TRANSPORT IN PLANTS

• Role of the phloem

  Transports sucrose and amino acids between the leaves and other parts of the plant for immediate use (growing regions) or for storage.
• Role of the xylem

  Transports water and mineral ions from the roots into the leaves of the plant to replace the water that has been lost due to transpiration.
• Direction of phloem transport

  Occurs in both directions, up and down
• Direction of xylem transport

  Occurs in one direction only, up
• Water absorption in plants

  Water is taken up by plants through root hair cells. These are

  specialised cells with a very large surface area to absorb water via osmosis. If the rate of transpiration increases, then the rate of water uptake will also increase as the plant attempts to replenish the loss
• Transpiration

  Transpiration is the evaporation of water from the surface of a plant. It is a consequence of gaseous exchange, as the stomata are open when this process occurs.
• Factor affecting transpiration: Light intensity

  An increase in light intensity leads to an increased rate of photosynthesis, so more stomata open to allow gaseous exchange to occur. This means more water can evaporate, leading to an increased rate of transpiration
• Factor affecting transpiration: Temperature

  An increase in temperature allows the water molecules to move faster, resulting in evaporation happening at a faster rate and therefore the rate of transpiration increases.

  The rate of photosynthesis also increases, meaning more stomata

  are open for gaseous exchange, so more water evaporates and

  the rate of transpiration increases
• Factor affecting transpiration: Air movement (wind)

  If more air on the surface of the leaf is blown away, then the concentration of water vapour surrounding the leaf will be lower. This will mean there will be a steeper concentration gradient resulting in diffusion happening faster. This will increase the rate of transpiration
• Factor affecting transpiration: Humidity

  If the relative humidity is high, then there will be a reduced concentration gradient between the concentrations of water vapour inside and outside the leaf, resulting in a slower rate of diffusion. This will decrease the rate of transpiration