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  • Transpiration
    • What is transpiration?
      • The loss of water vapour from the upper parts of the plant (particularly the leaves)
      • Water enters the leaves in the xylem and passes to the mesophyll cells by osmosis.
        • The spongy mesophyll cells have large air spaces between them that help the water vapour to diffuse through the leaf tissue
          • As water vapour collects in these air spaces, the water vapour potential rises. Once the water vapour potential inside the leaf is higher that outside, water molecules will diffuse out of the leaf
      • Open stomata provide an easy route for the water vapour to leave the leaf. The stomata are open during the day to allow gaseous exchange for photosyntesis.
      • 3 process involved in transpiration:
        • osmosis from the xylem to mesophyll cells
        • evaporation from the surface of the mesophyll cells into the intercellular spaces
        • diffusion of water vapour from the intercellular spaces out through the stomata
    • What is the transpiration stream?
      • As water leaves the xylem in the leaf, it must be replaced from below. Water moves up the xylem from the roots to replace the water lost
      • This movement of water up the stem is useful to the plant in a number of ways:
        • water is required in the leaves for photosynthesis
        • water is required to enable cells to grow and elongate
        • water keeps the cells turgid
        • evaporation of water can keep the plant cool
      • Measured using a potometer (only estimates)
        • Measures water uptake rather than water lost
    • Factors that affect the rate of transpiration
      • Number of leaves - a plant with more leaves has a larger surface area over which water vapour can be lost
      • Number, size and position of stomata - if the leaves have many large stomata, then water vapour is lost more quickly. If the stomata are on the lower surface, water vapour loss is slower
      • Presence of cuticle - a waxy cuticle reduces  evaporation from the leaf surface.
      • Temperature -  a higher temperature will increase the rate of water loss in three ways. It will:
        • increase the rate of evaporation from the cell surfaces so that the water vapour potential in the leaf rises
        • increase the rate of diffusion through the stomata because the water molecules have more kinetic energy
        • decrease the relative water vapour potential in the air, allowing more rapid diffusion of molecules out of the leaf
      • Relative humidity -  higher relative humidity i the air will decrease the rate of water loss. This is because there will be a smaller water vapour potential gradient between the air spaces in the leaf and the air outside
      • Air movement or wind - air moving outside the leaf will carry away water vapour that has just diffused out of the leaf. This will maintain a high water vapour potential gradient
      • Water availability - if there is little water in the soil, then the plant cannot replace the water that is lost. Water loss in plants is reduced when stomata are closed or when the plants shed leaves in winter
    • What happens if the plant loses too much water?
      • If water loss by transpiration is greater than water uptake from the soil, the plant cells will lose turgidity. Non-woody plants will wilt and eventually die. The leaves of woody plants will also wilt and the plant will eventually die


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