9.1

• Plant leaves are the primary organ of photosynthesis.
• • Photosynthesis involves the synthesis of carbohydrates using light energy
  • Carbon dioxide is used as a raw material
  • Oxygen is produced as a waste product
  • Exchange of these two gases(carbon and oxygen) must take place to sustain photosynthesis
• • Absorption of carbon dioxide is essential for photosynthesis
  • The waxy cuticle has very low permeability to carbon dioxide, so pores through the epidermis are needed. These pores are called stomata
• If stomata allow carbon dioxide to be absorbed, they will usually allow water vapour to escape, which is a problem. (this is waterloss).

  • This is an intractable problem for plants and other organisms: having gas exchange without water loss.
• • The loss of water vapour from the leaves and stems of plants is called transpiration
• Plants minimize water losses through stomata using guard cells
• Guard cells are cells found in pairs, one on either side of a stoma
• • Guard cells control the aperture of the stoma and can adjust from wide open to fully closed
• • Stomata are found in nearly all groups of land plants for at least part of the plant's life cycle
  • The exception is a group called liverworts
• Water leaving through stomata by transpiration is replaced by water from xylem
• • Water in the xylem climbs the stem through the pull of transpiration combined with the forces of adhesion and cohesion
• Water moves from soil into roots by osmosis due to active transport of minerals into the roots
• Once the water is in the root, it travels to the xylem through cell walls (the apoplast pathway) and through cytoplasm (the symplast pathway)