Photosynthesis: Energy Capture in Plants | 光合作用：植物中的能量捕获

📚 Photosynthesis: Energy Capture in Plants | 光合作用：植物中的能量捕获

Photosynthesis is the most important chemical reaction on Earth. It is the process by which green plants, algae and some bacteria use sunlight to convert carbon dioxide and water into glucose and oxygen. This reaction forms the foundation of almost every food chain and supplies the oxygen in the air we breathe.

光合作用是地球上最重要的化学反应。它是绿色植物、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过程。这一反应构成了几乎所有食物链的基础，并为我们呼吸的空气提供氧气。

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1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis is an endothermic reaction: it takes in energy from the surroundings in the form of light. The light energy is absorbed by a green pigment called chlorophyll, which is found in the chloroplasts of plant cells, particularly in the palisade mesophyll layer of leaves.

光合作用是一种吸热反应：它以光的形式从周围环境中吸收能量。光能被一种叫做叶绿素的绿色色素吸收，叶绿素存在于植物细胞的叶绿体中，尤其是叶片栅栏组织层中。

During the reaction, light energy is converted into chemical energy stored inside glucose molecules. This glucose is then used by the plant for respiration, for building biomass (such as cellulose and starch), and for making other substances like amino acids and proteins.

在反应过程中，光能被转化为化学能，储存在葡萄糖分子中。随后，植物利用这些葡萄糖进行呼吸作用、构建生物质（如纤维素和淀粉），并合成氨基酸和蛋白质等其他物质。

Because the product glucose contains more stored energy than the reactants, photosynthesis requires a continuous input of energy from sunlight to proceed.

由于产物葡萄糖所含的储存能量多于反应物，光合作用需要持续从阳光中输入能量才能进行。

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2. The Word Equation | 文字方程式

The word equation summarises photosynthesis in a simple way. It shows which substances are used up and which substances are produced.

文字方程式以简单的方式概括了光合作用，标明哪些物质被消耗、哪些物质被生成。

Carbon dioxide + Water → Glucose + Oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

The arrow means ‘using light energy and chlorophyll’. The reactants enter the leaf from the air (carbon dioxide through stomata) and the roots (water through root hair cells), while the products leave or stay in the plant (oxygen diffuses out; glucose is stored or used).

箭头上方隐含着“利用光能和叶绿素”的条件。反应物进入叶片（二氧化碳通过气孔）和根部（水通过根毛细胞），而产物则离开或留在植物体内（氧气扩散出去；葡萄糖被储存或利用）。

A more precise version of the word equation includes the condition above or below the arrow: ‘Carbon dioxide + Water → (light energy, chlorophyll) Glucose + Oxygen’. Examiners expect you to mention both light and chlorophyll as necessary conditions.

更精确的文字方程式会在箭头旁注明条件：“二氧化碳 + 水 →（在光能和叶绿素作用下）葡萄糖 + 氧气”。考官希望你同时提到光和叶绿素这两个必要条件。

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

For IGCSE Edexcel Science, you must be able to recall the balanced symbol equation for photosynthesis and use it in calculations of mass or gas volumes.

在 IGCSE Edexcel 科学课程中，你必须能够记住光合作用的平衡符号方程式，并运用它进行质量或气体体积的计算。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

The numbers before the formulas (6, 1, 1, 6) are balancing coefficients. There are 6 carbon atoms, 12 hydrogen atoms and 18 oxygen atoms on each side of the arrow, so the equation is balanced. The subscript numbers inside a formula, such as CO₂, cannot be changed.

化学式前的数字（6、1、1、6）是配平系数。箭头两侧各有6个碳原子、12个氢原子和18个氧原子，因此方程式是平衡的。化学式内部的角标数字（如 CO₂）不能更改。

Note that water appears as a reactant in the overall equation, although some of the oxygen gas released actually comes from the splitting of water molecules during the light-dependent stage. This is a common exam trap: the oxygen produced in photosynthesis comes from water, not from carbon dioxide.

注意，虽然总方程式中水是反应物，但实际释放的氧气部分来自光反应阶段水分子的分解。这是一个常见的考试陷阱：光合作用产生的氧气来自水，而不是二氧化碳。

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4. Requirements of Photosynthesis | 光合作用的条件

For photosynthesis to occur, four key requirements must be present at the same time. A lack of any one of them will slow or stop the process.

光合作用要发生，必须同时具备四个关键条件。缺少任何一个条件都会减慢或停止这一过程。

• Light energy: absorbed by chlorophyll and used to drive the reaction. Natural sunlight or artificial light both work.

  光能：由叶绿素吸收并驱动反应。自然阳光或人造光均可。

• Carbon dioxide: enters the leaf through stomata by diffusion. It is the carbon source for glucose.

  二氧化碳：通过气孔扩散进入叶片，是合成葡萄糖的碳源。

• Water: absorbed by root hair cells and transported up the xylem. It supplies hydrogen and releases oxygen.

  水：由根毛细胞吸收，经木质部向上运输，提供氢并释放氧气。

• Chlorophyll: the green pigment located in chloroplasts, which traps light energy. Without chlorophyll, photosynthesis cannot start.

  叶绿素：位于叶绿体中的绿色色素，负责捕获光能。没有叶绿素，光合作用无法启动。

Temperature is also important because photosynthesis is controlled by enzymes, but it is not consumed in the reaction. The table below summarises the role of each factor.

温度也很重要，因为光合作用受酶控制，但酶并不在反应中被消耗。下表总结了各因素的作用。

Factor | 因素	Role in Photosynthesis | 在光合作用中的作用
Light | 光	Provides energy to combine CO₂ and H₂O | 提供能量将二氧化碳和水合成
Chlorophyll | 叶绿素	Absorbs light energy; transfers it to the reaction | 吸收光能并将其传递给反应
Carbon dioxide | 二氧化碳	Source of carbon atoms for glucose | 葡萄糖中碳原子的来源
Water | 水	Source of hydrogen; source of released oxygen | 氢的来源；也是释放氧气的来源

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

Photosynthesis is essential for life on Earth for several distinct reasons, and these reasons are frequently tested in structured questions.

光合作用对地球生命至关重要，原因有多个方面，这些原因也常常出现在结构化考题中。

• Food production: glucose produced by plants is the starting point of all food chains. Herbivores eat plants; carnivores eat herbivores. Even humans rely directly or indirectly on plant carbohydrates.

  食物生产：植物产生的葡萄糖是所有食物链的起点。草食动物吃植物，肉食动物吃草食动物。人类也直接或间接依赖植物的碳水化合物。

• Oxygen supply: photosynthesis releases oxygen into the atmosphere. This oxygen is used by almost all organisms for aerobic respiration.

  氧气供应：光合作用向大气释放氧气。几乎所有生物都利用这些氧气进行有氧呼吸。

• Carbon dioxide removal: photosynthesis removes CO₂ from the atmosphere, helping to regulate the carbon cycle and reduce the greenhouse effect.

  二氧化碳清除：光合作用从大气中移除二氧化碳，有助于调节碳循环并缓解温室效应。

• Biomass and fossil fuels: the carbon fixed in glucose becomes plant biomass, which can later form fossil fuels such as coal over millions of years.

  生物质和化石燃料：固定在葡萄糖中的碳转变为植物生物质，经过数百万年可形成煤等化石燃料。

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6. Limiting Factors: The Core Concept | 限制因素：核心概念

A limiting factor is any condition that is in shortest supply and therefore controls the rate of a reaction. When the limiting factor is increased, the rate increases; when another factor becomes the new limiting factor, the rate stops rising.

限制因素是指供应不足、从而控制反应速率的任何条件。当限制因素提高时，速率随之提高；当另一个因素成为新的限制因素时，速率便不再上升。

For photosynthesis, the three main limiting factors are light intensity, carbon dioxide concentration and temperature. These are tested together on a single graph, and you must be able to interpret the plateau.

光合作用的三大限制因素是光照强度、二氧化碳浓度和温度。它们常常画在同一张图上考查，你必须能够解释曲线趋于平缓的部分。

Rate of photosynthesis = f(light, CO₂, temperature)

光合作用速率 = f(光、二氧化碳、温度)

At the plateau of a graph, the listed factor is no longer limiting: some other factor has taken over as the limiting factor. For example, if light intensity is doubled but the rate stays constant, then light is not the limiting factor anymore.

在曲线的平台区，所列因素已不再是限制因素：其他因素接管了限制作用。例如，若光照强度加倍但速率保持不变，则光已不再是限制因素。

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7. Light Intensity and the Inverse Square Law | 光照强度与平方反比定律

Light intensity strongly affects the rate of photosynthesis. In the laboratory, we usually change light intensity by moving a lamp closer to or further away from the plant.

光照强度强烈影响光合作用速率。在实验室中，我们通常通过移动灯与植物之间的距离来改变光照强度。

As the distance between a plant and a light source increases, the light intensity decreases. The relationship is not linear: doubling the distance makes the light intensity four times weaker.

随着植物与光源之间距离的增加，光照强度下降。这种关系并非线性：距离加倍会使光照强度减弱为原来的四分之一。

light intensity ∝ 1/d²

光照强度 ∝ 1/d²

Here, d is the distance from the lamp to the plant. This is called the inverse square law. If the distance is tripled, the light intensity falls to one ninth (1/9) of the original value. You should be able to carry out such calculations.

这里的 d 是灯到植物之间的距离。这称为平方反比定律。若距离增至三倍，光照强度降至原来的九分之一（1/9）。你应该能进行此类计算。

• At low light intensity, light is the limiting factor: increasing light raises the rate proportionally.

  光照强度较低时，光是限制因素：增加光照，速率按比例提高。

• At high light intensity, the rate plateaus because temperature or CO₂ becomes limiting.

  光照强度较高时，速率趋于平缓，因为温度或二氧化碳成为限制因素。

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8. Temperature and Enzyme Activity | 温度与酶活性

Photosynthesis involves many enzyme-controlled reactions, particularly the Calvin cycle stages that build glucose. As temperature increases, kinetic energy increases, so enzymes work faster and the rate rises.

光合作用涉及许多酶促反应，尤其是合成葡萄糖的卡尔文循环阶段。随着温度升高，分子动能增加，酶的催化速度加快，反应速率上升。

Most plants have an optimum temperature for photosynthesis between 25 °C and 35 °C. Above this optimum, enzymes begin to denature: the active site changes shape irreversibly and the rate falls sharply.

大多数植物光合作用的最适温度在 25 °C 到 35 °C 之间。超过最适温度后，酶开始变性：活性位点发生不可逆的形状改变，速率急剧下降。

• Below optimum: rate doubles approximately for every 10 °C rise, until limited by other factors.

  低于最适温度：每升高 10 °C，速率大约翻倍，直到受其他因素限制。

• Above optimum: denaturation of enzymes (especially RuBisCO) causes the rate to drop steeply.

  高于最适温度：酶（尤其是 RuBisCO）变性导致速率骤降。

• Very high temperatures (above about 45 °C) stop photosynthesis almost completely.

  极高温度（约 45 °C 以上）几乎完全停止光合作用。

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9. Carbon Dioxide Concentration | 二氧化碳浓度

Carbon dioxide is the raw material for glucose synthesis, so its concentration directly affects the rate in normal conditions. Atmospheric air contains only about 0.04% CO₂; therefore CO₂ is often the limiting factor in nature.

二氧化碳是合成葡萄糖的原料，因此在正常条件下其浓度直接影响速率。空气仅含约 0.04% 的二氧化碳，因此自然界中二氧化碳常常是限制因素。

Increasing CO₂ concentration causes the rate to increase steeply at first, then level off. When CO₂ is abundant, light or temperature becomes the limiting factor.

提高二氧化碳浓度时，速率起初急剧上升，随后趋于平缓。当二氧化碳充足时，光或温度变成限制因素。

In commercial greenhouses, growers may burn paraffin in a heater: this releases both heat and carbon dioxide, boosting the photosynthetic rate and increasing crop yield. This is a classic ‘application of biology’ question.

在商业温室中，种植者可能会燃烧石蜡加热器：这同时释放热量和二氧化碳，从而提高光合作用速率、增加作物产量。这是经典的“生物学应用”考题。

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10. Chlorophyll and Mineral Deficiency | 叶绿素与矿质缺乏

Plants need certain minerals from the soil to make chlorophyll. A shortage of these minerals reduces the amount of chlorophyll, and so reduces the rate of photosynthesis.

植物需要从土壤中吸收某些矿物质才能合成叶绿素。缺乏这些矿物质会减少叶绿素含量，从而降低光合作用速率。

Magnesium ions (Mg²⁺) are essential for chlorophyll synthesis. Nitrate ions (NO₃⁻) are needed to make amino acids and proteins, including the enzymes used in photosynthesis. A plant lacking magnesium shows yellowing of leaves, a condition called chlorosis.

镁离子（Mg²⁺）是合成叶绿素所必需的。硝酸根离子（NO₃⁻）用于合成氨基酸和蛋白质，包括光合作用所需的酶。缺镁的植物叶片变黄，这种症状称为缺绿病（黄化病）。

• Magnesium deficiency: chlorophyll cannot be produced; leaves turn yellow; photosynthesis slows.

  缺镁：无法产生叶绿素；叶片变黄；光合作用减慢。

• Nitrate deficiency: protein and enzyme synthesis fails; plant growth is stunted; old leaves yellow first.

  缺氮（硝酸根）：蛋白质和酶合成受阻；植株生长矮小；老叶先变黄。

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11. Investigating the Rate: Pondweed Experiment | 测定速率：金鱼藻实验

The classic IGCSE experiment uses pondweed (Elodea or Cabomba) to measure the rate of photosynthesis by counting oxygen bubbles released per minute, or by collecting the gas in a measuring cylinder.

IGCSE 经典实验使用金鱼藻（Elodea 或 Cabomba）测定光合作用速率，方法是每分钟计数释放的氧气气泡数，或用量筒收集释放的气体。

• Place a lamp at a fixed distance from the pondweed; count bubbles for one minute and record the result.

  将灯放置在距金鱼藻固定距离处；计数一分钟内的气泡数并记录结果。

• Move the lamp further away (increasing distance d) and repeat. Calculate light intensity using 1/d².

  将灯移远（增大距离 d）并重复实验。用 1/d² 计算光照强度。

• Control variables: keep temperature constant, keep the concentration of sodium hydrogencarbonate (NaHCO₃) solution constant to provide CO₂, and use the same plant.

  控制变量：保持温度恒定、保持碳酸氢钠（NaHCO₃）溶液的浓度恒定以提供二氧化碳，并使用同一株植物。

• If using bubbles, results are only qualitative. Collecting gas with a syringe or data logger gives precise volume measurements.

  若用气泡计数，结果仅为定性数据。使用注射器或数据记录仪收集气体可获得精确的体积测量值。

In the exam, you may be asked to identify a control variable or to explain why the bubble count is not completely accurate (bubble size varies, oxygen dissolves in water, etc.).

在考试中，你可能会被要求指出控制变量，或解释为何气泡计数不完全准确（气泡大小不一、氧气溶于水等因素）。

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12. Practical Applications: Greenhouses | 实际应用：温室

Knowledge of limiting factors is used commercially to maximise plant growth and crop yield in greenhouses. Each artificial control targets a specific limiting factor.

限制因素的知识被商业地用于温室中最大化植物生长和作物产量。每一项人工控制都针对特定的限制因素。

Greenhouse Control | 温室调控	Limiting Factor Addressed | 应对的限制因素
Artificial lamps | 人工光源	Light intensity, especially in winter or at night | 光照强度，尤其是冬季或夜间
Paraffin heaters | 石蜡加热器	Temperature and CO₂ concentration | 温度和二氧化碳浓度
Ventilation / fans | 通风/风扇	Maintains CO₂ supply and prevents overheating | 维持二氧化碳供应并防止过热
Watering systems | 灌溉系统	Water availability | 水分供应

Farmers must weigh the cost of these controls against the increase in yield. For example, extra lighting is only worthwhile if light is currently the limiting factor; heating is only worthwhile if temperature is the limiting factor. This is termed ‘economic yield’ and appears in exam contexts.

农民必须权衡这些控制的成本与产量增加的收益。例如，额外照明仅在当前光为限制因素时才值得；加热仅在温度为限制因素时才值得。这称为“经济产出”，常出现在考题情境中。

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