Carbohydrates

Question 1

What elements do all carbohydrates contain?

Answer 1

Carbon, hydrogen and oxygen

Question 2

Give the formula for glucose, a hexose sugar

Answer 2

C₆H₁₂O₆

Question 3

Give the chemical equation for glucose + fructose

Answer 3

Glucose + Fructose -> Sucrose + water
C₆H₁₂O₆ + C₆H₁₂O₆ -> C₁₂H₂₂O₁₁ + H₂O

Question 4

What bond is formed from a condensation reaction?

Answer 4

Glycosidic bond

Question 5

Explain the difference between a condensation and a hydrolysis reaction

Answer 5

A condensation reaction forms a glycosidic bond and a molecule of water, whilst a hydrolysis reaction requires a molecule of water to break the glycosidic bond

Question 6

What is an isomer?

Answer 6

A variant of a molecule, like α-glucose and β-glucose

Question 7

What is the difference between an α-glucose and a β-glucose molecule?

Answer 7

An α-glucose molecule has its hydroxyl group below the hydrogen, whilst the β-glucose has its hydroxyl group above the hydrogen

Question 8

Describe the biochemical test for reducing sugars

Answer 8

Benedicts solution is added and heated. A red colour change shows a positive test

Question 9

What happens if there is no colour change when testing for sugars?

Answer 9

Take a fresh sample and boil with a dilute acid. This hydrolyses the bonds in the sugar. Neutralise this then with an alkali and reheat with benedicts solution. If a positive test now occurs the sugar is non-reducing

Question 10

What are the issues with the benedicts test?

Answer 10

It is non specific as it doesn’t tell us which reducing sugar is present. It also is qualitative as a colour change is used to determine the result

Question 11

What is the use for a colorimeter in the benedicts test?

Answer 11

The colorimeter quantifys the results meaning that it accurately identifies what the colour change is