3.4 Testing for Carbohydrates

Question 1

What are reducing sugars?

Answer 1

Saccharides (sugars) that donate electrons resulting in the reduction (gain of electrons) of another molecule.

Question 2

All monosaccharides and some disaccharides are examples of what type of sugars?

Answer 2

Reducing sugars.

Question 3

What is used to test for a reducing sugar?

Answer 3

Benedict’s reagent, an alkaline solution of copper(II)sulfate.

Question 4

What is the first step when testing for a reducing sugar?

Answer 4

Place the sample to be tested in a boiling tube. If it is not in a liquid form, grind it up or blend it in water.

Question 5

What is the second step when testing for a reducing sugar?

Answer 5

Add an equal volume of benedict’s reagent.

Question 6

What is the third step when testing for a reducing sugar?

Answer 6

Heat the mixture gently in a boiling water bath for five minutes.

Question 7

What is a positive test for reducing sugars?

Answer 7

Brick-red precipitate.

Question 8

Why does the brick-red colour appear in a positive test for reducing sugars?

Answer 8

Reducing sugars will react with the copper ions in Benedict’s reagent, resulting in the addition of electrons to the blue Cu2+ ions, reducing them to brick-red Cu+ ions.

Question 9

How is the test for reducing sugars qualitative?

Answer 9

Concentration of the reducing sugars present in the solution depends on the amount of brick-red precipitate and blue unchanged copper ions left in the boiling tube.

Question 10

What does a green colour indicate in a benedict’s test?

Answer 10

Low concentration of reducing sugars.

Question 11

What does a yellow/amber/orange colour indicate in a benedict’s test?

Answer 11

Medium concentration of reducing sugars.

Question 12

What does a red colour indicate in a benedict’s test?

Answer 12

High concentration of reducing sugars.

Question 13

What is the most common non-reducing sugar?

Answer 13

Sucrose.

Question 14

How could you test for non-reducing sugars?

Answer 14

Boil the sample with hydrochloric acid before testing for reducing sugars.

Question 15

Why is the non-reducing sugar sample boiled with hydrochloric acid first?

Answer 15

The acid hydrolyses the non-reducing sugar to reducing sugars to be able to carry out the Benedict’s test on a non-reducing sugar.

Question 16

What is used to test for starch?

Answer 16

Iodine test.

Question 17

How is an Iodine test for starch carried out?

Answer 17

Mix the sample with a few drops of iodine dissolved in potassium iodide solution.

Question 18

What is a positive test for starch?

Answer 18

Solution changes colour from yellow/brown to purple/black.

Question 19

What is a negative test result for starch?

Answer 19

Solution remains yellow/brown.

Question 20

What are reagent strips used to test?

Answer 20

Reducing sugars, most commonly glucose.

Question 21

What is the advantage of using reagent test strips?

Answer 21

The use of colour-coded chart, where the concentration of the sugar can be determined.

Question 22

What is a colorimeter?

Answer 22

A piece of equipment used to quantitatively measure the absorbance, or transmission, of light by a coloured solution.

Question 23

Using a colorimeter, what is seen in a more concentrated solution?

Answer 23

More light will be absorbed and less light will be transmitted.

Question 24

What are biosensors?

Answer 24

A device that uses biological components to determine the presence and concentration of molecules such as glucose.

Question 25

What are the main components in a biosensor?

Answer 25

• Sample (analyte)
• Transducer
• Data processor

Question 26

What happens in a biosensor?

Answer 26

• Molecular recognition, protein or a single strand of DNA is immobilised to a surface. This will bind to the specific molecule under investigation.
• Transduction, detects the change due to the binding and produces a response.
• Display, a visible, qualitative or quantitative signal is seen