Transport in plantsEdexcel A-Level Biology B: Subtopic test

1. 1
   A botanist stains transverse and longitudinal sections of a sunflower stem and examines them with a light microscope. The stain turns lignin red. In the vascular bundles, some tubes have thick red walls with rings and small gaps, and contain no cell contents. Other tubes have thin walls, are separated by perforated end plates and lie beside small cells with dense cytoplasm.

   (a)

   Which feature of mature xylem vessels allows water to move upwards in a continuous column?

   [1 mark]

   • AThe cells are separated by perforated sieve plates
   • BThe cells have lost their end walls and cell contents, forming a hollow tube
   • CThe walls are made only of thin cellulose
   • DEach vessel is linked to a companion cell that pumps water

   (b)

   Which statement about phloem tissue is correct?

   [1 mark]

   • ASieve tube elements have thick lignified walls and no cytoplasm
   • BCompanion cells have no nucleus and few mitochondria
   • CSieve tube elements have few organelles, and companion cells with many mitochondria supply them with ATP
   • DPhloem transports mainly water and mineral ions upwards

   (c)

   Explain how the lignified walls and the small gaps seen in the xylem tubes help xylem to transport water.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Researchers place the cut end of a root in a solution containing a dye whose molecules are too large to cross cell surface membranes but pass freely through cell walls. The root cortex cells are joined by cytoplasmic strands. The endodermis, the layer around the central vascular tissue, has a waterproof band of suberin in its cell walls.

   (a)

   By which route does the dye travel through the cortex?

   [1 mark]

   • AThe apoplast pathway, through the cell walls and spaces between cells
   • BThe symplast pathway, through the cytoplasm of connected cells
   • CThrough the vacuoles of adjacent cells
   • DThrough the lumen of xylem vessels in the cortex

   (b)

   Water moving by the symplast pathway passes from one cortex cell to the next through

   [1 mark]

   • Apits in lignified walls
   • Bsieve plates
   • Cair spaces between the cells
   • Dplasmodesmata, the cytoplasmic strands linking cells

   (c)

   Suggest why the dye is seen throughout the cortex but not beyond the endodermis.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A research team studies a coast redwood tree 90 m tall. When they drill a small hole through the bark into a mature xylem vessel, no sap spurts out. Instead, a little air is drawn into the hole. Over a hot afternoon the trunk diameter is measured with sensitive instruments and is found to shrink slightly.

   (a)

   Explain how these observations support the cohesion-tension model of water transport.

   [3 marks]

   (b)

   Explain how the water column is pulled up such a tall tree without breaking, and why an air bubble in one vessel does not stop water transport in the whole trunk.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class studies water and sucrose transport in privet shoots. In one investigation they use a potometer to measure water uptake by a leafy shoot. In a second discussion they consider how sucrose made in the leaves is moved to the roots, where it is stored.

   (a)

   Describe how a potometer could be used to compare the rate of water uptake by a shoot in still air and in moving air from a fan, and explain the result expected.

   [6 marks]

   (b)

   Evaluate the mass-flow hypothesis as an explanation of the movement of sucrose from the leaves to the roots.

   [6 marks]

   Total for question 4: 12 marks

End of questions