Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is one of the most important biological processes on Earth. It is the way green plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

光合作用是地球上最重要的生物过程之一。绿色植物、藻类和某些细菌通过它将光能转化为化学能，利用二氧化碳和水生成葡萄糖和氧气。

In the IGCSE Edexcel Science syllabus, photosynthesis is a core topic covering the process, the balanced chemical equation, limiting factors, and practical investigations. This article provides a comprehensive revision guide to help you master the key concepts.

在IGCSE Edexcel科学课程中，光合作用是一个核心主题，涵盖过程、平衡化学方程式、限制因素和实验探究。本文提供全面的复习指南，帮助你掌握关键概念。

1. The Definition of Photosynthesis | 光合作用的定义

Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using energy from light. The raw materials are carbon dioxide and water, and the products are glucose and oxygen.

光合作用是植物利用光能，从原料中制造碳水化合物的过程。原料是二氧化碳和水，产物是葡萄糖和氧气。

This process occurs in the chloroplasts of plant cells, particularly in the palisade mesophyll cells of the leaf. Chlorophyll, the green pigment, absorbs light energy to drive the reaction.

该过程发生在植物细胞的叶绿体中，尤其是叶片的栅栏组织细胞。叶绿素是绿色色素，它吸收光能来驱动反应。

Occurs in plants, algae, and some bacteria
Takes place in chloroplasts
Requires light energy absorbed by chlorophyll
Produces glucose and oxygen

发生在植物、藻类和某些细菌中
在叶绿体中进行
需要叶绿素吸收的光能
产生葡萄糖和氧气

2. The Balanced Chemical Equation | 平衡化学方程式

The overall equation for photosynthesis can be written in words and symbols. It is important to remember the correct coefficients and the states of matter.

光合作用的总体方程式可以用文字和符号表示。记住正确的系数和状态非常重要。

Carbon dioxide + Water → Glucose + Oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The balanced symbol equation is:

平衡符号方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Note that the arrow points to the right, and light energy is written above the arrow. Some versions include ‘light energy’ as a condition:

注意箭头指向右边，光能写在箭头上方。有些版本将“光能”作为条件写出：

6CO₂ + 6H₂O ⇢ (light) C₆H₁₂O₆ + 6O₂

6CO₂ + 6H₂O ⇢ (光) C₆H₁₂O₆ + 6O₂

3. The Two Stages of Photosynthesis | 光合作用的两个阶段

Photosynthesis consists of two main stages: the light-dependent reaction and the light-independent reaction (the Calvin cycle).

光合作用包括两个主要阶段：光依赖反应和光不依赖反应（卡尔文循环）。

Light-dependent reaction: This occurs in the thylakoid membranes of the chloroplasts. Light energy is absorbed by chlorophyll, splitting water into oxygen, hydrogen ions, and electrons. Oxygen is released as a by-product.

光依赖反应：发生在叶绿体的类囊体膜上。光能被叶绿素吸收，将水分解为氧气、氢离子和电子。氧气作为副产物释放。

Light-independent reaction: This occurs in the stroma of the chloroplasts. Carbon dioxide is combined with hydrogen (from the light-dependent stage) to form glucose. This stage does not directly require light, but it depends on the products of the light-dependent stage.

光不依赖反应：发生在叶绿体的基质中。二氧化碳与来自光依赖阶段的氢结合生成葡萄糖。这个阶段不直接需要光，但依赖于光依赖阶段的产物。

Feature	Light-dependent	Light-independent
Location	Thylakoid membranes	Stroma
Requires light?	Yes	No (but needs ATP and NADPH from light stage)
Inputs	Water, light, chlorophyll	CO₂, hydrogen ions, electrons
Outputs	Oxygen, ATP, reduced NADP	Glucose, ADP, NADP

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

The leaf is the main photosynthetic organ of a plant. Its structure is adapted to maximise light absorption and gas exchange.

叶片是植物主要的光合作用器官。其结构适应于最大限度吸收光和进行气体交换。

Waxy cuticle: reduces water loss
Upper epidermis: transparent, allows light through
Palisade mesophyll: packed with chloroplasts, main site of photosynthesis
Spongy mesophyll: air spaces for gas exchange
Stomata: pores that allow CO₂ in and O₂ out

角质层：减少水分蒸发
上表皮：透明，允许光透过
栅栏组织：富含叶绿体，光合作用的主要场所
海绵组织：有气隙，便于气体交换
气孔：允许CO₂进入和O₂排出的孔隙

5. Limiting Factors | 限制因素

A limiting factor is something that determines the rate of photosynthesis. If a factor is in short supply, it limits the rate even if other factors are plentiful. The three main limiting factors are light intensity, carbon dioxide concentration, and temperature.

限制因素是决定光合作用速率的条件。如果某个因素供应不足，即使其他因素充足，它也会限制速率。三大主要限制因素是光强度、二氧化碳浓度和温度。

Light intensity: As light intensity increases, the rate of photosynthesis increases until a plateau is reached, where other factors become limiting.

光强度：随着光强度增加，光合作用速率增加，直到达到平台期，此时其他因素成为限制因素。

Carbon dioxide concentration: Increasing CO₂ concentration generally increases the rate, but only if light and temperature are not limiting.

二氧化碳浓度：增加CO₂浓度通常能提高速率，但仅在光和温度不受限制的情况下。

Temperature: Photosynthesis is controlled by enzymes, so it increases with temperature up to an optimum (around 30-35 °C). Above this, enzymes denature and the rate drops sharply.

温度：光合作用受酶控制，因此温度升高速率增加，直至最适温度（约30–35 °C）。超过后，酶变性，速率急剧下降。

Rate ∝ light intensity (within the linear range)

速率 ∝ 光强度（在线性范围内）

6. How to Investigate Photosynthesis Rate | 如何探究光合作用速率

A common practical is to measure the rate of oxygen production using pondweed (Elodea) in a test tube. By counting bubbles or collecting gas, you can compare rates under different conditions.

一个常见的实验是使用水生植物（如伊乐藻）在试管中测量氧气的产生速率。通过计数气泡或收集气体，可以在不同条件下比较速率。

Method steps:

方法步骤：

Place a piece of pondweed in a beaker of water with sodium hydrogencarbonate (to supply CO₂).
Shine a lamp at a fixed distance and count the number of oxygen bubbles per minute.
Repeat with different light intensities (move the lamp closer/farther).
Control temperature by keeping a heat shield or using a water bath.
Record results in a table and plot a graph.

将一段水生植物放入含有碳酸氢钠（提供CO₂）的水中。
在固定距离处用灯照射，并计数每分钟氧气气泡的数量。
改变光强度（将灯移近/移远）重复实验。
通过使用隔热屏或水浴来控制温度。
记录结果并绘制图表。

Alternatively, you can use a gas syringe to collect the oxygen and measure its volume over time.

或者，你可以使用注射器收集氧气并测量其体积随时间的变化。

7. The Effect of Light Intensity | 光强度的影响

When investigating light intensity, you can change the distance between the lamp and the plant. The light intensity follows the inverse square law:

在研究光强度时，你可以改变灯与植物之间的距离。光强度遵循平方反比定律：

Light intensity = 1 / (distance)²

光强度 = 1 / (距离)²

So if the distance is halved, the light intensity becomes four times greater. This is a calculation question that often appears in exams.

因此，如果距离减半，光强度变为原来的四倍。这是考试中常见的计算题。

At low light intensities, the rate increases linearly with intensity. As intensity increases further, the rate levels off because other factors (e.g. CO₂ concentration) become limiting.

在低光强度下，速率随强度线性增加。随着强度进一步增加，速率趋于平稳，因为其他因素（如CO₂浓度）成为限制因素。

8. Uses of Glucose in Plants | 植物中葡萄糖的用途

The glucose produced during photosynthesis is used by the plant for several purposes:

光合作用产生的葡萄糖在植物中有多种用途：

Respiration: Glucose is broken down to release energy.
Cellulose: Glucose is converted into cellulose for cell walls.
Starch: Excess glucose is stored as starch in leaves, roots, or seeds.
Sucrose: Transported in the phloem to other parts of the plant.
Proteins: Combined with nitrate ions to form amino acids and proteins.
Fats and oils: Stored as energy reserves in seeds and fruits.

呼吸作用：葡萄糖被分解以释放能量。
纤维素：葡萄糖转化为纤维素构成细胞壁。
淀粉：多余的葡萄糖以淀粉形式储存在叶片、根或种子中。
蔗糖：在韧皮部中运输到植物其他部位。
蛋白质：与硝酸根离子结合形成氨基酸和蛋白质。
脂肪和油：作为能量储备储存在种子和果实中。

9. The Importance of Photosynthesis | 光合作用的重要性

Photosynthesis is essential for life on Earth because it:

光合作用对地球生命至关重要，因为它：

Produces oxygen, which is needed for respiration by most organisms.
Produces carbohydrates, the foundation of most food chains.
Removes carbon dioxide from the atmosphere, helping to regulate the greenhouse effect.
Provides raw materials for fossil fuels over geological timescales.

产生氧气，大多数生物呼吸所需。
产生碳水化合物，是大多数食物链的基础。
从大气中清除二氧化碳，有助于调节温室效应。
在地质时间尺度上提供化石燃料的原料。

In agriculture, understanding limiting factors helps farmers maximise crop yields by controlling light, CO₂, and temperature in greenhouses.

在农业中，了解限制因素帮助农民通过控制温室中的光、CO₂和温度来最大化作物产量。

10. Common Exam Mistakes and Tips | 常见考试错误与建议

Many students lose marks on photosynthesis questions due to careless errors. Here are some tips:

许多学生在光合作用问题上因粗心错误而丢分。以下是一些建议：

Always write the balanced equation with the correct coefficients (6,6,1,6).
Remember the arrow notation: light energy is not a product or reactant, so write it above the arrow.
When answering questions about limiting factors, state that a factor ‘limits’ the rate only when it is below the optimum.
Use the inverse square law for distance/light intensity calculations.
In practical questions, mention controlling temperature as a variable.

始终写出正确系数的平衡方程式（6,6,1,6）。
记住箭头写法：光能不是反应物或产物，应写在箭头上方。
回答限制因素问题时，指出只有在某因素低于最适值时才“限制”速率。
对于距离/光强度计算，使用平方反比定律。
在实验题中，提到控制温度作为变量。

Also, be careful with units: temperature in °C, light intensity in lux or arbitrary units, and rate in bubbles per minute or cm³ per minute.

还要注意单位：温度用°C，光强度用勒克斯或任意单位，速率用每分钟气泡数或每分钟cm³。

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Photosynthesis | 光合作用