Biological molecules

Question 1

Define monomer and 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 & 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

glycosidic bond

Question 7

What is a disaccharide

Answer 7

2 monomers
1 chemical bond

Question 8

Name 3 disaccharides. Describe how they form.

Answer 8

condensation reaction forms glycosidic bond between 2 monosaccharides
● maltose: glucose + glucose
● sucrose: glucose + fructose
● lactose: glucose + galactose

Question 9

Describe the structure and functions of starch

Answer 9

storage polymer of 𝛼-glucose in plant cells
● insoluble = no osmotic effect on cells
● large = does not diffuse out of cells

Question 10

Describe the structure and functions of glycogen.

Answer 10

main storage polymer of 𝛼-glucose in animal cells ( but also found in plant cells)
● glycosidic bonds

● branched = many terminal ends for hydrolysis
● insoluble = no osmotic effect & does not diffuse out of cells
● compact

Question 11

Describe the structure and functions of cellulose

Answer 11

polymer of 𝛽-glucose gives rigidity to plant cell walls (prevents bursting under turgor pressure, holds stem up)
● glycosidic bonds
● straight-chain, unbranched molecule
● H-bond crosslinks between parallel strands form microfibrils = high tensile strength

Question 12

Describe the Benedict’s test for reducing sugars

Answer 12

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

Question 13

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

Answer 13

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 14

Describe the test for starch

Answer 14

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

Question 15

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

Answer 15

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