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First exams 2025

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Plant Hormones (HL) (HL IB Biology)

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Cara Head

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Cara Head

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Phytohormones in Plants

  • Phytohormones are plant hormones that regulate their growth, development, reproductive processes, longevity, and even death
  • There are many chemicals which act as phyohormones in plants, some examples include:
    • Auxins which result in cell elongation
    • Abscisic acid which suppresses the growth of plants
    • Cytokinins which increase the rate of cell division
    • Ethylene which promotes fruit ripening
    • Gibberellin which control cell elongation, seed germination, flowering and dormancy
    • Brassinosteroids which regulate growth, development, and responses to stresses
  • Plant hormones are sometimes referred to as plant growth regulators

Maintaining Phytohormone Concentration Gradients

  • Auxins are a group of plant hormones that influence many aspects of plant growth
    • A common auxin is known as IAA (indole-3-acetic acid)
  • In shoots, auxin is produced in cells at the growing tip before moving away into the surrounding tissues
  • Auxin has an important role in regulating shoot growth
    • In shoots, auxin causes cells to elongate, leading to stem growth
      • Note that in roots, auxin inhibits cell growth; the opposite effect to that in shoot cells
      • Note that at very high concentrations, auxin can also inhibit shoot growth

Auxin efflux carriers

  • Auxin enters cells by simple diffusion, however, to exit the cell (and therefore move to the next cell), it requires membrane proteins called auxin efflux carriers to exit the cell
    • The term 'efflux' refers to an outward flow of a substance; in this case auxin is pumped out of one cell and into another
    • Efflux carriers are a type of protein called PIN3 proteins
  • Plant cells can distribute auxin efflux carriers on one side of the cell to encourage one way movement of auxin
  • The process requires ATP so is a type of active transport
  • These efflux carriers or pumps are important in establishing an auxin gradient across a stem or root in response to a stimulus such as light or gravity
    • E.g. Light is thought to affect the expression of genes that code for the PIN3 protein efflux pumps; light shining on one side of a stem more than the other can therefore lead to an uneven distribution of efflux pumps, creating an auxin gradient

Cell Growth by Auxin

  • Light affects the growth of plant shoots in a response known as phototropism
  • The concentration of auxin determines the rate of cell elongation within the stem
    • A higher concentration of auxin causes an increase in the rate of cell elongation 
    • If the concentration of auxin is not uniform across the stem then uneven cell growth can occur
  • When light shines on a stem from one side, auxin is transported, by PIN3 proteins, from the illuminated side of a shoot to the shaded side
  • An auxin gradient is established, with more auxin on the shaded side and less on the illuminated side
  • The higher concentration of auxin on the shaded side of the shoot causes a faster rate of cell elongation, and the shoot bends towards the source of light

Effect of Auxin Diagram

_-auxin-and-phototropism-in-shoots

Higher concentrations of auxin on the shaded side of a stem increases the rate of cell elongation so that the shaded side grows faster than the illuminated side

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

Cell growth by auxin diagram

promotion-of-cell-growth-by-auxin

The role of auxin (IAA) in controlling growth by elongation

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Cara Head

Author: Cara Head

Expertise: Biology

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding