1. 1
   A student placed a leafy potted plant inside a sealed transparent plastic bag, covering the pot and soil separately so no soil water could evaporate directly, and left it in bright sunlight. After an hour, small droplets of liquid had formed on the inside of the bag.

   (a)

   A student placed a leafy potted plant inside a sealed plastic bag and left it in sunlight. After an hour, small droplets of liquid had formed on the inside of the bag. What is the best explanation for this observation?

   [1 mark]

   • AWater vapour lost from the leaves by transpiration condensed on the bag
   • BCarbon dioxide released by the plant condensed on the bag
   • CGlucose produced by photosynthesis leaked out of the leaves
   • DOxygen released by respiration condensed on the bag

   (b)

   Through which structures does the water vapour mainly leave the leaves in this investigation?

   [1 mark]

   • AXylem vessels
   • BStomata
   • CRoot hairs
   • DLenticels on the stem

   (c)

   Explain, in terms of evaporation and diffusion, how water vapour is lost from a leaf and reaches the air spaces inside it before diffusing out.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A researcher used a potometer to measure the rate of water uptake by a leafy shoot under three conditions: still air, a fan blowing air across the leaves, and the fan combined with a heater. The fastest rate of water uptake was recorded when the fan and heater were used together.

   (a)

   Which explanation best accounts for this result?

   [1 mark]

   • AOnly temperature affects transpiration rate; wind speed has no effect
   • BIncreased wind speed decreases transpiration while temperature has no effect
   • CIncreased wind speed and increased temperature both increase the rate of transpiration
   • DHeating and moving air both reduce the water potential gradient, slowing transpiration

   (b)

   The researcher then repeated the investigation in an environment with very high humidity. What effect would high humidity have on the rate of transpiration, compared with dry air at the same temperature and wind speed?

   [1 mark]

   • ATranspiration stops completely regardless of humidity level
   • BTranspiration rate increases because humid air diffuses more quickly into the stomata
   • CTranspiration rate is unaffected by humidity because stomata always remain fully open
   • DTranspiration rate decreases because the concentration gradient of water vapour between the leaf and the air is reduced

   (c)

   Explain why increasing wind speed around a leaf increases its rate of transpiration.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A gardener left two identical potted plants outdoors on a hot, windy day: one in direct sunlight in an exposed spot, and one in a shaded, sheltered corner. By the evening, the plant in direct sunlight had wilted noticeably while the sheltered plant had not.

   (a)

   Explain why the plant in direct sunlight and wind wilted first.

   [3 marks]

   (b)

   Explain, in terms of cell structure and water movement, what is happening inside the leaf and stem cells of the wilted plant, and describe how the plant could recover if watered promptly.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class built a model to explain how a tall oak tree manages to move water more than twenty metres upward from its roots to its highest leaves without any pump, focusing on the properties of water and the structure of the xylem.

   (a)

   Describe the mechanism by which water moves upwards through the xylem of a tall tree, from the roots to the topmost leaves, explaining the role of transpiration pull and the properties of water molecules in this process.

   [6 marks]

   (b)

   Explain why cutting a hole in the trunk of the tree, exposing the xylem to air, would disrupt the upward movement of water, and discuss why very tall trees are more vulnerable to this kind of disruption than short plants.

   [6 marks]

   Total for question 4: 12 marks

End of questions