plants

• Levels of organization in plants


  • Cells
  • Tissues
  • Organs
  • Organ systems
• Leaf


  An organ that, along with a stem and roots, forms an organ system for transport of substances around the plant
• Photosynthesis


  1. Requires carbon dioxide and water

  2. Water transported from soil by roots and xylem

  3. Carbon dioxide diffuses into leaf through stomata

  4. Photosynthesis occurs in palisade mesophyll layer
• Stomata


  • Tiny holes in the lower epidermis of the leaf
  • Allow carbon dioxide to diffuse into the leaf
  • Controlled by guard cells to balance carbon dioxide intake and water loss
• Palisade mesophyll


  • Layer where most photosynthesis occurs
  • Packed with chloroplasts
• Xylem and phloem


  • Transport water and nutrients around the plant
• Cuticle


  • Waxy waterproof layer on top of leaf to reduce water loss
• Guard cells are turgid (well hydrated)


  Stomata open wider
• Guard cells are flaccid (dehydrated)


  Stomata close
• Guard cells


  • Sensitive to light, close stomata at night
• Auxins


  Plant hormones that control growth at the ends of shoots and roots
• Meristem tissue


  • Plant stem cells at growing tips of roots and shoots
  • Can differentiate into many cell types
• How auxins work


  1. Produced in tips

  2. Dissolve in cell solution

  3. Diffuse backwards along shoot or root
• Most stomata are on the underside of leaves to reduce water evaporation
• Auxins in shoots


  Stimulate growth, causing shoots to grow
• Auxins in roots


  Inhibit growth
• Phototropism


  • Response to light
• Geotropism/Gravitropism


  • Response to gravity
• Auxins accumulate on


  • Shaded side rather than sunny side
  • Lower side rather than upper side
• Shoots


  • Positively phototropic (grow towards light)
  • Negatively geotropic (grow away from ground)
• Roots

  • Negatively phototropic (grow away from light)
  • Positively gravitropic (grow down towards ground)
• Normally all plant roots are underground so don't detect light
• If roots get exposed to light

  • Auxins accumulate on lower shaded side
  • Upper side grows faster
  • Root curves downwards