OCR A specification for A level biology summary of plant hormones. Helped obtain A* in 2022 exams. Contains information on abiotic stress, herbivory, nastic movements, tropisms, and plant hormones and their domestic uses.
Plants increase their chances of survival by responding to changes in their environment.
e.g. growing towards light to maximise photosynthesis, sensing gravity to grow in the right
direction, climbing plants sense touch so they can nd things to climb and reach sunlight
• These are all tropisms. Plants are also more likely to survive if they respond to abiotic stress
and herbivory.
Abiotic Stress is anything natural that is potentially harmful to a plant but is not living. e.g.
drought. Plants can respond to abiotic stress, such as by responding to extreme cold by
producing their own antifreeze (carrots)
Herbivory is when plants are eaten by animals including insects. Plants have chemical defences
against herbivory. e.g. they can produce toxic chemicals in response to being eaten. e.g.
• alkaloids: chemicals with bitter tastes, noxious smells or poisonous characteristics that deter
or kill herbivores.
• Tannins: these taste bitter and in some herbivores they can bind to proteins in the gut and
make the plant hard to digest. Both of these things deter the animals from eating the plant.
Some plants also produce pheromones in response to herbivory. Pheromones are signalling
chemicals that produce a response in other organisms. e.g.
• Some plants release alarm pheromones into the air in response to herbivore grazing which can
cause nearby plants to detect these chemicals to start making chemical defences such as
tannins.
• When corn plants are being eaten by caterpillars, they produce pheromones which attract
parasitic wasps which then lay their eggs in the caterpillar, which eventually kills them.
Other plants are able to fold up in response to being touched:
The plant Mimosa pudica is touched, a signal spreads around the whole leaf, causing it to fold up.
This may knock o small insects feeding on the plant and may scare o animals trying to eat it.
Caused by rapid uptake of water at the base of each lea et, adjacent cells lose water and
collapse. Not related to the direction of the stimulus so cannot be classed as a tropism. Non
directional responses are known as ‘nastic movements’
Tropisms:
A tropism is the response of a plant to a directional stimulus. Plants respond to stimuli by
regulating their growth. Positive tropism is towards the stimulus and negative tropism is away
from the stimulus.
Phototropism, geotropism, hydrotropism, thigmotropism, chemotropism
• Phototropism- the growth of a plant in response to light. Shoots grow towards light and roots
grow away from light.
• Geotropism- the growth of a plant in response to gravity. Shoots are negatively geotropic and
grow away from gravity and roots are positively geotropic and grow towards gravity.
Plant Hormones:
Di erent to animals: produced by unspecialised cells, work in low concentrations and with one
another.
Auxins, gibberellins, ethene, ABA
Germination- mobilisation of food reserve and promotion of growth
Dormancy- a stage in which the seed will not germinate
Gibberellins: stimulates germination, stem growth and owering. Stimulates cell elongation and
promotes cell division. Produced when the seed absorbs water and stimulates the production of
an enzyme to break down food stores. Inhibited by ABA so relative levels determine whether or
not germination occurs. Works in synergism with auxins
Synergism- when two factors’ actions are greater than both of their actions added together.
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