CIE A Level Biology (9700) exams from 2022

Revision Notes

7.2.2 Transpiration in Plants

Transpiration Explained

  • The movement of water through a plants xylem is largely due to the evaporation of water vapour from the leaves and the cohesive and adhesive properties exhibited by water molecules
  • It is the gradient in water potential that is the driving force permitting the movement of water from the soil (high water potential), to the atmosphere (low water potential), via the plant’s cells
  • Plants are constantly taking water in at their roots and losing water via the stomata (in the leaves)
  • Around 99% of the water absorbed is lost through evaporation from the plant’s stem and its leaves in a process called transpiration
  • Transpiration refers to the loss of water vapour from a plant to its environment by diffusion and the transpiration stream refers to the movement of water from the roots to the leaves
  • The advantage of transpiration is that:
    • It provides a means of cooling the plant via evaporative cooling
    • The transpiration stream is helpful in the uptake of mineral ions
    • The turgor pressure of the cells (due to the presence of water as it moves up the plant) provides support to leaves (enabling an increased surface area of the leaf blade) and the stem of non-woody plants

Movement of water through leaves

  • Certain environmental conditions (eg. low humidity, high temperatures) can cause a water potential gradient between the air inside the leaves (higher water potential) and the air outside (lower water potential) which results in water vapour diffusing out of the leaves through the stomata (transpiration)
  • The water vapour lost by transpiration lowers the water potential in the air spaces surrounding the mesophyll cells
  • The water within the mesophyll cell walls evaporates into these air spaces resulting in a transpiration pull
  • This transpiration pull results in water moving through the mesophyll cell wall (apoplastic pathway) or out of the mesophyll cytoplasm (symplastic pathway) into the cell wall
  • The pull from the water moving through the mesophyll cells results in water leaving the xylem vessels through pits (non-lignified areas), which then causes water to move up the xylem vessels (due to the cohesive and adhesive properties of the water). This movement is called transpiration stream

The role of the stomata

  • Transpiration is mainly controlled by the pairs of guard cells that surround stomata (plural, stoma is singular)
  • Guard cells open the stomata when they are turgid and close the stomata when they lose water
  • When the stomata are open there is a greater rate of transpiration and of gaseous exchange
  • When the stomata close transpiration and gaseous exchange decrease
  • As stomata allow gaseous exchange (CO2 in and O2 out) they are generally open during the day

Exam Tip

Remember that water vapour diffuses through the stomata and water evaporates from the mesophyll cells into the air spaces in the leaf. Transpiration and transpiration pull/stream are different – transpiration is the loss of water vapour from the leaves or stem, whereas transpiration pull/stream is the movement of water through the xylem tissue and mesophyll cells.

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.
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