Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is one of the most important biological processes on Earth. It is the way in which green plants, algae and some bacteria convert light energy into chemical energy, producing the food and oxygen that nearly all living organisms depend on. This revision guide covers everything you need for the Edexcel IGCSE Science examination, including the equations, limiting factors, leaf adaptations, experiments and the uses of glucose.

光合作用是地球上最重要的生物过程之一。绿色植物、藻类和某些细菌通过光合作用，将光能转化为化学能，制造出几乎所有生物都依赖的食物和氧气。本复习指南涵盖 Edexcel IGCSE 科学考试所需的全部核心知识，包括方程式、限制因素、叶片结构适应性、实验以及葡萄糖的用途。

1. What Is Photosynthesis? | 什么是光合作用

Photosynthesis is the process by which green plants make glucose using carbon dioxide and water, in the presence of light and chlorophyll. Light energy is trapped by chlorophyll, a green pigment found in chloroplasts, and is used to convert the raw materials into a sugar called glucose. Oxygen is released as a waste product.

光合作用是绿色植物利用二氧化碳和水，在光照和叶绿素存在的条件下制造葡萄糖的过程。叶绿素是存在于叶绿体中的绿色色素，它吸收光能，并利用这些能量将原料转化为葡萄糖。氧气作为副产物被释放出来。

The process can be summarised in two ways — as a word equation and as a balanced chemical equation. You must be able to write and interpret both for the Edexcel examination.

这个过程可以用两种方式概括——文字方程式和配平的化学方程式。在 Edexcel 考试中，你必须能够正确书写并解释这两种方程式。

Word equation: carbon dioxide + water → glucose + oxygen
文字方程式：二氧化碳 + 水 → 葡萄糖 + 氧气
Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
配平化学方程式：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Light energy (trapped by chlorophyll) + 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

光能（由叶绿素捕获）+ 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Note that the light energy and chlorophyll are not substances in the equation — they are conditions required for the reaction to occur. Many students forget to mention them in the word equation or when describing the process. Make sure you always refer to light and chlorophyll as essential requirements.

请注意，光能和叶绿素并不是方程式中的物质，而是反应发生所需的条件。许多学生在书写文字方程式或描述这一过程时忘记提及它们。请务必始终指出光照和叶绿素是必要条件。

2. Photosynthesis Is an Endothermic Reaction | 光合作用是吸热反应

In IGCSE Chemistry, reactions are classified as endothermic or exothermic. Photosynthesis is an endothermic reaction because it takes in energy from the surroundings — in this case, light energy. This energy is stored in the chemical bonds of the glucose molecules.

在 IGCSE 化学中，反应分为吸热反应和放热反应。光合作用属于吸热反应，因为它从周围环境中吸收能量——在这里指光能。这些能量以化学键的形式储存在葡萄糖分子中。

This is an important crossover idea between biology and chemistry. When you compare photosynthesis with respiration, remember: respiration is exothermic (releases energy), while photosynthesis is endothermic (stores energy).

这是生物与化学之间的重要交叉知识点。比较光合作用与呼吸作用时，请记住：呼吸作用是放热反应（释放能量），而光合作用是吸热反应（储存能量）。

Photosynthesis: light energy → chemical energy (stored in glucose)

光合作用：光能 → 化学能（储存于葡萄糖中）

3. Why Is Photosynthesis Important? | 光合作用的重要性

Photosynthesis is fundamental to life on Earth for two main reasons. First, it produces glucose, which is the starting point for almost all food chains. Herbivores eat plants, and carnivores eat herbivores, so every organism ultimately depends on photosynthesis for its energy. Second, it releases oxygen into the atmosphere, which is essential for aerobic respiration in most living organisms.

光合作用对地球生命至关重要，主要有两个原因。第一，它产生葡萄糖，这几乎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，因此所有生物最终都依赖光合作用获取能量。第二，它向大气中释放氧气，而氧气是大多数生物进行有氧呼吸所必需的。

Photosynthesis also removes carbon dioxide from the atmosphere, helping to regulate its concentration. This is linked to the carbon cycle, a topic that frequently appears in the IGCSE specification. Plants act as carbon sinks during photosynthesis, locking carbon into organic compounds.

光合作用还从大气中移除二氧化碳，有助于调节其浓度。这与碳循环密切相关，碳循环是 IGCSE 考试大纲中经常出现的主题。植物在光合作用中充当碳汇，将碳固定在有机化合物中。

In addition, photosynthesis is the ultimate source of fossil fuels. Coal, oil and natural gas formed from the remains of ancient plants and plankton that originally stored energy through photosynthesis millions of years ago.

此外，光合作用是化石燃料的最终来源。煤、石油和天然气是由古代植物和浮游生物的残骸形成的，这些生物在数百万年前通过光合作用储存了能量。

4. Leaf Structure and Its Adaptations | 叶片结构与其适应性

The leaf is the main photosynthetic organ of a plant. Its structure is perfectly adapted to maximise the rate of photosynthesis. The Edexcel specification expects you to relate the structure of a leaf to its functions, so you should be able to name each tissue and explain how it helps the leaf carry out photosynthesis efficiently.

叶片是植物进行光合作用的主要器官。其结构完美适应了最大化光合作用速率的需求。Edexcel 考试大纲要求你将叶片的结构与其功能联系起来，因此你需要能够说出每种组织的名称，并解释它如何帮助叶片高效地进行光合作用。

Adaptation \| 适应性特征	Function \| 功能
Large, flat surface area	Absorbs maximum light; wide surface for gas exchange
面积大而扁平	吸收最多光照；提供宽广的气体交换表面积
Thin structure	Short diffusion distance for carbon dioxide to reach chloroplasts
叶片较薄	缩短二氧化碳扩散到达叶绿体的距离
Chloroplasts in palisade mesophyll cells	Chlorophyll traps light energy; palisade layer receives most light
栅栏组织细胞中含有叶绿体	叶绿素捕获光能；栅栏层接收最多光照
Tiny holes called stomata (mainly on lower surface)	Allow carbon dioxide in and oxygen out; open and close via guard cells
气孔（主要分布在叶片下表皮）	允许二氧化碳进入、氧气逸出；通过保卫细胞控制开闭
Network of veins (xylem and phloem)	Xylem delivers water; phloem transports glucose away
叶脉网络（木质部和韧皮部）	木质部输送水分；韧皮部将葡萄糖运走
Transparent upper epidermis	Allows light to pass through to photosynthetic cells below
透明的上表皮	让光线穿透到达下方的光合作用细胞
Spongy mesophyll with air spaces	Allows gases to diffuse quickly between stomata and palisade cells
海绵组织带有细胞间隙	使气体在气孔与栅栏细胞之间快速扩散

A common examination question asks you to compare the palisade mesophyll and spongy mesophyll. The palisade cells are tall, tightly packed and contain the vast majority of chloroplasts, making them the main site of photosynthesis. The spongy mesophyll cells are more loosely arranged, with fewer chloroplasts, but they create air spaces that facilitate gas circulation.

常见的考试题目会要求你比较栅栏组织和海绵组织。栅栏细胞呈长柱状、排列紧密，含有绝大多数叶绿体，是光合作用的主要场所。海绵组织细胞排列较为疏松，叶绿体较少，但其细胞间隙有助于气体循环。

5. The Limiting Factors of Photosynthesis | 光合作用的限制因素

A limiting factor is a condition whose absence or low concentration restricts the rate of a reaction. In photosynthesis, the three main limiting factors are light intensity, carbon dioxide concentration and temperature. Water shortage can also limit photosynthesis, but it is rarely named as the most important limiting factor in the Edexcel specification because plants typically wilt well before water becomes the immediate bottleneck.

限制因素是指其缺失或浓度过低会限制反应速率的条件。在光合作用中，三个主要限制因素是光照强度、二氧化碳浓度和温度。缺水也会限制光合作用，但在 Edexcel 大纲中，水很少被列为首要限制因素，因为植物在水分成为直接瓶颈之前通常会先萎蔫。

Light Intensity | 光照强度

As light intensity increases, the rate of photosynthesis increases linearly at first, because more light energy is available to drive the reaction. However, beyond a certain point, the rate plateaus. At this stage, light is no longer the limiting factor — something else, such as carbon dioxide concentration or temperature, is limiting the rate.

随着光照强度增加，光合作用速率首先呈线性上升，因为有更多光能可用于驱动反应。然而，超过某一阈值后，速率趋于平稳。此时，光照不再是限制因素——其他条件，如二氧化碳浓度或温度，成为新的限制因素。

Carbon Dioxide Concentration | 二氧化碳浓度

Carbon dioxide is a raw material of photosynthesis, so raising its concentration generally increases the rate — up to a plateau. In the atmosphere, CO₂ is only about 0.04%, so it is often the limiting factor in natural conditions. Farmers exploit this in greenhouses by burning paraffin, which releases both heat and carbon dioxide.

二氧化碳是光合作用的原料，因此提高其浓度通常会增加反应速率——直至达到平台期。大气中二氧化碳仅占约 0.04%，因此在自然条件下它常常是限制因素。农民在温室中利用这一点，通过燃烧石蜡来同时释放热量和二氧化碳。

Temperature | 温度

Photosynthesis is controlled by enzymes, so temperature has a significant effect on its rate. As temperature rises, the rate increases up to the optimum (usually around 30-40 °C for most plants), because particles have more kinetic energy and enzyme-substrate collisions happen more frequently. Above the optimum, the rate declines rapidly because the enzymes begin to denature — their active sites change shape and they can no longer catalyse the reaction.

光合作用受酶控制，因此温度对其速率有显著影响。随着温度升高，速率上升至最适温度（大多数植物约为 30-40 °C），因为粒子动能增加，酶与底物的碰撞更加频繁。超过最适温度后，速率迅速下降，因为酶开始变性——其活性位点形状改变，无法再催化反应。

You should remember that denaturation is permanent; it is not the same as the reversible slowing of enzymes at low temperatures. This distinction is a favourite exam question.

你需要记住，变性是不可逆的；这与低温下酶活性可逆降低不同。这一区别是考试中的高频考点。

6. Using Limiting Factors in Agriculture | 限制因素在农业中的应用

Understanding limiting factors allows farmers and gardeners to increase crop yields artificially. In greenhouses, multiple conditions can be controlled simultaneously. Glasshouses trap heat, artificial lighting extends the hours of photosynthesis, and carbon dioxide generators or paraffin heaters enrich the air with CO₂.

理解限制因素使农民和园丁能够人为提高作物产量。在温室中，可以同时控制多个条件。玻璃温室保温，人工照明延长光合作用时间，二氧化碳发生器或石蜡加热器增加空气中的 CO₂ 浓度。

Artificial lighting: allows photosynthesis to continue at night or during winter when natural light is limited.
人工照明：在夜间或冬季自然光照不足时，使光合作用得以继续进行。
Heating systems: keep the temperature near the optimum, especially in cold months.
加热系统：使温度保持接近最适温度，尤其在寒冷月份。
CO₂ enrichment: increases the concentration of carbon dioxide, boosting photosynthesis until another factor becomes limiting.
CO₂ 增施：提高二氧化碳浓度，促进光合作用，直到另一个因素成为限制因素为止。
Fertilisers: provide mineral ions such as nitrates, which plants need to make proteins and other molecules after glucose is formed.
肥料：提供硝酸盐等矿质离子，植物在生成葡萄糖后需要这些离子来制造蛋白质和其他分子。

In an exam, you may be asked to interpret graphs showing the effect of two factors, for example light intensity at two different CO₂ concentrations. Remember that the line with the higher CO₂ concentration reaches a higher plateau, proving that CO₂ is limiting at the higher light intensities.

在考试中，你可能会被要求解读显示两个因素影响的图表，例如两种不同 CO₂ 浓度下光照强度的影响。请记住，CO₂ 浓度较高的曲线达到更高的平台，这证明在较高光照强度下 CO₂ 是限制因素。

7. Investigating Photosynthesis: Core Experiments | 探究光合作用：核心实验

The Edexcel IGCSE course includes several required practicals related to photosynthesis. You should know the method, the controls and the expected results of each one.

Edexcel IGCSE 课程包含几个与光合作用相关的必做实验。你需要掌握每个实验的方法、对照设置和预期结果。

Testing a Leaf for Starch | 检测叶片中的淀粉

Starch is stored in leaves when glucose is produced faster than it is used. Iodine solution turns from brown to blue-black in the presence of starch. The procedure involves four steps: first, boil the leaf in water to kill cells and stop enzymatic activity; second, boil the leaf in ethanol to remove the green chlorophyll, so that colour changes are visible; third, rinse the leaf in cold water to soften it; fourth, add iodine solution and observe the colour change.

当葡萄糖的产生速度超过消耗速度时，叶片会储存淀粉。碘液遇淀粉会由棕色变为蓝黑色。实验步骤包括四步：第一，将叶片在沸水中煮以杀死细胞并终止酶活性；第二，将叶片在乙醇中煮沸以去除绿色叶绿素，使颜色变化可见；第三，用冷水冲洗叶片使其软化；第四，滴加碘液并观察颜色变化。

A standard exam question involves a variegated leaf — one that has both green and white areas. When tested for starch, only the green areas turn blue-black, because only those areas contain chlorophyll and can photosynthesise.

一个标准的考试题目涉及花斑叶——即同时具有绿色区域和白色区域的叶片。当检测淀粉时，只有绿色区域变为蓝黑色，因为只有这些区域含有叶绿素并能进行光合作用。

Using Pondweed to Measure the Rate | 使用金鱼藻测量光合作用速率

Pondweed (Elodea or Cabomba) is a water plant that produces oxygen bubbles during photosynthesis. A lamp is placed at different distances from the pondweed, and the number of bubbles released per minute is counted. The closer the lamp, the higher the light intensity and the faster the bubble production. Alternatively, the oxygen can be collected in a capillary tube or measuring cylinder to measure the volume of gas produced in a set time.

金鱼藻（Elodea 或 Cabomba）是一种水生植物，在光合作用过程中会释放氧气气泡。将灯放置在不同距离处，统计每分钟释放的气泡数量。灯越近，光照强度越高，气泡产生越快。或者，可以用毛细管或量筒收集氧气，测量单位时间内产生的气体体积。

You should know how to calculate and use the inverse square law relating light intensity to distance:

你需要知道如何计算和应用光照强度与距离之间的平方反比定律：

Light intensity ∝ 1 / d²

光照强度 ∝ 1 / d²（d 为灯与植物之间的距离）

For example, if the distance is doubled, light intensity falls to one quarter of its original value. This relationship explains why the rate of photosynthesis changes dramatically with small changes in distance.

例如，如果距离加倍，光照强度降至原来的四分之一。这一关系解释了为什么距离的微小变化会导致光合作用速率的大幅改变。

Controls and Fair Testing | 对照与公平实验

To make the pondweed experiment a fair test, you must keep the temperature constant (for example, by using a water bath), use the same piece of pondweed, and keep the concentration of sodium hydrogencarbonate the same. Sodium hydrogencarbonate is added to the water to provide a constant supply of carbon dioxide.

为了使金鱼藻实验成为公平测试，你必须保持温度恒定（例如使用水浴）、使用同一株金鱼藻，并保持碳酸氢钠浓度不变。在水中加入碳酸氢钠是为了提供稳定的二氧化碳来源。

8. What Happens to the Glucose Produced? | 产生的葡萄糖去向如何

Glucose produced by photosynthesis is used in several ways. The Edexcel specification expects you to list how plants use the products of photosynthesis, linking this to plant nutrition and growth.

光合作用产生的葡萄糖有多种用途。Edexcel 大纲要求你列出植物如何利用光合作用产物，并将其与植物营养和生长联系起来。

Respiration: glucose is broken down in respiration to release energy for cell activities.
呼吸作用：葡萄糖在呼吸作用中被分解，释放能量供细胞活动使用。
Starch storage: glucose is converted to starch and stored in roots, stems and leaves for later use. Starch is insoluble, so it does not affect the water balance of cells.
淀粉储存：葡萄糖转化为淀粉，储存于根、茎、叶中以备后用。淀粉不溶于水，因此不会影响细胞的水分平衡。
Cellulose: glucose is used to make cellulose, a structural polysaccharide found in plant cell walls.
纤维素：葡萄糖用于合成纤维素，这是植物细胞壁中的结构多糖。
Proteins: glucose combines with nitrate ions (absorbed from the soil) to form amino acids, which are then built into proteins.
蛋白质：葡萄糖与从土壤中吸收的硝酸根离子结合，形成氨基酸，再进一步合成蛋白质。
Fats and oils: glucose is converted into lipids for energy storage, especially in seeds.
脂肪和油：葡萄糖转化为脂质用于储能，尤其是在种子中。

This list is often tested directly in short-answer questions. Make sure you use the keyword “nitrates” when talking about protein formation — do not say that plants absorb amino acids from the soil; they make them themselves.

这个列表经常在简答题中直接考查。在谈到蛋白质合成时，务必使用关键词”硝酸盐”——不要说出植物从土壤中吸收氨基酸；氨基酸是植物自己合成的。

9. Photosynthesis and Respiration — A Comparison | 光合作用与呼吸作用的比较

Students often confuse photosynthesis with respiration. They are opposite processes, and the Edexcel exam frequently asks you to compare them in a table or short answer. Below is the key comparison you should memorise.

学生经常混淆光合作用与呼吸作用。它们是相反的过程，Edexcel 考试经常要求你用表格或简答来比较它们。以下是你需要牢记的关键比较。

Feature \| 特征	Photosynthesis \| 光合作用	Respiration \| 呼吸作用
Energy change	Endothermic — stores energy	Exothermic — releases energy
能量变化	吸热——储存能量	放热——释放能量
Requirements	Light, carbon dioxide, water, chlorophyll	Glucose (or other substrate) and oxygen
所需条件	光照、二氧化碳、水、叶绿素	葡萄糖（或其他底物）和氧气
Products	Glucose and oxygen	Carbon dioxide, water and energy
产物	葡萄糖和氧气	二氧化碳、水和能量
Where it occurs	Chloroplasts in plant cells	Mitochondria (aerobic) in all living cells
发生场所	植物细胞中的叶绿体	所有活细胞中的线粒体（有氧呼吸）
When it occurs	Only in the light	Continuously, day and night
发生时间	仅在光照条件下	持续进行，昼夜不停

A classic exam question states: “A plant is kept in a dark cupboard for 48 hours before an experiment. Explain why.” The answer is that this removes all starch from the leaves through respiration, ensuring that any starch detected later must have been produced by photosynthesis during the experiment. This process is called destarching.

一个经典考题是：”实验前将植物放在暗橱中 48 小时，请解释原因。”答案是：通过呼吸作用消耗掉叶片中所有淀粉，确保之后检测到的任何淀粉都必定是实验期间通过光合作用产生的。这个过程称为”脱淀粉”（destarching）。

10. Common Misconceptions and Exam Tips | 常见误区与考试技巧

Many students lose marks on photosynthesis questions because of small but repeated errors. Here are the most common pitfalls and how to avoid them.

许多学生在光合作用相关题目中因反复出现的小错误而失分。以下是最常见的陷阱以及如何避免它们。

Misconception: “Plants photosynthesise only during the day and respire only at night.” Correction: plants respire continuously, day and night. They photosynthesise only when light is available.
误区：”植物白天只进行光合作用，夜间只进行呼吸作用。”纠正：植物昼夜不停地呼吸。只有在有光照时才进行光合作用。
Misconception: “Oxygen is made during respiration and carbon dioxide during photosynthesis.” Correction: it is the reverse — photosynthesis produces oxygen, respiration produces carbon dioxide.
误区：”呼吸作用产生氧气，光合作用产生二氧化碳。”纠正：恰好相反——光合作用产生氧气，呼吸作用产生二氧化碳。
Misconception: “Temperature keeps increasing the rate of photosynthesis.” Correction: above the optimum temperature, enzymes denature and the rate falls sharply.
误区：”温度越高，光合作用速率越快。”纠正：超过最适温度后，酶变性，速率急剧下降。
Misconception: “Chlorophyll is an enzyme.” Correction: chlorophyll is a pigment that absorbs light; it is not an enzyme.
误区：”叶绿素是一种酶。”纠正：叶绿素是吸收光能的色素，不是酶。
Exam tip: when explaining a plateau on a graph, always state the next most likely limiting factor and explain why it limits the rate.
考试技巧：解释图表上的平台期时，始终指出下一个最可能的限制因素，并解释它为何限制反应速率。
Exam tip: use the exact phrases “light intensity”, “carbon dioxide concentration” and “temperature” in answers — do not write “sunlight amount” or “heat”.
考试技巧：在答案中使用准确术语”光照强度”、”二氧化碳浓度”和”温度”——不要写”阳光量”或”热量”。

Also remember that glucose is not the direct product used to test for photosynthesis; starch is. The test uses iodine solution, and the positive result is a blue-black colour, not “black” alone and not “purple”. Getting this vocabulary exactly right will earn you marks.

还要记住，用于检测光合作用产物的是淀粉，而不是葡萄糖本身。检测使用碘液，阳性结果为蓝黑色，而不是仅仅”黑色”或”紫色”。精确使用这些术语能帮你得分。

11. Quick Revision Summary | 快速复习总结

Use this checklist to confirm you are ready for the examination on photosynthesis.

使用这张清单来确认你已经为光合作用的考试做好准备。

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Photosynthesis | 光合作用