hormonal control

• how the stem responds to light: positive phototropism (grows towards the light)
• how the stem responds to gravity: negative geotropism (grows away from the ground)
• how the root responds to light: negative phototropism - the root grows away from the light
• how the root responds to gravity: positive geotropism (grows towards the ground)
• water tropism = hydrotropism
• auxins are plant hormones
• auxins are made in the tips of growing stems and roots
• auxins can diffuse into other parts of the stem or roots
• auxins change the rate of elongation in plant cells, controlling how long they become
• auxins in stems (gravity)

  • auxin produced at the tip of the stem accumulates at the bottom due to gravity
  • auxin promotes cell grown in the stem
  • the stem grows up (negative geotropism)
• auxin in the roots (gravity)

  • auxin produced at the tip of the root accumulates at the bottom due to gravity
  • auxin inhibits growth of cells in the root
  • the root grows towards the ground (positive geotropism)
• auxin in stems (Light)

  auxin is produced in the tip of the stem. light causes the auxin to move to the shaded side of the stem. this makes the shaded side grow faster so it bends towards the light
• if the tips of a stem have been removed:

  • no auxin because it's made by cells in the shoot tip
  • no cell elongation so no growth
• if the tips of a stem are covered:

  • auxin spreads evenly down both sides of the shoot tip
  • all cells elongate by the same amount on all sides
  • the stem grows straight up
• if the tips of the stem are exposed to light:

  • auxin moves away from the light so there's a higher concentration on the shaded side
  • cells elongate more on the shaded side
  • the stem bends towards the light
• advantages of the roots of a plant having a positive geotropic response:

  • it's anchored (has support)
  • can get more minerals (because minerals are found in the soil)
  • can get more water (because water is found in the soil)
• auxins are used as weed killers and as rooting powders
• ethene (plant hormone) controls cell division and ripening of fruits
• gibberellins are important in initiating seed germination
• stimuli (gravity and light) cause the redistribution of auxin
• FSH (Follicle Stimulating Hormone):

  • Produced by the pituitary gland.
  • Stimulates the ovaries to develop eggs and produce oestrogen.
• Oestrogen:

  • Produced by the ovaries.
  • Stimulates the thickening of the uterine lining (endometrium) in preparation for pregnancy.
  • Inhibits the release of FSH (negative feedback) and stimulates the release of LH
• LH (Luteinizing Hormone):

  • Also produced by the pituitary gland.
  • Triggers ovulation — the release of a mature egg from the ovary.
• Progesterone:

  • Maintains the thickened uterine lining to support a fertilised egg.
  • If pregnancy doesn’t occur, progesterone levels drop, leading to menstruation.
• • Day 1 to 5: Menstruation (shedding of the uterine lining).
  • Day 6 to 14: Follicular phase (FSH and oestrogen increase, egg develops).
  • Day 14: Ovulation (LH surge causes egg release).
  • Day 15 to 28: Luteal phase (progesterone maintains the uterine lining).
• • FSH stimulates the maturation of follicles in the ovary and triggers oestrogen production.
  • Oestrogen promotes the build-up of the uterus lining and, when it rises, inhibits FSH to prevent too many eggs from maturing. It also stimulates a surge in LH.
  • LH causes ovulation (release of the egg around day 14).
  • Progesterone is released after ovulation by the empty follicle (now called the corpus luteum) and maintains the uterus lining. It also inhibits FSH and LH.
  • If the egg isn’t fertilised, progesterone levels drop, the lining breaks down, and menstruation occurs.