4. Transpiration


Transpiration stream.

  • plants lose water from their leaves by transpiration. This draws water up the stem in constant stream.
  • the root is lined with a layer of cells (epidermis). Many of these cells feature a long outgrowth called a root hair, which increases the SA for osmosis.
  • The water then has to move through the main bulk of the root (the cortex) to reach the xylem. It can travel via the symplast pathway, moving between cells through the plasmodesmata.
  • Or, it can travel through the cortex via the apoplast pathway, moving by osmosis through the cellular matrix of the cell walls until it reaches the endodermis.
  • The endodermis separates the root cortex from the stele, which contains the xylem. The apoplast pathway here is blocked by a suberin coating on the walls (casparian *****).
  • The casparian ***** gives the plant more control over what enters the xylem, as water must go through the cell membrane of the endodermal cells, which is selectively permeable .
  • water enters the xylem and begins to rise up the root towards the stem. It is drawn upwards by cohesion of wtaer molecules to each other, as water leaves the plants by transpiration.
  • When the water reaches the leaf it moves out of the xylem into surrounding cells by osmosis. Water vapour enters the air spaces in the spongy mesophyll.
  • Water vapour diffuses out of the plant via the stomata.
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= the loss of water vapour from the upper parts of the plant - particularly the leaves.

Transpiration involves 3 processes:

  • osmosis (xylem to mesophyll cells)
  • evaporation (surface of mesophyll cells)
  • diffusion (water vapour through stomata).
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why does the plant use the transpiration stream?

  • for photosynthesis
  • for plant growth and cell elongation
  • plant turgidity
  • carries mineral ions
  • evaporation cools the plant.
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Factors effecting the rate of transpiration:

  • number of leaves
  • number, size, position of stomata
  • light
  • temperature
  • relative humidity
  • air movement/wind
  • water availability
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Transpiration stream

  • As water leaves the leaf it is replaced below.
  • measured using a potometer
  • measures the water uptake.
  • If the transpiration rate is more than the uptake rate, then the plant loses turgidity and will wilt then die.
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