Polysachharides

Question 1

Glycogen and cellulose are both carbohydrates.

Describe two differences between the structure of a cellulose molecule and a glycogen molecule.

Answer 1

Cellulose is made up of β-glucose (monomers) and glycogen is made up of α-glucose (monomers);

2. Cellulose molecule has straight chain and glycogen is branched;
3. Cellulose molecule has straight chain and glycogen is coiled;
4. glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has only 1,4- glycosidic bonds;

Question 2

Starch is a carbohydrate often stored in plant cells.

Describe and explain two features of starch that make it a good storage molecule

Answer 2

Insoluble (in water), so doesn’t affect water potential;

2. Branched / coiled / (α-)helix, so makes molecule compact;

OR

Branched / coiled / (α-)helix so can fit many (molecules) in small area;

3. Polymer of (α-)glucose so provides glucose for respiration;
4. Branched / more ends for fast breakdown / enzyme action;
5. Large (molecule), so can’t cross the cell membrane

Question 3

Starch and cellulose are two important plant polysaccharides.

The following diagram shows part of a starch molecule and part of a cellulose molecule.

(a) Explain the difference in the structure of the starch molecule and the cellulose molecule shown in the diagram above.

just diagram of glucoses of polysaccharides bonded

Answer 3

Starch formed from α-glucose but cellulose formed from β-glucose;

2. Position of hydrogen and hydroxyl groups on carbon atom 1 inverted.

Question 4

Explain how cellulose molecules are adapted for their function in plant cells

Answer 4

Long and straight chains;

2. Become linked together by many hydrogen bonds to form fibrils;
3. Provide strength (to cell wall).

3

Question 5

Describe how the structures of starch and cellulose molecules are related to their functions.

_______________

Answer 5

Helical/ spiral shape so compact;

2. Large (molecule)/insoluble so osmotically inactive;

Accept: does not affect water potential/ψ.

3. Branched so glucose is (easily) released for respiration;

Ignore: unbranched.

4. Large (molecule) so cannot leave cell/cross cell-surface membrane;

Cellulose (max 3)

5. Long, straight/unbranched chains of β glucose;
6. Joined by hydrogen bonding;

Note: references to ‘strong hydrogen bonds’ disqualifies this mark point.

7. To form (micro/macro)fibrils;
8. Provides rigidity/strength;

Question 6

not heating the straw above 90°C? (line 9).

5

Straw consists of three main organic substances – cellulose, hemicellulose and lignin.

Cellulose molecules form chains which pack together into fibres. Hemicellulose is a small

molecule formed mainly from five-carbon (pentose) sugar monomers. It acts as a cement

holding cellulose fibres together. Like hemicellulose, lignin is a polymer, but it is not a

carbohydrate. It covers the cellulose in the cell wall and supplies additional strength. In

addition to these three substances, there are small amounts of other biologically important

polymers present.

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The other main component of straw is water. Water content is variable but may be determined

by heating a known mass of straw at between 80 and 90°C until it reaches a constant mass.

The loss in mass is the water content.

Since straw is plentiful, it is possible that it could be used for the production of a range of

organic substances. The first step is the conversion of cellulose to glucose. It has been

suggested that an enzyme could be used for this process. There is a difficulty here, however.

The lignin which covers the cellulose protects the cellulose from enzyme attack.

Use information from the passage and your own knowledge to answer the following questions.

Answer 6

only water given off below 90 °C;
(above 90°C) other substances straw burnt / oxidised / broken down; and lost as gas / produce loss in mass;

Question 7

Describe the structure of a cellulose molecule and explain how cellulose is adapted for its function in cells.

Answer 7

1. made from β-glucose;
2. joined by condensation / removing molecule of water / glycosidic bond;
3. 1 : 4 link specified or described;
4. “flipping over” of alternate molecules;
5. hydrogen bonds linking chains / long straight chains;
6. cellulose makes cell walls strong / cellulose fibres are strong;
7. can resist turgor pressure / osmotic pressure / pulling forces;
8. bond difficult to break;
9. resists digestion / action of microorganisms / enzymes;