Module 5.5 Part 6


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  • Module 5.5 Plant Hormones
    • Plant Hormones
      • Differ from animal ormones
        • produced from a specific
        • produced by unspecialised cells
        • vary depending on circumstances
      • e.g. flowering or germination
    • Phototropic mechanism
      • Coordinate by the hormone IAA
        • type of auxin
        • synthesised in meristem and passed down the stem
        • switches genes on/off
        • stimulates extension growth
          • stimulates proteins in cell wall aka expansins
            • (expansins make cell walls more flexible)
    • Geotropism in shoots
      • negative tropism in plant stems
      • auxins accumulate on the lower side
      • e.g. roots grow down
    • Apical Dominance
      • inhibition of lateral buds by chemicals produced by the optical bud
        • auxin's growing at the apex of plants stems causes the stem to grow upwards but also inhibits growth of lateral buds (side buds)
      • once the apex is remove lateral buds grow
      • cytokines promote lateral bud growth only if tip is removed
    • Gibberellins
      • plant hormones involved in growth
        • produced in young leaves/seeds/root tips
      • stimulates germination, stem growth and flowering
      • stimulates cell elongation and promotes cell division
      • works synergistically with auxin's
      • synergism
        • the interaction or cooperation of two or more organisations to produce a combined effect greater than the sum of their separate effects
    • hormones and leaf loss
      • Deciduous
        • plants lose leaves when very hot or very dry
          • reduced water loss
          • even in winter when water might be frozen
          • no photosynthesis
      • leaf loss is controlled by hormones
        • abscission layer (bottom of leaves/stalks develop)
          • layer of parenchyma cells within walls
        • ethene breaks down cell walls in abscission layers
          • leaver fall
        • abscission
          • the shedding of an old or mature organ froma plant
      • as leafs get older, conc. of auxins decreases
        • allows for leaf loss in response to ethene
    • Hormones and stomatal closure
      • Abscisic Acid (ABA)
        • plant hormone that controls stomatal closure
        • produced in roots in response to a decrease in soil water potential
        • translocated to leaves
      • Guard cells lose water and become less turgid through the following steps
        • 1.
          • ABA binds to receptors on the cell surface of guard ells (causes a cascade of reaction)
            • 2.
              • Ca2+ channels open, influx of Ca2+ info guard cells (ph of cytoplasm is raised)
                • 3.
                  • causes k+, NO-3 and CL- ions to leave the cell
                    • 4.
                      • water potential of the cell increases
                        • 5.
                          • water removed from guard cells by osmosis

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