MAKING SENSE OF TROPISMS IGCSE Biology 3 3

MAKING SENSE OF TROPISMS IGCSE Biology 3. 3 Chemical Coordination in Plants Brought to you by Mr. Exham. com Copyright © 2014 Henry Exham

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Copyright © 2014 Henry Exham 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?

Copyright © 2014 Henry Exham Plants respond to their environment • Its not any animals that can respond to their environments. Mimosa Pudica Venus Fly Trap Click on the images to watch the video about each plant

Copyright © 2014 Henry Exham Tropisms • A growth response of a plant to directional stimuli is called a tropism. • Grows towards stimuli – POSITIVE • Grows away from stimuli – NEGATIVE

Copyright © 2014 Henry Exham Phototropism A tropism where the stimuli is light is called phototropism. WHY? • The shoot of a plant grows towards the light to maximise photosynthesis. • Called positive phototropism. Cress seeds grown on a window sill

Positive Phototropism

Copyright © 2014 Henry Exham Phototropism

Copyright © 2014 Henry Exham Geotropism • If there is no light under the soil, how does a germinating seed know which way to grow?

Copyright © 2014 Henry Exham Geotropism A tropism where the stimuli is gravity is called geotropism. WHY? • The root of a plant grows towards gravity to uptake water & mineral ions • Called positive geotropism.

Positive Geotropism

POSITIVE PHOTOTROPISM Stems grow upwards NEGATIVE GEOTROPISM Stems grow upwards NEGATIVE PHOTOTROPISM Roots grow downwards POSITIVE GEOTROPISM Roots grow downwards

Copyright © 2014 Henry Exham The ROLE of HORMONES • AUXIN = growth hormone produced by plants PRODUCED AT: Tip of the shoot – diffuse downwards ROLE OF AUXIN: It causes cell elongation & division.

Copyright © 2014 Henry Exham 1. Auxin is produced at the tip of the shoot and causes it to grow. 2. 3. Auxin diffuses AWAY from the light into the shadowed side The shaded side elongates faster than the illuminated side and the shoot bends towards the light