3.4 Testing for Carbohydrates

Question 1

What is a reducing sugar?

Answer 1

Saccharides that donate electrons resulting in the reduction of another molecule

Question 2

What is a non-reducing sugar?

Answer 2

A sugar which cannot serve as a reducing agent

Question 3

What is an example of a non-reducing sugar?

Answer 3

Sucrose, all polysaccharides

Question 4

What does a molecule need to have to be a reducing sugar?

Answer 4

A free OH group on the anomeric carbon

Question 5

How do you identity the anomeric carbon?

Answer 5

Find a carbon which is bonded directly to 2 different oxygen atoms, if this carbon has an OH group than it is a reducing sugar as it can donate electrons to the Benedict’s solution

Question 6

What is the test for a reducing sugar?

Answer 6

Benedict’s

Question 7

What is an example of a reducing sugar?

Answer 7

All monosaccharides, some disaccharides such as maltose and lactose

Question 8

Give the steps for Benedict’s test

Answer 8

1. Place 4cm3 of the sample into a boiling tube, if it is not a liquid, grind it up or blend with water
2. Use a dropping pipette add 2cm3 of Benedict’s reagent
3. Record observations of each sample immediately after adding Benedict’s reagent
4. Heat the mixture gently in boiling water bath for 5 minutes
5. After 5 minutes record your observations, if reducing sugar is present, the mixture will change for a blue solution to a brick red precipitate

Question 9

Give the steps for the test of non-reducing sugars

Answer 9

1. Put 4cm3 of sample into a boiling tube
2. Using a dropping pipe the add 2cm3 of 2moldm3 of dilute hydrochloric acid
3. Heat the mixture gently in a boiling water bath for 5 minutes (acid will hydrolyse disaccharides into monosaccharides)
4. Neutralise the mixture by adding sodium hydrogencarbonate to the sample until no more effervescence is observed
5. Retest this mixture using the test for reducing sugars
6. If non-reducing sugars were present at the start, the minute will change form blue to brick red

Question 10

If a brick-red precipitate is produced in the Benedict’s test what does this indicate?

Answer 10

Very high concentration of reducing sugar

Question 11

If a yellow/orange precipitate is produced in the Benedict’s test what does this indicate?

Answer 11

Medium concentration of reducing sugar

Question 12

If a green precipitate is produced in the Benedict’s test what does this indicate?

Answer 12

Low concentration of reducing sugar

Question 13

If a blue precipitate is produced in the Benedict’s test what does this indicate?

Answer 13

No reducing sugar is present

Question 14

What is the test for starch?

Answer 14

Iodine test

Question 15

What test is used for the presence of starch?

Answer 15

Iodine test

Question 16

What is a positive result for the iodine test?

Answer 16

yellow/brown to blue-black

Question 17

What are the steps for the Iodine test?

Answer 17

1. Place 2cm3 of food sample into a test tube
2. Add a couple of drops of iodine solution and shake
3. If starch is present, solution will turn from yellow/brown to blue/black

Question 18

What are all the methods used to test for glucose concentration?

Answer 18

-Benedict’s test
-Reagent strips
-Colorimetry
-Biosensors

Question 19

How do biosensors work to test for glucose concentration?

Answer 19

-Analyte is the compound under investigation
-Molecular recognition - a protein or single strand of DNA is immobilised to the surface (eg. glucose test strip), this will interact with or bind to the molecule under investigation
-Transduction - this interaction will cause a change in a transducer and produces a response (such as release of an immobilised dye on a test trip or an glucose testing machine)
-Display - this then produced a visible qualitative or quantitative signal such as a particular colour on a test strip or a reading on a test machine

Question 20

How do you calculate Rf value?

Answer 20

Distance moved by solute / distance moved by the solvent

Question 21

What is the stationary phase in chromatography?

Answer 21

-Where the molecules can’t move
eg. chromatography paper (in paper chromatography), glass or plastic plate (in thin-layer chromatography)

Question 22

What is the mobile phase in chromatography?

Answer 22

-Where the molecules can move
eg. liquid solvent such as ethanol, water

Question 23

What are the two types of chromatography?

Answer 23

Paper, Thin-layer

Question 24

How can a biosensor detect chemicals in a solution?

Answer 24

-Uses a biological molecule, such as an enzyme to detect a chemical
-Biological molecules produce a signal, which is converted to a electrical signal by a transducer
-Electrical signal is then processed and can be used to work out other information

Question 25

How is a biosensor used to determine glucose concentration?

Answer 25

-Uses enzyme glucose oxidase and electrodes -Enzyme catalyses the oxidation of glucose at electrodes, this creates a charge, which is converted into a electrical signal by the electrodes (the transducer) -Electrical signal is then processed to work out initial concentration of glucose