Tropisms and Plant Responses: KS3 Biology Guide

What is a tropism?

A tropism is a directional growth response in a plant caused by a stimulus from a particular direction. The word comes from the Greek tropos, meaning "turning". Unlike an animal's nervous response, a tropism involves differential growth — cells on one side of a shoot or root grow faster or more slowly than cells on the other side, causing the organ to bend.

Tropisms are described as:

• Positive — the organ grows towards the stimulus
• Negative — the organ grows away from the stimulus

Plants respond to several different stimuli. The main ones at KS3 are light, gravity, water, and touch.

What is phototropism?

Phototropism is the growth response of a plant to light. Shoots show positive phototropism — they grow towards the light source. This maximises the light available for photosynthesis.

The classic demonstration is placing a seedling in a box with a hole cut in one side. The shoot bends towards the hole as it grows, even if the box is rotated periodically, showing the response is to the direction of light rather than to gravity or any other cue.

Real-world example: sunflowers (and other tall plants) orient their leaves and stems to track the sun across the sky, maximising photosynthesis throughout the day.

What is gravitropism?

Gravitropism (also called geotropism) is the growth response to the force of gravity.

• Shoots are negatively gravitropic — they grow upwards, away from gravity. Even if you place a seedling on its side in the dark (removing the light stimulus), the shoot will curve upward.
• Roots are positively gravitropic — they grow downward, into the soil. This anchors the plant and directs roots towards water held deeper in the soil.

Real-world example: after a storm bends a young tree's stem sideways, gravitropism and phototropism both act on the shoot, which curves back upward over the following days.

What is hydrotropism?

Hydrotropism is the growth response to water. Roots show positive hydrotropism — they grow towards regions of higher water concentration in the soil. This allows roots to seek out moisture when the soil is unevenly hydrated.

Real-world example: tree roots can grow great distances through soil towards underground water sources. In urban environments, roots may grow towards cracked water pipes, which is why pipes near mature trees are more vulnerable to damage.

What is thigmotropism?

Thigmotropism is the growth response to touch or contact with a solid object. Climbing plants show positive thigmotropism in their tendrils or stems, which wrap around supports as they grow.

Real-world examples:

• Pea tendrils curl around a cane or fence wire when they make contact, allowing the plant to climb without spending energy on a thick, self-supporting stem
• Ivy stems and roots attach to walls by producing adhesive pads in response to contact with a surface

What are the positive and negative responses for different organs?

Note: roots are generally considered insensitive to light at the levels involved in phototropism experiments, though they may show a very weak negative phototropic response in extreme conditions.

What is auxin and how does it control tropisms?

Auxin (also written as IAA — indole-3-acetic acid) is a plant hormone produced at the growing tip (apical meristem) of shoots. It diffuses back down the shoot and promotes cell elongation — cells exposed to auxin grow longer.

How auxin explains phototropism in shoots:

When light hits a shoot from one side:

• Auxin moves away from the bright side and accumulates on the shaded side
• Cells on the shaded side are exposed to higher auxin concentrations and elongate more
• The shoot curves towards the light because the shaded side grows faster

Why roots respond differently to auxin:

Roots are much more sensitive to auxin than shoots. Concentrations of auxin that promote elongation in shoots actually inhibit elongation in roots. This reversal of effect explains gravitropism:

• In a horizontal root, auxin accumulates on the lower side (gravity pulls it down)
• The lower side of the root has too much auxin and its cells are inhibited
• The upper side has less auxin and grows faster
• The root curves downward — positive gravitropism

This single hormone therefore produces opposite effects in shoots and roots due to the difference in sensitivity between the two tissues.

Frequently asked questions

Q: What is the difference between a tropism and a taxis?

A tropism is a growth response — the plant grows differently on each side, causing a permanent change in direction. A taxis is a movement response by an organism (typically a single-celled organism or animal) that moves bodily towards or away from a stimulus. Tropisms change the direction of growth; taxes change the direction of movement. Plants show tropisms; bacteria and some invertebrates show taxes.

Q: Why don't plants need a nervous system to respond to stimuli?

Plants respond to stimuli by producing and redistributing chemical signals (hormones like auxin), which travel through plant tissue by diffusion and active transport. This is slower than a nervous impulse but sufficient for the gradual growth responses plants need. Animals require rapid, coordinated responses — catching prey or escaping predators — which need a nervous system to transmit signals in milliseconds. Plant responses can operate on timescales of hours to days.

Q: Can you demonstrate phototropism at home?

Yes. Place a seedling (a bean or sunflower seedling works well) in front of a window. Over several days you will see the stem lean towards the light. To see it more dramatically, put the seedling inside a shoebox with a hole in one end and leave the box near a light source. Check after two or three days — the shoot will be growing towards the hole. Rotating the box will cause the shoot to change direction over the following days.

Q: Is auxin the only hormone involved in plant responses?

At KS3 level, auxin is the main hormone you need to know. Plants also produce other hormones including gibberellins (which promote stem elongation and seed germination), cytokinins (which promote cell division), abscisic acid (which triggers stomatal closure and seed dormancy), and ethylene (which promotes fruit ripening). These are studied in more detail at GCSE and A Level.

Q: Why do roots grow downward even when planted upside down?

If a seed is planted with the root pointing up, positive gravitropism will cause the root to curve and grow downward within a day or two. Auxin accumulates on the lower side of the root (due to gravity), inhibiting elongation there while the upper side grows freely, causing the root tip to curve towards the ground. This ensures roots always grow into the soil regardless of how the seed lands.

Let Professor Pi guide you through tropisms and plant responses — visit aitutors.me.