OCR A2 Biology F215: Plant Responses

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  • Plants need to respond to their environment to avoid abiotic stress (such as water shortage).


  • Is...a directional growth response in which the direction of the response is determined by the direction of the external stimulus.
  • POSITIVE tropism grows toward the direction of the stimulus, and NEGATIVE tropism grows away from the stimulus.
  • Phototropism: Shoots grow towards light, to increase rate of photosynthesis.
  • Geotropism: Shoots grow upwards and roots grow downwards towards the pull of gravity.
  • Chemotropism: Pollen tubes grow towards chemicals given off by the ovary in a flower.
  • Thigmotropism: Shoots grow towards other plants by responding to touch.

Plant hormones

  • ...coordinate plant growth. Hormones are produced in one part of a plant and are transported to target cells by active transport, diffusion and in the xylem/phloem.
  • Cytokinins - promote mitosis in the meristem
  • Auxins - promote cell elongation in shoots and inhibit growth of lateral buds and leaf fall.
  • Gibberellins - promote cell elongation in stems.
  • Abscisic acid - stimulates production of ethene, and causes stomatal closure when plants are stressed by low water availability.
  • Ethene - stimulates leaf fall and promotes fruit ripening.


  • A shoot will grow towards a light stimulus because cells in




thanks for this

Lucy McFarlane


okar wrote:

thanks for this


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