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 one of the most important biological processes on Earth, as it supplies oxygen and forms the base of nearly every food chain.

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。它是地球上最重要的生物过程之一，因为它提供氧气，并构成几乎所有食物链的基础。

1. The Word Equation and Chemical Equation | 文字方程式与化学方程式

The overall process of photosynthesis can be summarised using a word equation. Carbon dioxide and water react, in the presence of light and chlorophyll, to produce glucose and oxygen.

光合作用的全过程可以用一个文字方程式来总结。二氧化碳和水在光照和叶绿素的作用下反应，生成葡萄糖和氧气。

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The balanced chemical equation shows the same relationship using symbols and numbers:

配平的化学方程式使用符号和数字展示了同样的关系：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Here, six molecules of carbon dioxide and six molecules of water produce one molecule of glucose and six molecules of oxygen. The energy captured from sunlight is stored in the chemical bonds of glucose.

在这里，六个二氧化碳分子和六个水分子生成一个葡萄糖分子和六个氧分子。从阳光中捕获的能量储存在葡萄糖的化学键中。

2. The Structure of the Leaf and Photosynthesis | 叶片结构与光合作用

Leaves are highly adapted organs for photosynthesis. Their broad, flat shape provides a large surface area to absorb sunlight. The internal structure contains several specialised tissues that carry out different functions.

叶片是为光合作用高度适应的器官。它们宽而扁平的形状提供了大的表面积来吸收阳光。内部结构包含几种执行不同功能的特化组织。

Epidermis: A transparent outer layer that allows light to pass through; the lower epidermis contains stomata for gas exchange. | 表皮：透明的外层，允许光线透过；下表皮含有气孔用于气体交换。
Palisade mesophyll: A layer of tall, closely packed cells containing many chloroplasts; this is the main site of photosynthesis. | 栅栏叶肉：一层高而紧密排列的细胞，含有许多叶绿体；这是光合作用的主要场所。
Spongy mesophyll: A loosely packed layer with air spaces that allow gases to diffuse freely around the cells. | 海绵叶肉：疏松排列的细胞层，具有气隙，使气体能在细胞周围自由扩散。
Xylem and phloem: Vascular tissues that transport water and dissolved substances to and from the leaf. | 木质部和韧皮部：维管组织，负责将水分和溶解物质运输进出叶片。

Chloroplasts contain the green pigment chlorophyll, which absorbs light energy and uses it to drive the reaction. The chlorophyll is located in the thylakoid membranes inside the chloroplast.

叶绿体含有绿色色素叶绿素，它吸收光能并利用它驱动反应。叶绿素位于叶绿体内的类囊体膜上。

3. The Role of Chlorophyll and Light Energy | 叶绿素和光能的作用

Light energy is absorbed by chlorophyll and converted into chemical energy in the form of ATP and reduced NADP. This energy is then used to convert carbon dioxide into glucose during the second stage of photosynthesis.

光能被叶绿素吸收，并转化为ATP和还原型NADP形式的化学能。然后，在光合作用的第二阶段，这种能量被用来将二氧化碳转化为葡萄糖。

In IGCSE Biology, you are not expected to know the detailed biochemical pathways, but you must understand that light is required for the initial splitting of water (photolysis) and for producing ATP.

在IGCSE生物中，不需要掌握详细的生化途径，但必须理解光对于水的初始分解（光解）和产生ATP是必需的。

Chlorophyll is essential because it captures light energy. Without chlorophyll, photosynthesis cannot occur even if light is present. This is why plants appear green – they reflect green light while absorbing red and blue light most effectively.

叶绿素是必需的，因为它捕获光能。没有叶绿素，即使有光，光合作用也不能发生。这就是为什么植物看起来是绿色的——它们反射绿光，而最有效地吸收红光和蓝光。

4. The Light-Dependent Reaction and the Light-Independent Reaction | 光依赖反应与光独立反应

Photosynthesis takes place in two main stages. The light-dependent reaction requires light and occurs in the thylakoids. The light-independent reaction does not require light directly and occurs in the stroma of the chloroplast.

光合作用分为两个主要阶段。光依赖反应需要光，发生在类囊体中。光独立反应不直接需要光，发生在叶绿体基质中。

Light-dependent reaction: H₂O → O₂ + H⁺ + e⁻ (using light energy)

光依赖反应：H₂O → O₂ + H⁺ + e⁻（利用光能）

The light-independent reaction uses the ATP and reduced NADP produced in the light-dependent reaction to reduce carbon dioxide and build glucose.

光独立反应利用光依赖反应中产生的ATP和还原型NADP来还原二氧化碳并合成葡萄糖。

Light-independent reaction: CO₂ → glucose (using ATP and reduced NADP)

光独立反应：CO₂ → 葡萄糖（利用ATP和还原型NADP）

In most IGCSE exam questions, the key point is that oxygen comes from the photolysis of water, not from carbon dioxide. This can be tested using an isotope experiment (¹⁸O labelling), but more simply, you should remember the equation.

在大多数IGCSE考试题目中，关键点是氧气来自水的光解，而不是来自二氧化碳。这一点可以通过同位素实验（¹⁸O标记）来验证，但更简单的是，你应该记住方程式。

5. The Rate of Photosynthesis and Limiting Factors | 光合作用速率和限制因素

The rate of photosynthesis is affected by several environmental factors. The three main limiting factors are light intensity, carbon dioxide concentration and temperature. At any given moment, the rate is limited by the factor that is in shortest supply.

光合作用速率受多种环境因素影响。三个主要限制因素是光强度、二氧化碳浓度和温度。在任何给定时刻，速率受供应最不足的那个因素限制。

Light intensity: As light intensity increases, the rate increases until another factor becomes limiting. | 光强度：随着光强度增加，速率增加，直到另一个因素成为限制因素。
Carbon dioxide concentration: CO₂ is a raw material; increasing its concentration raises the rate up to a plateau. | 二氧化碳浓度：CO₂是一种原料；增加其浓度会使速率上升到一个平台期。
Temperature: As temperature rises, the rate increases until the optimum, then falls rapidly as enzymes denature. | 温度：随着温度升高，速率增加直到最适温度，然后随着酶变性而迅速下降。

You may be asked to identify the limiting factor from a graph. The part of the curve that is horizontal is limited by a factor that is not being changed on the x-axis.

你可能会被要求从图表中识别限制因素。曲线水平的部分受一个不在x轴上变化的因素限制。

6. Investigating Photosynthesis: Experiments | 探究光合作用：实验

Several classic experiments can be used to test the rate of photosynthesis. One common method is to count bubbles of oxygen produced by pondweed (Elodea or Cabomba) under different light intensities.

有几种经典实验可用于测试光合作用速率。一种常见方法是数水草（伊乐藻或卡邦巴）在不同光强度下产生的氧气气泡数量。

To measure the rate accurately:

为了准确测量速率：

Place a test tube containing pondweed and water under a lamp at different distances. | 将装有水草和水的试管放在距离不同的灯下。
Use a ruler to measure the distance or a light meter to measure light intensity. | 用尺子测量距离或用光度计测量光强度。
Count the number of oxygen bubbles in a fixed time (e.g. one minute). | 在固定时间内（例如一分钟）计数氧气气泡的数量。
Repeat for each distance and record results in a table. | 对每个距离重复试验并记录结果在表格中。

Another method uses a gas syringe to collect the volume of oxygen produced, which is more accurate than counting bubbles. The experiment can also be repeated at different temperatures or with different concentrations of sodium hydrogencarbonate (as a source of CO₂).

另一种方法使用气体注射器收集产生的氧气体积，这比数气泡更准确。该实验还可以在不同温度或不同浓度的碳酸氢钠溶液（作为CO₂来源）下重复进行。

When investigating the effect of temperature, the temperature must be controlled using a water bath. The light intensity must be kept constant, and the same plant should be used throughout.

当研究温度的影响时，必须使用水浴控制温度。光强度必须保持恒定，并且整个过程中应使用同一株植物。

7. The Importance of Photosynthesis in Ecosystems | 光合作用在生态系统中的重要性

Photosynthesis is fundamental to life on Earth. It provides the oxygen needed for aerobic respiration, and it produces glucose that plants use for growth, storage and respiration. The glucose can be converted into starch for storage, or combined with nitrates to make amino acids and proteins.

光合作用是地球上生命的基础。它提供了有氧呼吸所需的氧气，并产生植物用于生长、储存和呼吸的葡萄糖。葡萄糖可以转化为淀粉储存，或与硝酸盐结合合成氨基酸和蛋白质。

All animals ultimately depend on photosynthesis for food. Even carnivores rely on herbivores that eat plants. Photosynthesis also removes carbon dioxide from the atmosphere, helping to regulate the greenhouse effect.

所有动物最终都依赖光合作用来获取食物。甚至食肉动物也依赖于吃植物的食草动物。光合作用还从大气中去除二氧化碳，有助于调节温室效应。

In the carbon cycle, photosynthesis is the key process that fixes carbon dioxide into organic compounds. Without it, atmospheric CO₂ would build up, and organic life would not be sustainable.

在碳循环中，光合作用是将二氧化碳固定为有机化合物的关键过程。没有它，大气中的CO₂会积聚，有机生命将无法持续。

8. Adaptations of Plants for Efficient Photosynthesis | 植物对高效光合作用的适应

Plants exhibit many adaptations at the leaf, cellular and molecular levels to maximise photosynthesis.

植物在叶片、细胞和分子水平上表现出许多适应特征，以最大化光合作用。

Large surface area: Broad leaves capture more sunlight. | 大的表面积：宽大的叶片捕获更多阳光。
Thin leaves: Short diffusion distance for carbon dioxide to reach chloroplasts. | 薄的叶片：二氧化碳到达叶绿体的扩散距离短。
Transparent epidermis: Allows light to reach photosynthetic cells. | 透明的表皮：让光线到达光合细胞。
Many chloroplasts in palisade cells: High concentration of chlorophyll near the upper surface. | 栅栏细胞中有许多叶绿体：靠近上表面有高浓度的叶绿素。
Stomata: Pores that allow gas exchange, especially carbon dioxide intake and oxygen release. | 气孔：允许气体交换的孔隙，特别是二氧化碳的摄入和氧气的释放。
Air spaces in spongy mesophyll: Allow rapid diffusion of carbon dioxide. | 海绵叶肉中的气隙：允许二氧化碳快速扩散。
Xylem and phloem: Efficient transport of water and sugars. | 木质部和韧皮部：有效运输水分和糖类。

Some plants, such as those in shaded environments, have larger chloroplasts or more chlorophyll per cell to absorb low light levels. Others, such as cacti, have adapted to reduce water loss while still carrying out photosynthesis.

一些植物，例如生长在阴凉环境中的植物，每个细胞有更大的叶绿体或更多的叶绿素，以吸收低水平的光。另一些植物，如仙人掌，已经适应在减少水分损失的同时仍进行光合作用。

9. Transport Systems Related to Photosynthesis | 与光合作用相关的运输系统

Water absorbed by roots must reach the chloroplast-containing cells in the leaf. This transport occurs through the xylem vessels. Water is pulled up by transpiration, which is driven by evaporation through the stomata.

根部吸收的水必须到达叶片中含叶绿体的细胞。这种运输通过木质部导管发生。水被蒸腾作用拉动，而蒸腾作用由气孔的蒸发驱动。

Glucose produced by photosynthesis is converted to sucrose and transported in the phloem to other parts of the plant – roots, stems, flowers and fruits. This movement of sugars is called translocation.

光合作用产生的葡萄糖转化为蔗糖，并在韧皮部中运输到植物的其他部位——根、茎、花和果实。这种糖的移动称为转运。

If the phloem is damaged, sugars cannot reach the roots, and the plant will eventually die. Photosynthesis depends on both transport systems working correctly.

如果韧皮部受损，糖无法到达根部，植物最终会死亡。光合作用依赖于两个运输系统正常工作。

10. Typical Exam Questions and Common Mistakes | 典型考试题目和常见错误

In Edexcel IGCSE Science exams, photosynthesis appears in multiple-choice, short-answer and extended-response questions. You may be asked to interpret graphs, describe experiments, or explain why a factor limits the rate.

在爱德思IGCSE科学考试中，光合作用出现在选择题、简答题和扩展回答题中。你可能会被要求解读图表、描述实验或解释某个因素为什么限制速率。

Common mistakes students make include:

学生常犯的错误包括：

Writing the incorrect word equation (e.g. missing glucose or using starch instead). | 写出错误的文字方程式（例如漏掉葡萄糖或用淀粉代替）。
Stating that oxygen comes from carbon dioxide rather than water. | 声称氧气来自二氧化碳而不是水。
Confusing limiting factors with each other. | 混淆各个限制因素。
Describing an experiment without a valid control or without stating what is measured. | 描述实验时没有有效的对照或没有说明测量什么。
Using vague terms like “energy” without specifying that it is light energy. | 使用模糊术语如“能量”而不具体说明是光能。

For a high mark, you should use precise scientific language. For example, say “the rate of photosynthesis decreases because the enzyme involved begins to denature above the optimum temperature” rather than “the plant gets too hot.”

要获得高分，你应该使用精确的科学语言。例如，说“光合作用速率下降是因为相关酶在最适温度以上开始变性”，而不是说“植物太热了”。

11. Summary and Revision Tips | 总结与复习建议

Photosynthesis is a central topic in IGCSE Science. Make sure you can write the word equation and the balanced symbol equation without hesitation, draw a standard leaf cross-section and label the key tissues, and explain how light, CO₂ and temperature affect rate.

光合作用是IGCSE科学的核心主题。确保你能毫不犹豫地写出文字方程式和配平的符号方程式，能画出标准叶片横截面并标注关键组织，并解释光、CO₂和温度如何影响速率。

Practice past-paper questions that ask you to describe an experiment. Use the three-part structure: apparatus and method, measurements and controlled variables, results and conclusion.

练习要求你描述实验的历年真题。使用三部分结构：装置和方法、测量和受控变量、结果和结论。

Remember that photosynthesis is an endothermic reaction – it takes in energy in the form of light. It is the reverse of aerobic respiration in terms of the overall equation, although the pathways are different.

记住光合作用是吸热反应——它以光的形式吸收能量。从总体方程式来看，它是有氧呼吸的逆反应，但途径不同。

By mastering photosynthesis, you build a strong foundation for understanding ecosystems, the carbon cycle, plant transport and energy transformations in biology. Good luck with your revision!

掌握光合作用，你就为理解生态系统、碳循环、植物运输和生物中的能量转化奠定了坚实基础。祝你复习顺利！

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Photosynthesis | 光合作用