CIE A Level Biology (9700) 2019-2021

Revision Notes

15.2.2 The Role of Auxin in Elongation Growth

The Role of Auxin in Elongation Growth

  • Plant hormones (also known as plant growth regulators) are responsible for most communication within plants
  • Auxins are a type of plant growth regulator that influence many aspects of growth, including elongation growth which determines the overall length of roots and shoots
  • The principle chemical in the group of auxins made by plants is IAA (indole 3-acetic acid) and this chemical is often simply referred to as ‘auxin’
  • Auxin (IAA) is synthesised in the growing tips of roots and shoots (ie. in the meristems, where cells are dividing)
  • Growth in these meristems occurs in three stages:
    • cell division by mitosis
    • cell elongation by absorption of water
    • cell differentiation
  • Auxin (IAA) is involved in controlling growth by elongation

Controlling growth by elongation

  • Auxin molecules bind to a receptor protein on the cell surface membrane
  • Auxin stimulates ATPase proton pumps to pump hydrogen ions from the cytoplasm into the cell wall (across the cell surface membrane)
  • This acidifies the cell wall (lowers the pH of the cell wall)
  • This activates proteins known as expansins, which loosen the bonds between cellulose microfibrils
  • At the same time, potassium ion channels are stimulated to open
  • This leads to an increase in potassium ion concentration in the cytoplasm, which decreases the water potential of the cytoplasm
  • This causes the cell to absorb water by osmosis (water enters the cell through aquaporins)
  • This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins)
  • The cell elongates


Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.

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