Evaporation and transpration

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  • Evaporation and transpiration
    • water loss from leaves
      • stomata - small openings across a leaf surface
        • stomata can be opened when air enters leaves
          • CO2 from atmosphere diffuses into air spaces and then into cells down a concentration gradient
            • at same time, oxygen from photosynthesis is removed from leaf by diffusion in surrounding air
          • size of stomata and their opening and closing is controlled by guard cells
      • loss of water vapour is called transpiration
        • as water evaporates from leaf surface, more water is pulled up through the xylem
          • this is the transpiration stream: constant movement of water molecules through xylem from roots to leaves
            • driven by evaporation of water from leaves
              • any affecting rate of evaporation will affect rate of transpiration
    • most water vapour lost by plants is lost from leaves
      • usually takes place by diffusion through stomata when they are open
        • one of the main reason plants need to keep their stomata closed
          • reduce transpiration

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