Subject: Biology | Level: GCSE | Exam Board: WJEC
Master the chemical messengers that control plant growth and responses. This topic covers the vital mechanisms of phototropism and gravitropism, plus the commercial applications of auxins, gibberellins, and ethene that frequently appear in 6-mark questions.
Revision Notes & Key Concepts
Revision Podcast Transcript
Welcome to the GCSE Biology Revision Podcast. I'm your host, and today we're diving into one of the most fascinating topics in the entire specification — Plant Hormones, topic 4.4. Whether you're revising for AQA, Edexcel, or OCR, this episode has got you covered. So grab a pen, maybe a highlighter, and let's get into it. Plants might seem like they just sit there doing nothing, but they're actually incredibly responsive organisms. They can detect light, gravity, water, and touch — and they respond to all of these using chemical messengers called hormones. Today we're going to cover the three main plant hormones you need to know: auxins, gibberellins, and ethene. We'll look at how they work, where they're produced, and — crucially — how examiners will test you on them. By the end of this episode, you'll be able to answer any plant hormone question that comes your way. Let's go. Section one: Auxins and Phototropism. Auxins are probably the most important plant hormone for your exam. They're produced at the tip of plant shoots — called the apical meristem — and they control cell elongation. Here's the key thing to understand: auxins cause cells to elongate, meaning they get longer. When auxin is present in higher concentrations on one side of a shoot, that side grows faster, causing the shoot to bend. Now, phototropism is the growth response of a plant to light. Shoots show positive phototropism — they grow toward the light. Here's how it works step by step, and this is exactly what examiners want you to explain. Step one: Light hits the shoot tip from one side — let's say the left. Step two: Auxin is produced at the tip and begins to migrate — it moves away from the light, toward the shaded side on the right. Step three: The shaded side now has a higher concentration of auxin. Step four: The higher auxin concentration causes the cells on the shaded side to elongate more. Step five: Because the right side grows faster than the left side, the shoot bends to the left — toward the light. Remember: the shoot bends TOWARD the light because the shaded side grows FASTER. This trips up so many candidates in the exam. They say the shoot bends away from the light — that's wrong. The shoot bends toward the light. The mechanism involves the shaded side growing more, which pushes the tip toward the light. Now let's talk about gravitropism, which is also called geotropism. This is the growth response of a plant to gravity. Roots show positive gravitropism — they grow downward, toward gravity. Shoots show negative gravitropism — they grow upward, away from gravity. Here's the clever bit: auxin affects roots and shoots differently. In shoots, high auxin concentration promotes elongation. But in roots, high auxin concentration actually inhibits elongation — it slows growth down. This is because roots are much more sensitive to auxin than shoots are. So in a root placed horizontally: auxin accumulates on the lower side due to gravity. The lower side of the root has more auxin, which inhibits elongation there. The upper side has less auxin, so it elongates more. The root therefore bends downward — toward gravity. Positive gravitropism. This is a Higher Tier concept that examiners love to test with a diagram. If you see a diagram of a horizontal root or shoot, you need to be able to explain which side has more auxin and what effect that has on growth. Section two: Gibberellins. Gibberellins are a group of hormones produced in young leaves, seeds, and roots. Their main roles are: promoting seed germination, stimulating stem elongation, and triggering flowering in some plants. For your exam, the most important application of gibberellins is in agriculture. Gibberellins are used to produce seedless grapes — they're sprayed onto grape vines to increase the size of the fruit and prevent seeds from forming. They're also used to control the height of crop plants. Some dwarf varieties of wheat have mutations that affect gibberellin production — understanding this has been crucial in developing high-yield crop varieties. The key phrase for gibberellins is stem elongation. If an exam question asks you to identify which hormone promotes stem elongation, the answer is gibberellin. Section three: Ethene. Ethene is a gas — it's actually the simplest alkene, which links nicely to your Chemistry specification. As a plant hormone, ethene controls fruit ripening and leaf drop, which is called abscission. The commercial application of ethene is brilliant for exam questions. Fruit is often picked unripe and transported in sealed containers. Ethene gas is then pumped into the container to trigger ripening just before the fruit reaches the supermarket. This is why bananas are often green when they arrive at the warehouse but yellow by the time you buy them. Ethene also causes leaves to fall in autumn — it weakens the cells at the base of the leaf stalk, causing the leaf to detach. This is called leaf abscission. A common exam question asks you to evaluate the use of ethene in the food industry. You need to be able to give advantages — fruit can be transported safely without bruising — and disadvantages — it can be difficult to control the ripening process precisely. Section four: Exam Tips and Common Mistakes. Right, let's talk exam technique. Plant hormones questions appear in almost every GCSE Biology paper, so getting these right is essential. Common mistake number one: Confusing the effect of auxin on shoots versus roots. Remember — high auxin promotes elongation in shoots but inhibits elongation in roots. Roots are more sensitive to auxin. A useful way to remember this: think of roots as being "auxin-sensitive" — even a little too much auxin stops them growing. Common mistake number two: Saying the shoot bends away from the light. No! The shoot bends TOWARD the light. The mechanism is: auxin moves to the shaded side, shaded side grows faster, shoot bends toward the light source. Common mistake number three: Forgetting that ethene is a gas. Examiners sometimes ask candidates to explain how ethene travels — it diffuses through the air as a gas, unlike auxin and gibberellin which are transported through plant tissues. Common mistake number four: Not using the command word properly. If the question says "explain", you must give a reason — use the word "because" or "therefore" to link your points. If it says "describe", just say what happens. If it says "evaluate", you must give both advantages and disadvantages and reach a conclusion. For a 6-mark "explain" question on phototropism, examiners are looking for: auxin produced at shoot tip; auxin moves to shaded side; higher concentration of auxin on shaded side; cells on shaded side elongate more; shoot bends toward light. That's five marking points — you need to make sure you hit all of them clearly and in a logical order. Section five: Quick-Fire Recall Quiz. Cover up your notes and try to answer these out loud. Ready? Question one: Which hormone is responsible for phototropism in shoots? — Auxin. Question two: In gravitropism, does high auxin concentration promote or inhibit elongation in roots? — Inhibit. Question three: Name two commercial applications of plant hormones. — Rooting powder using auxin; ripening fruit using ethene; growing seedless grapes using gibberellin — any two of those. Question four: What is the term for the growth response of a plant to gravity? — Gravitropism, or geotropism. Question five: Which hormone is a gas? — Ethene. How did you do? If you got all five, brilliant — you're in great shape. If you missed any, go back and re-read those sections before your exam. Section six: Summary and Sign-Off. Let's bring it all together. Plant hormones are chemical messengers that coordinate growth and development in plants. Auxins control phototropism and gravitropism — they cause cell elongation in shoots but inhibit elongation in roots. Gibberellins promote seed germination, stem elongation, and flowering. Ethene controls fruit ripening and leaf drop, and has important commercial applications. For your exam: always explain the mechanism, not just the outcome. Use precise terminology — auxin, elongation, concentration gradient. And remember the key difference between how auxin affects shoots versus roots. You've got this. Good luck with your revision, and I'll see you in the next episode. Keep going — every mark counts.
Key Terms & Definitions
- Tropism
- A directional growth response of a plant to an environmental stimulus.
- Phototropism
- The growth of a plant in response to light.
- Gravitropism (Geotropism)
- The growth of a plant in response to gravity.
- Auxin
- A plant hormone produced in the tips of shoots and roots that controls cell elongation.
- Gibberellins
- A group of plant hormones that promote seed germination, stem elongation, and flowering.
- Ethene
- A gaseous plant hormone that controls fruit ripening and leaf abscission.
Worked Examples
Worked Example
Question: A student placed a seedling in a box with a hole on one side to let light in. After three days, the shoot had bent towards the light. Explain the mechanism that caused the shoot to bend towards the light. [4 marks]
Solution: Step 1: Auxin is produced at the tip of the shoot. Step 2: The auxin diffuses/moves to the shaded side of the shoot (away from the light). Step 3: This causes a higher concentration of auxin on the shaded side. Step 4: Auxin promotes cell elongation in shoots, so the cells on the shaded side grow faster/longer than the illuminated side, causing the shoot to bend towards the light.
Worked Example
Question: Evaluate the use of artificial plant hormones in agriculture. [6 marks]
Solution: Step 1: State an advantage of auxins. E.g., Auxins can be used as selective weedkillers to kill broad-leaved weeds without harming narrow-leaved crops, which increases crop yield. Step 2: State an advantage of ethene. E.g., Ethene allows fruit to be picked unripe so it is less likely to be damaged during transport, and then ripened precisely when needed for sale. Step 3: State an advantage of gibberellins. E.g., Gibberellins can be used to produce seedless fruits or increase the size of fruits like grapes. Step 4: State a disadvantage. E.g., Synthetic hormones can wash into nearby water sources, potentially harming wild plant species and disrupting local ecosystems. Step 5: State another disadvantage. E.g., Using rooting powders to create clones reduces genetic variation, making the entire crop susceptible to the same diseases. Step 6: Provide a conclusion. E.g., Overall, while artificial plant hormones significantly increase food production and reduce transport losses, their use must be carefully managed to prevent environmental damage and loss of biodiversity.
Worked Example
Question: Describe the effect of gravity on the growth of a root. [3 marks]
Solution: Step 1: Roots show positive gravitropism, meaning they grow downwards towards gravity. Step 2: Gravity causes auxin to accumulate on the lower side of the root. Step 3: In roots, a high concentration of auxin inhibits cell elongation, so the upper side grows faster, causing the root to bend downwards.
Practice Questions
Question: Name the plant hormone that is a gas at room temperature. [1 mark]
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Question: A gardener wants to grow new plants from cuttings of an existing plant. Explain how they can use plant hormones to ensure the cuttings grow successfully. [3 marks]
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Question: Some weedkillers contain synthetic plant hormones. Explain how these weedkillers increase crop yield. [4 marks]
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Question: A student investigates the effect of light on the growth of seedlings. They place one seedling in a box with light coming from all directions, and another in a box with light coming only from a hole on the right side. Compare the growth of the two seedlings after 3 days. [3 marks]
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Question: Explain why a root placed horizontally in the soil will eventually grow downwards. You must refer to the role of hormones in your answer. [5 marks]
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