2.2.3 Monosaccharides

Question 1

Reducing and Non-Reducing Sugars

Answer 1

Sugars can be classified as reducing or non-reducing
This classification is dependent on their ability to donate electrons

Question 2

Reducing Sugars

Answer 2

Reducing sugars can donate electrons (the carbonyl group becomes oxidised)
They sugars become the reducing agent
- thus reducing sugars can be detected using Benedict’s test as they reduce the soluble copper sulphate to insoluble brick-red copper oxide
Examples of reducing sugars include:
- glucose
- fructose
- galactose
Fructose and galactose have the same molecular formula as glucose however they have a different structural formula
The different arrangement of atoms in these monosaccharides gives them slightly different properties

Question 3

Non-Reducing Sugars

Answer 3

Non-reducing sugars cannot donated electrons, therefore they cannot be oxidised
- to be detected, non-reducing sugars must first be hydrolysed to break the disaccharide into its two monosaccharides before the Benedict’s test can be carried out
An example of a non-reducing sugar is sucrose

Question 4

Types of Monosaccharides

Answer 4

There are different types of monosaccharides formed from molecules with varying numbers of carbon atoms
For example:
- trioses (3C) e.g. glyceraldehyde
- pentose (5C) e.g. ribose
- hexoses (6C) e.g. glucose

Question 5

Glucose

Answer 5

The most well-known carbohydrate monomer is gluco0se
Glucose has the molecular formula C6H12O6
Glucose is the most common monosaccharide and is of central importance to most forms of life
- the main function of glucose is an energy source
- it’s is the main substrate used in respiration, releasing energy from the production of ATP
- glucose is soluble and so can be transported in water
Glucose exists in 2 structurally different forms:
- alpha glucose
- beta glucose
It is therefore it known as a isomer
- this is a structural variety results in different functions between carbohydrates

Question 6

Polysaccharides Formed From Glucose

Answer 6

Different polysaccharides are formed from the two different isomers of glucose:
- starch (alpha glucose)
- glycogen (alpha glucose)
- cellulose (beta glucose)

Question 7

Ribose and Deoxyribose

Answer 7

Sugars that contain 5 carbon molecules are described as pentose sugars
Ribose and deoxyribose are important pentose sugars found in the nucleotides that make up RNA and DNA
Ribose and deoxyribose are very similar in terms of structure
- deoxyribose has lost one oxygen atom at carbon number 2