8.3 Transpiration

2026 Syllabus Objectives

  1. Describe transpiration as the loss of water vapour from leaves
  2. State that water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaves through the stomata as water vapour
  3. Investigate and describe the effects of variation of temperature and wind speed on transpiration rate
  4. Explain how water vapour loss is related to: the large internal surface area provided by the interconnecting air spaces between mesophyll cells and the size and number of stomata
  5. Explain the mechanism by which water moves upwards in the xylem in terms of a transpiration pull that draws up a column of water molecules, held together by forces of attraction between water molecules
  6. Explain the effects on the rate of transpiration of varying the following factors: temperature, wind speed and humidity
  7. Explain how and why wilting occurs

What is Transpiration? 🌿

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Transpiration is the loss of water vapour from leaves. This is a continuous process that occurs in plants and plays a crucial role in water movement through the plant.

The Process of Water Loss

Water movement during transpiration follows this pathway:

  1. Liquid water is present in the mesophyll cells of the leaf
  2. Water evaporates from the surfaces of the mesophyll cells into the air spaces within the leaf
  3. Water vapour then diffuses out of the leaves through the stomata into the surrounding air

Key Point: Water changes from liquid (inside cells) to gas (water vapour) during transpiration, and exits through the stomata.

The Role of Mesophyll Cells in Transpiration 💧

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The spongy mesophyll tissue in a plant leaf is very important in helping to keep water moving through the plant. The structure of these cells creates ideal conditions for transpiration:

Large Internal Surface Area

  • The combined surface area of all the spongy mesophyll cells is very large
  • This surface is in contact with the interconnecting air spaces in the leaf
  • The large surface area allows for rapid evaporation of water from the cell surfaces into the air spaces

Water Movement in Mesophyll Cells

  1. Liquid water moves into the mesophyll cells from the xylem vessels by osmosis
  2. Much of this water then evaporates from the cell walls
  3. Water ends up as water vapour in the interconnecting air spaces
  4. Water vapour then diffuses out through the stomata into the air surrounding the leaf

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