Starch, cellulose and plant fibresEdexcel International A Level Biology: Subtopic test

1. 1
   A food technology student extracts two polysaccharides from plant material. Substance P is extracted from potato tuber cells, where the plant uses it as an energy store. Substance Q is extracted from the cell walls of flax stems, where it gives the stems their strength. Both substances are polymers of glucose.

   (a)

   Which row correctly gives the monomer and the type of glycosidic bond that joins the monomers in substance Q (cellulose)?

   [1 mark]

   • Aα-glucose joined by 1,4 glycosidic bonds
   • Bβ-glucose joined by 1,4 glycosidic bonds
   • Cβ-glucose joined by 1,6 glycosidic bonds
   • Dα-glucose joined by 1,6 glycosidic bonds

   (b)

   Which statement about substance P (starch) is correct?

   [1 mark]

   • AIt contains only unbranched chains of α-glucose
   • BIt is soluble, so it changes the water potential of the tuber cells
   • CIts straight chains are held together in microfibrils by hydrogen bonds
   • DIt is a mixture of amylose, which is helical, and amylopectin, which is branched

   (c)

   Explain two ways in which the structure of starch makes it suitable as an energy store in potato tuber cells.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Hemp stems contain bundles of long sclerenchyma fibres that are used to make rope. The wall of each fibre cell is made of cellulose microfibrils, and lignin is deposited in the wall during secondary thickening.

   (a)

   Which statement best describes the bonding that holds cellulose chains together within a microfibril?

   [1 mark]

   • AHydrogen bonds between hydroxyl groups on adjacent parallel chains
   • BGlycosidic bonds between adjacent parallel chains
   • CIonic bonds between oppositely charged groups on adjacent chains
   • DDisulfide bridges between adjacent chains

   (b)

   What is the function of lignin in the walls of sclerenchyma fibres?

   [1 mark]

   • AIt stores energy as a respiratory substrate
   • BIt lets water pass freely through the wall into the cell
   • CIt strengthens and waterproofs the wall, making it rigid
   • DIt joins neighbouring cells together as calcium pectate

   (c)

   Explain why sclerenchyma fibres have a high tensile strength.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student compared the tensile strength of fibres from two plants, X and Y. A fibre sample was clamped at its upper end, masses were hung from its lower end, and more mass was added until the sample snapped. Sample X had a diameter of 0.60 mm and snapped at a mean mass of 2.5 kg. Sample Y had a diameter of 0.40 mm and snapped at a mean mass of 1.9 kg. Assume that each sample has a circular cross-section and use a gravitational field strength of 9.81 N kg⁻¹.

   (a)

   Describe how the student should carry out this investigation so that the comparison between X and Y is valid and the results are reliable.

   [3 marks]

   (b)

   Calculate the tensile strength (force per unit cross-sectional area) of X and of Y, and state which plant fibre is stronger.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A company makes rope and carrier bags from oil-based plastics. It is considering replacing the rope with hemp rope, made from bundles of sclerenchyma fibres from hemp stems, and replacing the carrier bags with film made from starch extracted from maize.

   (a)

   Explain how the structure of cellulose, and its arrangement in the walls of sclerenchyma fibres, make hemp fibres strong enough to be used for rope.

   [6 marks]

   (b)

   Evaluate the use of hemp fibres and starch-based film in place of oil-based plastics, in terms of sustainability.

   [6 marks]

   Total for question 4: 12 marks

End of questions