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A-Level Biology OCR A

Water Xylem Transpiration Leaf Move Results Mesophyll Cells

The cohesion-tension theory explains that water evaporation from mesophyll cells lowers water potential, drawing water from xylem vessels by osmosis, and water's cohesion and adhesion in the xylem allow its upward movement against gravity.

La teoría de la cohesión-tensión explica que la evaporación de agua de las células mesófilas reduce el potencial hídrico, atrayendo agua desde los vasos del xilema por ósmosis, y la cohesión y adhesión del agua en el xilema permiten su ascenso contra la gravedad.

Front Explain how transpiration results in water moving through the plant (cohesion-tension theory).
Back  Cohesion-tension theory:
  • Water molecules evaporate from surface of mesophyll cells into air spaces in leaf and move out of stomata into surrounding air by diffusion down conc gradient
  • Loss of water lowers water potential of mesophyll cell so water moves into it from an adjacent cell by osmosis along both pathways
  • Repeated across leaf to xylem, water moves out of xylem by osmosis into cells of leaf
  • Water mols form hydrogen bonds with carbs in walls of xylem vessels - adhesion, they also form them with each other - cohesion resulting in combined effect - capillary action allowing water to move against force of gravity
  • Water is drawn up xylem in continuous stream to replace water lost by evaporation - transpiration pull
  • Transpiration pull results in tension in xylem helping to move water across roots from the soil

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