Transcript Communication - Groby Bio Page

F215 control, genomes and environment
Module 4 – responding to the environment
Learning Outcomes
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Explain why plants need to respond to their
environment in terms of the need to avoid
predation and abiotic stress.
Define the term tropism.
Explain how plant responses to environmental
changes are coordinated by hormones, with
reference to responding to changes in light
direction
Outline the role of hormones in leaf loss in
deciduous plants
Plant Responses
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Plants have evolved a wide range of
responses to a large variety of stimuli,
this helps them to
 Survive long enough to reproduce
 Avoid stress
 Avoid being eaten
Sensitivity in plants
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A plants responses to the external
environment are mainly growth responses
Tropism
 Growth response to a stimulus
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Positive Tropic
 Responding towards a stimulus
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Negative Tropic
 Responding away from a stimulus
Examples of Tropisms
Tropism
Phototropism
Geotropism
Chemotropism
Thigmotropism
What is it and why is it
important?
Tropisms
Phototropism
 Shoots grow towards the light - photosynthesis
 Geotropism
 Roots grow towards the pull of gravity – anchor,
water, minerals
 Chemotropism
 Pollen tubes grow down the style attracted by
chemicals - fertilisation
 Thigmotropism
 Response to touch e.g. Shoots of climbing plants
wind round solid structures – support
 https://www.youtube.com/watch?v=yFSpKqiI-H8
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Hormones
Chemical messengers that coordinate a
plants response to an environmental stimuli
 Act on target cells/tissues by binding to
receptors in the membrane
 More commonly known as plant growth
regulators
 Move around the plant by
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 active transport,
 diffusion
 mass flow
Examples
Hormone
Effect
Auxins
Promotes cell elongation, inhibit growth in
side shoots, inhibit leaf fall (abscission)
Cytokinins
Promotes cell division
Gibberellins
Promotes seed germination and growth of
stems
Inhibit seed germination and growth,
causes stomatal closure when low water
availability
Promotes fruit ripening
Abscisic acid
Ethene
Tropisms
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Slow responses resulting in directional
growth
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“is a directional growth response in
which the direction of the response is
determined by the direction of the
external stimulus”
Plant Growth
Roots and shoots
get longer
Roots and shoots
get wide
Gives rise to side
shoots
Shoots get longer
Auxin (IAA)
Produced at the apex
Travels by diffusion or active transport to
zone of elongation
 Growth is proportional to concentration of
auxin
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Auxin action – how cells
elongate
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Auxin increase stretchiness of cell wall
 Promotes the active transport of hydrogen
ions by ATPase enzyme
 Build up of hydrogen ions in the cell walls
 The Low pH activates enzymes that break
cross-linkages between molecules in walls
 Cell takes up water by osmosis, cell swell
and become longer
 Permanent effect
Phototropism
Phototropism is the
response of plant
organs to the
direction of light.
 A shoot shows
Positive
phototropism
 GCSE knowledge:
what causes
positive
phototropism?

Phototropism
What causes phototropism?
How light causes redistribution of auxin is uncertain
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2 enzymes (phototropin 1 and phototropin 2)
activated by blue light.
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Active phototropin on the light side and the
phototropin in the rest of the shoot is progressively
less active.
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The gradient is thought to cause the redistribution
of auxin.
Plant movements – non directional
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Nastic Movements
 Usually brought about by changes in
turgidity in cells
 Rapid responses
 examples
▪ Venus fly trap shutting
▪ Leaves closing
▪ Petals closing
https://www.youtube.com/w
atch?v=BLTcVNyOhUc
Leaf Abscission
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Trees in temperate countries shed their
leaves in autumn.
Survival advantage
 Reduces water loss through leaf surfaces
 Avoids frost damage
 Avoid fungal infections through damp,
cold leaf surfaces
 Plants have limited photosynthesis in
winter
 How do leaves fall?
Abscission (leaves being shed)
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Cytokinins stop leaves senescing (aging turning brown), by ensuring they have a
good supply of nutrients, when levels drop
senescence starts
Senescence lowers auxin production
Cells in the abscission zone become more
sensitive to ethene
Lower auxin concentration also increases
ethene concentration
Ethene raises cellulase production which
digests cell walls in abscission zone
Abscission