AQA A Level Biology复习笔记6.1.3 Indoleacetic Acid (IAA)

Indoleacetic Acid (IAA)

 

  • Indoleacetic acid (IAA) is a specific growth factor found in plants
  • 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

     

  • IAA is involved in controlling growth by elongation

Controlling growth by elongation

  • IAA molecules bind to a receptor protein on the cell surface membrane
  • IAA 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) which is then stored in the vacuole
  • This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins)
  • The cell elongates

 

Role-of-IAA-in-Elongat-1Role-of-IAA-in-Elongat-2

The role of IAA in the elongation of cells

Phototropism

  • Phototropism affects shoots and the top of a stem
  • The concentration of IAA determines the rate of cell elongation within the region of elongation
  • If the concentration of IAA is not uniform on either side of a root or shoot then uneven growth can occur
  • When the shoots grow towards the light it is known as positive phototropism
    • It is described as positive because growth occurs towards the stimulus

     

  • In shoots higher concentrations of IAA results in a greater rate of cell elongation
    • Experiments have shown that IAA moves from the illuminated side of a shoot to the shaded side
    • The higher concentration of IAA on the shaded side of the shoot causes a faster rate of cell elongation
    • This causes the shoot to bend towards the light

     

Phototropism-mechanism

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

 

Gravitropism

  • Gravitropism affects roots
  • When the roots grow towards gravity it is known as positive gravitropism
  • In roots, higher concentrations of IAA results in a lower rate of cell elongation
  • Some plants cells are able to detect gravity
    • Columellar cells near the root tip possess heavy organelles called amyloplasts
    • Amylopasts are densely packed with starch and so they sink to the bottom of the cell
    • When a root is moved from the vertical plane to the horizontal plane these organelles fall to where the bottom of the cell is

     

  • IAA is actively transported to the region in the root tip where the amyloplasts have sunk
    • The larger concentration of IAA at the lower side of the root inhibits cell elongation
    • As a result, the lower side grows at a slower rate than the upper side of the root
    • This causes the root to bend downwards

     

Gravitropism-mechanism

 

IAA is transported towards the amyloplasts. It inhibits cell elongation in the roots causing the lower side to elongate at a slower rate than the upper side.

 

Exam Tip

You may see IAA referred to as auxin by some other exam boards. IAA is a particular type of auxin so be careful as different exam boards will want you to use different terms!

Investigating the Effect of Indoleacetic Acid on Root Growth

 

  • In shoots higher concentrations of IAA results in a greater rate of cell elongation
    • Shoots show positive phototropism and negative gravitropism

     

  • The opposite is true for roots, higher concentrations of IAA results in a lower rate of cell elongation
    • Roots show negative phototropism and positive gravitropism

     

  • Experiments can be carried out to investigate the effect of IAA on shoot and root growth in seedlings

Apparatus

  • Seedlings (of the same age and plant species)
  • Cutting tile
  • Scalpel
  • Light source
  • Lightproof container
  • Blocks of agar (all the same volume)
  • Marker/pen
  • Test tubes
  • Water

Method

  • Use the scalpel to cut a 1cm section from the root tip of each seedling
  • Mark the root tips at 2mm marks
  • Divide the root tips into three groups and place them in test tubes of water
    • The water helps to keep the plant tissue alive

     

  • Group A receives treatment 1
    • Remove the ends of the root tips using the scalpel
    • Transfer root cuttings with the end removed to an agar block
    • A uniform light source is present

     

  • Group B receives treatment 2
    • Transfer intact root tips to an agar block
    • A light-proof container is placed over the seedlings to prevent light from entering

     

  • Group C receives treatment 3
    • Transfer intact root tips to an agar block
    • Apply a directional light source to one side of the root tips

     

  • Leave all the roots in their treatment conditions for 3 hours
  • Use the 2mm marker lines to determine if growth has occurred
  • Note if the growth has been even on both sides

Results and analysis

  • In group A (tips removed) the roots grow evenly on both sides
    • IAA is synthesised in the root tips so removing them means that no IAA is produced
    • There is no inhibition of cell elongation

     

  • In group B (no light) the roots grow slightly less than group A but evenly on both sides
    • There is an equal concentration of IAA on both sides of the root tip
    • The inhibition of cell elongation is equal on both sides of the root tip
    • The roots do not grow as long as those in group A due to the presence of IAA

     

  • In group C (directional light) the cells on the illuminated side of the root grow longer than those on the shaded side
    • There is a greater concentration of IAA on the shaded side
    • This results in greater inhibition of cell elongation on the shaded side
    • So the illuminated side grows at a faster rate
    • The roots bend away from the light – negative phototropism

     

Limitations

  • Although the same species of plant is being used the plants will still have differences
    • Certain genotypes may be more prone to bending or have slightly different sensitivities to IAA

     

  • The 2mm marks used for measuring growth may get smudged or move
    • If the root is mishandled the marks can be altered, which will affect the results

     

  • The evenness of growth can be hard to determine using the naked eye
    • Only general comments can be made about whether there has been even growth on both sides of the roots

     

IAA-effect-on-roots

The different treatments produce different levels of growth in the root tips. The IAA molecules inhibit cell elongation in roots.

 

 

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