Plant hormones: Homeostasis and response: Biology: GCSE (9:1)

• Auxins


  Plant hormones that coordinate and control cell growth in response to light and gravity

  
• Tropism


  When an organism such as a plant responds to a stimulus by turning or growing in a certain direction

  
• Phototropism


  A change in the direction of a plant in response to light

  
• Gravitropism or geotropism

  A change in the direction of a plant in response to gravity

  
• Positive phototropism


  Most plant shoots grow towards a light source to increase the rate of photosynthesis

  
• Negative phototropism


  Plant roots grow away from light sources as darker conditions such as the soil are more likely to provide moisture and minerals

  
• Positive gravitropism


  Plant roots grow towards the centre of gravity to increase the chances of obtaining water and minerals from the soil

  
• Negative gravitropism


  Plant shoots grow away from the centre of gravity to increase exposure to sunlight for photosynthesis

  
• Tips of shoots and roots

  Known as apical meristems, where auxins are produced in plants

  
• Effect of auxins in shoots

  Auxins accumulate on the shaded side of plant shoots and promote cell elongation, causing the shoots to grow towards light

  
• Effect of auxins in roots

  Auxins accumulate on the lower side of plant roots and inhibit cell growth, causing the roots to grow towards gravity

  
• Cell elongation in shoots

  Shaded cells stretch more in plant shoots so that the shaded side becomes longer than the light side, resulting in the shoots growing towards light

  
• Inhibition of cell growth in roots

  Cells on the lower side of plant roots stretch and grow less than cells on the upper side, causing the roots to grow down towards gravity

  
• Diffusion

  Auxin molecules are originally equally distributed from the tips of shoots and roots via diffusion

  
• Unequal distributions

  Gravity and light causes auxins to diffuse unevenly and move towards a shaded side or a side closer to the centre of gravity

  
• Foil covering plant shoot tips

  Plant shoots grow upwards but do not grow towards light if foil covers plant tips, suggesting the tips of shoots are sensitive to light
• Foil covering plant shoot stems


  Plant shoots continue to grow towards light if foil covers stems, suggesting the stems and lower parts of shoots are not sensitive to light