Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae and some bacteria use sunlight energy to convert carbon dioxide and water into glucose and oxygen. This process forms the foundation of almost all life on Earth by providing food and oxygen.

光合作用是绿色植物、藻类和某些细菌利用太阳能，将二氧化碳和水转化为葡萄糖和氧气的过程。这一过程为地球上的几乎所有生命提供了食物和氧气，是生命的基础。

1. Why Photosynthesis Matters | 光合作用的重要性

Photosynthesis is essential because it produces the organic compounds that plants use as food, which are then passed along food chains to animals and other organisms. It also releases oxygen into the atmosphere, which most living things need to respire.

光合作用至关重要，因为它能产生植物用作食物的有机化合物，这些化合物随后沿着食物链传递到动物和其他生物。光合作用还向大气释放氧气，而大多数生物都需要氧气进行呼吸。

Additionally, photosynthesis removes carbon dioxide from the atmosphere, helping to regulate the greenhouse effect and maintain a stable global climate.

此外，光合作用会从大气中吸收二氧化碳，有助于调节温室效应，维持稳定的全球气候。

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

The overall chemical change during photosynthesis can be summarised by a balanced symbol equation. Carbon dioxide and water react in the presence of light and chlorophyll to produce glucose and oxygen.

光合作用过程中的整体化学变化可以用一个平衡的符号方程式来概括。二氧化碳和水在光照和叶绿素存在的条件下反应，生成葡萄糖和氧气。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

In words: carbon dioxide + water → glucose + oxygen. The coefficients (6) ensure that the number of atoms of each element is the same on both sides.

用文字表达：二氧化碳 + 水 → 葡萄糖 + 氧气。系数（6）确保反应前后各元素的原子数目相同。

3. The Word Equation | 文字方程式

For IGCSE exams, you need to recall both the word equation and the symbol equation for photosynthesis.

在IGCSE考试中，你需要记得光合作用的文字方程式和符号方程式。

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

These equations show the reactants (carbon dioxide and water) and the products (glucose and oxygen). The reaction is endothermic because it absorbs light energy.

这些方程式显示了反应物（二氧化碳和水）以及产物（葡萄糖和氧气）。该反应是吸热的，因为它吸收光能。

4. Where Does Photosynthesis Happen? | 光合作用发生在哪里？

Photosynthesis occurs inside chloroplasts, which are organelles found mainly in the palisade mesophyll layer of leaves. Chloroplasts contain chlorophyll, a green pigment that absorbs light energy.

光合作用发生在叶绿体内部，叶绿体是主要存在于叶片栅栏组织层中的细胞器。叶绿体含有叶绿素，这是一种吸收光能的绿色色素。

The structure of a chloroplast is highly adapted: the thylakoid membranes increase the surface area for light absorption, and the stroma contains enzymes for the Calvin cycle (though this detail is only needed at A-level).

叶绿体的结构高度适应其功能：类囊体膜增大了吸收光能的表面积，基质中含有卡尔文循环所需的酶（但这一细节仅需要在A-level阶段掌握）。

5. The Structure of a Leaf | 叶片的结构

Leaves are organs specialised for photosynthesis. They have several adaptations that allow them to absorb light and exchange gases efficiently.

叶片是专门进行光合作用的器官。它们具有多种适应性结构，能够高效地吸收光和进行气体交换。

Waxy cuticle – reduces water loss but is transparent to light.

蜡质角质层 – 减少水分蒸发，但能让光透过。
Upper epidermis – thin and transparent, allowing light to reach the palisade layer.

上表皮 – 薄而透明，使光能到达栅栏组织层。
Palisade mesophyll – closely packed cells full of chloroplasts, where most photosynthesis occurs.

栅栏组织 – 细胞排列紧密，富含叶绿体，是光合作用的主要场所。
Spongy mesophyll – contains air spaces for gas diffusion.

海绵组织 – 含有气室，便于气体扩散。
Stomata – pores in the lower epidermis that allow CO₂ in and O₂ out.

气孔 – 下表皮上的微小孔隙，允许二氧化碳进入、氧气排出。

6. Chlorophyll and Light Absorption | 叶绿素与光吸收

Chlorophyll absorbs red and blue light most strongly, and reflects green light, which is why leaves appear green. The absorbed light energy is used to split water molecules and to generate ATP.

叶绿素对红光和蓝光的吸收最强，而反射绿光，因此叶片呈现绿色。吸收的光能用于分解水分子并生成ATP。

Some plants have leaves with different pigments, such as carotene, which can absorb different wavelengths of light. This increases the range of light that can be used for photosynthesis.

有些植物的叶片含有不同的色素，如胡萝卜素，可以吸收不同波长的光，这增加了可用于光合作用的光谱范围。

7. Factors Affecting Photosynthesis | 影响光合作用的因素

The rate of photosynthesis can be affected by several environmental factors. The most important ones are light intensity, carbon dioxide concentration and temperature.

光合作用的速率可能受到多种环境因素的影响。其中最重要的是光照强度、二氧化碳浓度和温度。

Light intensity: As light intensity increases, the rate rises until another factor becomes limiting.

光照强度：随着光照强度增加，光合速率上升，直到另一个因素成为限制因素。
Carbon dioxide concentration: Increasing CO₂ concentration generally speeds up photosynthesis until the stomata reach their limit.

二氧化碳浓度：增加二氧化碳浓度通常会加快光合作用，直到气孔达到其扩散极限。
Temperature: Photosynthesis is controlled by enzymes, which work best at an optimum temperature (about 25–35°C for most plants). Above this, the rate drops as enzymes denature.

温度：光合作用受酶控制，酶在适宜温度（大多数植物约25–35°C）下活性最高。超过这一温度，酶会变性，速率下降。

8. Limiting Factors | 限制因子

A limiting factor is a condition that prevents the rate of photosynthesis from increasing, even when other factors are available in surpluss. In any particular situation, only one factor is usually limiting at a time.

限制因子是指即使其他条件充足，也阻止光合作用速率继续提高的因素。在特定情况下，通常只有一个因素处于限制状态。

For example, at night light intensity is the limiting factor; in winter temperature may be limiting; in closed greenhouses farmers may enrich the air with CO₂ to avoid that factor limiting growth.

例如，在夜间，光照强度是限制因子；在冬季，温度可能是限制因子；在封闭的温室中，农民可能会增加空气中的二氧化碳浓度，以避免该因素限制植物生长。

The concept of limiting factors is often tested with graphs. A typical graph shows the rate levelling off as one factor increases, indicating that another factor has become limiting.

限制因子的概念经常通过曲线图来考查。典型曲线显示，随着某一因素增加，速率趋于平稳，说明另一个因素已成为限制因子。

9. Investigating Photosynthesis | 探究光合作用

There are two common experiments for IGCSE:

IGCSE中有两个常见实验：

Testing a leaf for starch: A destarched plant is placed in sunlight for a few hours. A leaf is then boiled in ethanol to remove chlorophyll, washed, and iodine solution is added. A blue-black colour confirms starch was produced.

检验叶片中的淀粉：将经过暗处理（去除淀粉）的植物放在阳光下数小时。随后将叶片放入乙醇中煮沸以除去叶绿素，冲洗后滴加碘液。出现蓝黑色说明产生了淀粉。
Pondweed experiment: A piece of Canadian pondweed is placed in a beaker of water with a lamp at different distances. The number of oxygen bubbles released per minute is counted as a measure of the rate of photosynthesis.

金鱼藻实验：将一段金鱼藻放入装有水的烧杯中，将灯放置在离水藻不同距离处。通过计算每分钟释放的氧气气泡数量，来测量光合作用速率。

Both experiments show that light intensity and CO₂ concentration affect photosynthesis, and they allow you to plot rate against the independent variable.

这两个实验都表明光照强度和二氧化碳浓度会影响光合作用，并且可以让你绘制速率随自变量变化的曲线图。

10. The Uses of Glucose | 葡萄糖的用途

Glucose produced during photosynthesis is used by plants in several ways:

光合作用产生的葡萄糖在植物体内有多种用途：

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

呼吸作用：葡萄糖被分解以释放能量，供细胞活动使用。
Starch: Excess glucose is stored as insoluble starch in leaves, roots and stems.

淀粉：多余的葡萄糖以不溶性淀粉的形式储存于叶片、根和茎中。
Sucrose: Glucose is converted to sucrose and transported through the phloem to other parts of the plant.

蔗糖：葡萄糖可转化为蔗糖，通过韧皮部运输到植物其他部位。
Cellulose: Glucose molecules join together to form cellulose, a key component of cell walls.

纤维素：葡萄糖分子连接形成纤维素，这是细胞壁的重要成分。
Proteins: Combined with nitrate ions absorbed from the soil, glucose can be used to build amino acids and proteins.

蛋白质：葡萄糖与从土壤中吸收的硝酸根离子结合，可用于合成氨基酸和蛋白质。

11. Summary | 总结

Photosynthesis is a vital endothermic reaction that converts light energy into chemical energy stored in glucose. It involves the raw materials carbon dioxide and water, and produces glucose and oxygen as by-products.

光合作用是一个重要的吸热反应，将光能转化为储存在葡萄糖中的化学能。它利用二氧化碳和水作为原料，并产生葡萄糖和氧气作为副产物。

The process happens in chloroplasts, is limited by light, CO₂ concentration and temperature, and provides food and oxygen for almost all living organisms.

这个过程发生在叶绿体中，其速率受光照、二氧化碳浓度和温度的限制，并为几乎所有生物提供食物和氧气。

Remember the equation, the leaf adaptations, and the factors that affect photosynthesis – these are key skills required for Edexcel IGCSE Science exams.

记住方程式、叶片适应性以及影响光合作用的因素——这些是Edexcel IGCSE科学考试中需要掌握的关键技能。

Published by TutorHao | Science Revision Series | aleveler.com

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Photosynthesis | 光合作用