How Water Move Up a Plant

• Transpiration

  Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through the stomata of the leaves
• Transpiration Experiment

  • petroleum jelly prevents gas exchange

  
• Transpiration pull

  • Transpiration pull is the suction force due to transpiration

  
• Water Movement Inside a Leaf

  • Water evaporates from the thin film of moisture around the mesophyll cells
  • Water vapour diffuses out into the atmosphere. This is transpiration
  • More water moves out of the mesophyll cells
  • Water is drawn from the cells deeper inside the leaf by osmosis
  • Water is eventually drawn from the xylem vessels, and this creates a transpiration pull

  
• Measuring rate of transpiration with spring balance

  • rate of transpiration = loss in mass (g)/ time taken (hours)

  
• Using a Potometer

  • A potometer measures the rate of water absorption by a plant

  • The rate of transpiration can be measured assuming the rate of absorption is proportional to it

  • The effect of different environmental conditions can also be determined using a potometer

  • Rate of transpiration (cm3 /min) = Difference in volume (cm3)/Time taken (min)
• Factors That Affect the Rate of Transpiration

  • Wind
  • Temperature
  • light
  • humidity
• Factors That Affect the Rate of Transpiration
- Wind


  • Wind blows away the water vapour that diffuses out of a leaf, rate of transpiration increases as wind increases
• Factors affecting rate of transpiration - temperature

  • When temperature increases, water evaporates from the cell surfaces more quickly, rate of transpiration increases with increasing temperature
• factors affecting rate of transpiration- light intensity


  • When there is light, the stomata open wider, rate of transpiration increases with increasing light intensity
• factors affecting rate of transpiration - humidity

  • The amount of moisture in the surrounding air affects the concentration gradient of water vapour between the leaf and the surrounding air, rate of transpiration decreases with increasing humidity
• Potometer Experiment


  1. insert the shoot through the hole in the cork of the potometer, smear petroleum jelly around the region that the shoot passes through the cork, making it airtight
  2. open the tap of the reservoir to fill the graduated capillary tube with water
  3. close the tap when the tube is full, the rate of movement of the water column gives the rate of absorption by the shoot
  4. record the distance moved by the water column after 5, 10, 15, 20 and 25 minutes, repeat 2 more times
  5. plot a graph of the distance moved by the water column against the time, and determine the rate of transpiration