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 & 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

Molecular formula: C6H12O6

• Glucose
• Fructose
• Galactose

Question 6

Name the type of bond formed when monosaccharides react

Answer 6

• (1,4 or 1,6) glycosidic bond
• 2 monomers = 1 chemical bond = disaccharide
• Multiple monomers = many chemical bonds = polysaccharide

Question 7

Name 3 disaccharides, describe how they form

Answer 7

Condensation reaction forms glycosidic bond between 2 monosaccharides, molecular formula: C12H22O11

• Maltose: glucose + glucose
• Sucrose: glucose+ fructose
• Lactose: glucose + fructose

Question 8

Describe the structure and functions of starch

Answer 8

Storage polymer of α-glucose in plant cells

• Insoluble = no osmotic effect on cells
• Large = does not diffuse out of cells
• Made from amylose and amylopectin

Question 9

What is amylose and amylopectin in starch?

Answer 9

Amylose:
- 1,4 glycosidic bonds
- Helix with intermolecular H-bonds = compact
Amylopectin:
- 1,4 & 1,6 glycosidic bonds
- Branched = many terminal ends for hydrolysis into glucose

Question 10

Describe the structure and functions of glycogen

Answer 10

Main storage polymer of α-glucose in animal cells (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 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)

• 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 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 electric water bath at 100°C 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 (sucrose) into their monomers by adding 1cm3 of HCL, heat in a 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: colour 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-xis), concentration (x-axis)
3. Record absorbance or % transmission values of unknown samples, use calibration curve to read off concentration

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-xis), concentration (x-axis)
3. Record absorbance or % transmission values of unknown samples, use calibration curve to read off concentration