Water transport in plantsAQA A-Level Biology: Subtopic test

1. 1
   A student examined a longitudinal section of the stem of a sunflower with a light microscope. The mature xylem vessels contained no cytoplasm, had no end walls between neighbouring vessel elements, and had walls thickened with lignin.

   (a)

   Which feature of mature xylem vessels allows water to move up the stem as a continuous column?

   [1 mark]

   • ASieve plates between neighbouring cells
   • BLiving cytoplasm that pumps water upwards
   • CNo end walls between neighbouring vessel elements, forming a hollow tube
   • DVery thin walls that allow water to leave easily

   (b)

   What is the function of lignin in the walls of xylem vessels?

   [1 mark]

   • AIt allows water to move through the cell walls by active transport
   • BIt strengthens the walls, so that the vessels do not collapse inwards
   • CIt stores starch for use in the growing shoot
   • DIt provides the energy needed to move water upwards

   (c)

   Explain why the walls of xylem vessels need to be strong.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A scientist fitted a sensitive instrument to the trunk of a pine tree and recorded its diameter over several days. The diameter was smallest in the afternoon, when the air was warm, dry and sunny, and largest in the early morning. The change in diameter was about 0.5 mm.

   (a)

   Which process is mainly responsible for pulling water up the xylem in a tall tree?

   [1 mark]

   • AEvaporation of water from the leaves, which creates tension in the xylem
   • BRoot pressure, which pushes water up from the roots
   • CActive transport of water by the xylem vessels
   • DCapillary movement of water through the dead cells only

   (b)

   Which property of water molecules allows a column of water to be pulled up the xylem without breaking?

   [1 mark]

   • AThey have a low specific heat capacity
   • BThey are hydrophobic and repel the xylem walls
   • CThey are non-polar and so move freely
   • DThey are attracted to each other by hydrogen bonds (cohesion)

   (c)

   Explain why the trunk diameter was smaller in the afternoon than in the early morning.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student set up a potometer with a leafy shoot from a laurel plant cut under water, and measured the distance moved by an air bubble in a capillary tube of internal radius 0.5 mm. In still air, the bubble moved 12 mm in 5 minutes. When a fan blew air across the leaves, the bubble moved 30 mm in 5 minutes.

   (a)

   Calculate the rate of water uptake in the moving air, in mm³ min⁻¹.

   [3 marks]

   (b)

   Explain why the rate of water uptake was higher in moving air, and why this supports the idea that water is pulled up the xylem rather than pushed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Scientists studying water movement in trees recorded the following observations. First, across many trees, the rate of water movement up the stem is strongly and positively correlated with the rate of transpiration from the leaves (r = +0.94). Second, when a xylem vessel in a transpiring tree is cut, air is drawn into the vessel rather than water flowing out. Third, trunk diameter decreases during the day, when transpiration is fastest, and recovers at night.

   (a)

   Explain how the cohesion-tension theory accounts for the movement of water from the roots to the leaves.

   [6 marks]

   (b)

   Evaluate how far the three observations support the cohesion-tension theory.

   [6 marks]

   Total for question 4: 12 marks

End of questions