igcse biology - chemical coordination in plants
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MAKING SENSE OF TROPISMS
IGCSE Biology3.3 Chemical Coordination in
Plants
Copyright © 2014 Henry Exham
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This PowerPoint is protected under copyright.It is designed for educational use. Either personal study or to be presented to a class. It may be edited or duplicated for these purposes only.It must not be shared or distributed online in any format.Some images used are under a separate creative commons license, these are clearly marked.
Copyright © 2014 Henry Exham
Brought to you by MrExham.com
Learning Objectives • Do you understand that plants respond
to stimuli?• Can you give an example of positive
phototropism?• How do plant roots and stems respond to
gravity?
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Plants respond to their environment
• Its not any animals that can respond to their environments.
Click on the images to watch the video about each plant
Venus Fly TrapMimosa Pudica
Copyright © 2014 Henry Exham
Tropisms• 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.
Copyright © 2014 Henry Exham
Tropisms• 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.
Copyright © 2014 Henry Exham
Phototropism• 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.
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Cress seeds grown on a window sill
Geotropism• If there is no light under the soil, how does a
germinating seed know which way to grow?
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Geotropism• 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.
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Hydrotropism• The roots of some species show positive
hydrotropism.
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What causes these responses?
• 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.
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What causes phototropism?• Auxin is produced at the tip of the shoot and
causes it to grow.
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What causes phototropism?• When light comes from one direction, the
auxins get transported to the shaded side.
Copyright © 2014 Henry Exham
What causes phototropism?• The shaded side elongates faster than the
illuminated side and the shoot bends towards the light
Copyright © 2014 Henry Exham
Terms and Conditions
This PowerPoint is protected under copyright.It is designed for educational use. Either personal study or to be presented to a class. It may be edited or duplicated for these purposes only.It must not be shared or distributed online in any format.Some images used are under a separate creative commons license, these are clearly marked.
Copyright © 2014 Henry Exham
Brought to you by MrExham.com
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