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Summary - Plant and animal responses
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The whole of the content needed for module 5 of biology A OCR exam board, in depth with diagrams
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Plant responsesPlant responses to their environment:
Types of stimuli
Responding to the environment helps plants to survive longer so they can reproduce. Plants
show speci c responses to threats such as herbivores, they use the following chemical
defences:
Chemical defence Where E ect
Tannins Found in the upper epidermis - Toxic to microorganisms and large
herbivores, make the leaf taste bad
- Prevent in ltration by pathogenic
microorganisms
Alkaloids Growing tips and owers, peripheral - Derived from amino acids
cell layers, stems and roots - Act as a feeding deterrent in animals
and make the plant taste bitter
Pheromones It’s a hormone - A ect the physiology or behaviour of
another member of the same species
- Could be toxic to insects and trigger
other chemical defences in plants
Types of response
Tropisms are directional growth responses of plants, it could be towards the stimulus (positive
tropisms), or away from the stimulus (negative tropisms)
There’s 5 di erent types of tropisms:
1. Phototropism, light
- Positively phototrophic
- Shoots grow towards the light, maximises photosynthesis
2. Geotropism, gravity
- Positively geotropic in the roots, they grow towards gravity
- Negatively geographic in the shoots, grow away from gravity
3. Chemotropism, chemicals
- Response to certain chemicals in a plant
- Pollen tubes grow down as they’re attracted to chemicals towards the ovary where
fertilisation takes place
4. Thigmotropism, touch
- Where shoots of growing plants wind round other plants to gain support
- Helps with access to light
5. Hydrotropism, moisture
- Root tips grow towards damper areas of soil
- Increases availability to water
Non-directional responses to external stimuli are called nastic responses it may be an adaption
to protect from herbivorous insects
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, Control of responses
- Hormones coordinate plant responses,
when the hormones reach their target cells
they bind to receptors on the plasma
membrane
- Speci c hormones have speci c shapes
that have complimentary shapes to the
membranes of particular cells
- The speci c binding makes sure the
hormones can only act on the correct
tissues, and they can amplify or cancel out
each other’s e ects
- Hormones can in uence cell division, cell
elongation or cell di erentiation
Hormone E ects
Cytokinins - Promote cell division
- Overcome apical dominance, and promote cell
expansion
- Delay leaf senescence (ageing)
Abscisic acid - Inhibits seed germination and growth
- Causes stomatal closure when there’s water
stress
Auxins - Promote cell elongation
- Inhibits growth of side shoots
- Inhibit leaf abscission (leaf falling)
Gibberellins - Promote seed germination and growth of stems
Ethene - Promotes fruit ripening
Hormones move around the plant in the following ways:
- Active transport
- Di usion
- Mass ow in the phloem sap or in the xylem vessels
Controlling plant growth
Auxins
- Responsible for plant growth
- If you break the shoot, apex, of the plant then the side branches begin to grow from the lateral
buds that were previously dormant
- Research suggests that the apical bud prevent lateral buds from growing, so when the tip is
removed the auxin levels drop and the buds begin to grow
- The scientists applied a ring of auxin transport inhibitor below the apex of the shoot, meaning
the lateral buds grow
The results:
- Normal auxin levels in the lateral buds inhibits growth, but low auxin levels promote growth
- Abscisic acid inhibits bud growth, high auxin in the shoot keeps abscisic levels high in the
bud. When the apex is removed, abscisic levels drop and the bud starts to grow
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