Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants and algae convert carbon dioxide and water into glucose and oxygen using light energy. It is a central topic in the Edexcel IGCSE Science course, and exam questions frequently test the equation, limiting factors, leaf structure, and the pondweed investigation. A clear understanding of this reaction also connects to topics such as respiration, plant transport, and agriculture.

光合作用是绿色植物和藻类利用光能，将二氧化碳和水转化为葡萄糖和氧气的过程。这是 Edexcel IGCSE 科学课程的核心内容，考试题目经常考查反应方程式、限制因素、叶片结构以及金鱼藻实验。深入理解这一反应，还能将呼吸作用、植物运输和农业等专题联系起来。

1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis is a chemical reaction that takes place in the chloroplasts of plant cells. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy. The energy is then used to join carbon dioxide and water together to make glucose, a sugar that the plant uses as food.

光合作用是发生在植物细胞叶绿体中的化学反应。叶绿体含有绿色色素叶绿素，它可以吸收光能。植物利用这些能量将二氧化碳和水合成葡萄糖，葡萄糖是植物所需的养料。

Photosynthesis is an endothermic reaction because it takes in energy from the surroundings. In this case, the energy is light energy from the Sun. The reaction is essential for almost all life on Earth because it produces oxygen and provides the food that animals, including humans, depend on.

光合作用是一种吸热反应，因为它从周围环境中吸收能量。这里的能量来自太阳的光能。光合作用对地球上几乎所有的生命至关重要，因为它产生氧气，并提供包括人类在内的动物所依赖的食物。

In the Edexcel IGCSE specification, you must be able to state the definition of photosynthesis, identify the reactants and products, and describe at least one practical method for measuring the rate of photosynthesis.

在 Edexcel IGCSE 教学大纲中，你需要能够写出光合作用的定义、指出反应物和产物，并描述至少一种测量光合速率的方法。

2. The Chemical Equation | 化学方程式

The word equation for photosynthesis must be learned exactly. It shows the two reactants, carbon dioxide and water, and the two products, glucose and oxygen.

光合作用的文字方程式需要准确记忆。它列出了两种反应物（二氧化碳和水）以及两种产物（葡萄糖和氧气）。

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The balanced symbol equation is shown below. You should be able to write and balance it in an examination, and note that the arrow indicates that light energy is required.

配平的符号方程式如下。你应该能在考试中写出并配平它，注意箭头上方表示需要光能。

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 numbers in front of the formulas, called coefficients, show the ratio of reactants to products.

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。化学式前面的数字称为系数，表示反应物与产物之间的比例关系。

3. Light and Chlorophyll | 光与叶绿素

Chlorophyll is the green pigment found inside chloroplasts. It is most concentrated in the palisade mesophyll layer near the upper surface of the leaf. Chlorophyll absorbs light energy, mainly in the red and blue regions of the visible spectrum, and transfers that energy into the reactions of photosynthesis.

叶绿素是叶绿体内的绿色色素。它主要集中在靠近叶片上表皮的栅栏叶肉组织中。叶绿素主要吸收可见光谱中红光和蓝光区域的光能，并将这些能量传递到光合作用反应中。

Without light, photosynthesis cannot occur. As the light intensity increases, the rate of photosynthesis also increases, but only up to a certain point called the saturation point. Beyond this point, increasing light intensity has no effect, because another factor such as carbon dioxide concentration or temperature becomes limiting.

没有光，光合作用就无法进行。随着光照强度增大，光合速率也随之升高，但只能升高到所谓饱和点为止。超过饱和点后，继续增强光照不再影响速率，因为此时另一因素（如二氧化碳浓度或温度）变成了限制因素。

Light intensity falls with distance according to the inverse square law: light intensity ∝ 1/d². This means that if the distance between a lamp and a plant is doubled, the light intensity drops to one quarter of its original value. This explains why moving a lamp closer to a plant has such a strong effect on the rate.

光照强度随着距离的增加按平方反比定律减弱：光照强度 ∝ 1/d²。也就是说，如果灯与植物之间的距离加倍，光照强度将降为原来的四分之一。这解释了为什么把灯移近植物会对速率产生显著影响。

4. Limiting Factors | 限制因素

A limiting factor is a condition that, when present in too small an amount, prevents the rate of photosynthesis from increasing. The three main limiting factors in IGCSE Biology are light intensity, carbon dioxide concentration, and temperature.

限制因素是指当某条件供应不足时，会阻止光合速率继续升高的因素。在 IGCSE 生物中，三大主要限制因素是光照强度、二氧化碳浓度和温度。

Limiting Factor 限制因素	Why It Matters 为何重要	Greenhouse Solution 温室对策
Light intensity 光照强度	Light supplies the energy for the reaction 光为反应提供能量	Use artificial lighting 使用人工照明
Carbon dioxide 二氧化碳	CO₂ is one of the reactants CO₂ 是反应物之一	Burn paraffin or add CO₂ 燃烧石蜡或补充二氧化碳
Temperature 温度	Photosynthesis is controlled by enzymes 光合作用由酶控制	Use heaters or ventilators 使用加热器或通风设备

When answering exam questions about graphs of photosynthesis rate, describe the pattern in three parts: the initial steep rise, the plateau, and the reason for the plateau. The plateau always indicates that the factor you changed is no longer limiting, and that another factor has taken over as the limiting factor.

回答关于光合速率曲线图的考题时，要从三部分描述图形：最初快速上升段、平台段，以及平台段出现的原因。平台段表明你所改变的因素不再是限制因素，另一个因素已经取而代之成为新的限制因素。

5. Measuring the Rate of Photosynthesis | 测量光合速率

The rate of photosynthesis can be estimated in several ways. The most common methods in the school laboratory are measuring the volume of oxygen produced, counting the number of bubbles released per minute, measuring the uptake of carbon dioxide, or measuring the increase in plant biomass over time.

光合速率可以通过多种方式估计。学校实验室最常用的方法是测量产生的氧气体积、数每分钟释放的气泡数、测量二氧化碳的吸收量，或者测量一段时间内植物生物量的增加量。

In a typical investigation, a piece of pondweed such as Elodea is placed in water that contains dissolved sodium hydrogencarbonate, which acts as a source of carbon dioxide. A lamp is placed at a fixed distance from the pondweed, and the number of bubbles produced in one minute is recorded. The lamp is then moved to different distances to change the light intensity.

在典型的实验中，将一段金鱼藻（如 Elodea）放入含有溶解碳酸氢钠（作为二氧化碳来源）的水中。将灯放在距金鱼藻固定距离处，记录一分钟内产生的气泡数。然后移动灯到不同距离，以改变光照强度。

The bubble-counting method is quick, but it is not very accurate because bubbles are not all the same size. A better method is to collect the gas in a measuring cylinder or a capillary tube and measure the actual volume of gas produced per unit time.

数气泡的方法很快，但不够准确，因为气泡大小并不相同。更好的方法是用量筒或毛细管收集气体，测量单位时间内实际产生的气体体积。

6. The Pondweed Investigation | 金鱼藻实验

Here are the key steps of the pondweed investigation, which is a required practical for Edexcel IGCSE Science:

以下是金鱼藻实验的关键步骤，这是 Edexcel IGCSE 科学课程要求的实践内容：

Cut the stem of the pondweed at an angle under water to avoid damaging the tissue.
在水下斜切金鱼藻的茎，以避免损伤组织。
Place the pondweed in a beaker of water containing dissolved sodium hydrogencarbonate.
将金鱼藻放入含有碳酸氢钠溶液的水烧杯中。
Count the number of oxygen bubbles released in one minute.
数一分钟内释放的氧气气泡数量。
Repeat the count with the lamp at different distances from the plant.
在灯距植物不同距离处重复计数。
Keep the temperature constant by using a water bath around the beaker.

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Photosynthesis | 光合作用