📚 Plant Hormones: OCR GCSE Biology | 植物激素：OCR GCSE 生物考点精讲
Unlike animals, plants do not have a nervous system, yet they must still respond to their environment. They do so using chemical messengers called plant hormones. These hormones coordinate growth, development, and responses to stimuli such as light and gravity. In OCR GCSE Biology, you need to understand the roles of key hormones—auxins, gibberellins, and ethene—and how they regulate processes like tropisms, seed germination, and fruit ripening.
与动物不同，植物没有神经系统，但它们仍须对环境作出反应。植物利用称为植物激素的化学信使来实现这一目的。这些激素协调生长、发育以及对光和重力等刺激的响应。在 OCR GCSE 生物学中，你需要理解关键激素——生长素、赤霉素和乙烯——的作用，以及它们如何调控向性、种子萌发和果实成熟等过程。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones are chemical substances produced in one part of a plant and transported to another part, where they bring about a specific response. They work at very low concentrations and can have different effects depending on the target tissue and the concentration present. The main hormones you need to know for your OCR exam are auxins (such as indoleacetic acid, IAA), gibberellins, and ethene (ethylene).
植物激素是在植物某一部位产生并运输到另一部位，在那里引发特定反应的化学物质。它们在极低浓度下发挥作用，并且根据靶组织和浓度不同，效果可能不同。在 OCR 考试中你需要了解的主要激素包括生长素（如吲哚乙酸 IAA）、赤霉素和乙烯。
These hormones allow plants to adapt to their environment by controlling growth direction, timing of germination, flowering, and fruit ripening. Understanding them is fundamental to topics such as tropisms and commercial crop production.
这些激素通过控制生长方向、萌发时间、开花和果实成熟，使植物能够适应环境。理解它们对于向性反应和商业作物生产等主题至关重要。
2. Tropisms: Growth Responses to Stimuli | 向性：对刺激的生长反应
A tropism is a directional growth response in which a plant grows towards or away from a stimulus. The two main tropisms you must learn are phototropism (response to light) and gravitropism (also called geotropism, response to gravity).
向性是一种定向生长反应，植物向着或背着刺激方向生长。你必须掌握的两种主要向性是向光性（对光的反应）和向地性（也称向重力性，对重力的反应）。
In phototropism, shoots grow towards light (positive phototropism) to maximise light absorption for photosynthesis, while roots generally show little phototropic response. In gravitropism, roots grow downwards in the direction of gravity (positive gravitropism) to anchor the plant and reach water and minerals, while shoots grow upwards (negative gravitropism) to access light.
在向光性中，茎向着光生长（正向光性）以最大限度地吸收光进行光合作用，而根通常很少表现出向光性反应。在向地性中，根顺着重力方向向下生长（正向地性）以固定植物并获取水分和矿物质，而茎向上生长（负向地性）以接触光线。
3. The Role of Auxins in Tropisms | 生长素在向性中的作用
Auxins are a group of plant hormones that promote cell elongation in shoots and inhibit cell elongation in roots at higher concentrations. In OCR, you need to explain how uneven distribution of auxin causes tropisms. Auxins are produced in the tips of shoots and roots and move downwards by diffusion and active transport.
生长素是一类植物激素，在茎中促进细胞伸长，而在较高浓度下抑制根中的细胞伸长。OCR 要求你解释生长素的不均匀分布如何导致向性。生长素在茎尖和根尖产生，并通过扩散和主动运输向下移动。
When a shoot is illuminated from one side, auxin accumulates on the shaded side. This causes the cells on the shaded side to elongate more than those on the illuminated side, bending the shoot towards the light. In roots, high auxin concentration inhibits elongation, so when auxin accumulates on the lower side of a horizontal root, the cells on that side elongate less, causing the root to bend downwards.
当茎单侧受光时，生长素在背光侧积累。这导致背光侧的细胞比向光侧伸长更多，使茎弯向光。在根中，高浓度生长素抑制伸长，因此当水平放置的根向地侧积累生长素时，该侧细胞伸长较少，导致根向下弯曲。
4. Auxin and Phototropism in Detail | 生长素与向光性的详细机制
In phototropism, light causes a redistribution of auxin in the shoot tip. The exact mechanism is debated, but the accepted idea is that auxin is transported laterally away from the light, leading to a higher concentration on the dark side. This stimulates cell expansion on the dark side, causing the shoot to bend towards the light source.
在向光性中，光引起茎尖生长素的重新分布。确切机制仍有争议，但公认的观点是生长素从光源侧横向运输离开，导致黑暗侧浓度更高。这刺激了黑暗侧细胞扩张，使茎弯向光源。
You can investigate phototropism by placing seedlings in a box with a small hole on one side, allowing light to enter from one direction only. Over several days, the shoots will bend towards the hole. Control experiments with foil caps over the tips show that the tip is essential for detecting light and triggering auxin redistribution.
你可以通过将幼苗放入一侧有小孔的盒子中，只允许光从单一方向进入，来研究向光性。几天后，茎会弯向孔。用锡箔帽盖住茎尖的对照实验表明，尖端对于检测光并触发生长素重新分布至关重要。
5. Auxin and Gravitropism in Shoots and Roots | 生长素与茎和根的向地性
When a plant is laid on its side, auxin accumulates on the lower side in both shoots and roots due to gravity. In the shoot, this high auxin concentration promotes cell elongation, so the lower side grows faster, bending the shoot upwards against gravity. This is negative gravitropism.
当植物水平放置时，重力使生长素在茎和根的下侧积累。在茎中，高浓度生长素促进细胞伸长，因此下侧生长更快，使茎向上弯曲对抗重力，这是负向地性。
In roots, the same high auxin concentration inhibits cell elongation. Therefore, the cells on the upper side elongate more, and the root bends downwards in the direction of gravity. This is positive gravitropism. It’s important to remember that the concentration-response curve differs between root and shoot tissues.
在根中，相同的高浓度生长素却抑制细胞伸长。因此，上侧的细胞伸长更多，根向下弯曲顺着重力方向，这是正向地性。记住根和茎组织的浓度-反应曲线不同非常重要。
6. Auxins and Apical Dominance | 生长素与顶端优势
Apical dominance is the phenomenon where the main, central shoot of a plant grows more strongly than side shoots. This is controlled by auxin produced at the tip of the main shoot. The auxin moves downwards and inhibits the growth of lateral buds further down the stem.
顶端优势是指植物主茎比侧枝生长更旺盛的现象。这是由主茎顶端产生的生长素控制的。生长素向下移动并抑制茎下部侧芽的生长。
If you remove the shoot tip (pruning), auxin production stops, allowing side shoots to grow. This principle is used in horticulture to produce bushy plants. Cytokinins from roots promote lateral bud growth and interact antagonistically with auxin.
如果去掉茎尖（修剪），生长素停止产生，侧枝得以生长。这一原理被用于园艺中以培育茂密的株型。来自根的细胞分裂素促进侧芽生长，并与生长素发生拮抗作用。
7. Gibberellins: Overview and Seed Germination | 赤霉素：概述与种子萌发
Gibberellins are plant hormones that stimulate stem elongation, seed germination, and flowering. For OCR, focus on their role in seed germination. When a seed absorbs water, the embryo produces gibberellins, which diffuse to the aleurone layer, a protein-rich layer inside the seed.
赤霉素是刺激茎伸长、种子萌发和开花的植物激素。在 OCR 考试中，重点掌握其在种子萌发中的作用。当种子吸水后，胚产生赤霉素，扩散到种子内部的糊粉层（富含蛋白质的层）。
Gibberellins trigger the aleurone cells to synthesise the enzyme amylase. Amylase then breaks down starch stored in the endosperm into glucose. This glucose is used by the embryo for respiration to provide energy for growth. Without gibberellins, the seed would not mobilise its food reserves effectively.
赤霉素触发糊粉层细胞合成淀粉酶。淀粉酶随后将胚乳中储存的淀粉分解为葡萄糖。胚利用这些葡萄糖进行呼吸作用，为萌发提供能量。如果没有赤霉素，种子将无法有效动用自己的营养储备。
8. Gibberellins in Stem Elongation and Flowering | 赤霉素在茎伸长和开花中的作用
Gibberellins also promote stem elongation by stimulating cell division and cell elongation. Dwarf varieties of plants often have a genetic mutation that reduces gibberellin production; applying gibberellin can restore normal growth. This demonstrates the direct effect of the hormone.
赤霉素还通过刺激细胞分裂和伸长来促进茎伸长。矮化品种的植物通常具有减少赤霉素产生的基因突变；施加赤霉素可以恢复正常生长，这证明了该激素的直接效应。
Additionally, in some long-day plants, gibberellins are involved in triggering flowering when day length increases. They may work together with a hormone called florigen. However, OCR mainly expects you to link gibberellins to germination and stem growth.
此外，在一些长日照植物中，赤霉素参与在日照时间延长时诱导开花。它们可能与一种称为成花素的激素共同作用。然而 OCR 主要要求你联系赤霉素与种子萌发和茎生长。
9. Ethene and Fruit Ripening | 乙烯与果实催熟
Ethene (ethylene) is a gaseous plant hormone that stimulates fruit ripening, abscission (leaf drop), and flower wilting. It is unique because it is a gas at room temperature. During ripening, ethene triggers the conversion of starch to sugar, softening of cell walls, and colour changes in fruit.
乙烯是一种气态植物激素，能促进果实成熟、脱落（叶落）和花凋谢。它的独特之处在于室温下为气体。在成熟过程中，乙烯触发淀粉转化为糖、细胞壁软化和果实颜色变化。
A classic practical involves placing unripe bananas in a bag with a ripe banana. The ripe banana releases ethene, which accelerates ripening of the unripe ones. This is an example of a positive feedback loop, as ripening fruits produce more ethene, causing more ripening.
一个经典实验是将未熟香蕉与一根熟香蕉一起放入袋中。熟香蕉释放乙烯，加速未熟香蕉的成熟。这是正反馈循环的例子，因为成熟果实产生更多乙烯，导致更多成熟。
10. Commercial Uses of Plant Hormones | 植物激素的商业用途
Plant hormones and synthetic analogues are widely used in agriculture and horticulture. Understanding these applications is a common exam topic. Each hormone has specific commercial roles based on its natural function.
植物激素及其合成类似物在农业和园艺中广泛使用。理解这些应用是常考主题。每种激素根据其自然功能都有特定的商业用途。
Key Uses:
主要用途：
- Auxins are used as rooting powders to encourage root growth on stem cuttings, and as selective weedkillers that cause uncontrolled growth in broad-leaved weeds.
- 生长素用作生根粉，促进插条生根；也用作选择性除草剂，导致阔叶杂草生长失控。
- Gibberellins are applied to delay fruit senescence, increase fruit size, and promote uniform flowering and malting in barley for brewing.
- 赤霉素用于延缓果实衰老、增大果实个头、促进一致开花和大麦发芽酿酒。
- Ethene is used to ripen fruits such as bananas and citrus after transport, ensuring they are ready for sale at the same time.
- 乙烯用于催熟运输后的香蕉和柑橘，确保它们同时可售。
11. Practical Investigation: Effects of Light on Seedlings | 实验探究：光对幼苗的影响
OCR may ask you to design or interpret an experiment into phototropism. A typical setup uses cress or mustard seedlings in petri dishes with light coming from one direction. You can vary factors like light intensity or covering the tip with foil, the zone of elongation with a collar, etc.
OCR 可能要求你设计或解释向光性实验。典型的装置使用培养皿中的水芹或芥菜幼苗，并从单一方向提供光照。你可以改变因素如光照强度、用锡箔盖住尖端、用套筒包住伸长区等。
Variables to control include temperature, moisture, and type of seed. You should measure the angle of curvature with a protractor. Experiments show that the tip is the site of light perception and auxin production, while the region below the tip is where bending occurs.
需控制的变量包括温度、水分和种子类型。你应该用量角器测量弯曲角度。实验证明尖端是感知光和产生生长素的部位，而尖端以下的区域是弯曲发生的地方。
12. Key Terms and Summary | 关键术语与总结
To excel in this topic, ensure you can define tropism, phototropism, gravitropism, apical dominance, and describe the roles of auxin, gibberellin, and ethene. Use the concentration-response idea correctly: auxin promotes elongation in shoots but inhibits it in roots at high concentrations. Practice explaining the experiments clearly.
要掌握本主题，确保你能定义向性、向光性、向地性、顶端优势，并描述生长素、赤霉素和乙烯的作用。正确使用浓度-反应概念：在茎中生长素促进伸长，而在根中高浓度抑制。练习清晰解释实验。
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