📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants, algae and some bacteria use sunlight to convert carbon dioxide and water into glucose and oxygen. It is the foundation of nearly all life on Earth, providing both food and oxygen.
光合作用是绿色植物、藻类和一些细菌利用阳光，将二氧化碳和水转化为葡萄糖和氧气的过程。它是地球上几乎所有生命的基础，既提供食物，也提供氧气。
1. The Meaning and Importance | 含义与重要性
Photosynthesis is an endothermic reaction. It stores light energy as chemical energy in glucose. The glucose can then be used for respiration, or converted into starch, cellulose, fats and proteins.
光合作用是吸热反应。它将光能储存在葡萄糖中的化学能里。葡萄糖随后可用于呼吸作用，或转化为淀粉、纤维素、脂肪和蛋白质。
The oxygen released during photosynthesis comes from the splitting of water molecules. This process continuously replenishes atmospheric oxygen, which is essential for aerobic organisms.
光合作用释放的氧气来自水分子的分解。这一过程不断补充大气中的氧气，对有氧生物至关重要。
2. The Word Equation | 文字方程式
The overall process can be summarised by the following word equation:
整个过程可以用以下文字方程式概括：
Carbon dioxide + Water → Glucose + Oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
In the presence of light and chlorophyll, carbon dioxide from the air and water from the soil are transformed.
在光和叶绿素存在下，来自空气中的二氧化碳和来自土壤中的水发生转化。
3. The Chemical Equation | 化学方程式
The balanced chemical equation shows the exact number of atoms involved:
配平的化学方程式显示了参与反应的确切原子数目：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Here, six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.
在这里，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。
Note that this equation summarises many smaller steps. In reality, photosynthesis takes place in two main stages: the light-dependent and light-independent reactions.
请注意，这个方程式概括了许多更小的步骤。实际上，光合作用分两个主要阶段：光反应阶段和暗反应阶段。
4. Light-Dependent Reactions | 光依赖反应
These reactions occur in the thylakoid membranes of chloroplasts. They need light energy directly. During these reactions, light splits water into hydrogen and oxygen, producing ATP and reduced NADP.
这些反应发生在叶绿体的类囊体膜上。它们直接需要光能。在这些反应中，光将水分解为氢和氧，并产生ATP和还原型NADP。
Oxygen is released as a waste product from this stage. The hydrogen is taken up by a carrier molecule to be used in the next stage.
此阶段释放出氧气作为废物。氢被载体分子携带，供下一阶段使用。
5. Light-Independent Reactions | 光不依赖反应（暗反应）
These reactions take place in the stroma of the chloroplast. They do not require light directly, but they rely on the ATP and reduced NADP produced in the light-dependent stage.
这些反应发生在叶绿体的基质中。它们不直接需要光，但依赖光反应阶段产生的ATP和还原型NADP。
Carbon dioxide is fixed by a five-carbon compound, eventually forming glucose. If the light-dependent reactions stop, the light-independent reactions will also stop because they lack ATP.
二氧化碳被五碳化合物固定，最终形成葡萄糖。如果光反应停止，暗反应也会因为缺少ATP而停止。
6. Factors Affecting Photosynthesis | 影响光合作用的因素
Photosynthesis is controlled by several environmental factors. The three main factors are light intensity, carbon dioxide concentration and temperature.
光合作用受多种环境因素控制。三种主要因素是光强度、二氧化碳浓度和温度。
The rate of photosynthesis can be measured using the volume of oxygen produced per unit time, or the rate at which carbon dioxide is taken in.
光合作用速率可以通过单位时间内产生的氧气体积或二氧化碳吸收速率来测定。
7. Light Intensity | 光强度
As light intensity increases, the rate of photosynthesis increases linearly at first, provided other factors are not limiting. However, beyond a certain point, light is no longer the limiting factor, and increasing it further has no effect.
随着光强度增加，如果其他因素不受限制，光合作用速率起初线性增加。但超过某一点后，光不再成为限制因素，继续增加光强度不再产生作用。
A practical method to investigate this is to place an aquatic plant such as Cabomba under different distances from a lamp. The closer the lamp, the higher the light intensity, and the faster bubbles of oxygen are produced.
一个实际的研究方法是将水生植物（如卡彭巴草）放在距离台灯不同距离处。灯越近，光强度越高，氧气气泡产生得越快。
8. Carbon Dioxide and Temperature | 二氧化碳与温度
Carbon dioxide is a raw material. If its concentration increases, the rate of photosynthesis rises until another factor becomes limiting. A typical concentration of 1% to 5% can greatly increase growth in enclosed greenhouses.
二氧化碳是原料。如果其浓度增加，光合作用速率会上升，直到另一个因素成为限制因素。封闭温室中1%到5%的典型浓度可显著促进生长。
Temperature affects the enzymes involved in photosynthesis. The rate increases with temperature up to the optimum, usually around 30–35 °C for many plants. Above that, enzymes denature and the rate falls sharply.
温度影响光合作用中的酶。速率随温度升高而增加，直到最适温度，通常许多植物在30–35 °C左右。超过该温度，酶会变性，速率急剧下降。
9. Limiting Factors and Natural Conditions | 限制因素与自然条件
A limiting factor is any factor that is in short supply, preventing the rate of photosynthesis from increasing further. In the early morning, light may be the limiting factor; at midday, carbon dioxide may limit; and in winter, temperature may limit.
限制因素是任何供应短缺的因素，阻止光合作用速率进一步增加。清晨，光可能是限制因素；正午，二氧化碳可能限制；冬天，温度可能限制。
Farmers can manipulate these factors in greenhouses. For example, adding artificial lighting, burning natural gas to produce CO₂, or using heaters can optimise yields.
农民可以在温室中调节这些因素。例如，增加人工光照、燃烧天然气产生CO₂，或使用加热器来优化产量。
10. Leaf and Chloroplast Adaptations | 叶片和叶绿体的适应
Leaves are adapted to maximise photosynthesis. They are broad and thin, giving a large surface area and short diffusion distance for carbon dioxide. Stomata allow gas exchange, while the waxy cuticle reduces water loss.
叶片适应于最大化光合作用。它们宽而薄，提供大表面积和较短的二氧化碳扩散距离。气孔允许气体交换，而蜡质角质层减少水分散失。
Chloroplasts contain chlorophyll, which absorbs red and blue light most effectively. The internal membranes increase the surface area for the light-dependent reactions.
叶绿体含有叶绿素，最有效地吸收红光和蓝光。内部膜增加了光依赖反应的表面积。
11. Testing for Photosynthesis | 检验光合作用
Starch tests are commonly used to show that photosynthesis has occurred. The leaf is boiled in ethanol to remove chlorophyll, then treated with iodine. A blue-black colour indicates starch is present.
常用淀粉检验证明光合作用发生。将叶片在乙醇中煮沸以去除叶绿素，然后加入碘液。蓝黑色表明淀粉存在。
To test the need for light, a plant is destarched in darkness, then a leaf is covered with a stencil to block light. After sufficient daylight and a starch test, only the exposed areas turn blue-black.
为检验光的需求，先将植物在黑暗中饥饿处理，然后用模板遮住部分叶片阻挡光线。充足光照和淀粉检验后，只有暴露部分变蓝黑色。
12. Summary | 总结
Photosynthesis is an essential but complex process. The equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ represents the overall change. Its rate is affected by light, carbon dioxide, temperature, and internal factors such as chlorophyll content.
光合作用是一个基础但复杂的过程。方程式6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂代表了整体变化。其速率受光、二氧化碳、温度以及叶绿素含量等内部因素影响。
Understanding limiting factors allows humans to increase crop production in controlled environments. This concept appears frequently in IGCSE Edexcel Science examinations, so you should be able to explain graphs, identify limiting factors, and describe practical experiments.
理解限制因素使人类能够在受控环境中提高作物产量。这个概念在IGCSE Edexcel科学考试中经常出现，因此你应该能够解释图表、识别限制因素并描述实际实验。
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