Photosynthesis and Limiting Factors: KS3 CIE Answers | 光合作用与限制因素：KS3 CIE 答案与解析

📚 Photosynthesis and Limiting Factors: KS3 CIE Answers | 光合作用与限制因素：KS3 CIE 答案与解析

Photosynthesis is one of the most important processes on Earth. It is how green plants make their own food using light energy. In KS3 CIE Science, you need to explain the word equation, describe leaf adaptations, and interpret graphs showing limiting factors. This article gives clear explanations and model answers to common questions.

光合作用是地球上最重要的过程之一。它是绿色植物利用光能制造自身养料的方式。在 KS3 CIE 科学课程中，你需要解释文字方程式、描述叶片适应结构，并解读显示限制因素的曲线图。本文提供清晰的讲解和常见题型的标准答案。

1. What Photosynthesis Means | 什么是光合作用

Photosynthesis is the process by which green plants convert light energy into chemical energy stored in glucose. It takes place in the chloroplasts of plant cells. The plant uses carbon dioxide from the air and water from the soil to make glucose and oxygen.

光合作用是绿色植物将光能转化为储存在葡萄糖中化学能的过程。它发生在植物细胞的叶绿体中。植物利用空气中的二氧化碳和土壤中的水分来制造葡萄糖和氧气。

Because plants make their own food, they are called producers. Animals cannot photosynthesise, so they must eat plants or other animals to get energy. This makes photosynthesis the foundation of nearly all food chains.

由于植物能自己制造食物，它们被称为生产者。动物不能进行光合作用，因此必须吃植物或其他动物来获取能量。这使得光合作用成为几乎所有食物链的基础。

2. Word and Chemical Equations | 文字方程式与化学方程式

The word equation for photosynthesis is:

光合作用的文字方程式为：

carbon dioxide + water → glucose + oxygen

This reaction only happens in the presence of light energy and chlorophyll. Light provides the energy needed, and chlorophyll absorbs that light.

该反应只有在光能和叶绿素存在时才会发生。光提供所需能量，叶绿素吸收光能。

The balanced chemical equation is:

配平的化学方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Do not write ‘light’ or ‘chlorophyll’ as reactants. They are conditions, not substances used up in the reaction. In an exam, saying that light and chlorophyll are needed is good, but they should not appear on the left side of the equation as raw materials.

不要将 ‘光’ 或 ‘叶绿素’ 写成反应物。它们是条件，不是反应中消耗的物质。在考试中，说明需要光和叶绿素是好的，但它们不应作为原料出现在方程式的左侧。

3. Where Photosynthesis Happens | 光合作用发生在哪里

Photosynthesis happens inside chloroplasts. Chloroplasts contain a green pigment called chlorophyll. Chlorophyll absorbs light energy, especially in the red and blue parts of the spectrum. Green light is reflected, which is why leaves look green.

光合作用发生在叶绿体内部。叶绿体含有一种叫做叶绿素的绿色色素。叶绿素吸收光能，尤其是光谱中的红光和蓝光部分。绿光被反射，因此叶子看起来是绿色的。

In a leaf cross-section, the palisade mesophyll cells near the upper surface contain the most chloroplasts. This arrangement helps the leaf capture as much light as possible. The spongy mesophyll below has air spaces that allow carbon dioxide to move easily through the leaf.

在叶片横切面中，靠近上表面的栅栏叶肉细胞含有最多的叶绿体。这种排列有助于叶片尽可能多地捕获光。下方的海绵叶肉组织有气腔，使二氧化碳能够在叶片内轻松移动。

4. How Leaves Are Adapted | 叶片如何适应光合作用

Large surface area to absorb more sunlight.

大面积表面积，可吸收更多阳光。
Thin structure so carbon dioxide can diffuse quickly to photosynthesising cells.

叶片薄，二氧化碳可快速扩散到进行光合作用的细胞。
Chloroplasts packed in palisade cells near the upper surface to capture light efficiently.

叶绿体密集分布在靠近上表面的栅栏细胞中，以高效捕获光。
Stomata on the lower surface allow gas exchange while reducing water loss.

下表皮的气孔允许气体交换，同时减少水分流失。
A network of veins called xylem and phloem supplies water and removes glucose.

由木质部和韧皮部组成的叶脉网络输送水分并运走葡萄糖。

These adaptations do not work alone. Together they ensure that light is absorbed, carbon dioxide reaches the cells, water is supplied, and products are carried away. This makes the leaf a highly efficient organ for photosynthesis.

这些适应结构并非单独起作用。它们共同确保光被吸收、二氧化碳到达细胞、水分被输送、产物被运走。这使叶片成为进行光合作用的高效器官。

5. Defining Limiting Factors | 限制因素的定义

A limiting factor is something that slows down a process because it is in short supply. For photosynthesis, the main limiting factors are light intensity, carbon dioxide concentration, and temperature. Even if the other factors are plentiful, the rate of photosynthesis cannot increase beyond the limit set by the factor that is scarcest.

限制因素是指因为供应不足而减慢某一过程的因素。对光合作用而言，主要限制因素是光照强度、二氧化碳浓度和温度。即使其他因素充足，光合作用速率也无法超过由最稀缺因素所设定的上限。

Think of a factory making cars. If the factory has many workers but only one robot, the robot is the limiting factor. Adding more workers will not increase the number of cars made. In the same way, adding more light will not help photosynthesis if carbon dioxide is the factor in short supply.

想象一个制造汽车的工厂。如果工厂有许多工人但只有一台机器人，那么机器人就是限制因素。增加更多工人并不会增加汽车产量。同样，如果二氧化碳是供应不足的因素，增加更多光照也不会帮助光合作用。

6. Light Intensity as a Limiting Factor | 光照强度作为限制因素

At low light intensity, photosynthesis is slow because there is not enough energy to split water molecules and drive the reactions. As light intensity increases, the rate of photosynthesis increases. However, after a certain point, the rate stops rising and the graph plateaus.

在低光照强度下，光合作用较慢，因为没有足够能量分解水分子并驱动反应。随着光照强度增加，光合作用速率加快。然而，超过某一点后，速率停止上升，曲线趋于平稳。

At the plateau, light is no longer the limiting factor. Another factor such as carbon dioxide concentration or temperature is now limiting the rate. For example, on a very bright day in winter, the temperature may still prevent photosynthesis from reaching its maximum speed.

在平稳段，光不再是限制因素。二氧化碳浓度或温度等因素正在限制速率。例如，在冬季非常晴朗的日子里，温度仍可能阻止光合作用达到最大速度。

7. Carbon Dioxide Concentration | 二氧化碳浓度

Carbon dioxide is one of the raw materials for photosynthesis. If CO₂ concentration is low, the plant cannot make enough glucose. Increasing CO₂ concentration increases the rate of photosynthesis, but only up to a plateau.

二氧化碳是光合作用的原料之一。如果二氧化碳浓度低，植物就无法制造足够的葡萄糖。提高二氧化碳浓度会提高光合作用速率，但只能提高到某一平稳值。

In a greenhouse, farmers sometimes burn fuels or release extra CO₂ to raise the concentration. This can increase crop yield as long as light and temperature are not limiting. This is an example of using science to control limiting factors in commercial growing.

在温室中，农民有时燃烧燃料或释放额外的二氧化碳来提高浓度。只要光和温度不是限制因素，这就能提高作物产量。这是利用科学在商业种植中控制限制因素的一个例子。

8. Temperature and Enzymes | 温度与酶

Photosynthesis is controlled by enzymes. As temperature increases, enzyme activity increases, so the rate of photosynthesis rises. However, if the temperature becomes too high, the enzymes denature. Their active sites change shape, and the rate drops sharply.

光合作用受酶控制。随着温度升高，酶活性增强，因此光合作用速率上升。但如果温度过高，酶会变性。酶的活性位点改变形状，速率急剧下降。

For most plants, the optimum temperature for photosynthesis is around 20 °C to 30 °C. Above about 45 °C, many plant enzymes are damaged. This is why very hot, dry conditions can be harmful to plants even when light is abundant.

对大多数植物来说，光合作用的最适温度约为 20 °C 至 30 °C。超过约 45 °C 时，许多植物酶会被破坏。这就是为什么在炎热干燥的条件下，即使光照充足，植物也可能受到伤害。

9. Reading Limiting Factor Graphs | 解读限制因素曲线图

In an exam, you may be given a graph with rate of photosynthesis on the y-axis and a limiting factor on the x-axis. The graph usually rises steeply at first and then flattens out.

考试中，你可能会遇到光合作用速率在 y 轴、限制因素在 x 轴的曲线图。曲线通常先急剧上升，然后趋于平缓。

When the line is rising, the factor on the x-axis is the limiting factor. When the line becomes horizontal, something else is limiting the rate.

当曲线上升时，x 轴上的因素是限制因素。当曲线变为水平时，其他因素正在限制速率。

If you increase CO₂ concentration at the plateau point, the graph may rise again until a new plateau is reached. This shows that CO₂ is now the limiting factor. You should be able to explain this pattern using the idea of limiting factors.

如果在平稳点处提高二氧化碳浓度，曲线可能再次上升，直到达到新的平稳点。这表明二氧化碳现在是限制因素。你应该能够用限制因素的概念来解释这种模式。

10. Common Practical: Elodea Bubble Count | 常见实验：伊乐藻气泡计数

To investigate the effect of light intensity on photosynthesis, you can place a piece of pondweed such as Elodea in water and count the oxygen bubbles released per minute.

为了研究光照强度对光合作用的影响，你可以将一段水草（如伊乐藻）放入水中，然后数每分钟释放的氧气气泡数量。

You can change light intensity by moving a lamp closer to or further from the pondweed. The number of bubbles per minute is a simple measure of the rate of photosynthesis.

你可以通过将灯移近或远离水草来改变光照强度。每分钟气泡数量是光合作用速率的一种简单测量方法。

To make the experiment fair, keep the temperature, CO₂ concentration, and the type of pondweed the same. Use a fixed volume of water and wait for the pondweed to adjust before counting. Also place a glass screen between the lamp and the water to prevent the lamp from heating the water, because temperature could become another variable.

为使实验公平，应保持温度、二氧化碳浓度和水草种类不变。使用固定体积的水，并在计数前等待水草适应。此外，在灯和水之间放置一块玻璃挡板，以防止灯加热水，因为温度可能成为另一个变量。

rate of photosynthesis ≈ number of bubbles per minute

光合作用速率 ≈ 每分钟气泡数

11. Model Answers to Past-Paper Style Questions | 历年真题风格标准答案

Question 1: State the word equation for photosynthesis.

问题 1：写出光合作用的文字方程式。

Answer: carbon dioxide + water → glucose + oxygen (in the presence of light and chlorophyll)

答案：二氧化碳 + 水 → 葡萄糖 + 氧气（在光和叶绿素存在下）

Question 2: A plant is kept in a dark cupboard for 24 hours. Explain why it cannot make glucose.

问题 2：一株植物被放在黑暗的柜子里 24 小时。解释它为什么不能制造葡萄糖。

Answer: Photosynthesis requires light energy. In the dark, there is no light to provide energy, so carbon dioxide and water cannot be converted into glucose.

答案：光合作用需要光能。在黑暗中，没有光提供能量，因此二氧化碳和水无法转化为葡萄糖。

Question 3: The graph of light intensity against rate of photosynthesis rises and then levels off. Explain why the rate stops increasing.

问题 3：光照强度与光合作用速率的关系曲线先上升后趋于平稳。解释为什么速率不再增加。

Answer: At the plateau, light is no longer the limiting factor. Another factor, such as carbon dioxide concentration or temperature, is now limiting the rate of photosynthesis.

答案：在平稳段，光不再是限制因素。另一个因素（如二氧化碳浓度或温度）正在限制光合作用速率。

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Photosynthesis and Limiting Factors: KS3 CIE Answers | 光合作用与限制因素：KS3 CIE 答案与解析