Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. It is essential for life on Earth because it produces oxygen and forms the base of almost all food chains.

光合作用是绿色植物、藻类和某些细菌将光能转化为储存在葡萄糖中的化学能的过程。它对地球生命至关重要，因为光合作用产生氧气，并构成几乎所有食物链的基础。

1. The Word Equation | 文字方程式

The overall reaction of photosynthesis can be summarised using the following word equation:

光合作用的总体反应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 and water are converted into glucose and oxygen. The energy from sunlight is trapped by chlorophyll and used to drive the reaction.

在光照和叶绿素存在下，二氧化碳和水转化为葡萄糖和氧气。阳光中的能量被叶绿素捕获，并用于驱动该反应。

2. The Chemical Equation | 化学方程式

The balanced chemical equation for photosynthesis shows the exact number of atoms involved:

光合作用的平衡化学方程式显示了参与反应的确切原子数量：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Glucose (C₆H₁₂O₆) is a simple sugar that can be used immediately for respiration or stored as starch. The oxygen released comes from the splitting of water molecules, not from carbon dioxide.

葡萄糖（C₆H₁₂O₆）是一种单糖，可立即用于呼吸作用或储存为淀粉。释放的氧气来自水分子的分解，而不是来自二氧化碳。

3. The Structure of Leaf Cells | 叶片细胞的结构

Photosynthesis occurs mainly in the mesophyll cells of leaves. Palisade cells are tightly packed with chloroplasts and are the main site of photosynthesis.

光合作用主要发生在叶片的叶肉细胞中。栅栏细胞紧密排列，含有大量叶绿体，是光合作用的主要场所。

Chloroplasts contain chlorophyll, the green pigment that absorbs light energy.

叶绿体含有叶绿素，这种绿色色素能吸收光能。
Stomata allow carbon dioxide to enter and oxygen to exit the leaf.

气孔允许二氧化碳进入叶片，氧气离开叶片。
Guard cells control the opening and closing of stomata, balancing gas exchange with water loss.

保卫细胞控制气孔的开放与关闭，在气体交换与水分流失之间保持平衡。

4. Light-Dependent Processes | 光依赖过程

Photosynthesis is powered by light energy. In the thylakoid membranes of chloroplasts, light is absorbed by chlorophyll and splits water molecules into hydrogen and oxygen.

光合作用由光能驱动。在叶绿体的类囊体膜上，叶绿素吸收光能，将水分子分解为氢和氧。

H₂O → 2H⁺ + 2e⁻ + ½O₂

This stage is called the light-dependent reaction because it requires light directly. The hydrogen ions are used later, while oxygen is released as a by-product.

这一阶段被称为光依赖反应，因为它直接需要光。氢离子随后被利用，而氧气作为副产物释放出来。

5. Light-Independent Processes | 光不依赖过程

The second stage of photosynthesis does not need light directly. In the stroma of the chloroplast, carbon dioxide combines with hydrogen from the previous stage to form glucose.

光合作用的第二阶段不直接需要光。在叶绿体的基质中，二氧化碳与上一阶段产生的氢结合，形成葡萄糖。

6CO₂ + 12H⁺ → C₆H₁₂O₆ + 6H₂O

This stage is known as the Calvin cycle. It can continue in the dark as long as sufficient ATP and reduced NADP are supplied by the light-dependent reaction.

这一阶段被称为卡尔文循环。只要光依赖反应提供了足够的ATP和还原型NADP，它就可以在黑暗中继续进行。

6. Factors Affecting the Rate | 影响速率的因素

The rate of photosynthesis depends on several environmental factors. If any of these factors is below the optimum level, it can limit the rate.

光合作用的速率取决于多个环境因素。如果其中任何一个因素低于最适水平，它就可能限制速率。

Factor \| 因素	Effect \| 影响
Light intensity 光强度	Higher light intensity increases rate until saturation. 光强度升高，速率提高，直至饱和。
Carbon dioxide concentration 二氧化碳浓度	Increasing CO₂ concentration generally increases the rate up to a limit. 提高CO₂浓度通常会在一定限度内提高速率。
Temperature 温度	Enzyme activity rises with temperature until the optimum, then falls sharply above 40 °C. 酶活性随温度升高而增强，达到最适温度后，超过40 °C会急剧下降。
Water availability 水分供应	Water shortage causes stomata to close, limiting CO₂ entry. 缺水导致气孔关闭，限制CO₂进入。

7. Limiting Factor Principle | 限制因素原理

According to the law of limiting factors, at any given moment the rate of photosynthesis is limited by the factor that is present in the smallest amount relative to the optimum.

根据限制因素定律，在任何特定时刻，光合作用的速率都受相对最适水平而言含量最低的因素限制。

For example, if light intensity is low, increasing carbon dioxide concentration will not increase the rate because light is the limiting factor.

例如，如果光强度低，增加二氧化碳浓度不会提高速率，因为光是限制因素。

When all factors are at optimum, the rate plateaus. A graph of rate against light intensity shows a steep rise then a horizontal plateau.

当所有因素都处于最适状态时，速率趋于稳定。以速率为纵轴、光强度为横轴的曲线先急剧上升，然后进入水平平台期。

8. Measuring the Rate | 测量速率

The rate of photosynthesis can be measured by counting oxygen bubbles produced by an aquatic plant such as Elodea, or by collecting the gas in a measuring tube.

光合作用速率可以通过计数水生植物（如伊乐藻）产生的氧气气泡来测量，或者将气体收集在测量管中测量。

Use a lamp at different distances to change light intensity.

将灯放置在不同的距离以改变光强度。
Keep temperature constant using a water bath.

使用水浴保持温度恒定。
Control carbon dioxide concentration by adding sodium hydrogencarbonate.

通过添加碳酸氢钠来控制二氧化碳浓度。

Remember that distance affects light intensity according to the inverse square law: intensity ∝ 1/d².

请记住，距离根据平方反比定律影响光强度：光强度 ∝ 1/d²。

9. Importance of Photosynthesis | 光合作用的重要性

Photosynthesis provides the primary source of organic matter for almost all living organisms. It removes carbon dioxide from the atmosphere and releases oxygen, maintaining the balance of gases required for life.

光合作用为几乎所有生物提供了有机物质的主要来源。它从大气中去除二氧化碳并释放氧气，维持生命所需的气体平衡。

All fossil fuels – coal, oil and natural gas – originated from photosynthetic organisms millions of years ago. Therefore, photosynthesis is also a source of energy for human society.

所有化石燃料——煤、石油和天然气——都起源于数百万年前的光合生物。因此，光合作用也是人类社会的一种能源来源。

10. Exam Tips and Common Mistakes | 考试提示与常见错误

In IGCSE exams, students often lose marks by confusing the word equation or forgetting to mention the role of chlorophyll. Make sure you write ‘chlorophyll’ as a condition, not a reactant.

在IGCSE考试中，学生经常因混淆文字方程式或忘记提及叶绿素的作用而失分。请确保你写出“叶绿素”作为条件，而不是反应物。

Do not write CO₂ + H₂O → CH₂O + O₂ unless the question asks for the simplest ratio.

除非题目要求最简比例，否则不要写CO₂ + H₂O → CH₂O + O₂。
State that the oxygen produced comes from water, not carbon dioxide.

指出产生的氧气来自水，而不是二氧化碳。
For limiting factor questions, always quote evidence from the graph and explain why the rate plateaus.

对于限制因素问题，务必引用图表中的证据，并解释速率为何进入平台期。

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Photosynthesis | 光合作用