Photosynthesis: A Complete Guide for Edexcel IGCSE Science | 光合作用：Edexcel IGCSE 科学完全指南

📚 Photosynthesis: A Complete Guide for Edexcel IGCSE Science | 光合作用：Edexcel IGCSE 科学完全指南

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 reactions on Earth because it produces the oxygen we breathe and forms the base of almost 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化学反应之一，因为它产生我们呼吸的氧气，并构成几乎每一条食物链的基础。

1. The Word Equation | 光合作用的文字方程式

For Edexcel IGCSE, you must be able to write and interpret the word equation for photosynthesis. Carbon dioxide and water are the raw materials, and glucose and oxygen are the products.

对于 Edexcel IGCSE，你必须能够写出并解释光合作用的文字方程式。二氧化碳和水是原料，葡萄糖和氧气是产物。

Carbon dioxide + Water → Glucose + Oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The energy required for this reaction comes from sunlight, which is absorbed by chlorophyll in the chloroplasts. This makes photosynthesis an endothermic reaction.

该反应所需的能量来自阳光，阳光被叶绿体中的叶绿素吸收。这使得光合作用成为一种吸热反应。

2. The Balanced Chemical Equation | 配平的化学方程式

You should also know the balanced symbol equation. It shows the exact number of molecules involved in the reaction.

你还应知道配平的符号方程式。它显示了反应中涉及的分子的确切数量。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Here, six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.

在此反应中，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。

Note that the subscript numbers in CO₂ and H₂O represent the number of atoms in each molecule. The symbol equation must always be balanced, meaning the number of atoms of each element is the same on both sides.

注意，CO₂ 和 H₂O 中的下标数字表示每个分子中该原子的数量。符号方程式必须始终配平，即每种元素的原子数在两边相同。

3. The Role of Chlorophyll and Chloroplasts | 叶绿素与叶绿体的作用

Chlorophyll is a green pigment found in chloroplasts, mainly in the palisade mesophyll cells of leaves. It absorbs light energy, especially red and blue light, and uses that energy to drive the reactions of photosynthesis.

叶绿素是存在于叶绿体中的绿色色素，主要位于叶片的栅栏组织细胞中。它吸收光能，尤其是红光和蓝光，并利用该能量驱动光合作用的反应。

Chloroplasts also contain enzymes that catalyse the many steps of photosynthesis. Without these enzymes, the reaction would be far too slow to sustain life.

叶绿体还含有催化光合作用许多步骤的酶。如果没有这些酶，光合作用的速度会太慢，无法维持生命。

Leaves are adapted for efficient photosynthesis. They are broad and thin to provide a large surface area, and they have many chloroplasts. The upper epidermis is transparent to let light pass through, and stomata allow carbon dioxide to enter.

叶片为高效光合作用而特化。它们宽阔而薄，以提供巨大的表面积，并含有许多叶绿体。上表皮透明，使光能通过；气孔允许二氧化碳进入。

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

A limiting factor is something that slows down or stops a reaction when it is in short supply. For photosynthesis, the three main limiting factors are light intensity, carbon dioxide concentration and temperature.

限制因素是在供应不足时会减慢或停止反应的因素。对于光合作用，三个主要的限制因素是光照强度、二氧化碳浓度和温度。

As light intensity increases, the rate of photosynthesis increases proportionally, provided that no other factor is limiting. This is because more light energy means more photons are available for chlorophyll to absorb.

当光照强度增加时，光合作用速率成比例增加，前提是没有其他因素成为限制因素。这是因为更多的光能意味着叶绿素可以吸收更多的光子。

However, at a certain point, increasing light intensity no longer increases the rate. This happens because another factor, such as carbon dioxide or temperature, has become the new limiting factor. The graph levels off.

然而，在某个点之后，增加光照强度不再提高光合作用速率。这是因为另一个因素，如二氧化碳或温度，已成为新的限制因素。图中曲线趋于平缓。

5. Carbon Dioxide Concentration as a Limiting Factor | 二氧化碳浓度作为限制因素

Carbon dioxide is one of the raw materials of photosynthesis, so its concentration directly affects the rate. As CO₂ concentration increases, the rate of photosynthesis initially rises, then reaches a plateau.

二氧化碳是光合作用的原料之一，因此其浓度直接影响速率。随着 CO₂ 浓度增加，光合作用速率最初上升，然后达到一个平台期。

In greenhouses, farmers sometimes increase the CO₂ concentration by burning propane or using CO₂ generators. This can significantly increase crop yield, but only if light and temperature are also sufficient.

在温室中，农民有时通过燃烧丙烷或使用 CO₂ 发生器来提高二氧化碳浓度。这可以显著提高作物产量，但前提是光照和温度也足够。

In natural conditions, CO₂ concentration is usually around 0.04% in air. On a bright summer day, CO₂ is often the main limiting factor because light is abundant.

在自然条件下，空气中的 CO₂ 浓度通常在 0.04% 左右。在晴朗的夏日，光照充足，CO₂ 往往成为主要的限制因素。

6. Temperature as a Limiting Factor | 温度作为限制因素

Enzymes control photosynthesis, and enzyme activity is strongly affected by temperature. As temperature increases from cold conditions, the rate of photosynthesis increases because the enzymes gain kinetic energy and collisions happen more often.

酶控制着光合作用，而酶活性受温度影响很大。随着温度从低温条件下升高，光合作用速率增加，因为酶获得动能，分子碰撞更加频繁。

At very high temperatures, enzymes begin to denature. Their three-dimensional structure changes, and the active site is destroyed. This causes the rate of photosynthesis to fall sharply, even if light and CO₂ are plentiful.

在很高的温度下，酶开始变性。它们的三维结构改变，活性位点被破坏。这导致光合作用速率急剧下降，即使光照和 CO₂ 充足也是如此。

For most plants, the optimum temperature for photosynthesis is between 25°C and 35°C. This is why greenhouse growers often use heating in winter to keep temperatures near the optimum.

对大多数植物而言，光合作用的最适温度在 25°C 到 35°C 之间。这就是为什么温室种植者在冬季常使用加热设备以保持接近最适温度。

7. How to Measure the Rate of Photosynthesis | 如何测量光合作用速率

The rate of photosynthesis can be measured in several ways. The most common method for Edexcel IGCSE is to count the bubbles produced by an aquatic plant such as Elodea or Cabomba.

光合作用速率可以通过多种方式测量。对于 Edexcel IGCSE，最常见的方法是计数水生植物（如 Elodea 或 Cabomba）产生的气泡。

Alternatively, you can collect the oxygen produced in a gas syringe or measuring cylinder and record the volume per unit time. The rate can be calculated using the formula:

或者，你可以用气体注射器或量筒收集产生的氧气，并记录单位时间内的体积。速率可以用以下公式计算：

Rate = Volume of oxygen ÷ Time

速率 = 氧气体积 ÷ 时间

To investigate limiting factors, you can change the distance of a lamp from the plant. Because light intensity follows the inverse square law, moving the lamp twice as far away makes the light intensity four times weaker.

为了研究限制因素，你可以改变灯泡与植物之间的距离。由于光照强度遵循平方反比定律，把灯泡移到两倍远处会使光照强度减弱四倍。

Light intensity ∝ 1 / d²

光照强度 ∝ 1 / d²

8. The Uses of Glucose from Photosynthesis | 光合作用产生葡萄糖的用途

The glucose produced by photosynthesis is used by plants in several important ways.

光合作用产生的葡萄糖以多种重要方式被植物利用。

Respiration: Glucose is broken down in respiration to release energy for cellular processes.

呼吸作用：葡萄糖在呼吸作用中被分解，释放能量供细胞过程使用。
Starch storage: Excess glucose is stored as starch in roots, stems and leaves. Starch is insoluble, so it does not affect the water balance of cells.

淀粉储存：多余的葡萄糖以淀粉形式储存在根、茎和叶中。淀粉不溶于水，因此不会影响细胞的水分平衡。
Cellulose: Glucose is converted into cellulose for building strong cell walls.

纤维素：葡萄糖被转化为纤维素，用于构建坚固的细胞壁。
Proteins: Glucose combines with nitrate ions absorbed from the soil to form amino acids, which are then joined into proteins.

蛋白质：葡萄糖与从土壤中吸收的硝酸根离子结合，形成氨基酸，然后连接成蛋白质。
Fats and oils: Glucose can be converted into lipids for energy storage in seeds.

脂肪和油：葡萄糖可转化为脂质，用于种子中的能量储存。

9. Investigating the Need for Chlorophyll, Light and Carbon Dioxide | 探究叶绿素、光照和二氧化碳的必要性

A classic experiment uses a destarched plant to prove that photosynthesis requires light and chlorophyll. A plant is kept in the dark for 48 hours to remove all starch from its leaves, then partially covered with black paper and exposed to light.

经典实验使用去淀粉植物来证明光合作用需要光和叶绿素。将植物在黑暗中放置 48 小时以去除叶片中的所有淀粉，然后用黑纸部分遮盖并放在光下。

After several hours, the leaf is removed, boiled in ethanol to remove chlorophyll, washed with water, and tested with iodine solution. The exposed areas turn blue-black, while the covered areas stay orange-brown. This proves that light is necessary for starch formation.

数小时后，取下叶片，用乙醇煮沸以去除叶绿素，用水冲洗，然后用碘液测试。曝光区域变为蓝黑色，而遮光区域保持橙棕色。这证明了光照是淀粉形成所必需的。

The same method can be used with a variegated leaf, which has both green and white patches. Only the green patches produce starch, proving that chlorophyll is needed for photosynthesis.

同样的方法可用于斑叶，即同时具有绿色和白色斑块的叶片。只有绿色斑块产生淀粉，证明叶绿素是光合作用所必需的。

To show that carbon dioxide is required, soda lime can be used to absorb CO₂ from the air around one leaf, while another leaf is surrounded by normal air. After testing with iodine, only the leaf exposed to normal air shows a positive starch test.

为了证明二氧化碳是必需的，可以使用碱石灰吸收某一片叶片周围空气中的 CO₂，而另一片叶片周围保持正常空气。用碘液测试后，只有暴露在正常空气中的叶片呈阳性淀粉测试结果。

10. Minerals Needed for Healthy Plant Growth | 植物健康生长所需的无机盐

Plants do not only need photosynthesis to grow; they also require mineral ions from the soil. Two important ions appear frequently in Edexcel IGCSE exams: nitrates and magnesium.

植物生长不仅需要光合作用；它们还需要从土壤中吸收矿质离子。Edexcel IGCSE 考试中经常出现两种重要的离子：硝酸盐和镁。

Nitrate ions (NO₃⁻) are needed to make amino acids and proteins

硝酸根离子（NO₃⁻）用于合成氨基酸和蛋白质

If a plant lacks nitrate, its growth is stunted, and older leaves become yellow. This is because nitrogen is a key element in amino acids, DNA and proteins.

如果植物缺乏硝酸盐，其生长受阻，较老的叶片会变黄。这是因为氮是氨基酸、DNA 和蛋白质中的关键元素。

Magnesium ions (Mg²⁺) are needed to make chlorophyll

镁离子（Mg²⁺）用于合成叶绿素

A plant lacking magnesium cannot produce chlorophyll, so its leaves turn yellow. This condition is called chlorosis and greatly reduces the rate of photosynthesis.

缺乏镁的植物无法产生叶绿素，因此其叶片变黄。这种情况称为缺绿症，会大大降低光合作用速率。

11. Common Exam Mistakes and Tips | 常见考试错误与提示

Students often confuse photosynthesis with respiration. Remember that photosynthesis takes in CO₂ and releases O₂, while aerobic respiration takes in O₂ and releases CO₂. Photosynthesis happens only in the light, in plant cells with chlorophyll; respiration happens in all living cells all the time.

学生经常混淆光合作用与呼吸作用。记住：光合作用吸收 CO₂ 并释放 O₂，而有氧呼吸吸收 O₂ 并释放 CO₂。光合作用只在光照下、发生在含叶绿素的植物细胞中；呼吸作用则始终发生在所有活细胞中。

Another common error is writing “glucose” with the wrong formula or forgetting the 6 in front of CO₂ and O₂. Practise writing the balanced equation until it becomes automatic.

另一个常见错误是写错葡萄糖的分子式，或忘记 CO₂ 和 O₂ 前面的系数 6。反复练习配平方程式，直到能够熟练写出。

When answering questions about limiting factors, always identify the factor that is in shortest supply at that moment. If the graph is flat, state that a different factor is now limiting the rate.

回答关于限制因素的问题时，始终指出当前供应最短缺的因素。如果曲线平缓，说明是另一个因素现在限制了速率。

Finally, always use the correct units when calculating rates, usually cm³/min or bubbles/min, and show your working clearly in calculations.

最后，在计算速率时务必使用正确的单位，通常为 cm³/min 或 bubbles/min，并在计算中清楚展示解题步骤。

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Photosynthesis: A Complete Guide for Edexcel IGCSE Science | 光合作用：Edexcel IGCSE 科学完全指南