3.1.2 Carbohydrates

Question 1

Name the 3 hexose monosaccharides.

Answer 1

Glucose, fructose, galactose

Question 2

Name the type of bond formed when monosaccharides react.

Answer 2

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

Question 3

Name 3 disaccharides. Describe how they form.

Answer 3

Condensation reaction forms glycosidic bond between 2 monosaccharides.
Maltose: glucose + glucose
Sucrose: glucose + fructose
Lactose: glucose + galactose

Question 4

Draw the structure of α-glucose.

Answer 4

Question 5

Draw the structure of ß-glucose.

Answer 5

Question 6

Describe the structure and functions of starch.

Answer 6

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.
Amylose
- 1,4 glycosidic bonds
- helix with intermolecular hydrogen bonding = compact
Amylopectin
- 1,4 and 1,6 glycosidic bonds
- branched = many terminal ends for hydrolysis into glucose

Question 7

Describe the structure and functions of glycogen.

Answer 7

Main storage polymer of α-glucose in animal cells (but also found in plant cells)
- 1,4 glycosidic bonds
- branched = many terminal ends for hydrolysis
- insoluble - no osmotic effect and does not diffuse out of cells
- compact

Question 8

Describe the structure and functions of cellulose.

Answer 8

Polymer of ß-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°
- hydrogen bond crosslinks between parallel strands form microfibrils = high tensile strength

Question 9

Describe the Benedict’s test for reducing sugars.

Answer 9

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 minutes.
3. Positive result: colour change from blue to orange and brick-red precipitate forms.

Question 10

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

Answer 10

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 minutes.
3. Neutralise the mixture using sodium carbonate solution.
4. Proceed with the Benedict’s test as usual.

Question 11

Describe the test for starch.

Answer 11

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

Question 12

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

Answer 12

1. Make standard solution 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.