📚 IGCSE Science Topic 384: Photosynthesis | IGCSE科学主题384：光合作用
Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesise glucose from carbon dioxide and water. It is one of the most important biological processes on Earth and a core topic in Edexcel IGCSE Biology.
光合作用是绿色植物和一些其他生物利用阳光，将二氧化碳和水合成葡萄糖的过程。它是地球上最重要的生物过程之一，也是Edexcel IGCSE生物学的核心考点。
1. What is Photosynthesis? | 什么是光合作用？
Photosynthesis is an endothermic reaction that takes place in the chloroplasts of plant cells. The word 'photo' means light, and 'synthesis' means making, so photosynthesis literally means 'making with light'.
光合作用是一个吸热反应，发生在植物细胞的叶绿体中。单词‘photo’意思是光，‘synthesis’意思是制造，因此光合作用字面意思就是‘利用光制造’。
During photosynthesis, light energy is absorbed by chlorophyll, a green pigment found in chloroplasts. This energy is used to convert carbon dioxide and water into glucose and oxygen. Oxygen is released as a by-product.
在光合作用过程中，光能被叶绿体中的绿色色素——叶绿素吸收。这一能量被用于将二氧化碳和水转化为葡萄糖和氧气。氧气作为副产物释放出来。
Photosynthesis is essential for life on Earth because it produces not only the oxygen we breathe, but also the organic molecules that form the basis of most food webs.
光合作用对地球生命至关重要，因为它不仅产生我们呼吸的氧气，还产生构成大多数食物网基础的有机分子。
2. The Chemical Equation | 化学方程式
The overall balanced chemical equation for photosynthesis is:
光合作用的总平衡化学方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
In words, carbon dioxide and water react, in the presence of light energy and chlorophyll, to produce glucose and oxygen.
用文字表达即为：二氧化碳和水在光能和叶绿素的存在下反应，生成葡萄糖和氧气。
Note that the symbol equation must be balanced. The arrow indicates that light energy and chlorophyll are required, although they are not written as reactants.
注意，符号方程式必须配平。箭头表示需要光能和叶绿素，尽管它们不写在反应物一侧。
In Edexcel exams, you may be asked to write the word equation or identify the reactants and products. You should also be able to state that chlorophyll is a catalyst-like substance that speeds up the reaction without being used up.
在Edexcel考试中，你可能会被要求写出文字方程式或识别反应物和产物。你还应当能够说明叶绿素是一种类似催化剂的物质，能加快反应而不被消耗。
3. The Role of Light and Chlorophyll | 光照和叶绿素的作用
Light energy is the initial energy source for photosynthesis. Without light, photosynthesis cannot occur. The light energy is absorbed by chlorophyll, which raises electrons to a higher energy level, allowing the light-independent reactions to proceed.
光能是光合作用的初始能源。没有光，光合作用无法进行。光能被叶绿素吸收，使电子跃迁到更高的能级，使光不依赖反应得以进行。
Chlorophyll is located in the thylakoid membranes of chloroplasts. It appears green because it reflects green wavelengths of light while absorbing mainly red and blue light.
叶绿素位于叶绿体的类囊体膜上。它呈现绿色，是因为它反射绿光波长，而主要吸收红光和蓝光。
There are several pigments involved, including chlorophyll a, chlorophyll b, and carotenoids, but chlorophyll a is the primary pigment directly involved in the light-dependent reactions.
参与光合作用的色素有多种，包括叶绿素a、叶绿素b和类胡萝卜素，其中叶绿素a是直接参与光依赖反应的主要色素。
If a plant is kept in darkness, no photosynthesis occurs. If a leaf is partly covered with black paper, the covered region will not produce starch in a starch test, demonstrating that light is required.
如果植物保持在黑暗中，光合作用无法进行。如果用黑纸部分遮盖一片叶片，在淀粉检测中，被遮盖的区域将不会产生淀粉，从而证明光是必需的。
4. The Structure of the Leaf | 叶片的结构
The leaf is an organ specialised for photosynthesis. Its internal structure is adapted in several ways to maximise the rate of photosynthesis.
叶片是专门进行光合作用的器官。它的内部结构在多个方面经过适应，以最大化光合作用速率。
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Upper epidermis — a single layer of transparent cells that allows light to pass through to the palisade layer below.
上表皮——一层透明的细胞，允许光线穿过到达下方的栅栏层。
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Palisade mesophyll — elongated cells packed with chloroplasts, located near the top surface to capture maximum light.
栅栏组织——富含叶绿体的长形细胞，位于靠近上表面处，以捕获最大量的光。
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Spongy mesophyll — loosely packed cells with large air spaces, allowing carbon dioxide to diffuse to palisade cells and oxygen to exit.
海绵组织——排列疏松的细胞，具有大的气室，允许二氧化碳扩散到栅栏细胞，同时使氧气排出。
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Stomata — pores, mostly on the lower epidermis, that allow gas exchange and water loss by transpiration.
气孔——主要位于下表皮的小孔，允许气体交换和通过蒸腾作用失水。
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Veins (xylem and phloem) — transport water and minerals to the leaf and carry glucose away to other parts of the plant.
叶脉（木部和韧皮部）——将水和矿物质输送到叶片，并将葡萄糖运送到植物其他部位。
You should be able to label a leaf cross-section and explain how each adaptation relates to its function.
你应该能够标记叶片横截面，并解释每个结构与功能的关系。
5. Factors Affecting Photosynthesis | 影响光合作用的因素
The rate of photosynthesis is affected by several environmental factors. The three most important are light intensity, carbon dioxide concentration, and temperature.
光合作用速率受多种环境因素影响。最重要的三个因素是光照强度、二氧化碳浓度和温度。
Light intensity: As light intensity increases, the rate of photosynthesis increases because more light energy is available to excite electrons. However, this can only increase up to a certain point, when another factor becomes limiting.
光照强度：随着光照强度增加，光合作用速率增加，因为可获得更多光能来激发电子。然而，这个增加只能达到某一点，此时另一个因素成为限制因素。
Carbon dioxide concentration: CO₂ is a raw material for photosynthesis. Increasing CO₂ concentration initially increases the rate, but again, eventually a plateau is reached.
二氧化碳浓度：二氧化碳是光合作用的原料。增加CO₂浓度最初会提高速率，但同样，最终会达到平台期。
Temperature: Photosynthesis is controlled by enzymes. As temperature increases, the rate increases up to an optimum (usually around 35–40°C for most plants). Above this, enzymes denature and the rate rapidly decreases.
温度：光合作用由酶控制。随着温度升高，速率上升到最适点（通常对多数植物在35–40°C左右）。超过此点，酶变性，速率急剧下降。
Other factors include the availability of water and mineral ions, but these are rarely tested as direct rate-limiting factors.
其他因素包括水和矿质离子的可用性，但这些很少作为直接限制因素来考查。
6. Limiting Factors | 限制因素
A limiting factor is a factor that prevents the rate of photosynthesis from increasing, even if other factors are available in abundance. In any set of conditions, the factor at the lowest value relative to the plant's needs is the one that limits the rate.
限制因素是指即使其他因素充足，也阻止光合作用速率提高的因素。在任何一组条件下，与植物需求相比处于最低值的因素就是限制速率的因素。
For example, at night, light intensity is the limiting factor. In winter, temperature may be limiting. In a sealed greenhouse, CO₂ concentration often becomes limiting.
例如，在夜间，光照强度是限制因素。在冬季，温度可能成为限制因素。在密闭温室中，CO₂浓度常常成为限制因素。
Farmers use this knowledge to increase crop yields by controlling these factors. Greenhouses may use artificial lighting, CO₂ generators, and heating systems to optimise conditions.
农民利用这一知识通过控制这些因素来提高作物产量。温室可能会使用人工照明、CO₂发生器和加热系统来优化条件。
You may be asked to interpret graphs of rate against factor and identify where a point is limiting. For a given line at constant other factors, the plateau indicates that another factor is now limiting.
你可能会被要求解读速率随因素变化的图，并确定某点处的限制因素。在恒定其他因素的给定曲线上，平台表示现在有另一个因素在限制。
7. Uses of Glucose | 葡萄糖的用途
The glucose produced by photosynthesis is used by the plant in several ways. You should be able to list at least four uses.
光合作用产生的葡萄糖在植物中有多种用途。你应该能够列出至少四种。
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Respiration: Glucose is oxidised in respiration to release energy for the plant's metabolic processes.
呼吸作用：葡萄糖在呼吸作用中被氧化，释放能量供植物代谢过程使用。
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Starch storage: Glucose is converted into starch for storage in leaves, roots, and seeds. Starch is insoluble, so it does not affect water potential.
储存为淀粉：葡萄糖被转化为淀粉储存在叶片、根和种子中。淀粉不溶于水，因此不影响水势。
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Cellulose: Glucose is used to make cellulose for cell walls, providing structural support.
纤维素：葡萄糖用于合成细胞壁中的纤维素，提供结构支持。
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Amino acids and proteins: Combined with nitrate and sulfate ions absorbed from the soil, glucose is converted into amino acids, proteins, and other nitrogen-containing compounds.
氨基酸和蛋白质：葡萄糖与从土壤中吸收的硝酸根和硫酸根离子结合，转化为氨基酸、蛋白质和其他含氮化合物。
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Lipids: Glucose can be converted into fats and oils for energy storage, especially in seeds.
脂质：葡萄糖可以转化为脂肪和油脂，用于能量储存，尤其在种子中。
In exams, a common question asks: 'What does a plant do with the glucose it produces?' You must give specific answers, not just 'for growth'.
在考试中，常见问题问：‘植物如何利用它产生的葡萄糖？’你必须给出具体的答案，而不仅仅是‘用于生长’。
8. Investigating Photosynthesis | 光合作用的探究实验
You need to be aware of the standard experiments used to demonstrate and measure photosynthesis. The two most important are the starch test and the pondweed experiment.
你需要了解用于演示和测量光合作用的标准实验。两个最重要的实验是淀粉检测和水草实验。
Starch test: A leaf is first boiled in water to kill the cells, then boiled in ethanol to remove chlorophyll, and finally rinsed with water. When iodine solution is added, an area with starch turns blue-black. This shows that photosynthesis has produced starch in that part of the leaf.
淀粉检测：首先将叶片在沸水中煮沸以杀死细胞，然后在乙醇中煮沸以去除叶绿素，最后用水冲洗。当加入碘溶液时，含淀粉的区域会变成蓝黑色。这表明该部分叶片已进行光合作用并产生了淀粉。
Pondweed experiment: A piece of pondweed is placed in water with a light source at a fixed distance. The number of oxygen bubbles produced per minute gives an estimate of the rate of photosynthesis. The independent variable can be light intensity (by changing the distance), CO₂ concentration (by adding sodium bicarbonate), or temperature.
水草实验：将一根水草放入水中，光源固定在某一距离。每分钟产生的氧气泡数量用于估算光合作用速率。自变量可以是光照强度（通过改变距离）、CO₂浓度（通过添加碳酸氢钠）或温度。
You should know how to control variables, how to calculate the rate from bubble counts, and why it is better to use the cumulative volume of gas collected in a capillary tube rather than counting bubbles (bubbles can vary in size).
你应该知道如何控制变量，如何从气泡计数计算速率，以及为什么使用毛细管中收集的气体总体积比计数气泡更好（气泡大小可能不同）。
9. Common Exam Questions | 常见考点问题
Here are typical questions you may encounter in the Edexcel IGCSE paper, with guidance on how to answer them.
以下是在Edexcel IGCSE试卷中可能遇到的典型问题，以及如何回答的指导。
Q: Describe how the leaves of a plant are adapted for photosynthesis. (4 marks)
问：描述植物的叶如何适应光合作用。（4分）
A: Include: palisade layer contains many chloroplasts; transparent upper epidermis; large surface area; stomata allow gas exchange; spongy layer has air spaces for CO₂ diffusion.
答案：包括：栅栏层含大量叶绿体；上表皮透明；表面积大；气孔允许气体交换；海绵层有空隙利于CO₂扩散。
Q: Explain why, during a hot summer day, the rate of photosynthesis may decrease in the afternoon. (2 marks)
问：解释为什么在炎热的夏日午后，光合作用速率可能会下降。（2分）
A: High temperature causes stomata to close to reduce water loss, which reduces CO₂ uptake, so photosynthesis slows. Also, enzymes may begin to denature at very high temperatures.
答案：高温导致气孔关闭以减少水分流失，从而减少CO₂吸收，因此光合作用减慢。此外，在非常高的温度下酶可能开始变性。
Q: In an experiment, the rate of photosynthesis increased when light intensity was increased, but then stayed constant. Suggest why. (2 marks)
问：在实验中，光合作用速率随光照强度增加而增加，然后保持不变。请提出原因。（2分）
A: Another factor (e.g., CO₂ concentration or temperature) has become the limiting factor.
答案：另一个因素（如CO₂浓度或温度）已经成为了限制因素。
10. Summary | 总结
arbon dioxide, water, light, and chlorophyll. The equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
光合作用将光能转化为储存在葡萄糖中的化学能。它需要二氧化碳、水、光和叶绿素。方程式为6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。
The rate is affected by light intensity, CO₂ concentration, and temperature. A limiting factor stops the rate from increasing. Glucose can be used for respiration, starch, cellulose, amino acids, and lipids.
速率受光照强度、CO₂浓度和温度的影响。限制因素阻止速率提高。葡萄糖可用于呼吸、淀粉、纤维素、氨基酸和脂质。
Remember the leaf structure and the standard experiments. With this knowledge, you can tackle any photosynthesis question in your Edexcel IGCSE Science exam.
记住叶片结构和标准实验。有了这些知识，你就能应对Edexcel IGCSE科学考试中任何光合作用的问题。
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