Plant transport systems and transpiration

• Xylem


  Vessels that transport water and minerals up the plant
• Phloem


  Vessels that transport food substances (mainly sugars) both up and down the stem
• Transpiration

  The loss of water from the plant, mainly through the leaves
• Transpiration

  1. Caused by the evaporation and diffusion of water from the plant's surface, mainly the leaves

  2. Creates a slight shortage of water in the leaf, drawing more water up from the roots

  3. Leads to a constant transpiration stream of water through the plant
• Transpiration is a side-effect of the way leaves are adapted for photosynthesis, with stomata allowing gas exchange
• Benefits of transpiration stream

  • Keeps the plant cool
  • Provides water for photosynthesis
  • Creates turgor pressure to support the plant
  • Brings in minerals from the soil
• Phloem transports food substances up and down, while xylem only transports water and minerals up
• Sieve tube elements have no nucleus which means they cannot survive on their own
• Sieve tube elements in the phloem have no cytoplasm to make space for transportation of food molecules
• The xylem is made up of dead cells
• The movement of dissolved food in the phloem is called translocation
• The upper epidermis is the outer layer of a leaf which protects from diseases, pests and too much light
• The upper epidermis on leaves is covered by a waxy cuticle, which makes upper surface of the leaf waterproof to prevent water loss during dry conditions
• Most stomata are found on the lower surface of a leaf, where it is darker and cooler. This can slow down diffusion of water out of a leaf during dry conditions
• Plants in hot climates have smaller and less stomata on underside leaf and no stomata on upper epidermis. This prevents water loss
• The stomata can open and close automatically when there is less supply of water. The stomata opens or closes whether the guard cells are turgid or flaccid
• Guard cells

  Cells which control size of the stomata
• Thin outer walls and thick inner walls of the stomata make opening and closing function work properly
• Stomata are sensitive to light, so open at day and close at night, which allows them to conserve water while not missing out on photosynthesis