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Biology · Topic

Transport in plants and water relations

Define the process, then explain the direction of movement using the gradient.

2 original questions in the bank, worth 7 marks · above the two-per-topic floor

Drill this topic All transport in plants and water relations questions

The questions, with the examiner reasoning

Explain3 marksIGCSE

Explain how water reaches the leaves at the top of a 20 m tall tree.

Show the mark scheme and the examiner note

Answer: Transpiration from the leaves pulls water up the xylem; the pull is transmitted because water molecules are cohesive, and the narrow xylem walls support the column by adhesion.

  1. Evaporation from leaf surfaces creates tension (transpiration pull)
  2. Water molecules are cohesive, so the column does not break
  3. Adhesion to the xylem walls helps hold the column against gravity

Examiner technique: Three separate mechanisms earn three marks: transpiration pull, cohesion, and the xylem structure. Naming only one caps the answer at one mark.

Describe4 marksBoth boards

Figure 1 describes a cross-section of a leaf: an upper layer of tightly packed cells containing many chloroplasts; a lower layer of loosely packed cells with air gaps; a waxy layer across the top surface; and small openings on the lower surface, each bordered by two curved cells. Describe the structure of the leaf shown in Figure 1, and state one function of each of the four parts listed.

Show the mark scheme and the examiner note

Answer: Palisade layer: packed with chloroplasts for photosynthesis. Spongy layer: air gaps allow gas movement. Waxy cuticle: reduces water loss. Stomata with guard cells: allow carbon dioxide in and control water loss.

  1. Names the upper palisade layer and its role in photosynthesis (1 mark)
  2. Names the spongy layer and explains that the air gaps let gases diffuse (1 mark)
  3. Names the waxy cuticle and states that it reduces water loss (1 mark)
  4. Names the stomata or guard cells and states that they allow gas exchange and control water loss (1 mark)

Examiner technique: Describe means say what is there and what it does, in the order the figure presents it. Candidates lose marks by describing the whole leaf from memory instead of answering the figure in front of them.