📚 Plant Hormones: Key Concepts for IGCSE CIE Biology | 植物激素：IGCSE CIE 生物考点精讲
Plants may not have a nervous system, but they are far from passive organisms. They rely on chemical messengers called plant hormones (or plant growth regulators) to coordinate growth, development, and responses to environmental stimuli. Understanding how auxin, gibberellin, and ethene work is essential for success in the IGCSE CIE Biology exam. This article will guide you through the key concepts, mechanisms, and experimental evidence needed to tackle plant hormone questions with confidence.
植物虽然没有神经系统，但它们绝非被动的生物。它们依靠称为植物激素（或植物生长调节剂）的化学信使来协调生长、发育以及对环境刺激的响应。理解生长素、赤霉素和乙烯的作用机制，对于在 IGCSE CIE 生物考试中取得好成绩至关重要。本文将带你逐一梳理关键概念、作用机制和实验证据，让你自信应对植物激素相关的考题。
1. Introduction to Plant Hormones | 植物激素简介
Plant hormones are chemical substances produced in small amounts in one part of a plant and transported to another part, where they exert a specific effect on growth or development. Unlike animal hormones, they are not produced in specialised glands; instead, they are often synthesised in actively growing regions such as shoot tips, root tips, and developing seeds. The main plant hormones you need to know for CIE IGCSE are auxin, gibberellin, and ethene.
植物激素是在植物某一部位以微量产生、并被运输到另一部位，从而对生长或发育产生特定影响的化学物质。与动物激素不同，它们并非由专门的腺体产生，而通常在生长活跃的区域合成，如茎尖、根尖和发育中的种子。在 CIE IGCSE 考试中，你需要掌握的主要植物激素包括生长素、赤霉素和乙烯。
Plant hormones can either stimulate or inhibit growth, depending on their concentration and the tissue they act upon. They control tropisms (directional growth responses), seed germination, fruit ripening, and even leaf fall. The effects are often concentration-dependent: the same hormone can promote growth in shoots but inhibit it in roots.
植物激素可以根据其浓度和作用的组织不同而促进或抑制生长。它们控制向性（定向生长反应）、种子萌发、果实成熟甚至落叶。这些效应往往与浓度有关：同一种激素可能在茎中促进生长，而在根中却抑制生长。
2. Auxin: The Master Growth Regulator | 生长素：生长的主调控因子
Auxin is the most well-known plant hormone and is central to the control of directional growth. The most common natural auxin is indole-3-acetic acid (IAA). It is produced mainly in the tips of shoots and roots, in meristem cells that are actively dividing. From there it diffuses or is actively transported to the region of cell elongation, just behind the tip.
生长素是最广为人知的植物激素，是控制定向生长的核心。最常见的天然生长素是吲哚-3-乙酸（IAA）。它主要在茎尖和根尖的分生组织细胞中产生，这些细胞正进行活跃分裂。然后它扩散或被主动运输到紧靠尖端的细胞伸长区。
Auxin’s primary effect is to promote cell elongation by making the cell wall more stretchable, allowing the cell to take up water and expand. However, its effect is not universal: in shoots, auxin stimulates growth, while in roots, high concentrations of auxin actually inhibit growth. This differential sensitivity is key to understanding tropisms.
生长素的主要作用是促进细胞伸长，它使细胞壁更具延展性，让细胞吸水膨胀。但这种效应并非普遍适用：在茎中，生长素促进生长；而在根中，高浓度的生长素反而会抑制生长。这种不同的敏感性是理解向性的关键。
3. Phototropism: Bending Towards Light | 向光性：向光弯曲
Phototropism is a growth response in which a plant shoot bends towards a light source. This is a positive phototropic response that allows leaves to capture more light for photosynthesis. In shoots, light causes auxin to redistribute from the illuminated side to the shaded side.
向光性是指植物茎向光源弯曲的生长反应。这是一种正向光性反应，能让叶片捕获更多光能进行光合作用。在茎中，光照会导致生长素从向光一侧重新分布到背光一侧。
The higher concentration of auxin on the shaded side stimulates cells there to elongate more rapidly than those on the illuminated side. As a result, the shaded side grows faster, causing the shoot to bend towards the light. This is a classic example of how uneven auxin distribution leads to differential growth.
背光一侧生长素浓度更高，刺激该侧细胞比向光一侧伸长得更快。因此，背光侧生长更快，导致茎向光弯曲。这是生长素不均匀分布导致差异性生长的经典例子。
4. Mechanism of Phototropism in Shoots | 茎中向光性的机制
When a shoot is exposed to unilateral light (light from one side), phototropins (light-sensitive proteins) in the tip detect the direction of light. This triggers the lateral transport of auxin from the illuminated side to the shaded side. The exact mechanism involves active transport of auxin across cells, moving it against its concentration gradient, powered by ATP.
当茎受到单侧光照时，顶端的光敏蛋白（向光素）会感知光的方向。这会触发生长素从向光一侧横向运输到背光一侧。其确切机制涉及生长素借助 ATP 能量逆浓度梯度跨细胞的主动运输。
Once auxin accumulates on the shaded side, it moves downward into the elongation zone. Here, it activates enzymes that loosen the cell wall, allowing turgor pressure to push the cell to expand. This results in faster elongation on the shaded side and bending towards the light. This process can be summarised in a simple table:
一旦生长素在背光一侧积累，它便向下移动到伸长区。在这里，它激活酶来松动细胞壁，使膨压推动细胞扩张。这导致背光侧伸长更快、茎向光弯曲。这个过程可以用一个简单的表格来概括：
| Side (Shoot) | Auxin Concentration | Growth Rate | Result |
|---|---|---|---|
| Illuminated (向光侧) | Low | Slower | Short cells |
| Shaded (背光侧) | High | Faster | Long cells → bending towards light |
5. Gravitropism (Geotropism): Responding to Gravity | 向地性：对重力的响应
Gravitropism is a growth response to gravity. Roots show positive gravitropism (grow downwards in the direction of gravity), while shoots show negative gravitropism (grow upwards, against gravity). This ensures that roots anchor the plant and reach water and minerals, while shoots grow towards light.
向地性是对重力的生长反应。根表现出正向地性（顺着重力方向向下生长），而茎表现出负向地性（逆着重力方向向上生长）。这保证了根能固定植物并获取水分和无机盐，同时茎能朝向光生长。
Gravity is sensed by special starch-filled organelles called amyloplasts (statoliths) in root cap cells and in the endodermal cells of shoots. These dense bodies settle downwards, triggering a redistribution of auxin. However, the response in roots is opposite to that in shoots because of different sensitivity to auxin.
重力是由根冠细胞和茎内皮层细胞中充满淀粉的称为淀粉体（平衡石）的特殊细胞器感知的。这些致密的小体向下沉降，触发生长素的重新分布。但由于对生长素的敏感性不同，根的反应与茎相反。
6. Mechanism of Gravitropism in Roots and Shoots | 根与茎的向地性机制
In a horizontally placed root, gravity causes auxin to accumulate on the lower side. In roots, high auxin concentration inhibits cell elongation. Therefore, cells on the upper side (lower auxin) elongate faster than cells on the lower side (higher auxin). This causes the root to bend downwards, towards gravity.
在水平放置的根中，重力使生长素积聚在下侧。在根中，高浓度生长素抑制细胞伸长。因此，上侧细胞（生长素浓度较低）比下侧细胞（生长素浓度较高）伸长更快，导致根向下弯曲，朝向重力。
In a horizontally placed shoot, auxin also accumulates on the lower side due to gravity. However, in shoots, high auxin stimulates cell elongation. Thus, the lower side grows faster, pushing the shoot upwards, against gravity. This table compares root and shoot responses:
在水平放置的茎中，重力同样使生长素积聚在下侧。然而，在茎中，高浓度生长素促进细胞伸长。因此，下侧生长更快，推动茎向上弯曲，逆着重力方向。下面表格比较了根和茎的反应：
| Organ | Auxin accumulates on | Effect of high auxin | Growth result |
|---|---|---|---|
| Root | Lower side | Inhibits elongation | Upper side grows faster → bends downwards |
| Shoot | Lower side | Stimulates elongation | Lower side grows faster → bends upwards |
This opposite effect of auxin is a common exam point. Always remember: in roots, auxin is inhibitory at the concentrations reached; in shoots, it is stimulatory.
生长素的这种相反作用是常见的考点。一定要记住：在根中，达到的浓度对生长是抑制的；而在茎中，则是促进的。
7. Experimental Evidence for Auxin Function | 生长素功能的实验证据
Several classic experiments provide evidence for the role of auxin in tropisms. Darwin’s experiments (1880) with oat coleoptiles showed that the tip of the shoot is responsible for detecting light and that removing the tip prevents phototropism. Boysen-Jensen (1913) inserted a block of gelatine (permeable) or mica (impermeable) between the tip and the elongation zone, proving that a water-soluble chemical signal moves from the tip.
有几个经典实验为生长素在向性中的作用提供了证据。达尔文（1880）用燕麦胚芽鞘进行的实验表明，茎尖负责感知光照，除去尖端会阻止向光性。博伊森-詹森（1913）在尖端和伸长区之间插入一块明胶（可透）或云母（不可透），证明有一种水溶性化学信号从尖端向下传递。
Went (1928) later isolated this chemical and called it ‘auxin’. He placed excised coleoptile tips on agar blocks, allowing auxin to diffuse into the agar. He then placed the agar blocks asymmetrically on decapitated coleoptiles. If the block was placed on one side, that side grew faster, causing bending even in the dark. This demonstrated that auxin alone could induce differential growth.
温特（1928）后来分离出这种化学物质并命名为“生长素”。他将切下的胚芽鞘尖端放在琼脂块上，让生长素扩散到琼脂中。然后将琼脂块不对称地放在去尖端的胚芽鞘上。如果琼脂块被放在一侧，这一侧就长得更快，即便在黑暗中也引起弯曲。这证明仅生长素就能引发差异性生长。
For IGCSE, you should be able to interpret similar investigation scenarios, such as using light-proof foil over the tip to show the tip is the site of light detection, or using agar blocks containing auxin to replace the tip and restore phototropism.
在 IGCSE 中，你需要能够解释类似的探究情景，例如用不透光箔帽套住尖端以表明尖端是光感受部位，或者用含有生长素的琼脂块代替尖端来恢复向光性。
8. Gibberellin and Seed Germination | 赤霉素与种子萌发
Gibberellin is another important plant hormone that you need to know. It is involved in promoting seed germination, stem elongation, and flowering. In seed germination, gibberellin is produced in the embryo and triggers the production of enzymes (such as amylase) in the aleurone layer of cereal grains.
赤霉素是你需要知道的另一种重要植物激素。它参与促进种子萌发、茎的伸长和开花。在种子萌发过程中，赤霉素在胚中产生，并触发谷物糊粉层中酶（如淀粉酶）的产生。
These enzymes break down stored starch in the endosperm into glucose, which is then used by the growing embryo for respiration and growth. Without gibberellin, the seeds would not mobilise their food reserves and germination would be severely delayed or fail. This is why gibberellin is used commercially in the malting industry (e.g., to make barley seeds germinate quickly for beer production).
这些酶将胚乳中储存的淀粉分解为葡萄糖，然后供生长的胚用于呼吸和生长。没有赤霉素，种子就无法调动它们的食物储备，萌发会被严重延迟或失败。这就是为什么赤霉素被用于麦芽制造工业（例如，促使大麦种子快速萌发以用于啤酒生产）。
9. Ethene and Fruit Ripening | 乙烯与果实成熟
Ethene (C₂H₄) is a gaseous plant hormone that plays a major role in fruit ripening. It is produced in increasing amounts as fruits mature and acts as a trigger for the ripening process, which includes softening of the fruit, change in colour, and conversion of starch to sugars.
乙烯（C₂H₄）是一种气体植物激素，在果实成熟过程中起重要作用。随着果实成熟，它的产量不断增加，并作为成熟过程的触发因子，这包括果实软化、颜色变化以及淀粉向糖的转化。
Ethene stimulates a series of enzymatic reactions that cause these changes. An interesting aspect is that ripening is often autocatalytic: once ripening begins, ethene stimulates further ethene production, accelerating the process. This is why one ripe banana can cause other bananas nearby to ripen faster.
乙烯会刺激一系列酶促反应来引起这些变化。一个有趣的方面是，成熟往往是自催化的：一旦成熟开始，乙烯便刺激更多乙烯的产生，从而加速进程。这就是为什么一根熟香蕉会使旁边的其他香蕉更快成熟。
Commercially, ethene is used to ripen fruits such as bananas, tomatoes, and mangoes in a controlled manner during storage and transport. Inhibitors of ethene action can be used to delay ripening and extend shelf life.
在商业上，乙烯被用于在储存和运输过程中以可控方式催熟香蕉、番茄和芒果等水果。乙烯作用的抑制剂可用于延缓成熟并延长货架期。
10. Commercial Uses of Plant Hormones | 植物激素的商业应用
Understanding plant hormones has led to many practical applications in agriculture and horticulture. Auxin-based weedkillers (synthetic auxins like 2,4-D) selectively kill broad-leaved weeds by causing uncontrolled growth, while narrow-leaved cereal crops are less affected. Auxins are also used in rooting powders to stimulate adventitious root formation on stem cuttings.
对植物激素的了解衍生出许多农业和园艺上的实际应用。基于生长素的除草剂（合成生长素如 2,4-D）通过引起不可控制的生长来选择性杀死阔叶杂草，而窄叶的谷类作物受其影响较小。生长素还用于生根粉中，以促进茎插条上不定根的形成。
Gibberellin is used to improve fruit size in grapes, to delay senescence (aging) in citrus fruits, and to speed up malting in brewing. Ethene is used to synchronise fruit ripening and colour development. These applications often appear in exam questions as examples of how science can improve food production and reduce waste.
赤霉素被用于增大葡萄果实、延缓柑橘类水果的衰老，以及加速酿造中的麦芽生产。乙烯被用于同步果实成熟和色泽发育。这些应用常出现在考题中，作为科学如何改善食品生产和减少浪费的例子。
11. Summary and Exam Tips | 总结与考试技巧
Plant hormones are a popular topic in IGCSE Biology. Make sure you can clearly explain the mechanisms of phototropism and gravitropism in terms of auxin redistribution and differential growth. Remember the key difference: in shoots, auxin promotes growth; in roots, auxin inhibits growth at higher concentrations. Be ready to interpret experimental setups and predict the effects of removing the tip or applying auxin to one side.
植物激素是 IGCSE 生物中常考的话题。确保你能用生长素的重新分布和差异性生长来清晰解释向光性和向地性的机制。记住关键区别：在茎中，生长素促进生长；在根中，较高浓度的生长素抑制生长。做好解读实验装置并预测去除尖端或单侧施加生长素效果的准备。
For gibberellin and ethene, focus on their specific roles: gibberellin activates enzymes for starch breakdown during germination; ethene triggers fruit ripening. Memorising commercial uses will help you tackle application-style questions. Finally, don’t confuse tropisms with nastic movements—tropisms are directional and growth-related, while nastic movements (like Mimosa leaf folding) are rapid and reversible, often involving changes in turgor pressure, not growth.
对于赤霉素和乙烯，要关注它们的具体作用：赤霉素在萌发过程中激活酶来分解淀粉；乙烯触发果实成熟。记住商业用途有助于解答应用类问题。最后，不要混淆向性与感性运动——向性是定向的、与生长相关的，而感性运动（如含羞草的叶片闭合）是快速且可逆的，通常涉及膨压的变化，而非生长。
Use diagrams when revising: draw a shoot bending towards light, label auxin distribution; draw a root and shoot lying horizontally and indicate auxin gradients and the resulting bending directions. Active recall of these visuals will strengthen your understanding for the exam.
复习时多用图解：画出茎向光弯曲，标注生长素分布；画出水平放置的根和茎，标出生长素梯度以及由此产生的弯曲方向。对这些图像进行主动回忆将加深你对考试内容的理解。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导