Plant fibres, xylem and phloemEdexcel A-Level Biology A: Subtopic test

1. 1
   A student cuts a thin transverse section through a young sunflower stem, stains it with toluidine blue (which stains lignin blue-green and unlignified cellulose pink-purple) and examines one vascular bundle with a light microscope to identify the tissues.

   (a)

   The student identifies the tissues in one vascular bundle. Which row gives the correct order of the tissues from the outside of the stem inwards?

   [1 mark]

   • AXylem, phloem, sclerenchyma fibres
   • BSclerenchyma fibres, phloem, xylem
   • CPhloem, sclerenchyma fibres, xylem
   • DXylem, sclerenchyma fibres, phloem

   (b)

   Next to a large cell with perforated end walls the student sees a small cell with a nucleus, dense cytoplasm and many mitochondria. What is the small cell?

   [1 mark]

   • AA xylem vessel element
   • BA sclerenchyma fibre
   • CA companion cell
   • DA palisade mesophyll cell

   (c)

   Suggest why the xylem vessels stain strongly blue-green with toluidine blue but the sieve tubes stain pink-purple.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A rope manufacturer compares the strength of fibres extracted from hemp stems. A technician clamps a single fibre at its top end, hangs a mass holder from the lower end and adds 100 g masses one at a time until the fibre snaps. The distance between the clamp and the holder is the same for every fibre tested.

   (a)

   Which definition of tensile strength is correct?

   [1 mark]

   • AThe ability of a fibre to resist being compressed
   • BThe ability of a fibre to absorb water without swelling
   • CThe mass of a fibre per unit length
   • DThe ability of a fibre to resist stretching forces without breaking

   (b)

   One hemp fibre snaps when the total hanging mass is 3.5 kg. Taking the gravitational field strength as 9.8 N kg⁻¹, what is the force that broke the fibre?

   [1 mark]

   • A34 N
   • B3.5 N
   • C0.36 N
   • D340 N

   (c)

   Explain why the technician should use fibres of the same diameter when comparing fibres from different hemp plants.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Some trees lift water to the top of a trunk more than 30 m tall. Water is pulled upwards through the xylem by transpiration, which puts the water columns under tension (negative pressure). The wood of the trunk also has to support the weight of a large crown of branches.

   (a)

   Explain how the structure of xylem vessels adapts them to carrying water under tension.

   [3 marks]

   (b)

   Compare the structure and function of sclerenchyma fibres with those of xylem vessels in the trunk.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A textile company wants to replace some synthetic rope with rope made from plant fibres taken from the stems of flax and hemp. Its engineers need to understand why these fibres are so strong in tension and how to compare the fibres from different species reliably.

   (a)

   Explain how the arrangement of cellulose microfibrils and secondary thickening give flax and hemp fibres a high tensile strength.

   [6 marks]

   (b)

   Describe how the engineers could determine the tensile strength of fibres from flax and hemp so that the results can be compared reliably.

   [6 marks]

   Total for question 4: 12 marks

End of questions