Transpiration

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  • Transpiration
    • Potometers
      • In a lab, the rate of transpiration from a plant can be measured using a potometer
        • To correctly assemble the potometer, you should;
          • Set the apparatus up underwater, flooding the inside of the apparatus to make sure no air is trapped inside
          • Cut the leafy shoot underwater to prevent air from entering the xylem
          • Dry the leaves  to maximise the rate of transpiration
          • Insert the shoot into a bung and seal with vaseline to prevent air entering the system
          • Introduce a single air bubble into the capillary tube
        • As transpiration takes place from the leaves, the air bubble will move along the capillary tube. The distance moved by the air bubble per unit time is measured using a ruler
          • This measurement actually tells us the rate of water uptake, but is used to indicate the rate of transpiration
    • A potometer can be used to investigate how the following environmental factors affect the rate of transpiration
      • Light intensity
        • Affects transpiration by controlling the degree of stomatal opening for gas exchange for photosynthesis. Therefore, transpiration rate increases as the rate at which water molecules diffuse out of the stomata increases
      • Increase in temperature
        • A rise in temperature provides additional kinetic energy for the movement of water molecules
          • This additional energy accelerates the rate of evaporation of water from the walls of the mesophyll cells and, if the stomata are open, speeds up the rate of diffusion of water vapour into the surrounding atmosphere
            • The water potential of the atmosphere becomes lower as it's temperature is raised and it can hold more moisture
      • Humidity
        • Humidity is the % saturation of water molecules in air
          • The air inside the leaf is saturated with water vapour but the humidity of the atmosphere surrounding the leaf varies
            • The water potential gradient between leaf and atmosphere in warm temperature climates is always great and when stomata are open, water vapour rapidly diffuses from the leaf
      • Wind speed
        • Transpiration in still air results from the accumulation of a layer of saturated air at the surface of the leaves
          • This offers considerable resistance to the diffusion of water vapour through stomata and therefore reduces the rate of transpiration
            • Movement of the surrounding air reduces the thickness of this layer of saturated air and results in increased transpiration

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