📚 Photosynthesis | 光合作用
Photosynthesis is one of the most important chemical processes on Earth. Green plants, algae and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen. This process supplies food and oxygen for nearly all living organisms. In the Edexcel IGCSE Science (Double Award) specification, photosynthesis is a core topic that appears in both Paper 1 and Paper 2, and questions frequently test the word equation, the balanced symbol equation, limiting factors and experimental methods.
光合作用是地球上最重要的化学过程之一。绿色植物、藻类和某些细菌利用光能将二氧化碳和水转化为葡萄糖和氧气，为几乎所有生物提供食物和氧气。在 Edexcel IGCSE 科学（Double Award）大纲中，光合作用是核心专题，Paper 1 和 Paper 2 都会考查，题目常涉及文字方程式、配平的化学方程式、限制因素和实验方法。
1. What Is Photosynthesis? | 什么是光合作用？
The term ‘photosynthesis’ comes from the Greek words ‘photo’ (light) and ‘synthesis’ (making). It is the process by which plants manufacture glucose from carbon dioxide and water using light energy. The glucose produced can be used in respiration, stored as starch, converted into cellulose for cell walls, or turned into lipids and amino acids.
“光合作用”一词源自希腊语 “photo”（光）和 “synthesis”（制造），指植物利用光能，由二氧化碳和水合成葡萄糖的过程。生成的葡萄糖可用于呼吸作用、以淀粉形式储存、转化为构成细胞壁的纤维素，或转变为脂质和氨基酸。
- Energy is captured by chlorophyll, the green pigment found in chloroplasts. 能量由叶绿体中的绿色色素——叶绿素捕获。
- The reaction is endothermic because energy from light is transferred into chemical energy stored in glucose. 该反应是吸热反应，因为光能被转化为储存在葡萄糖中的化学能。
- Photosynthesis releases oxygen as a waste product, which maintains the oxygen level of the atmosphere. 光合作用释放氧气作为副产品，维持了大气中的氧气含量。
2. The Word Equation | 文字方程式
The word equation is the first thing you must memorise for the exam. Carbon dioxide and water are the reactants; glucose and oxygen are the products. Light energy and chlorophyll are not substances but conditions, so they are written above the arrow in the reaction.
文字方程式是考试中必须首先记住的内容。二氧化碳和水是反应物，葡萄糖和氧气是产物。光能和叶绿素并不是物质而是条件，因此书写时放在箭头上方。
Carbon dioxide + Water → Glucose + Oxygen
(in the presence of light and chlorophyll)
Note that the arrow should be written as → instead of =, and the conditions above the arrow must not be missed. Many Edexcel mark schemes award one mark for the correct reactants and products and one mark for the conditions.
注意箭头应写成 → 而不是等号，并且不能漏掉箭头上方的条件。许多 Edexcel 评分标准中，写出正确反应物和产物得一分，写出条件再得一分。
3. The Balanced Symbol Equation | 配平的化学方程式
The balanced symbol equation shows the same number of atoms of each element on both sides. Six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. This requires the subscripts in glucose, C₆H₁₂O₆, to be remembered exactly.
配平的化学方程式要求两侧每种元素的原子数目相等。六个二氧化碳分子与六个水分子反应，生成一个葡萄糖分子和六个氧分子。书写时必须准确记住葡萄糖的分子式 C₆H₁₂O₆ 中的下标。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Check the atom balance: on each side there are 6 carbon atoms, 18 oxygen atoms and 12 hydrogen atoms. A common exam question asks you to identify which reactant supplies the carbon and which reactant supplies the oxygen released. Carbon comes from CO₂, while the oxygen gas released comes from the splitting of water.
检查原子守恒：每侧有 6 个碳原子、18 个氧原子和 12 个氢原子。常见考题要求指出哪个反应物提供碳、以及放出的氧气来自哪个反应物。碳来自 CO₂，而释放的氧气来自水的分解。
4. Leaf Structure and Adaptations | 叶片结构与适应特征
Photosynthesis occurs mainly in the leaves, and the structure of a leaf is adapted to make the process as efficient as possible. The upper layer contains elongated palisade mesophyll cells that are packed with chloroplasts, while the spongy mesophyll below contains air spaces for gas diffusion.
光合作用主要发生在叶片中，叶片的结构使其尽可能高效。栅栏组织位于上层，细胞呈长柱状且充满叶绿体；海绵组织位于其下，细胞间有气室，便于气体扩散。
| Adaptation | 适应特征 | Function | 功能 |
|---|---|
| Large surface area | 大表面积 | Absorbs more light | 吸收更多光能 |
| Thin structure | 叶片薄 | Short diffusion distance for CO₂ | 缩短二氧化碳扩散距离 |
| Chlorophyll in chloroplasts | 叶绿体中的叶绿素 | Absorbs light energy | 吸收光能 |
| Stomata | 气孔 | Allow CO₂ in and O₂ out | 允许 CO₂ 进入、O₂ 排出 |
| Xylem and phloem in veins | 叶脉中的木质部与韧皮部 | Transport water and glucose | 运输水分和葡萄糖 |
5. The Two Stages of Photosynthesis | 光合作用的两个阶段
Although the detailed biochemistry is not required at IGCSE, you should know that photosynthesis takes place in two stages. The light-dependent stage requires light and happens in the grana (thylakoids) of the chloroplast. The light-independent stage does not need light directly and happens in the stroma.
虽然 IGCSE 阶段不要求掌握详细的生物化学过程，但应了解光合作用分两个阶段。光依赖反应需要光，发生在叶绿体的基粒（类囊体）中；光不依赖反应不直接需要光，发生在基质中。
| Feature | 特征 | Light-dependent stage | 光依赖反应 | Light-independent stage | 光不依赖反应 |
|---|---|---|
| Location | 场所 | Grana (thylakoids) | 基粒（类囊体） | Stroma | 基质 |
| Requires light | 是否需要光 | Yes | 需要 | No | 不需要 |
| Reactants | 反应物 | Water, light | 水、光 | CO₂, ATP, reduced NADP | 二氧化碳、ATP、还原型 NADP |
| Products | 产物 | O₂, ATP, reduced NADP | 氧气、ATP、还原型 NADP | Glucose | 葡萄糖 |
6. Factors Affecting the Rate | 影响光合作用速率的因素
The rate of photosynthesis is controlled by three main factors: light intensity, carbon dioxide concentration and temperature. Each factor has a similar effect: as the factor increases from a low value, the rate increases; but eventually the rate stops increasing because another factor becomes limiting.
光合作用速率受三个主要因素控制：光强度、二氧化碳浓度和温度。每个因素的影响方式类似：当该因素从低值增大时，速率随之上升；但最终速率不再上升，因为另一个因素开始起限制作用。
- Light intensity: more light gives more energy, so the light-dependent stage runs faster. 光强度越高，提供的光能越多，光依赖反应进行得越快。
- Carbon dioxide concentration: CO₂ is a raw material; higher concentration speeds up the light-independent stage. 二氧化碳是原料，浓度越高，光不依赖反应越快。
- Temperature: the reaction is controlled by enzymes, so rate rises with temperature up to an optimum of about 35-40 °C, then falls as enzymes denature. 光合作用由酶控制，温度升高，速率上升，最适温度约为 35-40 °C；超过后酶变性，速率下降。
7. Limiting Factors | 限制因素
A limiting factor is the factor whose shortage is currently slowing down the rate of photosynthesis. At any given moment, only one factor is the limiting factor. For example, at night light is limiting; in winter temperature is often limiting; on a bright summer day carbon dioxide concentration is usually limiting.
限制因素是指当前供应不足、因而减缓光合作用速率的因素。在任一时刻，只有一种因素起限制作用。例如，夜间光是限制因素；冬季常为温度限制；晴朗夏日通常是二氧化碳浓度限制。
In exams, you may be given a graph that levels off. You should be able to state that the rate is no longer increasing because the limiting factor has changed. For example, if light intensity is doubled but the rate stays the same, then CO₂ concentration or temperature must now be limiting.
考试中常给出趋于平缓的曲线图。你应能说明速率不再上升是因为限制因素改变了。例如，光强度加倍但速率不变时，说明此时二氧化碳浓度或温度已成为限制因素。
8. Experiment: Testing a Leaf for Starch | 实验：叶片淀粉检测
To prove that photosynthesis has occurred, test a leaf for starch. The procedure has four steps, and examiners often ask for them in order with full details.
为证明光合作用已经发生，可检测叶片中的淀粉。实验分四步，考官常要求按顺序完整写出每一步。
- Drop the leaf in boiling water for 30 seconds to kill the cells and stop metabolic reactions. 将叶片放入沸水中 30 秒，以杀死细胞并终止代谢反应。
- Heat the leaf in a boiling tube containing ethanol placed in a
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