More Related Content Similar to Tropic responses Similar to Tropic responses (20) More from Magdaléna Kubešová More from Magdaléna Kubešová (13) 2. Co-ordinated science LO:
• Define and investigate geotropism and
phototropism
• Explain the chemical control of plant growth
by auxins including geotropism and
phototropism in terms of auxins regulating
differential growth
3. Biology LO:
• Define gravitropism
• Define phototropism
• Investigate gravitropism and phototropism in shoots and root
• Explain phototropism and gravitropism of a shoot as examples of
the chemical control of plant growth
• Describe the use in weedkillers of the synthetic plant hormone 2,4-
D
• Explain the role of auxin in controlling shoot growth, limited to:
– auxin made in shoot tip (only)
– auxin spreads through the plant from the shoot tip
– auxin is unequally distributed in response to light and gravity
– auxin stimulates cell elongation
4. Combined sciences LO:
• Define and investigate geotropism and
phototropism
• Explain the chemical control of plant growth
by auxins including geotropism and
phototropism in terms of auxins regulating
differential growth
6. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
Plants can respond to their environment
6
• Its not only animals that can respond to their
environments.
Click on the images to watch the video about each plant
Venus Fly TrapMimosa Pudica
7. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
Tropisms
7
• These examples are very quick responses
which are not typical in plants.
• A typical response in a plant is a change in
growth.
• For example a plant may detect a certain
stimuli like light, and start growing towards it.
8. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
Tropisms
8
• A growth response of a plant to directional
stimuli is called a tropism.
– This is much slower as it involves cell division to
see the results.
– If it grows towards the stimuli it is a positive
tropism.
– If it grows away it is a negative tropism.
9. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
Phototropism = a response in which a plant grows
towards or away from the direction from which
light is coming
9
• Tropisms are given
different names dependent
on the stimuli.
• A tropism where the
stimuli is light is called
phototropism.
• The shoot of a plant grows
towards the light to
maximise photosynthesis.
• This is called positive
phototropism. Cress seeds grown on a window sill
10. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
Geotropism
10
• If there is no light under the soil, how does a
germinating seed know which way to grow?
11. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
Geotropism = a response in which a plant
grows towards or away from gravity
11
• If there is no light under the soil, how does a
germinating seed know which way to grow?
• Because the shoot is also negatively geotropic.
• So will detect gravity and grow in the opposite
direction.
• The roots are positively geotropic. So will grow
down to find mineral ions and water.
12. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
Hydrotropism
12
• The roots of some species show positive
hydrotropism.
13. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
What causes these responses?
13
• The plant produces many different plant
hormones (plant growth substances).
• The most important one of these is called
auxin.
• It’s produced at the tip of the shoot and
diffuses downwards.
• It causes cell elongation
and division.
14. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
What causes phototropism?
14
• Auxin is produced at the tip of the shoot and
causes it to grow.
15. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
What causes phototropism?
15
• When light comes from one direction, the
auxins get transported to the shaded side.
16. Copyright © 2015 Henry Exham
Chemical Coordination in Plants - Tropisms
What causes phototropism?
16
• The shaded side elongates faster than the
illuminated side and the shoot bends towards
the light
17. Copyright © 2015 Henry Exham
• Weedkillers (herbicide) are synthetic plant
hormones, similar to auxins.
• If these chemicals are sprayed on to plants they
can cause rapid, uncontrolled growth and
respiration, resulting in the death of the plant.
Some plant species are more sensitive than
others to synthetic plant hormones, so
weedkillers can be selective.
Many weedkillers kill only broad-leaved plants
(dicotyledons), leaving grasses
(moncotyledons) unharmed.
Effects of weedkillers
18. Copyright © 2015 Henry Exham
• Auxin (IAA) is actively transported into cells by
a transmembrane transporter and leaves the
cells by facilitated diffusion through a different
transporter.
• It turns out that the importer works fine for
2,4-D but that 2,4-D cannot leave the cell
through the exporter.
• It is the resulting accumulation of 2,4-D within
the cell that kills it.
Effects of weedkillers
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