Monomers, Polymers and Carbohydrates

Question 1

Define monomer. Give some examples.

Answer 1

Smaller units that join together to form larger molecules.
- monosaccharides (glucose, fructose, galactose)
- amino acids
- nucleotides

Question 2

Define polymer. Give some examples.

Answer 2

Molecules formed when many monomers join together.
- polysaccharides
- proteins
- DNA/RNA

Question 3

What happens in a condensation reaction?

Answer 3

A chemical bond forms between 2 molecules and a molecule of water is produced.

Question 4

What happens in a hydrolysis reaction?

Answer 4

A water molecule is used to break a chemical bond between 2 molecules.

Question 5

Name the 3 hexose monosaccharides.

Answer 5

• glucose
• fructose
• galactose
  All have the molecular formula C6H12O6

Question 6

Name the type of bond formed when monosaccharides react.

Answer 6

(1,4 or 1,6) glycosidic bond

Question 7

Name 3 disaccharides. Describe how they form.

Answer 7

Glycosidic bonds
- maltose: glucose + glucose
- sucrose: glucose + fructose
- lactose: glucose + galactose
all have the molecular formula C12H22O11

Question 8

Draw the structure of α-glucose.

Answer 8

HO — OH

Question 9

Draw the structure of β-glucose.

Answer 9

HO — H

Question 10

Describe the structure and functions of starch.

Answer 10

Storage polymer of α-glucose in plant cells
- insoluble = no osmotic effect on cells
- large = does not diffuse out of cells
Made from amylose:
- 1,4 glycosidic bonds
- helix with intermolecular H-bonds = compact
And Amylopectin:
- 1,4 and 1,6 glycosidc bonds
- branched = many terminal ends for hydrolysis into glucose.

Question 11

Describe the structure and functions of glycogen.

Answer 11

Main storage polymer of α-glucose in animal cells
- 1,4 & 1,6 glycosidic bonds
- branched = many terminal ends for hydrolysis
- insoluble = no osmotic effect & does not diffuse out of cells
- compact

Question 12

Describe the structure and functions of cellulose.

Answer 12

Polymer of β-glucose gives rigidity to plant walls
-1,4 glycosidic bonds
- straight chain, unbranched molecule
- alternate glucose molecules are rotated 180 degrees
- H-bond crosslinks between parallel strands form microfibrils = high tensile strength

Question 13

Describe the Benedict’s test for reducing sugars.

Answer 13

1. Add an equal volume of Benedict’s reagent to a sample
2. Heat the mixture in an electric water bath at 100 degrees for 5 mins
3. Positive result: colour change from blue to orange & brick-red precipitate forms.

Question 14

Describe the Benedict’s test for non-reducing sugars.

Answer 14

1. Negative result: Benedict’s reagent remains blue
2. Hydrolyse non reducing sugars e.g. sucrose into their monomers by adding 1cm3 of HCl. Heat in a boiling water bath for 5 mins.
3. Neutralise the mixture using sodium carbonate solution.
4. Proceed with the Benedict’s test as usual.

Question 15

Describe the test for starch.

Answer 15

1. Add iodine solution
2. Positive result: colour change from orange to blue-black.

Question 16

Outline how colorimetry could be used to give qualitative results for the presence of sugars and starch.

Answer 16

1. Make standard solutions with known concentrations. Record absorbance or % transmission values
2. Plot calibration curve: absorbance or % transmission (y-axis), concentration (x-axis)
3. Record absorbance or % transmission values of unknown samples. Use calibration curve to read off concentration.