Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the fundamental process by which green plants, algae, and some bacteria use sunlight to convert carbon dioxide and water into glucose and oxygen. It is central to life on Earth because it provides the chemical energy that most organisms depend on, and it also releases the oxygen we breathe.

光合作用是绿色植物、藻类和某些细菌利用阳光，将二氧化碳和水转化为葡萄糖和氧气的基本过程。它是地球上生命存在的基础，因为它提供了大多数生物体所依赖的化学能，同时释放出我们呼吸所需的氧气。

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1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis is an endothermic chemical reaction. It requires energy, which is absorbed from sunlight by a green pigment called chlorophyll, mainly found in the chloroplasts of plant cells.

光合作用是一种吸热化学反应。它需要能量，这些能量由一种称为叶绿素的绿色色素吸收，叶绿素主要存在于植物细胞的叶绿体中。

During photosynthesis, energy from light is transferred to chemical energy stored in glucose. This glucose can later be converted into starch for storage or used in respiration to release energy.

在光合作用中，光能被转化为储存在葡萄糖中的化学能。这些葡萄糖随后可以转化为淀粉储存，或用于呼吸作用释放能量。

The overall process can be written as a balanced chemical equation, but it actually happens in many small steps.

整个过程可以写成一个化学方程式，但实际上它分多个小步骤进行。

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2. The Word Equation | 文字方程式

For IGCSE Edexcel Science, you should memorise the word equation for photosynthesis:

对于IGCSE Edexcel科学，你需要牢记光合作用的文字方程式：

carbon dioxide + water →(light energy, chlorophyll) glucose + oxygen

二氧化碳 + 水 →(光能, 叶绿素) 葡萄糖 + 氧气

Light energy and chlorophyll are not reactants or products, but they are essential conditions for the reaction to occur.

光能和叶绿素既不是反应物也不是生成物，但它们是反应发生所必需的条件。

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3. The Chemical Equation | 化学方程式

The balanced chemical equation for photosynthesis is:

光合作用的平衡化学方程式为：

6CO₂ + 6H₂O →(light energy, chlorophyll) C₆H₁₂O₆ + 6O₂

This shows that six molecules of carbon dioxide combine with six molecules of water to produce one molecule of glucose and six molecules of oxygen.

这个方程式表明，六个二氧化碳分子与六个水分子结合，生成一个葡萄糖分子和六个氧分子。

Notice that the reaction is endothermic, meaning it absorbs energy. The energy from sunlight is stored in the chemical bonds of glucose.

注意该反应是吸热的，意味着它吸收能量。来自太阳光的能量被储存在葡萄糖的化学键中。

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4. Chlorophyll and Chloroplasts | 叶绿素与叶绿体

Chlorophyll is the pigment that absorbs light, mainly red and blue wavelengths. It reflects green light, which is why leaves appear green.

叶绿素是吸收光能的色素，主要吸收红光和蓝光。它反射绿光，因此叶子看起来是绿色的。

Chlorophyll is located in chloroplasts, which are organelles found in the palisade mesophyll cells of leaves. These cells are near the top of the leaf to capture maximum light.

叶绿素位于叶绿体内，叶绿体是叶片栅栏组织细胞中的细胞器。这些细胞靠近叶片顶部，以捕获最大量的光。

• Chlorophyll is essential for photosynthesis.
• Plants without chlorophyll cannot make their own food.

• 叶绿素是光合作用所必需的。
• 没有叶绿素的植物不能自己制造食物。

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5. Light-Dependent and Light-Independent Stages | 光依赖阶段与光不依赖阶段

Photosynthesis occurs in two main stages: the light-dependent stage and the light-independent stage (Calvin cycle).

光合作用主要分为两个阶段：光依赖阶段和光不依赖阶段（卡尔文循环）。

The light-dependent stage happens inside the thylakoid membranes of chloroplasts. Here, light energy is absorbed by chlorophyll and used to split water molecules into oxygen, protons, and electrons.

光依赖阶段发生在叶绿体的类囊体薄膜内。在这里，光能被叶绿素吸收，用于将水分子分解为氧气、质子和电子。

The oxygen is released as a waste product. The hydrogen and electrons are carried to the next stage.

氧气作为副产品释放出来。氢和电子被带入下一阶段。

The light-independent stage occurs in the stroma, the fluid part of the chloroplast. It uses carbon dioxide and the hydrogen carried from the light-dependent stage to produce glucose.

光不依赖阶段发生在叶绿体基质（液体部分）中。它利用二氧化碳和来自光依赖阶段的氢来合成葡萄糖。

For IGCSE, you only need a simple outline: light splits water, then carbon dioxide combines with hydrogen to make glucose.

对于IGCSE，你只需要简单了解：光分解水，然后二氧化碳与氢结合生成葡萄糖。

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6. Limiting Factors | 限制因素

The rate of photosynthesis is affected by several factors. A limiting factor is any factor that is in short supply and therefore slows down the reaction.

光合作用的速率受多种因素影响。限制因素是指供应不足并因此减慢反应速率的任何因素。

The main limiting factors are:

主要限制因素有：

• Light intensity | 光照强度
• Carbon dioxide concentration | 二氧化碳浓度
• Temperature | 温度

If light intensity is increased but the temperature is very low, the rate will not increase because enzymes work slowly in cold conditions.

如果增加光照强度但温度很低，速率不会增加，因为酶在低温下工作效率低。

On a hot sunny day, the rate may be limited by the amount of carbon dioxide available, since plants use CO₂ quickly.

在炎热晴朗的日子里，速率可能受可用的二氧化碳量限制，因为植物会快速消耗CO₂。

You need to be able to interpret graphs showing how each factor affects the rate of photosynthesis.

你需要能够解释显示每个因素如何影响光合作用速率的图表。

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7. Investigating the Rate of Photosynthesis | 探究光合作用速率

In the lab, photosynthesis can be measured using submerged aquatic plants like Cabomba or Elodea. The number of oxygen bubbles produced per minute is a simple measure of the rate.

在实验室中，可以使用水生的沉水植物（如卡波姆或金鱼藻）来测量光合作用。每分钟产生的氧气气泡数量可作为速率的简单衡量标准。

The effect of light intensity can be investigated by placing a lamp at different distances from the plant. As the distance increases, the light intensity decreases according to the inverse square law:

可以通过将灯放在距离植物不同位置来探究光照强度的影响。随着距离增加，光照强度按照平方反比定律减小：

light intensity ∝ 1 / distance²

光照强度 ∝ 1 / 距离²

For example, moving the lamp from 10 cm to 20 cm away reduces the light intensity to one quarter of its original value.

例如，将灯从10厘米移到20厘米处，光照强度会降到原来的四分之一。

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8. The Structure of a Leaf | 叶片结构

The leaf is a highly specialised organ for photosynthesis. Its structure is adapted to maximise light capture, gas exchange, and water supply.

叶片是高度特化的光合作用器官。其结构适合最大化光捕获、气体交换和水分供应。

Layer | 层次	Function | 功能
Waxy cuticle | 角质层	Prevents water loss | 防止水分蒸发
Upper epidermis | 上表皮	Thin and transparent | 薄而透明
Palisade mesophyll | 栅栏组织	Contains many chloroplasts | 含大量叶绿体
Spongy mesophyll | 海绵组织	Large air spaces for gas exchange | 大空隙利于气体交换
Stomata | 气孔	Allow CO₂ in and O₂ out | 允许CO₂进入和O₂排出

The palisade cells are elongated and packed with chloroplasts near the top of the leaf. The spongy layer allows gases to diffuse around the leaf easily.

栅栏细胞呈长形，靠近叶片顶部充满叶绿体。海绵层使气体能在叶片内轻松扩散。

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9. Mineral Requirements for Photosynthesis | 光合作用所需的矿物质

Plants need more than just glucose to stay healthy. They absorb mineral ions from the soil through their roots. Two important minerals are nitrates and magnesium.

植物不仅需要葡萄糖来维持健康。它们通过根部从土壤中吸收矿物质离子。两种重要的矿物质是硝酸盐和镁。

Nitrates are needed to make amino acids, which build proteins. Enzymes and chlorophyll are proteins, so without nitrates, plants show stunted growth and yellow leaves.

硝酸盐用于制造氨基酸，进而构建蛋白质。酶和叶绿素是蛋白质，因此缺乏硝酸盐时，植物会生长迟缓、叶片发黄。

Magnesium is a central component of chlorophyll. Without magnesium, plants cannot produce chlorophyll, so leaves become pale and photosynthesis slows down.

镁是叶绿素的核心成分。没有镁，植物无法合成叶绿素，因此叶子变苍白，光合作用减慢。

This is why fertilisers often contain both nitrogen and magnesium compounds.

这就是为什么肥料通常含有氮化合物和镁化合物。

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10. The Importance of Photosynthesis Globally | 光合作用在全球的重要性

Photosynthesis is not only essential for plants; it is essential for almost every living organism on Earth.

光合作用不仅对植物至关重要，而且对地球上的几乎所有生物都至关重要。

It produces oxygen, which is needed by all aerobic organisms for respiration. It also removes carbon dioxide from the atmosphere, helping to regulate global levels of this greenhouse gas.

它产生氧气，所有需氧生物都需要氧气进行呼吸。它还从大气中去除二氧化碳，有助于调节这种温室气体的全球水平。

Glucose from photosynthesis forms the basis of almost all food chains. Herbivores eat plants, and carnivores eat herbivores, so the energy captured by photosynthesis flows through ecosystems.

光合作用产生的葡萄糖构成了几乎所有食物链的基础。草食动物吃植物，肉食动物吃草食动物，因此光合作用捕获的能量在生态系统中流动。

Even fossil fuels such as coal and oil came from plants and algae that lived millions of years ago. Their stored energy originally came from sunlight through photosynthesis.

甚至煤炭和石油等化石燃料也来自数百万年前的植物和藻类。它们储存的能量最初是通过光合作用来自阳光。

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11. Common Exam Questions and Tips | 常见考点与答题技巧

In the Edexcel IGCSE Science exam, photosynthesis questions often appear in Paper 1 and Paper 2. You may be asked to:

在Edexcel IGCSE科学考试中，光合作用题目常出现在Paper 1和Paper 2中。你可能会被要求：

• Write the word equation or balanced symbol equation.
• Identify limiting factors from graphs.
• Describe a practical to measure the rate of photosynthesis.
• Explain how leaf structure is adapted for photosynthesis.
• Explain why plants need nitrate and magnesium ions.

• 写出文字方程式或配平的符号方程式。
• 从图表中识别限制因素。
• 描述测量光合作用速率的实验。
• 解释叶片结构如何适应光合作用。
• 解释为什么植物需要硝酸根离子和镁离子。

Make sure you use scientific vocabulary precisely: “chlorophyll absorbs light energy” is better than “chlorophyll gives green colour”.

确保精确使用科学词汇：“叶绿素吸收光能”比“叶绿素使叶子变绿”更好。

When describing experiments, always mention the control variables you would keep constant, such as temperature, CO₂ concentration, and time.

描述实验时，一定要提及需要保持不变的对照变量，如温度、CO₂浓度和时间。

Finally, remember that respiration also occurs in plants continuously, so net gas exchange depends on light conditions.

最后，请记住植物也持续进行呼吸作用，因此净气体交换取决于光照条件。

Published by TutorHao | Science Revision Series | aleveler.com

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