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Phototropism is the growth response of plants to light, and gravitropism is the growth response to gravity. Free notes with key terms and 13 questions answered.
Plant Hormones and Growth Responses is part of Plant hormones (biology only) in GCSE AQA Higher Biology.
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GCSE AQA Higher
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Phototropism is the growth response of plants to light, and gravitropism is the growth response to gravity. Free notes with key terms and 13 questions answered.
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Use this lesson to stay inside Plant hormones (biology only) while connecting the detail back to the wider Homeostasis and response topic.
Read the core explanations from Plant Hormones and Growth Responses before testing yourself from memory.
Plants can't move like animals, but they still need to find light for photosynthesis and water for survival. How do they manage this? The answer lies in their ability to grow in specific directions - towards light and away from gravity for shoots, towards gravity and water for roots.
This directional growth isn't random. Plants produce chemical messengers called hormones that coordinate these responses. Think of hormones as the plant's internal communication system, telling different parts when and how to grow.
Trap: Many students think plants are passive - they're not. Plants actively respond to their environment through coordinated growth responses controlled by hormones.
The most important plant hormone for growth responses is auxin. Picture auxin as tiny chemical messengers travelling through the plant, telling cells exactly how much to grow and in which direction.
Here's the clever bit: auxin doesn't spread evenly through the plant. It accumulates more on one side than the other, creating an unequal distribution. This unequal distribution is the key to directional growth.
Trap: Students often think auxin makes cells divide more. Actually, auxin makes existing cells grow longer by stretching - it's about cell elongation, not cell division.
When you place a plant near a window, the shoot bends towards the light within days. This response is called phototropism, and it's controlled by auxin's unequal distribution.
Worked example: How Phototropism Works in Shoots
Explain how a plant shoot bends towards light from one side.
Answer: Unequal auxin distribution causes unequal growth rates, bending the shoot towards the light source.
The key insight is that auxin promotes growth in shoots, so the side with more auxin grows faster. This creates the bend towards the light.
Even in complete darkness, plant shoots grow upwards and roots grow downwards. This response to gravity is called gravitropism (or geotropism), and it ensures shoots reach the surface whilst roots anchor the plant.
Here's where it gets interesting: auxin affects shoots and roots differently. In shoots, auxin promotes growth. In roots, auxin inhibits (slows down) growth. This opposite effect is crucial for gravitropism.
Worked example: Gravitropism in Shoots vs Roots
Compare how auxin causes gravitropism in shoots and roots when a plant is placed on its side.
Answer: Same auxin distribution causes opposite growth responses: shoots bend up (negative gravitropism), roots bend down (positive gravitropism).
Trap: Students often forget that auxin has opposite effects in shoots (promotes growth) versus roots (inhibits growth). This is why the same hormone can cause both upward and downward bending.
The secret to all plant growth responses is unequal auxin distribution. Whether responding to light or gravity, auxin always ends up more concentrated on one side than the other. This creates unequal growth rates, which causes bending.
Think of it like this: imagine two people walking side by side, but one person walks faster. They'll naturally curve towards the slower person's side. Plant growth works the same way - the faster-growing side curves the plant towards the slower-growing side.
Plant hormone questions often ask you to explain how growth responses work. Examiners want to see that you understand the link between auxin distribution and growth rates.
Common exam mistakes: Don't say auxin 'moves towards' light or gravity. Auxin moves away from light (it's broken down by light) and is pulled downwards by gravity. The distribution is what matters, not the direction of movement.
Master the principle of unequal auxin distribution, and you'll be able to explain any plant growth response. Remember: same hormone, different effects in different plant parts, all leading to survival advantages.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Phototropism is the growth response of plants to light direction. Shoots show positive phototropism, growing towards light, while roots show negative phototropism, growing away from light. It is controlled by the unequal distribution of the plant hormone auxin.
Auxin is a plant hormone that promotes cell elongation by making cell walls more flexible, so cells absorb water and grow longer. It makes existing cells grow longer rather than divide, and its unequal distribution causes unequal growth rates, which makes shoots and roots bend.
Gravitropism (or geotropism) is the growth response of plants to gravity. Shoots show negative gravitropism, growing upwards away from gravity, while roots show positive gravitropism, growing downwards towards gravity, even in complete darkness.
Plant hormones are chemical messengers produced in small quantities that coordinate and control growth and responses to environmental stimuli in plants. Auxin is the most important plant hormone for growth responses such as phototropism and gravitropism.
When a plant is on its side, gravity makes auxin accumulate on the lower side of both shoot and root. In shoots auxin promotes growth, so the lower side grows faster and the shoot bends up; in roots auxin inhibits growth, so the lower side grows slower and the root bends down.
In shoots, auxin promotes growth, but in roots auxin inhibits (slows down) growth. So when auxin accumulates on the lower side, a shoot's lower side grows faster and bends up, while a root's lower side grows slower and bends down.
Positive phototropism is growth towards light, shown by shoots, while negative phototropism is growth away from light, shown by roots. Shoots grow towards light to position leaves for maximum light capture.
Positive gravitropism is growth towards gravity, shown by roots growing downwards, while negative gravitropism is growth away from gravity, shown by shoots growing upwards. Gravitropism is also called geotropism.
When light comes from one side, auxin accumulates on the shaded side of the shoot because auxin is broken down by light. Auxin promotes cell elongation in shoots, so the shaded side grows faster and the shoot curves towards the light, positioning leaves for maximum light capture.
Auxin makes existing cells grow longer rather than divide more: it promotes cell elongation by making cell walls more flexible, so cells absorb water and stretch. Directional growth comes from cell elongation, not cell division.
Auxin accumulates on the shaded side of a shoot because it is broken down by light, so more survives on the dark side. The shaded side then grows faster, bending the shoot towards the light.
When a plant is placed on its side, gravity makes auxin accumulate on the lower side of both the shoot and root. The shoot's lower side grows faster so it bends upwards, while the root's lower side grows slower so it bends downwards.
Plants need growth responses because they cannot move like animals but still need to find light for photosynthesis and water for survival. Hormones coordinate growth towards light and away from gravity in shoots, and towards gravity and water in roots.