AQA A Level Biology

Revision Notes

3.6.2 Transpiration

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

Transpiration in a plant, downloadable AS & A Level Biology revision notes

The loss of water vapour from the leaves of plants (transpiration) results in a lower water potential creating a concentration gradient between the roots and leaves causing water to move upwards

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
    • When rates of transpiration are high the walls of the xylem are pulled inwards by the faster flow of water

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

Transpiration explained (1), downloadable AS & A Level Biology revision notesTranspiration explained (2), downloadable AS & A Level Biology revision notes

Water movement through a leaf. Water enters the leaf as a liquid and diffuses out as water vapour through the stomata. This loss of water by evaporation and transpiration results in a water potential gradient between the leaves (low) and roots (high) causing water to move up the plant in a transpiration stream

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: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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