📚 Photosynthesis and Plant Nutrition | 光合作用与植物营养
Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. It is the ultimate source of food and oxygen for nearly all life on Earth, and it drives the global carbon cycle. In the Edexcel IGCSE Science specification, photosynthesis appears in both the biology content and the practical skills sections of the exam, so a clear understanding of the equation, leaf structure, limiting factors and the standard experiments is essential for top marks.
光合作用是绿色植物、藻类及某些细菌将光能转化为储存在葡萄糖中的化学能的过程。它是地球上几乎所有生命食物和氧气的最终来源，也驱动着全球碳循环。在 Edexcel IGCSE 科学考纲中，光合作用同时出现在生物内容部分和实验技能部分，因此透彻理解方程式、叶片结构、限制因素及标准实验，是获得高分的关键。
1. What Is Photosynthesis? | 什么是光合作用？
Photosynthesis is an endothermic reaction in which light energy is absorbed by chlorophyll and used to convert carbon dioxide and water into glucose and oxygen. The word ‘photosynthesis’ comes from ‘photo’ meaning light and ‘synthesis’ meaning to make. The energy transferred from the environment is stored in the chemical bonds of the glucose molecules.
光合作用是一个吸热反应，叶绿素吸收光能，并将其用于将二氧化碳和水转化为葡萄糖和氧气。”光合作用”一词源自英文 “photo”（光）和 “synthesis”（合成），意为”利用光来制造”。从环境中转移而来的能量储存在葡萄糖分子的化学键中。
Several key features are essential for photosynthesis to occur:
光合作用的发生必须具备以下几个关键条件：
- Chlorophyll, a green pigment found in chloroplasts, which absorbs light energy, mainly in the red and blue-violet parts of the spectrum | 叶绿素，一种存在于叶绿体中的绿色色素，主要吸收光谱中红光和蓝紫光区域的光能
- Carbon dioxide, which enters the leaf through stomata | 二氧化碳，通过气孔进入叶片
- Water, which is absorbed by root hairs and transported to the leaves through the xylem | 水，由根毛吸收并经由木质部运输至叶片
- Light energy, usually from the Sun | 光能，通常来自太阳
- A suitable temperature, since the reaction is controlled by enzymes | 适宜的温度，因为该反应由酶控制
Photosynthesis can be summarised as a two-stage idea: the light-dependent reactions trap light energy and split water, while the light-independent reactions use that energy to fix carbon dioxide into glucose. For IGCSE, you only need to know the overall process, but understanding that light provides the energy and enzymes control the reactions will help you explain experimental results.
光合作用可以概括为两个阶段：光依赖反应捕获光能并分解水，光不依赖反应则利用这些能量将二氧化碳固定为葡萄糖。对于 IGCSE 而言，你只需要掌握总体过程，但理解”光提供能量、酶控制反应”这一点，能帮助你解释实验现象。
2. The Word Equation and Symbol Equation | 文字方程式与配平符号方程式
You must be able to write both the word equation and the balanced symbol equation for photosynthesis. The word equation is:
你必须能够写出光合作用的文字方程式和配平的符号方程式。文字方程式为：
carbon dioxide + water → glucose + oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
The reaction takes place in the presence of light energy and chlorophyll, so in exam answers you should state both conditions. The balanced symbol equation is:
该反应需要在光能和叶绿素存在的条件下进行，因此在考试作答时应同时说明这两个条件。配平的符号方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Notice that the numbers in front of the formulas balance the atoms: six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms appear on each side. Glucose is C₆H₁₂O₆, and the six oxygen molecules are released as a waste product.
注意各化学式前的数字使原子数目守恒：每一边都有 6 个碳原子、12 个氢原子和 18 个氧原子。葡萄糖为 C₆H₁₂O₆，而 6 个氧分子作为副产物释放出来。
If state symbols are required, the equation can be written as:
如果题目要求状态符号，方程式可以写成：
6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)
A very common exam error is to confuse photosynthesis with respiration. Respiration is the process that releases energy from glucose, and its equation is the reverse of photosynthesis. Always check the direction of the arrow and the substances involved.
一个非常常见的考试错误是将光合作用与呼吸作用混淆。呼吸作用是从葡萄糖中释放能量的过程，其方程式与光合作用正好相反。务必检查箭头的方向和参与反应的物质。
3. Leaf Structure Adapted for Photosynthesis | 叶片结构与光合作用的适应
The leaf is the main photosynthetic organ of a plant. Its structure is highly adapted to maximise the rate of photosynthesis. You need to describe how each part of the leaf contributes to this process.
叶片是植物进行光合作用的主要器官。其结构高度特化，以最大化光合作用的速率。你需要能够描述叶片的每个部分如何为该过程作出贡献。
- Waxy cuticle: A transparent waterproof layer on the upper surface that reduces water loss while still letting light pass through | 角质层：上表面的一层透明防水结构，减少水分蒸发，同时仍然允许光线透过
- Upper epidermis: A transparent layer of cells that allows light to reach the mesophyll below | 上表皮：一层透明的细胞，使光线能够到达下方的叶肉组织
- Palisade mesophyll: Elongated cells packed tightly together near the top of the leaf, each containing many chloroplasts; this is where most photosynthesis occurs | 栅栏组织：位于叶片上部的长条形细胞，排列紧密，每个细胞含有大量叶绿体；这是光合作用发生的主要部位
- Spongy mesophyll: Loosely packed cells with large air spaces that allow carbon dioxide to diffuse rapidly through the leaf and oxygen to diffuse out | 海绵组织：排列疏松的细胞，具有较大的气室，使二氧化碳能快速扩散穿过叶片，并使氧气扩散出去
- Stomata: Pores, usually on the lower epidermis, that allow carbon dioxide to enter and oxygen and water vapour to leave; each pore is surrounded by two guard cells that control its opening and closing | 气孔：通常位于下表皮的小孔，允许二氧化碳进入，并让氧气和水蒸气逸出；每个气孔由两个保卫细胞环绕，控制其开闭
- Veins: Contain xylem, which brings water from the roots, and phloem, which transports the glucose made in the leaves to the rest of the plant | 叶脉：包含木质部（将水从根部输送上来）和韧皮部（将叶片制造的葡萄糖运输到植物其他部位）
Because the palisade layer contains the most chloroplasts, it is positioned at the top of the leaf where light intensity is highest. The air spaces in the spongy layer shorten the diffusion pathway for gases, which increases the rate of gas exchange during photosynthesis.
由于栅栏层含有最多的叶绿体，它位于光强最高的叶片顶部。海绵层中的气室缩短了气体的扩散路径，从而提高了光合作用过程中气体交换的速率。
4. Factors Affecting the Rate of Photosynthesis | 影响光合作用速率的因素
In the IGCSE exam you are expected to know the effect of four main factors on the rate of photosynthesis: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll.
在 IGCSE 考试中，你需要掌握四个主要因素对光合作用速率的影响：光照强度、二氧化碳浓度、温度以及叶绿素的含量。
- Light intensity: As light intensity increases, the rate of photosynthesis increases because more light energy is available to be absorbed by chlorophyll. However, beyond a certain point, further increases in light intensity have no effect because another factor is limiting the reaction | 光照强度：随着光照强度增加，光合作用速率加快，因为叶绿素能够吸收更多的光能。但超过某一程度后，继续增强光照不再提高速率，因为此时有另一个因素成为限制因素
- Carbon dioxide concentration: Carbon dioxide is a raw material of photosynthesis, so increasing its concentration increases the rate up to a limit; normal air contains only about 0.04% CO₂, which is why CO₂ is often the limiting factor in well-lit conditions | 二氧化碳浓度：二氧化碳是光合作用的原料，因此提高其浓度可以在一定范围内加快速率；普通空气中 CO₂ 仅占约 0.04%，因此在光照充足的条件下，CO₂ 常常成为限制因素
- Temperature: The reaction is controlled by enzymes, so increasing temperature increases the rate up to the optimum temperature (usually around 35–40 °C). Above this, the enzymes denature and the rate falls rapidly | 温度：该反应由酶控制，因此在达到最适温度（通常为 35–40 °C）之前，升高温度会加快速率。超过最适温度后，酶发生变性，速率迅速下降
- Amount of chlorophyll: Chlorophyll absorbs light energy; if the plant lacks chlorophyll due to magnesium deficiency or disease, the rate of photosynthesis is reduced | 叶绿素含量：叶绿素吸收光能；如果植物因缺镁或患病而缺乏叶绿素，光合作用速率会降低
You should also know how to measure the rate of photosynthesis experimentally. The rate can be estimated by measuring the volume of oxygen produced, the number of bubbles released per minute, the rate of carbon dioxide uptake, or the increase in biomass over time.
你还应该知道如何通过实验测定光合作用速率。可以通过测量产生的氧气体积、每分钟释放的气泡数、二氧化碳吸收速率或生物量随时间增加的量来估算速率。
5. Limiting Factors | 限制因素
A limiting factor is the factor that is in shortest supply and therefore determines the maximum rate of the reaction. In an experiment where light intensity is increased while temperature and CO₂ concentration are kept constant, the rate of photosynthesis initially rises, then plateaus. At the plateau, light is no longer the limiting factor; the limiting factor is now either temperature or carbon dioxide concentration.
限制因素是指在供应上最为短缺、从而决定反应最大速率的那种因素。在保持温度和 CO₂ 浓度恒定的情况下逐渐增强光照的实验中，光合作用速率先上升，然后趋于平稳。在平台期，光照不再是限制因素；此时限制因素变为温度或二氧化碳浓度。
The concept of limiting factors can be summarised as follows: the rate of photosynthesis is limited by the factor that is present in the lowest amount relative to what is needed. If you increase that single factor, the rate increases until a different factor becomes limiting.
限制因素的概念可以概括为：光合作用的速率受相对需求量而言供应量最低的那个因素限制。如果提高这个因素，速率就会增加，直到另一个因素成为新的限制因素。
This idea is used commercially in greenhouses. Growers may use artificial lighting to prevent light from being a limiting factor, add extra carbon dioxide by burning paraffin, and use heaters to keep the temperature near the optimum. These measures together produce a much higher rate of photosynthesis and therefore a larger crop yield.
这一原理在温室生产中得到实际应用。种植者使用人工光源防止光照成为限制因素，通过燃烧石蜡增加二氧化碳浓度，并使用加热器将温度维持在接近最适水平。这些措施共同作用，使光合作用速率大幅提高，从而获得更高的作物产量。
Rate of photosynthesis increases → then plateaus → raising the limiting factor raises the rate again
光合作用速率上升 → 然后趋于平稳 → 提高限制因素可使速率再次上升
6. Investigating Photosynthesis | 光合作用的实验探究
There are several classic experiments you must be able to describe and interpret. The first is the starch test, which proves that glucose has been made and stored as starch. A plant is first destarched by placing it in the dark for 24–48 hours, so that existing starch is used up. One leaf is then partially covered with black paper (or a stencil) and the plant is exposed to light for several hours. Finally, the leaf is tested for starch.
有几个经典实验你必须能够描述和解释。第一个是淀粉检测，用于证明叶片合成了葡萄糖并将其储存为淀粉。首先将植物在黑暗中放置 24–48 小时使其耗尽原有淀粉（脱淀粉处理）。然后用黑纸（或模板）部分遮盖一片叶子，让植物在光下照射数小时。最后对叶片进行淀粉检测。
The starch test procedure has four steps:
淀粉检测的步骤共有四步：
- Boil the leaf in water to kill the cells and break down cell membranes | 将叶片在沸水中煮沸，杀死细胞并破坏细胞膜
- Boil the leaf in ethanol (using a water bath, never direct heating) to remove the green chlorophyll, so colour changes are visible | 将叶片放入乙醇中隔水加热（切勿直接加热），以去除绿色叶绿素，使颜色变化可见
- Rinse the leaf in cold water to soften it again | 用冷水冲洗叶片使其重新变软
- Add iodine solution; a blue-black colour shows that starch is present | 滴加碘液；出现蓝黑色说明存在淀粉
The parts of the leaf that received light turn blue-black, while the shaded parts remain brown, proving that light is needed for photosynthesis.
受光的部分会变为蓝黑色，而遮光部分仍为棕色，从而证明光合作用需要光照。
The second experiment measures the volume of oxygen produced by an aquatic plant such as pondweed (Elodea). The cut stem is placed in a test tube of water, a light source is placed at a fixed distance, and the number of bubbles released per minute is counted, or a gas syringe collects the oxygen. By moving the lamp closer or further away, you can investigate the effect of light intensity on the rate of photosynthesis. It is important to keep temperature and CO₂ concentration constant to make it a fair test.
第二个实验是测量金鱼藻（Elodea）等水生植物产生氧气的体积。将切好的茎放入盛水的试管中，将光源置于固定距离，然后计算每分钟释放的气泡数，或用气体注射器收集氧气。通过移动灯的距离，可以探究光照强度对光合作用速率的影响。为了确保公平实验，必须保持温度和 CO₂ 浓度恒定。
A third investigation uses hydrogencarbonate indicator, which changes colour depending on the carbon dioxide concentration. In red light, a photosynthesising plant removes CO₂ and the indicator turns purple. In the dark, the plant respires and produces CO₂, turning the indicator yellow. At atmospheric CO₂ concentration the indicator remains red.
第三个实验使用碳酸氢盐指示剂，其颜色随二氧化碳浓度变化而变化。在红光下，进行光合作用的植物吸收 CO₂，指示剂变为紫色。在黑暗中，植物进行呼吸作用产生 CO₂，指示剂变为黄色。当 CO₂ 浓度为大气水平时，指示剂保持红色。
7. Uses of Glucose in the Plant | 葡萄糖在植物体内的用途
Immediately after photosynthesis, glucose is used for many purposes within the plant. You need to be able to list and explain these uses, as this is a frequent short-answer question.
光合作用产生的葡萄糖会立即被植物用于多种用途。你需要能够列举并解释这些用途，因为这是高频简答题。
- Respiration: Glucose is broken down in respiration to release energy for cell activities such as active transport and protein synthesis | 呼吸作用：葡萄糖在呼吸作用中被分解，释放能量供细胞进行主动运输和蛋白质合成等活动
- Starch storage: Glucose is converted into starch, which is insoluble and therefore does not affect the water potential of cells; starch can be stored in roots, stems and leaves | 储存为淀粉：葡萄糖被转化为淀粉，淀粉不溶于水，因此不会影响细胞的水势；淀粉可储存在根、茎和叶中
- Cellulose: Glucose molecules are joined together to form cellulose, a strong structural material for cell walls | 纤维素：葡萄糖分子聚合形成纤维素，作为细胞壁的坚固结构材料
- Sucrose transport: Glucose is changed into sucrose, which is soluble and can be transported in the phloem to other parts of the plant | 蔗糖运输：葡萄糖转化为蔗糖，蔗糖可溶于水并能通过韧皮部运输到植物其他部位
- Amino acids and proteins: Glucose combines with nitrate ions absorbed from the soil to form amino acids, which are then built into proteins | 氨基酸和蛋白质：葡萄糖与从土壤中吸收的硝酸根离子结合形成氨基酸，再进一步合成蛋白质
- Fats and oils: Glucose is converted into lipids for storage in seeds, providing an energy store for germination | 脂肪和油：葡萄糖转化为脂质储存在种子中，为萌发提供能量储备
The conversion of glucose into these different substances is an example of how the products of one metabolic process feed into other metabolic pathways in the plant.
葡萄糖转化为这些不同物质的过程，体现了植物中一个代谢过程的产物如何进入其他代谢途径。
8. Mineral Nutrition | 矿质营养
Plants also require mineral ions from the soil, which are absorbed by root hairs through active transport. In the Edexcel IGCSE specification, you need to know the roles of nitrate ions and magnesium ions, and the symptoms of their deficiency.
植物还需要通过根毛以主动运输方式从土壤中吸收矿质离子。在 Edexcel IGCSE 考纲中，你需要掌握硝酸根离子和镁离子的作用，以及它们缺乏时的症状。
Nitrate ions contain nitrogen, which is needed to make amino acids and proteins. A plant lacking nitrate shows stunted growth and yellowing of the older leaves, because proteins cannot be made and nitrogen is moved from older leaves to younger growing parts.
硝酸根离子含有氮元素，是合成氨基酸和蛋白质所必需的。缺硝酸盐的植物生长迟缓，老叶发黄，因为无法合成蛋白质，且氮元素会从老叶转移到幼嫩的生长部位。
Magnesium ions are a central component of the chlorophyll molecule. Without magnesium, the plant cannot make chlorophyll, so its leaves turn yellow in a condition called chlorosis. Because chlorophyll is needed to absorb light energy, photosynthesis is severely reduced.
镁离子是叶绿素分子的中心成分。没有镁，植物无法合成叶绿素，叶片因此变黄，这种症状称为失绿症。由于叶绿素是吸收光能所必需的，光合作用会严重减弱。
| Mineral | 矿质元素 | Use in plant | 在植物中的用途 | Deficiency symptom | 缺乏症状 |
| Nitrate | 硝酸盐 | Making amino acids and proteins | 合成氨基酸和蛋白质 | Stunted growth; older leaves turn yellow | 生长迟缓；老叶变黄 |
| Magnesium | 镁 | Making chlorophyll | 合成叶绿素 | Chlorosis; leaves turn yellow | 失绿症；叶片变黄 |
Farmers and gardeners add fertilisers containing these ions to the soil to replace the minerals removed by crops and to increase yield.
农民和园丁会施加含有这些离子的肥料，以补充作物带走并提高产量。
9. Common Mistakes and Exam Tips | 常见错误与考试技巧
Many students lose marks on photosynthesis questions because of avoidable errors. Here are the most common mistakes and how to avoid them.
许多学生因为可以避免的错误而在光合作用题目上丢分。以下是最常见的错误及避免方法。
- Writing the equation backwards: Photosynthesis uses carbon dioxide and water to make glucose and oxygen; respiration uses glucose and oxygen to make carbon dioxide and water. Check the arrow direction carefully | 把方程式写反：光合作用利用二氧化碳和水生成葡萄糖和氧气；呼吸作用利用葡萄糖和氧气生成二氧化碳和水。仔细检查箭头方向
- Forgetting light and chlorophyll: When writing the word or symbol equation, state that light energy and chlorophyll are needed. In the symbol equation, these conditions are not part of the formulas, so writing them in words nearby is essential | 遗漏光和叶绿素：写文字方程式或符号方程式时，要注明需要光能和叶绿素。在符号方程式中，这些条件并不体现在化学式里，因此必须在
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