H.3.5.6 Plant Growth Regulators
and Animal Hormones (E.S)
Objectives – What you will need to
know from this section
 Explain the mechanism of plant response to any one
external stimulus.
 Describe the feedback mechanism of any one animal
hormonal system.
Plant Regulators
 Plant growth regulators [hormones] are chemicals that
interact with one another to control a particular
development or response.
 Plants only grow at their tips,
in small regions of active cell
division called meristems—
the tips of shoots, roots and
side buds.
 Plant growth regulators are
produced in the meristems
and transported through
the vascular system of the
plant.
Plant Regulators
 IAA (Indoleacetic acid) is an auxin that is made in the
meristems of shoots, buds and root, and in the tips of
coleoptiles.
 The coleoptile is the sheath around
the leaves and shoot of grass
seedlings, such as corn or oats.
 It travels backwards from the tip and causes cells to
elongate (get longer) and it regulates the differentiation of
the vascular tissue (xylem and phloem).
 A tropism is a plant’s response to a stimulus coming from
one direction, e.g. sunlight, gravity.
 Phototropism is a
growth response of
a stem towards
light, so that it can
receive the
maximum amount
of light for
photosynthesis.
 If a plant receives light from one side only, the light causes the
auxin travelling down from the meristem to be redirected away
from the light and move down the ‘dark’ side of the stem (or
coleoptile).
Quicker growth
here due to more
auxin
 The extra auxin makes these cells elongate faster than cells
on the ‘bright’ side.
 This causes a bending of the stem towards the light.
APICAL
DOMINANCE
 Auxins are
responsible for
apical dominance,
where the main bud
inhibits the growth
of buds lower down
stem.
 This photograph
shows side buds
sprouting when the
main stem is cut off
[pruned].
LEARNING CHECK
• What is a meristem?
• What is IAA and give its function?
• Explain phototropism in terms of light and
IAA action.
• What is apical dominance.
• Explain apical dominance in terms of auxin.
Animal Hormones
 Homeostasis is the maintenance of a constant internal
environment and is achieved by ‘feedback mechanisms’.
Homeostasis is necessary if an organism is to be
independent of its surroundings, and if its metabolism is to
function efficiently.
Most homeostatic mechanisms work by negative feedback,
i.e. if there is a change away from the normal optimum value,
action is automatically taken to reverse this change.
The thermostat in an oven
is an example of how this
negative feedback system
works.
 In negative feedback, the response reduces the strength
of the original stimulus—very useful for controlling the
release of hormones.
There must be a sensor to detect the change, and an
effector to reverse the change.
Control of water, thyroxine & sugar levels are examples of
how homeostasis works in humans.
Control of Water Levels
 The kidney regulates the amount of water in the body by
varying the amount of urine produced.
 This is known as osmoregulation, and it is an example of
homeostasis.
 ADH [Anti-diuretic hormone]
controls whether the distal
tubule and collecting ducts
reabsorb water or not.
If you drink a great deal of water
 the hypothalamus in the brain detects the diluted
blood and turns off ADH production.
 Less water is reabsorbed, so more water is allowed
to escape to the bladder,
 and a larger volume of dilute urine is produced.
When the body is low on water
 ADH is secreted from the pituitary gland.
 More water is reabsorbed
 and only a small volume of urine is produced.
 When the body is low on water, ADH is secreted from the
pituitary gland. More water is reabsorbed and only a small
volume of urine is produced.
If Water Levels Fall in the Body…..
If Water Levels Rise in the Body …..
 When the body has excess water, ADH is NOT secreted from
the pituitary gland. Less water is reabsorbed and so a larger
volume of urine is produced.
LEARNING CHECK
•
•
•
•
•
What is homeostasis?
What is osmoregulation?
What is ADH?
Where is it made and where does it act?
What happens in the kidney if we take in a
lot of fluid?
• What happens in the kidney if we are short
of water in the body?
Control of Thyroxine Levels
 The regulation of thyroid hormone secretion depends on a
negative feedback loop between the pituitary and the thyroid
gland:
 High concentration of thyroxine --- inhibits pituitary ---> less
TSH --- > less thyroxine --> level drops
TSH = Thyroid Stimulating Hormone
 Low concentration of thyroxine ---> stimulates pituitary --->
more TSH ---> more thyroxine --> level rises
THYROXINE control – Feedback
Control of Sugar Levels
If Sugar Levels Rise in the Body …..
If Sugar Levels Fall in the Body …..
LEARNING CHECK
• What is meant by feedback?
• What is negative feedback?
• Can you find out what positive feedback is
and give an example from everyday life?
• What’s the difference between TSH and
Thyroxine?
• What is the difference between insulin and
glucagon?