A2. Sugars

Question 1

Monosaccharides
All carbohydrates contain the elements C, H and O. The monomers that carbohydrates are made from are monosaccharides, e.g. glucose, fructose and galactose.
Glucose is a hexose sugar-a monosaccharide with six carbon atoms in each molecule. There are two types of glucose, alpha (a) and beta (B) glucose-they’re _______ (molecules with the same molecular formula as each other, but with the atoms connected in a different way).

Answer 1

All carbohydrates contain the elements C, H and O. The monomers that carbohydrates are made from are monosaccharides, e.g. glucose, fructose and galactose.
Glucose is a hexose sugar-a monosaccharide with six carbon atoms in each molecule. There are two types of glucose, alpha (a) and beta (B) glucose-they’re isomers (molecules with the same molecular formula as each other, but with the atoms connected in a different way).

Question 2

Figure 1: Glucose isomers.

Answer 2

Question 3

Disaccharide formation
What is a disaccharide?
How are they formed?

Answer 3

A disaccharide is formed when two monosaccharides join together.
Monosaccharides are joined together by condensation reactions -a glycosidic bond forms between the two monosaccharides as a molecule of water is released.

Question 4

Two α-glucose molecules are joined together by a glycosidic bond to form maltose. Draw this out

Answer 4

Question 5

What are the three disaccharides are and from what monosaccharides are they made from?

Answer 5

Glucose + Glucose = Maltose
Glucose + Galactose = Lactose
Glucose + Fructose = Sucrose

Question 6

The Benedict’s test for sugars - Reducing sugars
Reducing sugars include all monosaccharides and some disaccharides, e.g. maltose and lactose.

What is the test?

Answer 6

Reducing sugars include all monosaccharides and some disaccharides, e.g. maltose and lactose.

1. Heat with Benedict’s reagent
2. If a brick-red precipatate forms then reducing sugar is present

Question 7

The Benedict’s test for sugars - Non-reducing sugars
If the result of the reducing sugars test is negative, there could still be a non-reducing sugar present. To test for non-reducing sugars, like sucrose, first you have to break them down into monosaccharides.

What’s the test?

Answer 7

If the result of the reducing sugars test is negative, there could still be a non-reducing sugar present. To test for non-reducing sugars, like sucrose, first you have to break them down into monosaccharides.

1. Heat sample with Benedict’s reagent and get a negative result
2. Heat with dilute hydrochloric acid then neutralise sample by adding sodium hydrogencarbonate.
3. Heat sample with Benedict’s reagent.
4. Brick red precipitate forms