Edexcel IGCSE Biology

Revision Notes

2.10.3 Response to Stimuli: Plants

Response to Stimuli: Plants

  • Plants need to be able to grow in response to certain stimuli
    • For example, plants need to be able to grow in response to light, to ensure their leaves can absorb light for photosynthesis
    • They also need to be able to grow in response to gravity, to ensure that shoots grow upwards and roots grow downwards
  • The directional growth responses made by plants in response to light and gravity are known as tropisms
    • A response to light is a phototropism and a response to gravity is a geotropism (or gravitropism)
  • If the growth is towards the stimulus, the tropism is positive and if the growth is away from the stimulus, the tropism is negative
    • As shoots grow upwards, away from gravity and towards light (so that leaves are able to absorb sunlight), shoots show a positive phototropic response and a negative geotropic response
    • As roots grow downwards into the soil, away from light and towards gravity (in order to anchor the plant and absorb water and minerals from the soil), roots show a negative phototropic response and a positive geotropic response

Geotropism and Phototropism Table

Gravitropism & phototropism table, , downloadable IGCSE & GCSE Biology revision notes

Auxins

  • Plants produce plant growth regulators (similar to hormones in animals) called auxins to coordinate and control directional growth responses such as phototropisms and geotropisms
  • Auxins are produced in the tips of the shoots and the roots; they diffuse to the cells below the tips and have the following effects:
    • In the shoots auxins promote cell elongation (growth); more auxin = more cell elongation = more growth
    • In the roots auxins inhibit cell elongation (growth); less auxin = less cell elongation = less growth
  • The distribution of auxin in the shoots is affected by light and gravity, whereas the distribution in the roots is primarily affected by gravity alone
    • If a shoot or root is placed on its side, auxins will accumulate along the lower side as a result of gravity; so the uppermost side has a lower auxin concentration
    • In the shoots, the lower side grows faster than upper side (more auxin = more cell elongation), so the shoot grows upwards
    • In the roots, the lower side grows slower than the upper side (as auxin inhibits cell elongation and growth in roots), so the root grows downwards
  • Unequal distributions of auxin cause unequal growth rates in plant roots and shoots

Geotropism in shoots and roots 1, downloadable IGCSE & GCSE Biology revision notes Geotropism in shoots and roots 2, downloadable IGCSE & GCSE Biology revision notes
The difference in the geotropic response of roots and shoots results from their different sensitivities to auxin

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