Carbohydrates

Question 1

What are carbohydrates?

Answer 1

Carbohydrates are organic compounds with the general formula (CH2O)n or Cn(H2O)n.

Question 2

How are carbohydrates classified?

Answer 2

Based on the number of glucose molecules:

1. Monosaccharides: have 1 sugar molecules, exp. Glucose.
2. Disaccharides: have 2 glucose molecules, exp. Maltose and Sucrose.
3. Oligosaccharides: have 3-10 glucose molecules, exp. Raffinose.
4. Polysaccharides: have >10 molecules, exp. Starch.

Question 3

How is starch tested for?

Answer 3

By heating up the substance and adding a few drops of iodine. If starch is present, the substance will turn dark blue.

Question 4

How are simple sugars (glucose and fructose eg) tested for?

Answer 4

By heating up the substance and adding a few drops of Fehling’s solution. If the colour turns from orange to dark red, then Cu2+ has been reduced to Cu+ and a simple sugar is present.

Question 5

How are disaccharides tested for?

Answer 5

By adding Benedict’s solution. Colour change from blue to red indicates the presence of disaccharides.

Question 6

Explain why sucrose does not reduce Fehling’s solution (i.e. no colour change in it).

Answer 6

Sucrose does not reduce Fehling’s solution because it, unlike glucose, does not contain a free aldehyde group to reduce the solution; this is why it is not a reducing sugar.

Question 7

Draw the open chain structure of d-glucose.

Answer 7

Here