6.1.3Organisms Respond to Changes in their Environments (A Level only)

Indoleacetic Acid (IAA) — AQA A Level Biology 7402 Revision Notes & Topical Questions

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Lesson summary

6.1.3 Indoleacetic Acid (IAA) — AQA A Level Biology 7402 notes: Explain how IAA brings about tropisms through unequal distribution and cell elongation, and interpret experimental evidence.A tropism is a directional growth response of part of a plant to a directional stimulus.

phototropic
gravitropic
acid growth hypothesis
Exam-style question 1
Exam-style question 2
Phototropism

Section 1 of 8Tropisms

A tropism is a directional growth response of part of a plant to a directional stimulus. Positive tropisms are towards the stimulus; negative are away from it.

Phototropic

Shoots are positively phototropic (grow towards light) and negatively gravitropic (grow away from gravity); roots are positively gravitropic and generally negatively phototropic.

Other tropisms include hydrotropism (water) and thigmotropism (touch, for example tendrils).

Tropisms increase the chance of survival: shoots reach light for photosynthesis, roots grow downwards to anchor the plant and absorb water and ions.

Section 2 of 8IAA and phototropism

Indoleacetic acid (IAA) is the most common auxin. It is made in the tips of shoots and roots and moves down the stem by diffusion and active transport. In a shoot illuminated from one side:

IAA moves laterally from the light side to the shaded side of the shoot tip.

The concentration of IAA is therefore higher on the shaded side.

IAA stimulates cell elongation in the shoot, so cells on the shaded side elongate faster than those on the lit side.

The shoot bends towards the light.

The mechanism of cell elongation is the acid growth hypothesis: IAA binds to receptors and activates proton pumps in the cell surface membrane, so H⁺ ions are actively transported into the cell wall.

IAA and phototropism

The lowered pH loosens the bonds between cellulose microfibrils, and activates wall-loosening enzymes.

The cell takes in water by osmosis and the cell's turgor pressure expands the wall, so the cell lengthens.

The same chemical has different effects in different tissues: high IAA in roots inhibits elongation.

Section 3 of 8IAA and gravitropism

In a horizontal shoot, IAA accumulates on the lower side.

In the shoot, the high concentration stimulates growth, so the lower side elongates more and the shoot bends upwards (negative gravitropism).

Inhibits

In a horizontal root, IAA also accumulates on the lower side,

but here it inhibits cell elongation,
so the upper side grows faster and the root curves downwards.
- (positive gravitropism)

The difference occurs because roots are more sensitive to IAA, which inhibits elongation at concentrations that stimulate shoots.

Dense starch-containing amyloplasts (statoliths) in cells settle at the lowest side of the cell and are thought to help detect gravity.

IAA also affects apical dominance, where the apical bud produces IAA that inhibits the growth of lateral buds. Removing the tip, as in pruning, removes the source of IAA and lateral shoots grow, giving a bushier plant.

Section 4 of 8Evidence from experiments

Classic experiments support the role of the tip and of a mobile chemical.

Removing the coleoptile tip stops the response to unilateral light; replacing the tip restores it.

Covering the tip with an opaque cap stops bending; covering the section below the tip does not.

A mica barrier inserted on the shaded side blocks transport and prevents bending,

whereas one on the lit side does not, supporting movement of a chemical to the shaded side.

An agar block placed on one side of a decapitated shoot, after exposure to IAA, causes bending away from the side with the block.

When interpreting such data, describe what happens, then explain it with IAA movement and elongation. Remember to include controls, such as a plain agar block without IAA.

Section 5 of 8Explaining results from IAA experiments

Many questions give a diagram of shoots with tips removed, covered or separated, and ask you to predict the direction of growth.

Use the rule: bending occurs towards the side with less IAA in a shoot,

because the side with more IAA elongates faster;
- and growth continues only if the tip remains able to supply IAA.

If the tip is removed, no growth response occurs; if an agar block containing IAA is placed on one side of the cut stump, that side elongates more,

so the shoot bends away from the block.

If a tip is placed in the dark and an agar block is placed beneath it, the IAA diffuses into the block equally; in unilateral light more IAA is found in the block on the shaded side.

Write a short explanation each time linking tip, transport, concentration and elongation.

Remember that IAA is not the only factor involved, as other growth factors modify the response.

Section 6 of 8Exam-style questions

Describe and explain how a shoot grows towards light. [5]5 marks
Explain why a root grows downwards but a shoot grows upwards although IAA accumulates on the lower side in both. [3]3 marks

Section 7 of 8Key terms

Term	Meaning
Tropism	Directional growth response to a directional stimulus.
Phototropism	Growth response to the direction of light.
Gravitropism	Growth response to gravity.
IAA	Indoleacetic acid, the main auxin.
Apical dominance	Inhibition of lateral buds by IAA from the apical bud.
Acid growth hypothesis	Idea that IAA activates proton pumps, acidifying the wall and loosening it to allow elongation.

Section 8 of 8Exam technique

Saying IAA is made by the shaded side. It is made in the tip and moves sideways.

Claiming IAA always stimulates growth; in roots it inhibits at higher concentrations.

Describing growth as cell division only; the key is elongation.

Writing that light destroys IAA is inaccurate; it redistributes it.

Last updated: 18 October 2026