Chp - 8 Transport in plants

• Xylem


  Transport, support
• Xylem


  • Thick cell wall (lignin)
  • One-way flow
  • Cells having no end walls between them
• Phloem


  • Thin cell wall (cellulose)
  • Two-way flow of sap
  • Cells with end walls and perforations
• Xylem transports water and minerals
• Phloem transports sucrose and amino acids
• Xylem has elongated tubes with lignin in the walls and cell walls are impermeable to water
• Xylem has no organelles or cytoplasm and no end walls between cells
• Water moves from a high water potential in the soil into the lower water potential inside the root hair cell down a water potential gradient
• Transpiration is the loss of water vapour from plant leaves by the evaporation of water at the surfaces of the mesophyll cells, followed by diffusion of water vapour through the stomata
• Conditions that affect transpiration rate are temperature, wind, light, and humidity
• Transpiration pull


  A force produced by the loss of water vapour from a leaf, which reduces the pressure at the top of xylem vessels
• Transpiration


  1. Water moves from xylem vessels into mesophyll cells

  2. Water evaporates from cell walls into air spaces

  3. Water vapour diffuses out through stomata
• Many open stomata


  Increases the rate of water vapour diffusion out of the leaf
• Transpiration


  Reduces the pressure in the xylem vessels
• Pressure difference between top and bottom of xylem

  Causes water to flow upwards
• Transpiration pull


  Similar to sucking a drink through a straw, reducing pressure at the top causes the liquid to flow upwards
• Water molecules have a strong tendency to stick to each other, forming a continuous column that does not break apart
• Humidity


  How much water vapour is present in air
• Wilting


  Losing more water than a plant can take up, so the cells lose their turgidity
• Translocation


  The movement of sucrose and amino acids in phloem from sources to sinks
• Source


  Part of a plant that releases sucrose or amino acids, to be transported to other parts
• Sink


  Part of a plant to which sucrose or amino acids are being transported, and where they are used or stored