GCSE Combined Science (AQA)

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Transpiration Stream

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Organisation Plant Tissues, Organs and Systems

Transpiration Stream

3:43 Transportation in Plants
Spec 4.2.3.2
Previous Translocation Next Root Hair Cells
  • Transpiration is defined as the evaporation of water from the surface of leaves.
  • This evaporation leads to water loss from the leaves, causing water to move up the plant in a stream.
  • Stomata, controlled by guard cells, open to allow carbon dioxide in, which also facilitates water evaporation from leaf cells.
  • Water evaporates into air gaps from the spongy mesophyll cells and diffuses out through the stomata.
  • Inside the leaf, water diffuses from areas of high water potential to areas of low water potential.
  • The loss of water from leaves draws water up from the xylem to replace it.
  • The xylem, strengthened by lignin, is a structure through which water and minerals travel, supporting the transpiration stream.
  • Water molecules in the xylem are pulled up in a continuous stream due to intermolecular forces, known as cohesion.
  • The transpiration effect continues down to the roots, where water lost at the leaves is replaced by osmosis from the soil into the root hair cells.
  • Osmosis is defined as the diffusion of water from an area of higher concentration to an area of lower concentration, facilitated by a partially permeable membrane like the cell membrane of root hair cells.

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