Transpiration

Movement of water up the stream

• in xylem

• transpiration stream

  • transpiration of water molecules through the stomata

  • cohesion and adhesion of water molecules

  • capillary action - water moving upwards against the force of gravity

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Transpiration

• the loss of water vapour from the stems and leaves of plants

• water is lost when it evaporates to the air outside the leaf via stomata which are found of the underside of plant leaves

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Cohesion - tension theory

1. water molecules move through the cells in the leaf from the vascular bundle to the mesophyll by osmosis

2. they then evaporate from the mesophyll cells into the air spaces

3. water moves by diffusion through the air spaces to the stomata down a concentration gradient

4. water vapour leaves the plant through the stomata - transpiration

5. the loss of water from the mesophyll cells reduces the water potential in that cell

6. draws in water from neighbouring mesophyll cells

7. cohesion and adhesion cause the water to rise up the xylem by capillary action to replace the water lost by evaporation

Evidence for the cohesion tension theory

• changes in the diameter of trees

  • high transpiration = smaller diameter

• when a xylem vessel is broken air is drawn in

  • water can no longer be drawn up as transpiration stream is broken