Transpiration

• Transpiration


  Loss of water vapor from the leaves
• Transpiration


  1. Water evaporates from the surfaces of the spongy mesophyll cells

  2. Water diffuses out of the leaves through the stomata

  3. Evaporation of water from the leaves creates suction

  4. Suction draws a column of water up the stem from the roots

  5. Water moves up the stem in xylem vessels
• Factors affecting transpiration rate


  • Temperature
  • Wind speed
• Investigating effects of temperature and wind speed on transpiration rate


  1. Thoroughly water a leafy plant

  2. Cover the pot with a polythene bag and seal around the stem

  3. Place the plant on a top pan balance

  4. Record its mass

  5. Leave for a set period of time and re-weigh

  6. Calculate the change in mass and percentage change in mass
• Increasing wind speed and temperature


  Increases the rate of transpiration
• When investigating effects of a variable, other factors like light intensity, wind speed and humidity should be kept constant
• Transpiration mechanism


  1. Mesophyll cells have a vacuole which exerts turgor pressure

  2. Turgor pressure forces water out of the cells

  3. Water evaporates from the outer surface into the air spaces

  4. Air in the spaces becomes saturated with water vapor

  5. Water vapor diffuses out of the leaf through the stomata

  6. Evaporated water is replaced by water from the xylem vessels

  7. Creates a transpiration pull that draws water up the stem from the roots
• As temperature increases


  Water molecules gain more kinetic energy, speeding up evaporation
• In still air


  The region surrounding the leaf becomes saturated with vapor
• In moving air

  Water vapor is carried away from the leaf, maintaining a steep concentration gradient
• In humid air


  There is less space for water molecules to diffuse into, slowing transpiration
• In dry air


  There is a greater difference in water molecule concentration inside and outside the leaf, speeding up diffusion
• Wilting


  1. Cell loses water, turgor pressure falls

  2. If water is scarce, transport in xylem decreases

  3. Water loss to evaporation cannot be replaced quickly enough

  4. Cells become flaccid, plant is said to be wilting