Detailed and comprehensive notes on control of plant growth (Edexcel biology A). Covers auxins, photoreceptors, photoperiodism and phototropism. [“A-Level Biology: Edexcel A Year 1 & 2 Complete Revision & Practice” (CGP, ISBN: 9781782942986), “Salters-Nuffield AS/A level Biology Student Book ...
Auxins
8.6 Understand how phytochrome and IAA bring about responses in plants to environmental cues,
including their effects on transcription.
Tropisms
• Plants increase their chances of survival by responding to changes in their environment.
o E.g. growing towards light to maximise light absorption for photosynthesis.
• Tropism: The response of a plant to a directional stimulus.
• Plants respond to directional stimuli by regulating their growth.
o Positive Tropism: Growth towards the stimulus.
o Negative Tropism: Growth away from the stimulus.
• Phototropism: The growth of a plant in response to light.
o Shoots are positively phototrophic and grow towards light.
o Roots are negatively phototrophic and grow away from light.
• Geotropism: The growth of a plant in response to gravity.
o Shoots are negatively phototrophic and grow upwards.
o Roots are positively phototrophic and grow downwards.
Growth Factors
• Plants cannot respond using neurons or hormones as they lack nervous or circulatory system.
• Plants respond to stimuli using growth factors.
• Growth Factors: Chemicals that speed up or slow down plant growth.
• Growth factors are produced in the growing regions of the plant (e.g. shoot tips, leaves) and move to
other parts of the plant.
• Auxins are growth factors that stimulate growth of shoots by cell elongation.
• High concentrations of auxins inhibit growth in roots.
Indoleacetic Acid (IAA)
• Indoleacetic acid (IAA) is an important auxin produced in the tips of shoots in flowering plants.
• When IAA enters the nucleus of a cell, it’s able to regulate the transcription of genes related to cell
elongation and growth.
• IAA is moved around the plant to control tropisms – it moves by diffusion and active transport for
short distances or via the phloem for long distances.
• As a result, different parts of the plant have different amounts of IAA.
• The uneven distribution of IAA leads to uneven growth of the plant.
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