Download - IGCSE Biology - Chemical Coordination in Plants

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.



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?


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


https://www.youtube.com/watch?v=BLTcVNyOhUc

https://www.youtube.com/watch?v=O7eQKSf0LmY

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.


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.


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.


Cress seeds grown on a window sill

PhototropismCopyright © 2014 Henry Exham

http://www.youtube.com/watch?v=tLYOBi5gaVs

Geotropism• If there is no light under the soil, how does a

germinating seed know which way to grow?


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.


Hydrotropism• The roots of some species show positive

hydrotropism.


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.


What causes phototropism?• Auxin is produced at the tip of the shoot and

causes it to grow.


What causes phototropism?• When light comes from one direction, the

auxins get transported to the shaded side.


What causes phototropism?• The shaded side elongates faster than the

illuminated side and the shoot bends towards the light


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.



Download - IGCSE Biology - Chemical Coordination in Plants

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