How does the effect of a high IAA concentration differ between shoot cells and root cells?

In shoots, a high concentration of IAA stimulates cell elongation, leading to faster growth on that side. Conversely, in roots, a high concentration of IAA inhibits cell elongation, causing that side to grow more slowly.

What is the difference between the transport of IAA over short distances versus long distances?

Short-distance transport occurs via diffusion and active transport between individual cells. Long-distance transport is achieved through the phloem, which carries the hormone from the growing tips to other plant tissues.

How does the production site of IAA differ from that of typical animal hormones?

Animal hormones are usually produced in specialized endocrine glands. IAA is produced by cells in the actively growing regions (meristems) of the plant, such as shoot and root tips, rather than a dedicated gland.

What is a common misconception regarding the effect of IAA on root growth during geotropism?

Students often wrongly assume that because IAA stimulates shoots, it must also stimulate roots. In reality, the high concentration of IAA on the lower side of a root inhibits growth, which is what allows the root to bend downwards.

Why is it incorrect to say that light 'destroys' IAA in phototropism?

Light does not destroy IAA; instead, it causes the hormone to be redistributed or transported from the illuminated side to the shaded side. This creates the concentration gradient necessary for directional bending.

What error occurs when explaining tropisms without mentioning the 'rate' of growth?

Simply stating that one side 'grows' is insufficient; bending is caused by a difference in the rate of elongation. You must specify that one side elongates faster or slower than the opposite side.

Define Indoleacetic acid (IAA).

IAA is a specific type of auxin, which is a plant growth factor that coordinates directional growth responses (tropisms) by regulating cell elongation through gene transcription.

What is meant by 'positive phototropism'?

Positive phototropism is a growth response where a plant organ, typically the shoot, grows towards a light source. This is driven by the accumulation of IAA on the shaded side of the stem.

What is the role of the phloem in the context of IAA?

The phloem acts as the long-distance transport system for IAA, allowing the hormone to move from the apical meristems where it is produced to the rest of the plant body to regulate growth.

How does IAA physically cause a cell to elongate?

IAA affects the transcription of specific genes that produce proteins responsible for increasing cell wall plasticity. This allows the cell to expand as it takes up water via osmosis.

The Effects of IAA | Pearson Edexcel A-Level Biology

The Effects of IAA

Summary

Indoleacetic acid (IAA) is a primary auxin that regulates plant growth by controlling cell elongation. Its effects are tissue-specific, stimulating growth in shoots to facilitate phototropism while inhibiting growth in roots to enable geotropism, thereby ensuring the plant responds optimally to environmental stimuli.

1. Definition & Core Concepts

Indoleacetic acid (IAA) is a naturally occurring plant growth factor belonging to the auxin group, which acts as a chemical messenger to coordinate growth responses.

Unlike animal hormones produced in specific glands, IAA is synthesized in the growing regions of the plant, such as the shoot and root tips, before being transported to target tissues.

A tropism is a directional growth response where the direction of the stimulus determines the direction of the growth; these can be positive (towards the stimulus) or negative (away from the stimulus).

Diagram comparing IAA effects in shoots and roots. In shoots, high IAA on the shaded side stimulates elongation towards light. In roots, high IAA on the lower side inhibits elongation, causing downward growth.

2. Underlying Principles of Action

IAA functions by moving into the nucleus of plant cells and altering gene transcription, specifically targeting genes that code for proteins involved in cell wall expansion.

The resulting proteins increase the plasticity of the cell wall, allowing the cell to take up more water and expand, a process known as cell elongation.

The specific response to IAA is determined by the tissue type and the concentration of the hormone, leading to the distinct behaviors observed in aerial versus underground organs.

3. Transport Mechanisms

Over short distances, IAA moves between adjacent cells through a combination of diffusion and active transport across cell membranes.

For long-distance coordination, IAA is loaded into the phloem, allowing it to be redistributed from the site of production in the tips to more distant parts of the plant.

Environmental stimuli like light and gravity trigger the lateral redistribution of IAA, creating an uneven concentration gradient across the diameter of the stem or root.

4. Tissue-Specific Responses

5. Exam Strategy & Tips