What is the primary difference between positive and negative phototropism?

Positive phototropism is growth toward a light source, typically seen in shoots to maximize photosynthesis. Negative phototropism is growth away from light, which is often observed in roots as they grow deeper into the soil.

How do the geotropic responses of shoots and roots differ?

Shoots exhibit negative geotropism by growing upwards against the pull of gravity to reach light. Roots exhibit positive geotropism by growing downwards with gravity to anchor the plant and absorb water.

Compare the speed and duration of plant tropisms versus animal nervous responses.

Plant tropisms are much slower because they rely on chemical diffusion and structural cell elongation. However, the resulting change is more permanent, whereas nervous responses are rapid but short-lived.

Why is it incorrect to say that auxin causes 'cell division' in phototropism?

Auxin causes cell elongation, which is the physical lengthening of existing cells. Cell division refers to the creation of new cells through mitosis, which is not the primary mechanism for the directional bending seen in tropisms.

What is a common misconception regarding the effect of light on auxin molecules?

Many believe light destroys auxin, but it actually causes auxin to redistribute or migrate to the shaded side of the shoot. This creates the concentration gradient necessary for uneven growth and bending.

What error is made when students forget to mention the 'shaded side' in phototropism explanations?

Without specifying the shaded side, the explanation fails to account for the unequal growth rates. It is the accumulation of auxin specifically on the side away from the light that causes the curvature.

Define the term 'Tropism' in the context of plant biology.

A tropism is a directional growth response of a plant toward or away from an external stimulus. These responses are vital for the plant to adapt its positioning for survival.

What is Auxin and where is it primarily produced?

Auxin is a plant growth regulator (hormone) that coordinates growth responses. It is primarily synthesized in the growing tips of shoots and roots.

What is the specific role of the 'region of cell elongation'?

This is the area located just below the shoot or root tip where cells physically lengthen in response to auxin. This elongation is what ultimately causes the plant to increase in size or bend.

How does uneven auxin distribution lead to the bending of a plant shoot?

When auxin accumulates on one side, it stimulates the cells on that side to elongate faster than the cells on the opposite side. This disparity in growth rate forces the shoot to curve toward the slower-growing (sunny) side.

Response to Stimuli: Plants | Pearson Edexcel IGCSE Biology

2. Structure & Function in Living Organisms

Response to Stimuli: Plants

Summary

Plants utilize specialized directional growth responses known as tropisms to adapt to environmental changes. These responses are primarily coordinated by the plant growth regulator auxin, which controls cell elongation to ensure optimal access to light for photosynthesis and gravity for structural anchoring and nutrient absorption.

1. Definition & Core Concepts

Tropisms are directional growth responses in plants that occur in reaction to external stimuli such as light or gravity. If the plant grows towards the stimulus, it is classified as a positive tropism, whereas growth away from the stimulus is a negative tropism.
A stimulus is a detectable change in the environment, such as light, temperature, or chemicals, that triggers a specific response. In plants, specialized cells detect these changes to ensure the organism's survival by adjusting its growth patterns.
Phototropism refers specifically to the growth response toward or away from a light source, while geotropism (or gravitropism) is the response to the Earth's gravitational pull. These mechanisms ensure that leaves reach sunlight and roots penetrate the soil.

2. Underlying Principles

The coordination of plant growth is managed by chemical messengers called auxins. Auxin is primarily synthesized in the growing tips of shoots and then diffuses downward to the region of cell elongation located just behind the tip.
Auxin functions by stimulating cell elongation, causing existing cells to increase in size rather than increasing the total number of cells through division. The concentration of auxin directly determines the rate at which these cells grow and expand.
The distribution of auxin is sensitive to external factors; for instance, light causes auxin to accumulate on the shaded side of a shoot. This differential concentration leads to uneven growth rates across the tissue, resulting in a directional bend.

Diagram showing auxin accumulation on the shaded side of a plant shoot, leading to cell elongation and bending towards the light source.

3. Methods & Techniques

To demonstrate phototropism, observe a plant placed near a window; the auxin will migrate to the side furthest from the light, causing that side to grow faster and pushing the shoot to curve toward the window. This ensures maximum light absorption for the process of photosynthesis.
In response to gravity, roots exhibit positive geotropism by growing downwards into the soil. This behavior is critical for anchoring the plant firmly and ensuring the root system can access essential water and mineral ions from the earth.
Shoots exhibit negative geotropism by growing upwards, away from the pull of gravity. This response allows the plant to emerge from the soil and position its leaves effectively to capture sunlight, even if the seed was planted in an orientation that was initially horizontal.

4. Key Distinctions

5. Exam Strategy & Tips