Carbohydrates

Question 1

What happens in a condensation reaction?

Answer 1

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

Question 2

What happens in a hydrolysis reaction?

Answer 2

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

Question 3

Name the 3 hexose monosaccharides

Answer 3

-Glucose
-fructose
-galactose

Question 4

Name the type of bond formed when monosaccharides react

Answer 4

Glycosidic

Question 5

What is maltose made of

Answer 5

Glucose + Glucose

Question 6

What is sucrose made of?

Answer 6

Glucose + fructose

Question 7

What is lactose made of?

Answer 7

Glucose + galactose

Question 8

Draw the structure of alpha glucose

Answer 8

OH group at the bottom

Question 9

Draw the structure of beta glucose

Answer 9

OH group on the top

Question 10

Describe the functions of starch

Answer 10

• Storage polymer of alpha glucose in plant cells
• Insoluble so no osmotic effect on cells
• Large so it does not diffuse out of the cell

Question 11

Describe the structure of starch

Answer 11

It is made up of amylose and amylopectin
Amylose:
- 1,4 Glycosidic bonds
- it is compact because its a helix with intermolecular hydrogen bonds
Amylopectin:
- 1,4 & 1,6 glycosidic bonds
- Branched so many terminal ends for hydrolysis into glucose

Question 12

Describe the function of glycogen

Answer 12

• Main storage polymer of alpha glucose in animal cells
• insoluble so no osmotic effect and does not diffuse out of cells

Question 13

Describe the structure of glycogen

Answer 13

• 1,4 and 1,6 glycosidic bonds
• Branched so many terminal ends for hydrolysis

Question 14

Describe the structure of cellulose

Answer 14

• 1,4 glycosidic bonds
• straight chain unbranched molecule
• alternate glucose molecules are rotated 180*
• hydrogen bonds crosslinks between parallel strands form microfibrils with high tensile strength

Question 15

Describe the functions of cellulose

Answer 15

• Polymer of beta glucose that gives rigidity to plant cell walls
• Prevents bursting under turgor pressure
• holds stem up

Question 16

Describe Benedict’s test for reducing sugars

Answer 16

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 17

Describe the Benedict’s test for non-reducing sugars

Answer 17

1. Negative result: Benedict’s reagent remains blue
2. Hydrolyse non-reducing sugars e.g. sucrose into their monomers by adding 1cm of HCI. 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 18

Describe the test for starch

Answer 18

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

Question 19

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

Answer 19

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.