Photosynthesis: The Green Machine | 光合作用：绿色工厂

📚 Photosynthesis: The Green Machine | 光合作用：绿色工厂

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. It provides the oxygen we breathe and the food nearly all living things depend on. For Edexcel IGCSE Science, you must understand the equation, the leaf adaptations, the factors that affect the rate, and the practical investigations used to measure it.

光合作用是绿色植物、藻类和一些细菌将光能转化为储存在葡萄糖中的化学能的过程。它提供了我们呼吸的氧气和几乎所有生物赖以生存的食物。在爱德思 IGCSE 科学考试中，你必须掌握其化学方程式、叶片结构适应性、影响速率的因素以及相关实验方法。

1. What Is Photosynthesis? | 什么是光合作用？

Photosynthesis takes place mainly in the leaves of green plants. Plants capture sunlight using a green pigment called chlorophyll, which is found in chloroplasts. The energy from sunlight is used to combine carbon dioxide and water to make glucose, a simple sugar that acts as a fuel and building material for the plant.

光合作用主要发生在绿色植物的叶片中。植物利用一种称为叶绿素的绿色色素来捕获阳光，叶绿素位于叶绿体内。来自阳光的能量被用来将二氧化碳和水合成为葡萄糖——一种为植物提供能量和构建材料的单糖。

The word ‘photosynthesis’ comes from Greek roots: ‘photo’ means light and ‘synthesis’ means making or building. The reaction is endothermic, meaning it absorbs energy from the surroundings. Without this energy input, the reaction simply would not happen.

“光合作用”一词源自希腊语词根：”photo”意为光，”synthesis”意为合成。该反应是吸热反应，即从周围环境吸收能量。没有能量输入，这个反应根本不会发生。

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

You must be able to write both the word equation and the balanced symbol equation for photosynthesis. This is one of the most frequently tested recall points in the Edexcel IGCSE examination.

你必须能够写出光合作用的文字方程式和配平的符号方程式。这是爱德思 IGCSE 考试中最高频的默写考点之一。

First, the word equation:

Carbon dioxide + Water →(light energy, chlorophyll)→ Glucose + Oxygen

The reactants are carbon dioxide and water. The products are glucose and oxygen. Light energy is the energy source and chlorophyll is the catalyst-like substance that makes the reaction possible.

反应物是二氧化碳和水，产物是葡萄糖和氧气。光能是能量来源，叶绿素是使反应得以发生的类催化物质。

Second, the balanced symbol equation:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Note that six carbon dioxide molecules and six water molecules produce one glucose molecule and six oxygen molecules. The arrow is usually labelled with ‘light energy’ above it and ‘chlorophyll’ below it. Count the atoms on both sides to check balance: 6 carbons, 12 hydrogens and 18 oxygens appear on each side.

请注意：六个二氧化碳分子和六个水分子生成一个葡萄糖分子和六个氧分子。箭头通常上方标注”光能”，下方标注”叶绿素”。数一数两侧的原子数来验证配平：每侧都有 6 个碳原子、12 个氢原子和 18 个氧原子。

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

A leaf is a natural solar panel. Its structure is highly adapted to maximise the rate of photosynthesis. Each layer of the leaf has a specific job, and examiners often ask you to link structure to function.

叶片是一片天然的太阳能电池板。它的结构高度适应于最大化光合作用速率。叶片的每一层都有特定功能，考官经常要求你将结构与功能联系起来。

Waxy cuticle: a transparent waterproof layer that reduces water loss without blocking light.

蜡质角质层：透明的防水层，减少水分流失而不阻挡光线。
Upper epidermis: a thin, transparent layer that lets light pass through to the palisade cells.

上表皮：薄而透明的细胞层，让光线透过到达栅栏细胞。
Palisade mesophyll: tall, tightly packed cells full of chloroplasts. Most photosynthesis happens here because it is closest to the light.

栅栏叶肉：高大且紧密排列的细胞，富含叶绿体。这里是光合作用发生最多的地方，因为它最接近光源。
Spongy mesophyll: loosely packed cells with large air spaces, allowing carbon dioxide to diffuse freely through the leaf.

海绵叶肉：排列疏松的细胞，具有较大的气隙，使二氧化碳能够在叶片中自由扩散。
Stomata and guard cells: pores on the lower surface that let CO₂ enter and O₂ leave; guard cells control their opening.

气孔和保卫细胞：下表面的小孔，允许 CO₂ 进入、O₂ 逸出；保卫细胞控制其开闭。

The upper surface of a leaf usually receives the most sunlight, which explains why palisade cells are concentrated there. Guard cells respond to light and water availability: they open during the day and close at night or when the plant is short of water.

叶片的上表面通常接收最多的阳光，这就是栅栏细胞集中在那里的原因。保卫细胞对光照和水分状况作出反应：白天开放，夜间或植物缺水时关闭。

4. Light Intensity: A Key Variable | 光照强度：关键变量

Light provides the energy that drives photosynthesis. As light intensity increases, the rate of photosynthesis increases. However, this is only true up to a certain point; eventually the rate plateaus because another factor becomes limiting.

光提供了驱动光合作用的能量。随着光照强度增加，光合作用速率也会增加。然而，这只在一定范围内成立；最终速率会趋于平稳，因为另一个因素变成了限制因素。

If you place a lamp at different distances from a plant, you will observe the rate rising as the lamp moves closer. This is because light intensity follows the inverse square law: when the distance is halved, the intensity becomes four times greater.

如果你把台灯放在距离植物不同远近的位置，你会观察到灯越近，速率越高。这是因为光照强度遵循平方反比定律：距离减半，强度变为原来的四倍。

5. Carbon Dioxide Concentration | 二氧化碳浓度

Carbon dioxide is one of the raw materials of photosynthesis. In normal air, CO₂ makes up only about 0.04% of the atmosphere. Increasing its concentration from this low level will generally increase the rate of photosynthesis, because the enzyme that fixes CO₂ is not fully saturated.

二氧化碳是光合作用的原料之一。在正常空气中，CO₂ 仅约占大气的 0.04%。从这个低水平开始提高 CO₂ 浓度通常会加快光合作用速率，因为固定 CO₂ 的酶尚未达到饱和。

In commercial greenhouses, growers sometimes release extra carbon dioxide from burners to boost crop growth. The graph of rate against CO₂ concentration rises steeply at first, then levels off. Once every active site of the fixing enzyme is occupied, extra CO₂ makes no further difference and another factor starts to limit the rate.

在商业化温室中，种植者有时会通过燃烧器释放额外的二氧化碳来促进作物生长。速率对 CO₂ 浓度的曲线先急剧上升，然后趋于平缓。一旦固定酶的活性位点全部被占用，额外的 CO₂ 就不再起作用，另一个因素开始限制速率。

6. Temperature and Enzymes | 温度与酶

Photosynthesis is controlled by enzymes, so temperature has a major effect on its rate. Between about 5°C and 35°C, the rate roughly doubles for every 10°C rise in temperature, because the reacting molecules gain kinetic energy and collide more frequently with the enzymes.

光合作用由酶控制，因此温度对其速率有重大影响。在约 5°C 至 35°C 之间，温度每升高 10°C，速率大约翻倍，因为反应分子获得更多动能，与酶的碰撞更加频繁。

Above the optimum temperature, around 35°C for most temperate plants, the rate falls sharply. The enzymes involved begin to denature: their active site changes shape, so the substrate can no longer bind. Once denatured, the enzyme is permanently damaged and photosynthesis stops for that reaction chain.

超过最适温度（大多数温带植物约为 35°C）后，速率急剧下降。相关酶开始变性：其活性位点形状改变，底物无法再结合。一旦变性，酶被永久破坏，该反应链的光合作用便停止了。

At very low temperatures, the rate is very low because molecules move slowly. Frost can also physically damage the chloroplast membranes. That is why plants grow much faster in warm spring weather than in winter.

在极低温度下，速率非常低，因为分子运动缓慢。霜冻还会物理损伤叶绿体膜。这就是为什么植物在温暖的春天比冬天生长得快得多。

7. Limiting Factors: The Concept | 限速因素：核心概念

A limiting factor is the factor whose shortage is slowing down the reaction at a given moment. For photosynthesis, the three main limiting factors are light intensity, carbon dioxide concentration and temperature. The rate is controlled by whichever of these is in the shortest supply.

限速因素是指在某一时刻因不足而减慢反应的因子。对光合作用而言，三大限速因素是光照强度、二氧化碳浓度和温度。速率由三者中供应最短缺的那个所控制。

Consider three everyday situations:

考虑三种日常情境：

At night, light is the limiting factor because no light energy is available at all.

夜间，光是限速因素，因为完全没有任何光能可用。
On a bright summer midday, CO₂ is often the limiting factor, since light and heat are plentiful.

在晴朗的夏季正午，CO₂ 通常是限速因素，因为光和热都很充足。
In winter, temperature is commonly the limiting factor, as light and CO₂ may be adequate but the enzymes work slowly.

在冬季，温度通常是限速因素，因为光和 CO₂ 可能足够，但酶的工作速度很慢。

In greenhouses, growers can control all three factors: adding artificial light, injecting CO₂ and heating the air. This allows year-round growing of tomatoes, peppers and other high-value crops.

在温室中，种植者可以控制全部三个因素：补充人工光照、注入 CO₂ 并加热空气。这使得番茄、辣椒等高价作物可以全年种植。

8. The Fate of Glucose | 葡萄糖的用途

Glucose produced by photosynthesis is a versatile chemical. Plants use it in many ways, and these are frequent exam questions. You should be able to name at least five uses with the correct storage form.

光合作用产生的葡萄糖是一种多用途化学物质。植物以多种方式利用它，这属于高频考题。你应当能说出至少五种用途及对应的储存形式。

Respiration: glucose is broken down in respiration to release energy for growth, transport and reproduction.

呼吸作用：葡萄糖在呼吸作用中被分解，释放能量以供生长、运输和繁殖。
Starch storage: excess glucose is converted to starch, which is insoluble and does not affect the water balance of the cell.

淀粉储存：多余的葡萄糖转化为淀粉，淀粉不溶于水，不会影响细胞的水分平衡。
Cellulose: glucose is polymerised into cellulose, which strengthens plant cell walls.

纤维素：葡萄糖聚合为纤维素，用于加固植物细胞壁。
Fats and oils: glucose can be converted into lipids for long-term energy storage, especially in seeds.

脂肪和油：葡萄糖可转化为脂质用于长期储能，尤其是在种子中。
Proteins: combined with nitrate ions from the soil, glucose is used to build amino acids and then proteins, including enzymes.

蛋白质：与土壤中的硝酸根离子结合，葡萄糖被用于合成氨基酸，再组装成蛋白质，包括酶。
Sucrose transport: glucose is converted to sucrose, which is easily transported in the phloem to other parts of the plant.

蔗糖运输：葡萄糖转化为蔗糖，蔗糖便于在韧皮部中运输到植物其他部位。

9. Mineral Ions from the Soil | 土壤中的矿质离子

Photosynthesis does not happen in isolation. Plants also require mineral ions absorbed from the soil through their roots. Two are especially important for this topic.

光合作用不是孤立发生的。植物还需要通过根部从土壤中吸收矿质离子。其中两种对这一主题尤其重要。

Nitrate ions, NO₃⁻, are needed to make amino acids, proteins and DNA. Without nitrate, a plant shows stunted growth and yellowing older leaves because it cannot build new proteins. Magnesium ions, Mg²⁺, are a central component of the chlorophyll molecule itself. A magnesium-deficient plant develops pale, yellow leaves because chlorophyll cannot be synthesised.

硝酸根离子 NO₃⁻ 用于合成氨基酸、蛋白质和 DNA。缺乏硝酸盐时，植物生长矮小，老叶发黄，因为它无法构建新蛋白质。镁离子 Mg²⁺ 是叶绿素分子的核心组成成分。缺镁的植物叶片苍白发黄，因为叶绿素无法合成。

10. Measuring the Rate: Pondweed Experiment | 测量速率：金鱼藻实验

The classic IGCSE investigation uses an aquatic plant called pondweed, usually Elodea or Cabomba. Because the plant lives underwater, the oxygen bubbles it produces are clearly visible and can be counted.

经典的 IGCSE 实验使用一种称为”金鱼藻”（通常为伊乐藻或水蕴草）的水生植物。由于该植物生活在水下，其产生的氧气气泡清晰可见且可以计数。

Method outline:

方法概要：

Place a piece of pondweed in a beaker of water with a small amount of sodium hydrogencarbonate dissolved in it, which provides a constant source of CO₂.

将一段金鱼藻放入含有少量碳酸氢钠溶液的水中，碳酸氢钠提供恒定的 CO₂ 来源。
Shine a lamp at the pondweed from a fixed distance and count the number of gas bubbles produced per minute. This is an estimate of the rate of photosynthesis.

将台灯以固定距离照射金鱼藻，统计每分钟产生的气泡数量。这用于估算光合作用速率。
Repeat at different distances and convert distance to relative light intensity using 1/d².

在不同距离下重复实验，并根据 1/d² 将距离换算为相对光照强度。

For greater accuracy, collect the gas in a test tube or gas syringe and measure its volume directly. Testing the collected gas with a glowing splint will show it relights, confirming it is oxygen. You must control the temperature and CO₂ concentration throughout the experiment so that only light intensity changes.

为了更准确，可将气体收集在试管或气体注射器中直接测量体积。用带火星的木条检验收集到的气体，木条复燃则证明是氧气。整个实验必须控制温度和 CO₂ 浓度不变，使其成为仅改变光照强度的单一变量实验。

11. Testing a Leaf for Starch | 检测叶片中的淀粉

Because starch is the storage product of photosynthesis, its presence in a leaf indicates that photosynthesis has occurred. The iodine test for starch is a standard practical that you must be able to describe in order.

因为淀粉是光合作用的储存产物，所以叶片中是否存在淀粉可以指示是否发生了光合作用。碘液检测淀粉是标准实验，你必须能按顺序描述其步骤。

The procedure has four steps:

该实验有四个步骤：

Boil the leaf in water for about 30 seconds. This kills the cells and breaks down cell membranes so that iodine can enter.

将叶片在沸水中煮约 30 秒。这能杀死细胞并破坏细胞膜，使碘液能够进入。
Boil the leaf in ethanol (using a water bath, never direct heat) to remove the chlorophyll. The leaf becomes pale and brittle.

将叶片放入乙醇中用水浴加热（切勿直接加热）以去除叶绿素。叶片会变得苍白且易碎。
Rinse the leaf in cold water to soften it and remove excess ethanol.

用冷水冲洗叶片使其软化并去除多余乙醇。
Spread the leaf on a white tile and add a few drops of iodine solution. A blue-black colour shows that starch is present.

将叶片平铺在白瓷板上，滴加几滴碘液。出现蓝黑色即说明有淀粉存在。

To test whether light is needed for photosynthesis, use a destarched plant, cover part of a leaf with black paper for 24 hours, then test the whole leaf. Only the exposed green areas turn blue-black, proving that light is essential.

为检验光合作用是否需要光，可使用已去除淀粉的植物，用黑纸遮住叶片的一部分约 24 小时，再检测整片叶。只有受光的部分变为蓝黑色，从而证明光是光合作用的必要条件。

12. Photosynthesis and the Global Scale | 光合作用与全球生态

Photosynthesis is not just a plant process; it is the engine of the biosphere. It maintains the balance of oxygen and carbon dioxide in the atmosphere, which has made animal life possible on Earth. The oxygen released by photosynthesis is the source of almost all the oxygen in the air we breathe.

光合作用不仅仅是植物的过程，它是整个生物圈的引擎。它维持着大气中氧气和二氧化碳的平衡，使地球上的动物生命成为可能。光合作用释放的氧气几乎是我们呼吸空气中所有氧气的来源。

Photosynthesis also forms the base of nearly every food chain. Primary producers, such as plants and algae, convert light energy into chemical energy that herbivores then consume. It is also crucial for climate: plants absorb CO₂, a major greenhouse gas, during photosynthesis. Maintaining healthy forests and phytoplankton populations helps slow the rate of global warming.

光合作用还是几乎所有食物链的基础。植物和藻类等初级生产者将光能转化为化学能，供草食动物消耗。它对气候也至关重要：植物在光合作用中吸收 CO₂——一种主要的温室气体。维护健康的森林和浮游植物种群有助于减缓全球变暖的速度。

In agriculture, understanding photosynthesis helps farmers and growers maximise crop yields, whether by selecting the right planting densities, controlling greenhouse environments, or applying the correct fertilisers that supply nitrate and magnesium ions.

在农业中，理解光合作用帮助农民和种植者最大化作物产量，无论是通过选择合理的种植密度、控制温室环境，还是施用提供硝酸根和镁离子的正确肥料。

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Photosynthesis: The Green Machine | 光合作用：绿色工厂