded chart, concentration can be determined.
• Useful for testing a person’s urine to see if they have diabetes.

Question 6

Quantitative method

Answer 6

• Colorimeter measures the strength of a coloured solution by seeing how much light passes through it. It measures absorbance.
• The more concentrated the solution is, the more light it will absorb and less light it will transmit.

Question 7

Finding unknown concentration using colorimeter

Answer 7

1. A filter was placed in the colorimeter.
2. The colorimeter was calibrated using distilled water.
3. Ben’s test was performed on a range of concentrations of glucose.
4. The resulting solutions were filtered to remove precipitate.
5. The % transmission of each of the solutions was measured using the colorimeter.
6. A calibration curve was plotted.
7. Repeat this with the unknown and use the graph to find the concentration.

Question 8

Use of Biosensors

Answer 8

• Device that uses a biological molecule, like an enzyme, to detect a chemical.
• The biological molecule produces a signal which is converted to an electrical signal by a Transducer.
• The electrical signal is then processed to work out other info.

Question 9

Glucose Biosensors

Answer 9

• Used to determine the concentration of glucose in a solution.
• Uses the enzyme Glucose Oxidase and electrodes.
• The enzyme catalyses the oxidation of glucose at the electrodes which creates a charge.
• This is converted to an electrical signal by the electrodes which are the transducers.
• The electrical signal is then processed to work out the initial glucose concentration.
• Display shows a visible, qualitative or quantitative signal.