3.1.2 Carbohydrates

Question 1

Monomer

Answer 1

Small units that can build up larger molecules

Question 2

Polymer

Answer 2

Long chain of repeating sub units called monomers, 3 or more monomers

Question 3

Dimer

Answer 3

2 monomers

Question 4

Condensation reaction

Answer 4

Forms a bond and removes a molecule of water

Question 5

Example of condensation reaction

Answer 5

Polypeptides from amino acids

(Allow a relevant example)

Question 6

Hydrolysis reaction

Answer 6

Bond broken through addition of water molecule

Question 7

Carbon containing compounds are ____________

Answer 7

Organic

Question 8

Saccharide

Answer 8

Sugar

Question 9

Isomers of glucose

Answer 9

Alpha and beta glucose

Question 10

Draw the structure of alpha glucose

Answer 10

See notes

Question 11

Draw the structure of beta glucose

Answer 11

See notes

Question 12

Difference between alpha and beta glucose

Answer 12

In beta glucose, the OH group is inverted

Question 13

Describe how to test for reducing sugars (3)

Answer 13

1. Add Benedict’s reagent
2. Heat in 95 degree water for 5 minutes
3. A positive result will show orange-brown

Question 14

Glucose + glucose =

Answer 14

Maltose

Question 15

Glucose and fructose =

Answer 15

Sucrose

Question 16

Glucose and galactose =

Answer 16

Lactose

Question 17

Explain how a disaccharide is formed (2)

Answer 17

1. Condensation reaction between two glucose monomers
2. Water molecule is removed and glycosidic bond forms

Question 18

Describe the test for non-reducing sugars (3)

Answer 18

(Disaccharide must be hydrolysed into monosaccharide)
1. Add dilute hydrochloric acid and place in boiling water for 5 minutes (hydrolyses the disaccharide)
2. Add in alkali (sodium hydrogen carbonate) to neutralise acid, so Benedict’s will work
3. Add Benedict’s reagent and add to 95 degree water and observe an orange/brown colour change

Question 19

Colorimeter - reducing sugar with lots of precipitate

Answer 19

Samples with high concentration of reducing sugar form lot’s of precipitate, so the colorimeter has a high absorbance and low transmission

Question 20

Describe the structure of starch (4)

Answer 20

Any four from
- Bonded by 1-6 glycosidic bonds
- Alpha glucose monomers
- Insoluble in water so doesn’t affect water potential or osmosis
- Large and insoluble so doesn’t diffuse out/in cell (good storage molecule)
- Unbranched and helical so lots can be stored (compact)
- When hydrolysed, forms alpha glucose monomers, used in respiration
- Branched ends can be acted on by enzymes

Question 21

State the two differences between starch and glycogen (2)

Answer 21

1. Glycogen found in animals
2. (Functions are the same however) glycogen is more highly branched, acted on faster by enzymes, important for organisms with high metabolic rate as rate of hydrolysis is faster

Question 22

Which isomer of glucose is cellulose formed from

Answer 22

Beta glucose

Question 23

Describe the structure of cellulose (3)

Answer 23

• Bonded by 1-4 glycosidic bonds
• Straight chains held together by weak hydrogen bonds forming microfibrils (strength and stability)
• Each adjacent glucose monomers rotates 180 degrees resulting in a straight chain

Question 24

Describe the chemical reactions involved in the conversion of polymers to monomers and monomers to polymers.

Give two named examples of polymers and their associated monomers to illustrate your answer (5)

Answer 24

1. A condensation reaction joins monomers together and forms a
   (chemical) bond and releases water;
2. A hydrolysis reaction breaks a (chemical) bond between
   monomers and uses water;
3. A suitable example of polymers and the monomers from which
   they are made;
4. and 4. Polymers must contain many monomers.
5. and 4: suitable examples include
   * amino acid and polypeptide, protein, enzyme,
   antibody or specific
   example
   * nucleotide and polynucleotide, DNA or RNA
   * Alpha glucose and starch/glycogen
   * Beta glucose and cellulose.

If neither specific carbohydrate example is given, allow
monosaccharide/glucose and polysaccharide.

3. and 4. Reject (once) reference to triglycerides.
4. A second suitable example of polymers and the monomers
   from which they are made;
5. Reference to a correct bond within a named polymer;

Reject reference to ester bond.

Question 25

Why are colorimeters effective in improving accuracy of experiments?

Answer 25

Quantitative values produced

Question 26

Following digestion and absorption of food, the undigested remains are processed to form faeces in the parts of the intestine below the ileum.

The faeces of people with constipation are dry and hard. Constipation can be treated by drinking lactulose. Lactulose is soluble, but is not digested or absorbed in the human intestine.

Use your knowledge of water potential to suggest why lactulose can be used to help people suffering from constipation (2)

Answer 26

1. (Lactulose) lowers the water potential of faeces / intestine / contents of the intestine
2. Water retrained / enters (due to osmosis) and softens the faeces;
   Accept descriptions of soft faeces, eg faeces is less dry / less hard

Question 27

A precipitate is produced in a positive result for reducing sugar in a Benedict’s test. A precipitate is solid matter suspended in solution.

A student carried out the Benedict’s test. Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution (2)

Answer 27

1. Filter and dry (the precipitate);
   Accept: correct reference to evaporation after filtration
2. Find mass/weight;

Question 28

During early pregnancy, the glycogen in the cells lining the uterus is an important source of energy source for the embryo.

Suggest how glycogen acts as a source of energy.

Do not include transport across membranes in your answer.

Answer 28

1. Hydrolysed (to glucose);
2. Glucose used in respiration;
   1. Ignore ‘Broken down’
   2. ‘Energy produced’ disqualifies mp2

Question 29

Describe the structure of glycogen (2)

Answer 29

1. Polysaccharide of alpha glucose
   OR
   Polymer of alpha glucose
2. (Joined by) glycosidic bonds
   OR
   Branched structure;

Question 30

E.coli has no cholesterol in its cell-surface membrane. Despite this, the cell maintains a constant shape.

Explain why (2)

Answer 30

1. Cell unable to change shape;
2. (Because) cell has a cell wall;
3. (Wall is) rigid / made of peptidoglycan / meurin

Question 31

Describe the biochemical test you would use to confirm the presence of lipid, non-reducing sugar and amylase in a sample (5)

Answer 31

Lipid
1.   Add ethanol/alcohol then add water and shake/mix
OR
Add ethanol/alcohol and shake/mix then pour into/add water;
Reject heating emulsion test.
Accept ‘Add Sudan III and mix’.

2.   White/milky emulsion
OR
emulsion test turns white/milky;
Ignore cloudy.
Reject precipitate.
Accept (for Sudan III) top (layer) red.

Non-reducing sugar
3.   Do Benedict’s test and stays blue/negative;
Ignore details of method for Benedict’s test for this mp.

4.   Boil with acid then neutralise with alkali;
Accept named examples of acids/alkalis.

5.   Heat with Benedict’s and becomes red/orange (precipitate);
Do not credit mp5 if no attempt at mp4.
For ‘heat’ ignore ‘warm’/’heat gently’/’put in a water bath’ but accept stated temperatures ≥ 60°C.
Heat must be stated again, do not accept using residual heat from mp4.
Accept ‘do the Benedict’s test’ if full correct method given elsewhere.
Accept ‘sodium carbonate, sodium citrate and copper sulfate solution’ for Benedict’s but must have all three if term ‘Benedict’s’ not used.

Amylase
6.   Add biuret (reagent) and becomes purple/violet/mauve/lilac;
Accept ‘sodium or potassium hydroxide and copper sulfate solution’ for ‘biuret’.
Reject heating biuret test.

7.   Add starch, (leave for a time), test for reducing sugar/absence of starch;

5 max

Question 32

Iodine solution stains fresh apple tissue black. When iodine solution is added to apples stored for a week, the stain is less black.

The water potential of apple juice decreases when apples are stored.

Suggest why the water potential of apple juice decreases when apples are stored (2)

Answer 32

1. Starch hydrolysed;
2. Maltose is soluble (so reduces Ψ)

OR

Starch is insoluble;

Accept glucose for maltose
Ignore sugar

Question 33

Galacto-oligosaccharides (GOS) are polymers of galactose.

Explain why GOS are described as polysaccharides (2)

Answer 33

Any two from:
1. Galactose is a monosaccharide/monomer;
2. (Polysaccharide is a) carbohydrate polymer;
3. (Several) monosaccharides/monomers/galactose
joined by condensation reactions
OR
(Several) monosaccharides/

Question 34

Galacto-oligosaccharides (GOS) are polymers of galactose.

Give two differences between the structures of GOS and lactose (2)

Answer 34

1. Lactose contains (alpha) glucose and GOS does
   not
   OR
   Lactose contains (alpha)glucose + galactose
   and GOS contains only galactose;
2. Lactose is a disaccharide and GOS is a
   polysaccharide;
3. Lactose has one glycosidic bond and GOS has
   many glycosidic bonds;