1.2 Carbohydrates

Question 1

(Carbohydrates) Name the 3 hexose monosaccharides.

Answer 1

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

Question 2

(Carbohydrates) Name the type of bond formed when monosacchardies react.

Answer 2

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

Question 3

(Carbohydrates) 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
All have molecular formula C12H22O11

Question 4

(Carbohydrates) Draw the structure of alpha glucose.

Answer 4

Correct correctly draw structure, with H above OH.

Question 5

(Carbohydrates) Draw the structure of beta glucose.

Answer 5

Correct drawn structure, with OH above H.

Question 6

(Carbohydrates) Describe the structure and functions of starch.

Answer 6

Storage of alpha glucose in plant cells.
- Insoluble = no osmotic effect on 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 7

(Carbohydrates) Describe the structure and functions of glycogen.

Answer 7

Main storage parliament of alpha 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 and does not diffuse out of cells
- Compact

Question 8

(Carbohydrates) Describe the structure and functions of cellulose.

Answer 8

Polymer of beta glucose, gives rigidity to plant cell walls (prevents bursting under turgor pressure, hold stem up).
- 1-4 glycosidic bonds
- Straight-chain, unbranched molecule
- Alternate glucose molecules are rotated 180°
- H-bond cross links between parallel strands for microfibrils = high tensile strength

Question 9

(Carbohydrates) Describe the Benedict test for reducing sugars. (3)

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

Question 10

(Carbohydrates) Describe the Benedict’s test for non-reducing sugars. (4)

Answer 10

1) Negative result: Benedict’s reagent remains blue.
2) Hydrolyse non-reducing sugars e.g. sucrose into the monomers by adding 1 cm³ of hydrochloric acid. Heat in a boiling water bath for 5 minutes.
3) Neutralise the mixture by adding sodium carbonate solution.
4) Proceeded with the Benedict’s test as usual.

Question 11

(Carbohydrates) Describe the test for starch. (2)

Answer 11

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

Question 12

(Carbohydrates) Outline how colorimetry could be used to give qualitative results for the presence of sugars and starch. (3)

Answer 12

1) Make standard solutions with known concentrations. Record absorbent or percentage transmission values.
2) Plot calibration curve: absorbance or percentage transmission (y-axis), concentration (x-axis).
3) Record absorbance or percentage transmission values of unknown samples. Use calibration curve to read off concentration.