Monomers, Polymers & 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

Condensation reaction forms glycosidic bond between 2 monosaccharides
- maltose: glucose + glucose
- sucrose: glucose + fructose
- lactose: glucose + galactose
All have molecular formula C12H22O11

Question 8

Describe the structure and functions of starch.

Answer 8

storage of a-glucose in plant cells
- insoluble = no osmotic effect of cells
- large = does not diffuse out of cells
made from amylose:
- 1,4 glycosidic bonds
- helix with intermolecular H-bonds = compact
made from amylopectin:
- 1,4 & 1,6 glycosidic bonds
- branched = many terminal ends for hydrolysis into glucose

Question 9

Describe the structure and functions of glycogen.

Answer 9

main storage polymer of a-glucose in animal cells (but also found in plant 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 10

Describe the structure and functions of cellulose.

Answer 10

polymer of B-glucose gives rigidity to plant cell walls (prevents bursting under turgor pressure, holds stem up)
- 1,4 glycosidic bonds
- straight, unbranched molecule
- alternate glucose molecules are rotated 180 degrees
- H-bond crosslinks between parallel strands form microfibrils + high tensile strength

Question 11

Describe the Benedict’s test for reducing sugars.

Answer 11

1. Add 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 brick-red precipitate

Question 12

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

Answer 12

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 mixture using sodium carbonate solution
4. Proceed with Benedict’s test as usual

Question 13

Describe the test for starch.

Answer 13

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

Question 14

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

Answer 14

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.