Monomers, polymers & Carbohydrates

Question 1

Define monomer

Answer 1

A small unit from which larger molecules are made.

Question 2

Give some examples of monomers

Answer 2

● monosaccharides (glucose, fructose, galactose)
● amino acids
● nucleotides

Question 3

Define polymer

Answer 3

molecules made from a large number of monomers

joined together

Question 4

Give some examples of polymers

Answer 4

● polysaccharides
● proteins
● DNA / RNA

Question 5

What happens in a condensation reaction?

Answer 5

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

Question 6

What happens in a hydrolysis reaction?

Answer 6

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

Question 7

Name the 3 hexose monosaccharides.

Answer 7

● glucose
● fructose
● galactose
all have the molecular formula C6H12O6

Question 8

Name the type of bond formed when

monosaccharides react.

Answer 8

(1,4 or 1,6) glycosidic bond

Question 9

Name 3 disaccharides. Describe how they form.

Answer 9

condensation reaction forms glycosidic bond between 2 monosaccharides
● maltose: glucose + glucose
● sucrose: glucose + fructose
● lactose: glucose + galactose
all have molecular formula C12H22O11

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 amylopectin:
● 1,4 & 1,6 glycosidic bonds
● branched = many terminal ends for hydrolysis into glucose
made from amylose:
● 1,4 glycosidic bonds
● helix with intermolecular H-bonds = compact

Question 11

Describe the structure and functions of glycogen.

Answer 11

main storage polymer of 𝛼-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 12

Describe the structure and functions of cellulose.

Answer 12

polymer of 𝛽-glucose gives rigidity to plant cell walls
(prevents bursting under turgor pressure, holds stem up)
● 1,4 glycosidic bonds
● straight-chain, unbranched molecule
● alternate glucose molecules are rotated 180°
● 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℃ 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.
2. Plot calibration curve: absorbance (y-axis), concentration (x-axis).
3. Record absorbance values of unknown samples. Use calibration curve to read off concentration.