Plants

• Plants have branching shapes, which gives them a relatively large surface area : volume ratio.
• The gas exchange surface in a plant leaf is the surface of the mesophyll cells. These are surrounded by air spaces that link to the outside air via stomata.
• Stomata can close, which limits the amount of water vapour lost from the gas exchange surface of the mesophyll cells.
• Suggest appropriate units the student should use to compare the distribution of stomata on leaves.

  Stomata per mm2 or cm2.
• The pieces of leaf tissue examined were very thin.

  Explain why this was important.

  Single layer of cells so light can pass through.
• Give two reasons why it was important that the student counted the number of stomata in several parts of each piece of leaf tissue.

  So the sample is representative, and to obtain a reliable mean.
• Suggest reasons why the rate of water uptake by a plant might not be the same as the rate of transpiration.

  • Water used for support/turgidity
  • Water used in photosynthesis
  • Water used in hydrolysis
  • Water produced during respiration
• Describe a method you could use to find the surface area of a leaf

  1. Draw around leaf on graph paper
  2. Count squares
  3. Multiply by 2 (for upper and lower leaf surface)