Transpiration

• The loss of water vapour from the leaves is called transpiration
• Transpiration causes the water to be 'pulled up' the xylem in the stem and roots in continuous flow called the transpiration stream
• The transpiration stream has several functions:

  • Supplies water to leaf cells for photosynthesis
  • Carries minerals dissolved in the water
  • Provides water to keep cells turgid
• Before the water can diffuse out of the leaves, the water leaves the xylem vessels by osmosis, moving from cell to cell down the water concentration gradient
• Some water is used in photosynthesis in the palisade cells but most of the water leaves the mesophyll cells and evaporates into the air spaces between the spongy mesophyll cells
• The water vapour then diffuses out of the leaf through the stomata
• Guard cells are found in the lower epidermis and they help control transpiration.
• Guard cells have a kidney shape and they are adapted to open and close stomata in the leaf
• When a plant has lots of water, the guard cells will fill with it and go plump and turgid. This makes the stomata open so gases can be exchanged for photosynthesis
• When a plant is short of water, the guard cells lose water and become flaccid, making the stomata close. This helps too much water vapour escaping
• Thin outer walls and thickened inner walls make the opening and closing work
• Guard cells are sensitive to light and close at night to save water without losing out on photosynthesis
• You usually find more stomata on the undersides of leaves than on top. The lower surfacse is shaded and cooler - so less water is lost through the stomata than if they were on the upper surface
• Guard cells are therefore adapted for gas exchange and controlling water loss within a leaf
• There are four main factors which affect the rate of transpiration:

  • Light intensity
  • Temperature
  • Humidity
  • Wind speed
• Light intensity: The rate of transpiration increases in the light, because of the opening of the stomata in the leaves, so that the leaf can photosynthesise
• Temperature: High temperture increases the rate of transpiration by increasing the rate of evaporation of water from the mesophyll cells
• Humidity: When the air around the plant is humid, this reduces the diffusion gradient between the air spaces in the leaf and the external air. The rate of transpiration therefore decreases in humid air and speeds up in dry air
• Wind speed: The rate of transpiration increases with faster air movements across the surface of a leaf. The moving air removes any water vapour which might remin near the stomata. This moist air would otherwise reduce the diffusion gradient and slow down diffusion