Carbohydrates

Question 1

Explain the test for reducing sugars (5)

Answer 1

1. Dissolve substance in distilled water
2. Add Benedict’s Reagant (alkaline)
3. Heat (water bath)
4. Positive test = green, yellow, orange-brown, brick red (in order of increasing concentration)
5. Negative test = blue (colour of Benedict’s reagant)

Question 2

State the monosaccharides that form: (3)

1. Maltose
2. Sucrose
3. Lactose

Answer 2

1. Glucose + glucose
2. glucose + fructose
3. Glucose + galactose

Question 3

Explain how glucose monosaccharides join and break (3)

Answer 3

1. Condensation reaction - molecule of water removed
2. Forms glycosidic bond
3. Hydrolysis reaction - molecule of water added

Question 4

Explain the test for non-reducing sugars (5)

Answer 4

1. do the normal test for reducing sugars (add Benedict’s and heat)
2. If negative result, add HCl acid to another sample and gently heat (water bath) to hydrolyse any non-reducing sugars to reducing sugars
3. Add sodium hydrogencarbonate solution to neutralise HCl acid. Test with litmus paper to check solution is alkaline (Benedict’s only works in alkaline conditions)
4. Re-test solution with Benedict’s reagant and heat (water bath)
5. If non-reducing sugar was initially present, test should now give a positive result

Question 5

Explain test for starch (3)

Answer 5

1. Add iodine solution
2. Positive = blue-black
3. Negative = remains yellow

Question 6

Explain how the structure of starch is suited to its function (6)

Answer 6

1. Main role of starch is energy storage in plants which it is suited for because:
2. It is insoluble and so doesn’t affect water potential, so water is not drawn into cells by osmosis
3. Being large and insoluble it doesn’t diffuse out of cells
4. Amylose form (1,4 glycosidic bonds) is coiled tightly and is compact so a lot can be stored in a small space
5. When hydrolysed it forms a-glucose which is easily transported and readily used in respiration
6. Amylopectin (1,6 glycosidic bonds where branches) is branched so has many ends for enzymes to act on simultaneously, so that glucose is released rapidly

Question 7

Explain how the structure of glycogen is suited to its function (6)

Answer 7

1. Glycogen is used for energy storage in animals
2. Insoluble so doesn’t affect water potential, water is not drawn into cells
3. Being insoluble, does not diffuse out of cells
4. Compact coils so a lot can be stored in a small space
5. It is more highly branched than both forms of starch (has many 1,6 glycosidic bonds where it branches) so that enzymes act simultaneously increasing the rate at which it can release glucose monomers
6. This suits the needs of animals as they need to release more energy from respiration for movement or the production of heat

Question 8

Explain how the structure of cellulose is suited to its function (4)

Answer 8

1. Cellulose provides structural support for plants and helps maintain turgour pressure in plant cells
2. Because cellulose is formed from B-glucose, it forms straight unbranching chains
3. Chains run parallel to each other and form cross-linkages with hydrogen bonds. Although individual hydrogen bonding is weak, many hydrogen bonds adds to provide strength to the molecule
4. Cellulose molecules are further grouped together to form microfibrils which then group to form fibres that provide more strength