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Summary Plant and Animal Responses Spec Notes
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Notes made using the OCR A specification for Plant and Animal Responses
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a) i) the types of plant responses- Response to abiotic stress and herbivory - eg) chemical defences (tannins,
alkaloids and pheromones)
- Folding in response to touch (Mimosa pudica)
- Range of tropism in plants
Types of Stimuli
- Tannins
● Toxic to microorganisms and larger herbivores
● In leaves - found in upper epidermis - make leaf bad to taste
● In roots - prevent infiltration by pathogenic microorganisms
- Alkaloids
● Derived from amino acids
● Feeding deterrent to animals - taste bitter
● Located in growing tips and flowers + peripheral cell layers of stems and roots
- Pheromones
● Chemicals which are released by one individual which can affect the behaviour or
physiology of another.
Types of Responses
Tropisms = directional growth responses of plants
- Phototropism
● Shoots grow towards light - positively phototropic - enables them to photosynthesise
- Geotropism
● Roots grow towards the pull of gravity - anchors them to soil + helps them take up
water needed for support, photosynthesis and too cool the plant
- Chemotropism
● On flowers, pollen tubes grow down the style attracted by chemicals, towards the
ovary where fertilisation can take place.
- Thigmotropism
● Shoots of climbing plants (ivy) wind around other plants or structures to gain support
- Non directional responses to external stimuli = nastic responses
- The sensitive plant = Mimosa pudica - responds to touch with a sudden folding of leaves -
example of thigmonasty
,ii) practical investigations into phototropism and geotropism
, b) The roles of plant hormones
- Role of hormones in leaf loss in deciduous plants, seed germination and
stomatal closure
- Hormones coordinate plant responses to environmental stimuli
- Plant hormones = chemical messengers
- Hormones move around the plant by
● Active transport
● Diffusion
● Mass flow in phloem sap or xylem vessels
Hormone Effects
Cytokinins Promote cell division
Delay leaf senescence
Overcome apical dominance
Promote cell expansion
Abscisic Acid Inhibits seed germination and growth
Causes stomatal closure when the plant is stressed by low water
availability
Auxins Promote cell elongation
Inhibit growth of side shoots
Inhibit leaf abscission
Gibberellins Promote seed germination and growth of stems
Ethene Promotes fruit ripening
c) The experimental evidence for the role of auxins in the control of apical dominance
- Auxins = plant hormones responsible for regulating plant growth
- If you break the shoot tip (apex) of a plant - the plants starts to grow side branches from
lateral buds
- Researchers suggested that auxin from the apical bud prevent lateral buds from growing -
when the tip is removed - auxin levels in the shoot drop and buds growth
- To test hypothesis - scientist applied auxin paste to cute end of shoot - lateral buds did not
grow
- Scientists suggested that a normal auxin level in lateral buds inhibits growth, whereas low
auxin level promotes growth
- Scientist think that two other hormones involved
● Abscisic acid - inhibits bud growth
- High auxin in the shoot may keep abscisic acid levels high in the bu
- When tip removed - abscisic levels drop and bud starts to grow
● Cytokinins promote bud growth
- Directly applying cytokinin to buds can override the apical dominance effect
- High levels of auxin make the shoot apex a sink for cytokinins produced in
roots = when apex removed - cytokinin spreads evenly around the plants
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