Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

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 processes on Earth and a central topic in the Edexcel IGCSE Science specification.

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过程之一，也是 Edexcel IGCSE 科学课程的核心主题。

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1. What is Photosynthesis? | 什么是光合作用？

Photosynthesis takes place mainly in the leaves of green plants, inside organelles called chloroplasts. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy, usually from the Sun.

光合作用主要发生在绿色植物的叶片中，具体位置是称为叶绿体的细胞器内。叶绿体含有绿色色素叶绿素，它吸收光能（通常来自太阳）。

During photosynthesis, carbon dioxide and water are combined to produce glucose and oxygen. The glucose is used by the plant for respiration or converted into starch, cellulose, and other organic molecules.

在光合作用过程中，二氧化碳和水结合生成葡萄糖和氧气。植物利用葡萄糖进行呼吸作用，或将其转化为淀粉、纤维素和其他有机分子。

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2. The Word and Symbol Equations | 文字与符号方程式

It is essential to know both the word equation and the balanced chemical symbol equation for photosynthesis. These appear frequently in Edexcel IGCSE papers.

掌握光合作用的文字方程式和配平的化学符号方程式至关重要。这两者在 Edexcel IGCSE 试卷中经常出现。

carbon dioxide + water → glucose + oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The balanced symbol equation uses the formula of glucose, C₆H₁₂O₆:

配平的符号方程式使用葡萄糖的分子式 C₆H₁₂O₆：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Note that light energy is required for this reaction, so it is often written above the arrow. In the exam, you must be able to write this equation from memory.

注意该反应需要光能，因此通常写在箭头上方。在考试中，你必须能够凭记忆写出这个方程式。

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

Photosynthesis is important for two main reasons: it makes glucose for the plant, and it releases oxygen into the atmosphere.

光合作用之所以重要主要有两个原因：它为植物制造葡萄糖，并向大气释放氧气。

• It is the primary source of organic matter for almost all food chains.

  它是几乎所有食物链中有机物质的主要来源。

• It removes carbon dioxide from the atmosphere, helping to regulate the global carbon cycle.

  它从大气中吸收二氧化碳，有助于调节全球碳循环。

• The oxygen produced is used by most living organisms for aerobic respiration.

  产生的氧气被大多数生物用于有氧呼吸。

Additionally, the glucose made during photosynthesis can be stored as starch, used to make cellulose for cell walls, or combined with nitrate ions to form amino acids and proteins.

此外，光合作用制造的葡萄糖可以储存为淀粉，用于构建细胞壁的纤维素，或与硝酸根离子结合形成氨基酸和蛋白质。

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4. Leaf Structure and Adaptation | 叶片结构与适应性

The internal structure of a leaf is highly adapted for efficient photosynthesis. Each tissue has a specific function.

叶片内部结构高度适应高效光合作用。每种组织都有特定功能。

Structure | 结构	Adaptation | 适应性功能
Waxy cuticle | 角质层	Reduces water loss / 减少水分蒸发
Upper epidermis | 上表皮	Thin and transparent to let light through / 薄而透明，便于透光
Palisade mesophyll | 栅栏组织	Tightly packed cells with many chloroplasts / 细胞排列紧密，含大量叶绿体
Spongy mesophyll | 海绵组织	Air spaces for gas diffusion / 有气隙便于气体扩散
Stomata and guard cells | 气孔和保卫细胞	Allow gas exchange and control water loss / 允许气体交换并控制水分蒸发

The palisade layer is found near the upper surface of the leaf, where light is strongest. The chloroplasts can also move within the cells to maximise light absorption.

栅栏组织位于叶片上表面附近，那里光线最强。叶绿体还可以在细胞内移动以最大化光吸收。

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5. Limiting Factors | 限制因素

A limiting factor is something that restricts the rate of a reaction when it is in short supply. For photosynthesis, the main limiting factors are light intensity, carbon dioxide concentration, and temperature.

限制因素是指当某种条件供应不足时，会限制反应速率的因素。对于光合作用，主要的限制因素是光强度、二氧化碳浓度和温度。

If one factor is increased while all others remain constant, the rate of photosynthesis increases until that factor is no longer limiting. Then another factor becomes limiting.

如果保持其他条件不变，仅增加某一个因素，光合作用速率会上升，直到该因素不再成为限制因素。此时另一个因素会成为新的限制因素。

In an exam, you may be asked to identify the limiting factor from a graph. The rule is simple: if the graph plateaus, the factor that is not being increased is limiting.

在考试中，你可能需要从图中识别限制因素。规则很简单：如果曲线趋于平缓，那么未被增加的那个因素就是限制因素。

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6. Light Intensity and Rate | 光强度与速率

As light intensity increases, the rate of photosynthesis increases proportionally, provided that other factors are not limiting. This is because more light energy allows more chlorophyll molecules to become excited.

随着光强度增加，光合作用速率成比例增加，前提是其他因素不构成限制。这是因为更多的光能使更多叶绿素分子被激发。

However, beyond a certain point, the rate stops increasing because light is no longer the limiting factor. At very high intensities, the rate may even decrease due to damage to chloroplasts or excessive heat.

然而，超过某个点后，速率停止增加，因为光不再是限制因素。在极高光强度下，由于叶绿体受损或过热，速率甚至可能下降。

The relationship between light intensity and distance from a light source is important in experiments. Light intensity is inversely proportional to the square of the distance:

在实验中，光强度与光源距离之间的关系很重要。光强度与距离的平方成反比：

light intensity ∝ 1/d²

光强度 ∝ 1/d²

This means if you double the distance, the light intensity becomes one quarter of its original value.

这意味着如果你将距离加倍，光强度会变为原来的四分之一。

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7. Carbon Dioxide Concentration and Temperature | 二氧化碳浓度与温度

Carbon dioxide is one of the raw materials for photosynthesis. Increasing CO₂ concentration from low to moderate levels raises the rate of photosynthesis. Eventually, the rate levels off as other factors become limiting.

二氧化碳是光合作用的原料之一。将二氧化碳浓度从低水平增加到中等水平会提高光合作用速率。最终，当其他因素成为限制因素时，速率会趋于平稳。

Temperature affects the rate of photosynthesis through its effect on enzymes. Each enzyme has an optimum temperature. For most plants, this is around 25–30 °C.

温度通过影响酶来影响光合作用速率。每种酶都有一个最适温度。对大多数植物而言，最适温度约为 25–30 °C。

As temperature increases, molecular collisions become more frequent, so the rate increases. Above the optimum, enzymes begin to denature. Denaturation changes the shape of the active site, so the enzyme no longer works. At very high temperatures, photosynthesis stops completely.

随着温度升高，分子碰撞更加频繁，因此速率增加。超过最适温度后，酶开始变性。变性改变了活性位点的形状，使酶不再起作用。在极高温度下，光合作用完全停止。

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8. Chlorophyll and Mineral Requirements | 叶绿素与矿质需求

Chlorophyll is the pigment that absorbs light energy. Plants cannot make chlorophyll without certain mineral ions, especially magnesium ions (Mg²⁺) and nitrate ions (NO₃⁻).

叶绿素是吸收光能的色素。植物没有某些矿质离子，特别是镁离子（Mg²⁺）和硝酸根离子（NO₃⁻），就无法合成叶绿素。

• Magnesium is a central component of the chlorophyll molecule.

  镁是叶绿素分子的中心成分。

• Nitrogen is needed for the synthesis of proteins and nucleic acids, and it is also part of chlorophyll.

  氮用于合成蛋白质和核酸，同时也是叶绿素的组成部分。

A plant deficient in magnesium will show chlorosis, which is yellowing of the leaves. This reduces the plant’s ability to photosynthesise, so growth is stunted.

缺镁的植物会出现失绿症，即叶片变黄。这会降低植物进行光合作用的能力，导致生长受阻。

Nitrate deficiency also causes poor growth and yellowing of older leaves, because nitrogen is mobile and can be moved to younger leaves.

缺氮同样会导致生长不良和老叶变黄，因为氮在植物体内可移动，会被转运到幼叶中。

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9. Investigating Photosynthesis | 探究光合作用

In the Edexcel IGCSE course, you need to know how to investigate the effect of light intensity or carbon dioxide concentration on the rate of photosynthesis. A common method uses an aquatic plant such as Cabomba or Elodea.

在 Edexcel IGCSE 课程中，你需要知道如何探究光强度或二氧化碳浓度对光合作用速率的影响。常用方法使用水生植物，例如水蕴草或伊乐藻。

The plant is placed in water with dissolved sodium hydrogencarbonate (which releases CO₂). A lamp is placed at different distances, and the number of oxygen bubbles released per minute is counted. The closer the lamp, the higher the light intensity.

将植物放入含有溶解碳酸氢钠（可释放 CO₂）的水中。将灯放在不同距离处，计算每分钟释放的氧气气泡数量。灯越近，光强度越高。

Alternatively, the volume of oxygen collected in a gas syringe or measuring tube can be recorded over a fixed time. The rate is usually calculated as:

或者，可以记录在规定时间内收集在气筒或量管中的氧气体积。速率通常计算为：

rate = volume of oxygen / time

速率 = 氧气体积 / 时间

Remember to keep temperature constant and to repeat measurements for reliability. In a controlled experiment, only one variable should be changed at a time.

记得保持温度不变，并重复测量以确保可靠性。在受控实验中，每次只能改变一个变量。

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10. Agricultural Applications | 农业应用

Farmers and gardeners can increase crop yield by controlling the limiting factors of photosynthesis. In greenhouses, this is easier than in open fields.

农民和园丁可以通过控制光合作用的限制因素来提高作物产量。在温室中，这比在开阔田地更容易实现。

• Adding artificial lighting extends the hours during which photosynthesis can occur.

  增加人工照明可延长光合作用进行的时间。

• Burning a paraffin heater provides both heat and carbon dioxide, which can increase the rate of photosynthesis in winter.

  燃烧石蜡加热器既能提供热量，又能提供二氧化碳，可在冬季提高光合作用速率。

• Maintaining an optimal temperature using ventilation or heating systems prevents enzymatic inefficiency.

  通过通风或加热系统维持最适温度，可防止酶效率低下。

• Applying fertilisers containing magnesium and nitrate ensures healthy chlorophyll production.

  施用含镁和硝酸盐的肥料，可确保叶绿素正常生成。

However, increasing one factor too far may make another factor limiting. For example, very high temperatures cause stomata to close, reducing CO₂ uptake and increasing water loss.

然而，过度增加某一因素可能会使另一因素成为限制因素。例如，极高温度会导致气孔关闭，减少 CO₂ 吸收并增加水分流失。

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