Photosynthesis in Plants | 植物的光合作用

📚 Photosynthesis in Plants | 植物的光合作用

Photosynthesis is one of the most fundamental biological processes on Earth. It is the process by which green plants, algae and some bacteria use sunlight, carbon dioxide and water to produce glucose and oxygen. This process not only sustains plant life but also provides food and oxygen for nearly all living organisms on the planet.

光合作用是地球上最基本的生命过程之一。绿色植物、藻类和某些细菌利用阳光、二氧化碳和水，制造葡萄糖和氧气。这个过程不仅维持着植物自身的生命，也为地球上几乎所有生物提供了食物和氧气。

1. The Overall Equation | 总体反应方程式

The overall equation for photosynthesis is best understood in two forms: the word equation and the chemical equation. The word equation tells us the reactants and products in simple language, while the chemical equation shows the exact numbers of atoms involved.

光合作用的总体方程式最好通过两种形式来理解：文字方程式和化学方程式。文字方程式用简单语言告诉我们反应物和生成物，而化学方程式则展示了反应中涉及的确切原子数目。

Carbon dioxide + Water → Glucose + Oxygen

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Notice that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The energy for this reaction comes from sunlight, which is absorbed by chlorophyll in the chloroplasts of plant cells.

注意，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。这个反应所需的能量来自阳光，而阳光由植物细胞叶绿体中的叶绿素吸收。

2. Where Photosynthesis Happens | 光合作用发生的场所

Photosynthesis takes place mainly in the leaves of green plants. Inside the leaf, there are specialised cells called palisade mesophyll cells, which contain large numbers of chloroplasts. These cells are located near the upper surface of the leaf, where they can capture the maximum amount of sunlight.

光合作用主要发生在绿色植物的叶片中。叶片内部有一种特化的细胞，叫作栅栏组织细胞，它们含有大量的叶绿体。这些细胞位于叶片靠近上表面的位置，以便捕获最多的阳光。

Chloroplasts contain chlorophyll, a green pigment that absorbs light energy, mainly in the red and blue-violet regions of the visible spectrum. The internal structure of a chloroplast includes thylakoid membranes, where the light-dependent reactions occur, and the stroma, a fluid-filled space where the light-independent reactions take place.

叶绿体中含有叶绿素，这是一种绿色色素，主要吸收可见光谱中红光和蓝紫光区的光能。叶绿体的内部结构包括类囊体膜，光依赖反应在此发生，以及基质，即充满液体的空间，光不依赖反应在此发生。

3. The Two Stages of Photosynthesis | 光合作用的两大阶段

Photosynthesis is not a single-step reaction. It occurs in two main stages: the light-dependent stage and the light-independent stage (also known as the Calvin cycle). Both stages are essential for the production of glucose.

光合作用不是一步完成的反应，它分为两个主要阶段：光依赖阶段和光不依赖阶段（也称为卡尔文循环）。这两个阶段对葡萄糖的生成都至关重要。

In the light-dependent stage, chlorophyll absorbs light energy and uses it to split water molecules into hydrogen ions and oxygen gas. This process, called photolysis, releases oxygen as a by-product. The energy from light is also used to produce ATP, a molecule that stores chemical energy.

在光依赖阶段，叶绿素吸收光能，并利用光能将水分子分解为氢离子和氧气。这个过程称为光解作用，氧气作为副产物释放出来。光能还会用于产生ATP，这是一种储存化学能的分子。

In the light-independent stage, the hydrogen ions produced in the first stage combine with carbon dioxide to form glucose. This stage does not require light directly, but it depends on the ATP and hydrogen produced in the light-dependent stage.

在光不依赖阶段，第一阶段产生的氢离子与二氧化碳结合形成葡萄糖。这个阶段不直接需要光，但它依赖光依赖阶段产生的ATP和氢离子。

4. The Role of Chlorophyll | 叶绿素的作用

Chlorophyll is the key pigment involved in photosynthesis. It is located in the thylakoid membranes of the chloroplast and is responsible for absorbing light energy. Without chlorophyll, photosynthesis would not be possible.

叶绿素是光合作用中的关键色素，位于叶绿体的类囊体膜上，负责吸收光能。没有叶绿素，光合作用就不可能发生。

Chlorophyll absorbs red and blue-violet light most efficiently, while green light is largely reflected. This is why leaves appear green to our eyes. The reflected green light is not used in photosynthesis; instead, it is simply bounced off the leaf surface.

叶绿素对红光和蓝紫光的吸收效率最高，而绿光大部分被反射。这就是为什么我们看到的叶片是绿色的。被反射的绿光并不用于光合作用，而是直接从叶片表面弹开。

For the IGCSE examination, you should remember that the absorption spectrum of chlorophyll has two peaks: one in the blue-violet region (around 430 nm) and another in the red region (around 660 nm).

在IGCSE考试中，你需要记住叶绿素的吸收光谱有两个峰值：一个位于蓝紫光区（约430纳米），另一个位于红光区（约660纳米）。

5. Limiting Factors | 限制因素

A limiting factor is any condition that, when present in short supply, limits the rate of a reaction. In photosynthesis, the three main limiting factors are light intensity, carbon dioxide concentration and temperature.

限制因素是指当供应不足时，会限制反应速率的任何条件。在光合作用中，三大主要限制因素是光照强度、二氧化碳浓度和温度。

At low light intensity, increasing the light intensity will increase the rate of photosynthesis. However, at high light intensity, light is no longer the limiting factor, and further increases in light will have little effect on the rate.

在低光照强度下，增加光照强度会提高光合作用速率。然而，在高光照强度下，光不再是限制因素，继续增加光照对速率的影响很小。

Similarly, carbon dioxide makes up only about 0.04% of the atmosphere. Increasing the concentration of carbon dioxide will increase the rate of photosynthesis until another factor becomes limiting.

同样，二氧化碳仅占大气的约0.04%。提高二氧化碳浓度会加快光合作用速率，直到另一个因素成为限制因素为止。

Limiting Factor	Effect on Photosynthesis	限制因素	对光合作用的影响
Light intensity	Increases rate up to saturation point	光照强度	在饱和点之前提高速率
CO₂ concentration	Increases rate up to a plateau	二氧化碳浓度	在平台期之前提高速率
Temperature	Optimum around 25-35°C, then falls	温度	最适温度约25-35°C，之后下降

6. Temperature and Enzyme Activity | 温度与酶活性

Photosynthesis involves many enzyme-controlled reactions, particularly in the Calvin cycle. These enzymes work best at an optimum temperature, which for most plants is between 25°C and 35°C.

光合作用涉及许多由酶控制的反应，尤其是在卡尔文循环中。这些酶在最适温度下活性最高，对大多数植物而言，最适温度在25°C到35°C之间。

If the temperature rises above the optimum, the enzymes begin to denature. Denaturation is a permanent change in the three-dimensional structure of an enzyme molecule, which causes it to lose its function. This means the rate of photosynthesis decreases rapidly at high temperatures.

当温度升高超过最适值时，酶开始变性。变性是指酶分子的三维结构发生永久性改变，从而导致其功能丧失。这意味着在高温下，光合作用的速率会迅速下降。

At low temperatures, enzyme activity is slow because the molecules have less kinetic energy and fewer successful collisions occur between enzymes and their substrates. Between 0°C and 10°C, for example, the rate of photosynthesis is very low. This is why plants in cold climates grow slowly in winter.

在低温下，酶的活性较慢，因为分子动能较低，酶与底物之间的有效碰撞较少。例如，在0°C到10°C之间，光合作用速率非常低。这就是为什么寒冷气候下的植物在冬季生长缓慢。

It is important to distinguish between the effect on enzymes and the effect on other processes. In the IGCSE exam, you may be asked to interpret graphs or suggest reasons why a greenhouse is heated or cooled. A common answer is to maintain the optimum temperature for enzyme activity.

重要的是区分对酶的影响与对其他过程的影响。在IGCSE考试中，你可能会被要求解释曲线图，或说明温室为什么要加热或降温。常见答案是维持酶活性的最适温度。

7. Investigating Photosynthesis | 探究光合作用

There are several practical methods to investigate the rate of photosynthesis. The most common is the pondweed method, where the water plant Elodea is placed in a beaker of water and exposed to light. The number of oxygen bubbles produced per minute is counted as a measure of the rate of photosynthesis.

研究光合作用速率有几种实验方法，最常见的是伊乐藻法。将水生植物伊乐藻放入装有水的烧杯中并置于光照下，计算每分钟产生的氧气气泡数，以此作为光合作用速率的指标。

The rate of bubble production can be compared under different conditions, such as varying light intensity, different light colours or different concentrations of dissolved carbon dioxide. A more accurate method involves collecting the gas in a capillary tube and measuring its volume over a fixed period of time.

气泡产生速率可以在不同条件下进行比较，例如改变光照强度、光色或水中溶解二氧化碳的浓度。更精确的方法是用毛细管收集气体，并在固定时间内测量其体积。

When a lamp is moved closer to the pondweed, light intensity increases. According to the inverse square law, light intensity is inversely proportional to the square of the distance from the light source. In other words, halving the distance quadruples the light intensity.

当灯移近伊乐藻时，光照强度增加。根据平方反比定律，光照强度与光源距离的平方成反比。换句话说，距离减半，光照强度变为原来的四倍。

8. Using Hydrogencarbonate Indicator | 使用碳酸氢盐指示剂

Another way to investigate photosynthesis is by using hydrogencarbonate indicator, a pH-sensitive solution. When plants photosynthesise, they take in carbon dioxide and produce oxygen. In an alkaline solution, the indicator turns purple; in a neutral solution, it is red; and in an acidic solution, it becomes yellow.

另一种研究光合作用的方法是使用碳酸氢盐指示剂，这是一种对pH敏感的溶液。当植物进行光合作用时，它们吸收二氧化碳并产生氧气。在碱性溶液中，指示剂变为紫色；在中性溶液中为红色；在酸性溶液中为黄色。

When a plant is placed in a sealed tube with hydrogencarbonate indicator in bright light, photosynthesis removes carbon dioxide from the surrounding air. This causes the solution to become more alkaline, and the indicator turns purple. In the dark, respiration dominates, carbon dioxide accumulates, the solution becomes acidic, and the indicator turns yellow.

将植物放入装有碳酸氢盐指示剂的密封试管中，在强光下，光合作用会吸收周围空气中的二氧化碳，导致溶液变得更碱性，指示剂变为紫色。在黑暗中，呼吸作用占主导，二氧化碳积累，溶液变酸性，指示剂变黄。

It is worth noting that plants both respire and photosynthesise continuously. The observed change in the indicator depends on the relative rates of these two processes. If the rate of photosynthesis exceeds the rate of respiration, carbon dioxide is removed from the air.

值得注意的是，植物同时持续进行呼吸作用和光合作用。观察到的指示剂颜色变化取决于这两个过程的相对速率。当光合作用速率超过呼吸作用速率时，二氧化碳会从空气中被移除。

9. Photosynthesis and Respiration | 光合作用与呼吸作用的关系

Photosynthesis and respiration are two opposing but complementary processes. Photosynthesis builds glucose from carbon dioxide and water using light energy, while respiration breaks down glucose to release energy in the form of ATP.

光合作用和呼吸作用是相反但又互补的两个过程。光合作用利用光能将二氧化碳和水合成葡萄糖，而呼吸作用分解葡萄糖并以ATP形式释放能量。

Photosynthesis uses carbon dioxide and water to produce glucose and oxygen, whereas respiration uses glucose and oxygen to produce carbon dioxide and water. In many ways, one process is the reverse of the other:

光合作用利用二氧化碳和水生成葡萄糖和氧气，而呼吸作用利用葡萄糖和氧气生成二氧化碳和水。在许多方面，这两个过程互为逆反应：

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

During the day, photosynthesis usually occurs at a faster rate than respiration, so plants release oxygen and absorb carbon dioxide. At night, photosynthesis stops because there is no light, but respiration continues, so plants absorb oxygen and release carbon dioxide.

白天，光合作用的速率通常高于呼吸作用，因此植物释放氧气并吸收二氧化碳。夜晚因为没有光，光合作用停止，但呼吸作用持续进行，植物吸氧排碳。

10. The Products of Photosynthesis and Their Uses | 光合作用产物及其用途

Glucose, the direct product of photosynthesis, is a simple sugar that can be used by plants in several ways. Some glucose is used immediately in respiration to release energy for cellular activities. The rest is stored or converted into other materials.

葡萄糖是光合作用的直接产物，是一种单糖，植物可以多种方式利用它。一部分葡萄糖立即用于呼吸作用，为细胞活动释放能量。其余部分被储存或转化为其他物质。

Glucose can be converted into starch for short-term storage, or into sucrose for transport around the plant. It can also be converted into cellulose, which is a structural component of plant cell walls, or into amino acids when combined with nitrate ions absorbed from the soil.

葡萄糖可以转化为淀粉进行短期储存，或转化为蔗糖在植物体内运输。它还可以转化为纤维素，即植物细胞壁的结构成分，或与从土壤中吸收的硝酸根离子结合，转化为氨基酸。

Oxygen, the by-product of photosynthesis, is not used by plants themselves. Instead, it is released into the atmosphere through stomata. The oxygen is essential for the survival of aerobic organisms, including human beings. In fact, the oxygen in Earth’s atmosphere has been produced largely by photosynthesis over billions of years.

氧气作为光合作用的副产物，植物自身并不使用，而是通过气孔释放到大气中。氧气对包括人类在内的需氧生物的生存至关重要。事实上，地球大气中的氧气主要是在数十亿年的时间里由光合作用产生的。

This close relationship between photosynthesis products and plant metabolism is frequently tested in IGCSE examinations. You may be asked to describe how the products of photosynthesis are used by a plant, so be prepared to mention starch, sucrose, cellulose, amino acids and respiration.

光合作用产物与植物代谢之间的密切关系在IGCSE考试中经常被考查。你可能会被要求描述植物如何利用光合作用产物，因此要准备好提及淀粉、蔗糖、纤维素、氨基酸和呼吸作用。

11. Factors Affecting Photosynthesis in Greenhouses | 影响温室光合作用的因素

Farmers and gardeners control the conditions in greenhouses to maximise the rate of photosynthesis and therefore crop yield. They commonly increase carbon dioxide concentration, provide artificial lighting and maintain optimal temperatures.

农民和园丁通过控制温室条件来最大化光合作用速率，从而提高作物产量。常见的做法包括增加二氧化碳浓度、提供人工照明以及维持最适温度。

Adding carbon dioxide to a greenhouse, for example by burning paraffin lamps, increases the concentration of CO₂ available for photosynthesis. Artificial lighting extends the hours of photosynthesis when natural daylight is insufficient, which is why many greenhouses use lamps during winter months.

例如，通过燃烧石蜡灯向温室中补充二氧化碳，可以提高光合作用可利用的CO₂浓度。人工照明在自然光照不足时延长光合作用的时间，这就是为什么许多温室在冬季使用灯具。

Heating is used to keep the greenhouse at the optimum temperature for enzyme activity. However, care must be taken not to raise the temperature too high, because a temperature above 40°C will cause enzymes to denature and photosynthesis to stop.

加热用于保持温室内处于酶活性的最适温度。然而，注意不要将温度调得太高，因为超过40°C会导致酶变性，光合作用停止。

Understanding limiting factors helps farmers identify which factor is slowing down photosynthesis and how to address it. For example, on a cloudy day, extra lighting may be more beneficial than adding more CO₂, because light is the limiting factor at that moment.

理解限制因素有助于农民判断哪个因素正在减缓光合作用，以及如何解决。例如，在阴天，补光可能比增施CO₂更有效，因为那一刻光就是限制因素。

12. Common Exam Questions and Tips | 常见考题与答题技巧

In the IGCSE examination, questions on photosynthesis often ask you to identify limiting factors, interpret graphs or suggest improvements to an experiment. Here are some key points to remember.

IGCSE考试中，光合作用相关题目通常要求你识别限制因素、解释曲线图或对实验提出改进建议。以下是一些需要记住的关键要点。

Always write the full equation of photosynthesis when asked: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Write the word equation as a minimum if you cannot remember the chemical symbols.
Remember that the light-dependent stage occurs in the thylakoids and the light-independent stage occurs in the stroma of the chloroplast.
When discussing limiting factors, mention that the rate is limited by the factor in shortest supply, and that increasing any other factor will have no effect.

在IGCSE考试中，当被要求书写光合作用方程式时，一定要写出完整方程式：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。如果记不住化学符号，至少要写出文字方程式。

记住光依赖阶段发生在类囊体中，光不依赖阶段发生在叶绿体基质中。讨论限制因素时，要提到速率受最匮乏因素的限制，且增加其他因素不会产生影响。

For practical questions, remember that the pondweed experiment measures the volume of oxygen produced, and that the distance between the lamp and the pondweed affects light intensity according to the inverse square law. When investigating the effect of temperature, the solution must be kept at a constant temperature using a water bath.

对于实验题，记住伊乐藻实验测量的是产生的氧气体积，灯与伊乐藻之间的距离根据平方反比定律影响光照强度。在研究温度的影响时，必须使用水浴维持温度恒定。

Finally, always compare variables in experiments: keep all other conditions constant while changing only the independent variable. This ensures that any change observed in the dependent variable is caused solely by the independent variable.

最后，实验中要始终控制变量：只改变自变量，保持其他所有条件恒定。这样才能确保观察到的因变量变化完全是由自变量引起的。

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Photosynthesis in Plants | 植物的光合作用