Plants

• tropism = directional growth of plant, in response to external stimuli.

  + -> grows towards stimuli or -
• sunlight causes production of auxins in tips of shoot

  (Light region causes Pr->Pfr

  acts as transcription factor to make auxin)
• Auxin diffuses laterally, then downwards

  towards shaded region of plant
• Auxin regulated response

  • Auxins diffuse into active region
  • Binds to protein receptors, on membranes of target cells
  • Activates intracellular secondary messenger signal molecules (SMSM)
  • Which activates TFs
  • TFs control transcription of auxin-regulated genes
  • That translate into proteins
  • That causes changes in:

  -cell elongation

  -cell division

  -differentiation

  
  

  SO PLANTS CAN GROW TOWARDS SUNLIGHT

  
• Cell elongation(need to loosen cell wall)

  • Auxins activate proton pump
  • Pumps H+ from cytoplasm into cell wall
  • Acidification
  • Lowers pH
  • Which activates expansins proteins
  • Break down hemicellulose + pectin
  • Cellulose cell wall can easily be stretched
  • ANDDDDD
  • H+ in cell wall
  • Makes a conc gradient
  • So, H2O from outside cell wall
  • Diffuses down conc gradient via osmosis into cell wall
  • Making cell turgid & swell -> cell elongates