Photosynthesis: How Plants Make Their Own Food — 光合作用：植物如何制造自己的食物

1. What Is Photosynthesis? The Word Equation and Where It Happens | 什么是光合作用：文字方程式及其发生场所

光合作用是绿色植物利用光能，把二氧化碳和水合成为葡萄糖并释放氧气的过程。这个反应只发生在植物的绿色部分，最主要的是叶片，因为只有叶片里含有大量叶绿素这种绿色色素。

Photosynthesis is the process by which green plants use light energy to combine carbon dioxide and water to make glucose and release oxygen. The reaction only happens in the green parts of a plant, mainly the leaves, because only these parts contain large amounts of the green pigment chlorophyll.

用一句话记住这个定义：植物把光能锁进葡萄糖的化学键里，供自己生长和呼吸使用。光合作用是一切食物链的起点，因为几乎所有生物的能量最终都来自太阳光。

Memorise the definition in one line: a plant traps light energy and locks it inside the chemical bonds of glucose, which it uses for growth and respiration. Photosynthesis is the starting point of every food chain, because the energy of almost all living things ultimately comes from sunlight.

2. The Word Equation and the Role of Chlorophyll | 文字方程式与叶绿素的作用

光合作用的文字方程式可以写成：二氧化碳 + 水（在光照和叶绿素的条件下）生成葡萄糖 + 氧气。叶绿素的作用是吸收光能，并把这份能量传递给反应，因此叶绿素就像一块收集阳光的天线。

The word equation for photosynthesis is: carbon dioxide + water (in the presence of light and chlorophyll) produces glucose + oxygen. Chlorophyll absorbs light energy and passes it into the reaction, so chlorophyll acts like an antenna that collects sunlight.

请注意，叶绿素本身在反应前后不会被消耗，它只负责吸收能量，所以它是一种催化剂式的色素而不是反应物。如果一株植物缺少叶绿素（例如白化的叶片），它就无法进行光合作用。

Note that chlorophyll is not used up in the reaction; it only absorbs energy, so it works as a light-absorbing pigment rather than a reactant. If a plant lacks chlorophyll (for example a variegated leaf with white parts), those parts cannot photosynthesise.

3. The Balanced Chemical Equation: Counting Atoms on Both Sides | 平衡化学方程式：数一数两边的原子

光合作用的平衡化学方程式是：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。六个二氧化碳分子和六个水分子，在光和叶绿素的帮助下，生成一个葡萄糖分子和六个氧气分子。

The balanced chemical equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Six molecules of carbon dioxide and six molecules of water, helped by light and chlorophyll, produce one molecule of glucose and six molecules of oxygen.

检查平衡的方法：左边有 6 个碳原子，右边葡萄糖里也有 6 个碳；左边有 12 个氢，右边也是 12 个；左边有 18 个氧（6×2 + 6×1），右边葡萄糖有 6 个氧加上氧气里的 12 个，总共也是 18 个。两边原子数完全相等，所以方程式平衡。

To check the balance: the left side has 6 carbon atoms and glucose on the right also has 6; the left has 12 hydrogen and the right also has 12; the left has 18 oxygen (6×2 plus 6×1), and the right has 6 oxygen in glucose plus 12 in oxygen gas, also 18 in total. The atoms match on both sides, so the equation is balanced.

4. How the Leaf Is Adapted for Photosynthesis: Structure Meets Function | 叶如何适应光合作用：结构与功能相适应

叶片是一台为光合作用量身定做的太阳能板。它又宽又平，表面积大，能尽可能多地接收阳光；它很薄，让二氧化碳和氧气能够快速扩散进出叶片。

A leaf is a solar panel custom-built for photosynthesis. It is broad and flat with a large surface area, so it can capture as much sunlight as possible, and it is thin, so carbon dioxide and oxygen can diffuse in and out quickly.

叶片内部的适应结构包括：上表皮透明，让阳光照进叶肉；叶肉细胞里含有大量叶绿体；海绵层有空气间隙，让气体自由流动；气孔开在下表皮，控制气体交换；叶脉输送水和葡萄糖。每一处结构都对应一种功能。

Adaptations inside the leaf include: a transparent upper epidermis that lets light reach the mesophyll; mesophyll cells packed with chloroplasts; air spaces in the spongy layer for free gas movement; stomata on the lower epidermis that control gas exchange; and veins that transport water and glucose. Each structure maps to a specific function.

5. Chloroplasts: The Tiny Factories Inside Mesophyll Cells | 叶绿体：叶肉细胞里的小工厂

光合作用真正发生的场所是叶绿体，它们是叶肉细胞内的微小细胞器。每个叶绿体里都有叶绿素，叶绿素吸收红光和蓝光，反射绿光，这就是植物看起来是绿色的原因。

The real site of photosynthesis is the chloroplast, a tiny organelle inside the mesophyll cells. Each chloroplast contains chlorophyll, which absorbs red and blue light and reflects green light, which is why plants look green.

叶肉细胞分成两层：靠近上表皮的栅栏组织排列紧密、叶绿体最多，是光合作用的主力；下方的海绵组织较松散，有空气间隙方便气体扩散。栅栏组织紧贴上表面，能优先捕获阳光。

Mesophyll cells form two layers: the palisade layer near the upper surface is tightly packed and has the most chloroplasts, making it the main engine of photosynthesis; the spongy layer below is looser with air spaces for gas diffusion. The palisade layer sits against the upper surface so it captures light first.

6. The Raw Materials: Where Carbon Dioxide and Water Come From | 原料从哪里来：二氧化碳和水的来源

二氧化碳通过气孔从空气中扩散进入叶片。气孔是下表皮上的小孔，由两个保卫细胞控制开闭。当气孔张开时，二氧化碳进入，同时氧气和水蒸气离开。

Carbon dioxide diffuses into the leaf from the air through the stomata. Stomata are tiny pores on the lower epidermis, each controlled by two guard cells. When a stoma opens, carbon dioxide enters while oxygen and water vapour leave.

水则从根部吸收，通过木质部导管一路输送到叶片。水既用于光合作用，也用来保持细胞坚挺。如果土壤缺水，气孔会关闭以减少水分流失，但这也会减慢光合作用。

Water is absorbed by the roots and transported up to the leaves through xylem vessels. Water is used both for photosynthesis and to keep the cells firm. If the soil runs dry, the stomata close to reduce water loss, but this also slows photosynthesis down.

7. The Products: Glucose and Oxygen, and How Plants Use Glucose | 产物：葡萄糖与氧气，以及植物如何利用葡萄糖

光合作用生成两种产物：葡萄糖是植物储存能量的形式，氧气则作为副产品释放到空气中。植物释放的氧气正是所有动物呼吸所依赖的气体。

Photosynthesis makes two products: glucose is the form in which the plant stores energy, and oxygen is released into the air as a by-product. The oxygen plants release is the very gas that all animals depend on for respiration.

植物把葡萄糖用于五个方面：呼吸释放能量；转化为纤维素构建细胞壁；转化为蛋白质用于生长（需要土壤中的硝酸盐）；以淀粉形式储存；以及转化为脂肪和油脂储存。淀粉是不溶于水的，所以它是植物理想的储存形式，因为溶解的葡萄糖会改变细胞的渗透压。

Plants use glucose for five things: respiration to release energy; conversion into cellulose for cell walls; conversion into protein for growth (using nitrate from the soil); storage as starch; and storage as fats and oils. Starch is insoluble in water, which makes it the ideal storage form, because dissolved glucose would change the osmotic balance of the cell.

8. Limiting Factors: Light, Carbon Dioxide and Temperature | 限制因素：光照、二氧化碳和温度

限制因素是指任何处于短缺状态、从而拖慢整个反应速率的条件。光合作用有三个主要限制因素：光照强度、二氧化碳浓度和温度。任何一个不足，都会限制反应速率，即使其他两个都很充足。

A limiting factor is any condition in short supply that slows down the whole rate of reaction. Photosynthesis has three main limiting factors: light intensity, carbon dioxide concentration and temperature. If any one of them is in short supply, it limits the rate even when the other two are plentiful.

在低光照下，光是限制因素，增加光照会提高光合速率；当光不再短缺时，二氧化碳或温度就会成为新的瓶颈。速率曲线先上升，然后变平，这个平台处就是另一个因素开始限制的地方。

At low light, light is the limiting factor, and adding more light raises the rate; once light is no longer scarce, carbon dioxide or temperature becomes the new bottleneck. The rate curve rises and then flattens, and the plateau is the point where another factor begins to limit.

温度通过影响酶来起作用：温度太低，酶工作慢；温度太高，酶会变性失效。因此光合作用有一个最适温度，通常在中等的温暖范围内，过高或过低都会降低速率。

Temperature works through enzymes: if it is too cold the enzymes work slowly, and if it is too hot the enzymes denature and stop working. Photosynthesis therefore has an optimum temperature, usually in a moderate warm range, and rates drop both above and below it.

9. Testing a Leaf for Starch: The Classic Investigation | 检验叶片中的淀粉：经典实验

淀粉检验证明光合作用是否发生。步骤是：先把叶片放进沸水杀死细胞；再放进热水浴中的乙醇里脱色；然后用热水冲洗使叶片变软；最后滴加碘液。变蓝黑色说明有淀粉，证明光合作用发生了。

The starch test shows whether photosynthesis has occurred. The steps are: place the leaf in boiling water to kill the cells; then place it in ethanol in a hot water bath to remove the green colour; rinse in hot water to soften it; finally add iodine solution. A blue-black colour shows starch is present, proving photosynthesis has taken place.

这个实验有两个安全要点：乙醇是易燃的，必须在热水浴中加热，绝不能直接放在明火上；碘液要小心使用，它会把皮肤和衣物染色。对照实验通常用一株先在黑暗中放置两天的植物，以确保叶片原有的淀粉已经被消耗掉。

This experiment has two safety points: ethanol is flammable and must be heated in a water bath, never over a naked flame; and iodine solution must be handled carefully because it stains skin and clothes. The control usually uses a plant kept in the dark for two days first, so any original starch has already been used up.

10. Investigating Light Intensity: The Pondweed Bubbles Experiment | 探究光照强度：伊乐藻气泡实验

伊乐藻（黑藻）实验测量不同光照强度下光合作用的速率。把一段伊乐藻放在水中，靠近一盏灯，数每分钟冒出的氧气泡数量，或者用量筒测量收集到的气体体积。灯离得越近，气泡越多，说明速率越快。

The pondweed experiment measures the rate of photosynthesis at different light intensities. A piece of pondweed is placed in water near a lamp, and the number of oxygen bubbles released per minute is counted, or the volume of gas collected is measured. The closer the lamp, the more bubbles, showing a faster rate.

为了控制变量，二氧化碳浓度通过在水中加入碳酸氢钠（小苏打）来保持充足，温度保持恒定，同一个伊乐藻段用于所有距离。改变的是灯与植物的距离，也就是光照强度。

To control the variables, carbon dioxide concentration is kept plentiful by adding sodium hydrogencarbonate (baking soda) to the water, temperature is kept constant, and the same piece of pondweed is used for every distance. The only thing changed is the distance from the lamp, which is the light intensity.

11. Photosynthesis, Respiration and the Carbon Cycle | 光合作用、呼吸作用与碳循环

光合作用和呼吸作用互为补充。光合作用吸收二氧化碳、释放氧气、储存能量；呼吸作用吸收氧气、释放二氧化碳、释放能量。白天植物同时进行两者，但通常光合作用更强，所以白天植物净释放氧气。

Photosynthesis and respiration are complementary. Photosynthesis takes in carbon dioxide, releases oxygen and stores energy; respiration takes in oxygen, releases carbon dioxide and releases energy. During the day a plant does both, but photosynthesis is usually stronger, so a plant is a net producer of oxygen in daylight.

在碳循环中，植物通过光合作用把空气中的二氧化碳固定成有机物；动物吃植物，把碳沿食物链传递；植物和动物呼吸以及分解者分解尸体时，又把二氧化碳释放回空气。燃烧化石燃料也在短时间内释放大量二氧化碳。

In the carbon cycle, plants fix carbon dioxide from the air into organic matter through photosynthesis; animals eat plants and pass the carbon along the food chain; and carbon dioxide returns to the air through plant and animal respiration and through decomposition of dead bodies by decomposers. Burning fossil fuels also releases large amounts of carbon dioxide in a short time.

12. Common Exam Questions and a Four-Step Answer Method | 常见考题与四步答题法

考试最常见的题型是要求描述光合作用实验的结果、解释限制因素图表、或说明叶片的一种适应结构。答题时先读清楚题目问的是描述还是解释，描述只需说发生了什么，解释则要给出原因。

The most common exam questions ask you to describe the results of a photosynthesis experiment, explain a limiting-factor graph, or state one adaptation of a leaf. When answering, first check whether the question asks you to describe or to explain: describe only says what happens, explain gives the reason why.

四步答题法：第一步，写出相关的方程式或定义；第二步，指出图中的数据或趋势；第三步，把数据与科学原理（如限制因素、扩散、叶绿素）联系起来；第四步，回到题目要求的结论。每写一个结论都要附上理由，因为评分标准里理由占分。

Use the four-step method: first, write the relevant equation or definition; second, quote the data or trend from the graph; third, link the data to the science (such as limiting factors, diffusion or chlorophyll); fourth, return to the conclusion the question asks for. Every conclusion should be backed by a reason, because reasons carry marks in the mark scheme.

13. Investigating Carbon Dioxide Concentration: Adding Sodium Hydrogencarbonate | 探究二氧化碳浓度：加入碳酸氢钠

要研究二氧化碳是不是限制因素，可以在水中加入不同量的碳酸氢钠，它会缓慢释放二氧化碳。用量越多，水中的二氧化碳浓度越高，光合速率越快，直到二氧化碳不再是限制因素为止。

To investigate whether carbon dioxide is the limiting factor, add different amounts of sodium hydrogencarbonate to the water; it slowly releases carbon dioxide. The more you add, the higher the carbon dioxide concentration, and the faster photosynthesis runs, until carbon dioxide is no longer limiting.

实验里同样要控制变量：保持灯的距离不变，保持温度不变，只改变碳酸氢钠的用量。数每单位时间内伊乐藻冒出的气泡数，气泡越多代表氧气产量越高，也就是光合速率越高。

The experiment must again control its variables: keep the lamp distance constant, keep the temperature constant, and change only the amount of sodium hydrogencarbonate. Count the bubbles released by the pondweed per unit time; more bubbles mean more oxygen produced, which means a faster rate of photosynthesis.

14. Investigating Temperature: Enzymes and the Optimum | 探究温度：酶与最适温度

温度实验把伊乐藻分别放进不同温度的水浴里，保持光照和二氧化碳不变，比较产氧速率。结果是一条钟形曲线：温度从低到高时速率上升，到达最适温度后开始下降，温度继续升高时速率急剧跌落。

The temperature experiment places pondweed in water baths at different temperatures, keeping light and carbon dioxide constant, and compares the rate of oxygen production. The result is a bell-shaped curve: the rate rises as temperature rises from cold, peaks at the optimum, then falls, and drops sharply as temperature rises further.

速率上升是因为温度升高让酶和分子运动更快，反应更容易发生；超过最适温度后，叶绿体里的酶开始变性，形状改变，无法再催化反应，所以速率迅速下降。这一点和人体酶的规律完全一致。

The rate rises because higher temperature makes enzymes and molecules move faster, so reactions happen more easily; above the optimum, the enzymes in the chloroplasts begin to denature, change shape, and can no longer catalyse the reaction, so the rate falls quickly. This follows exactly the same pattern as enzymes in the human body.

15. Variegated Leaves and the Need for Chlorophyll | 白斑叶与叶绿素的必要性

斑叶（白斑叶）是证明叶绿素必不可少的好材料。这种叶子的边缘是白色的，没有叶绿素，中间是绿色的。把植物放在光下数小时后取下叶子做淀粉检验，只有绿色部分变蓝黑，白色部分保持黄褐色。

A variegated leaf is a good material for proving that chlorophyll is essential. The edges of such a leaf are white with no chlorophyll, while the middle is green. After the plant has been in the light for several hours, remove a leaf and run the starch test: only the green parts turn blue-black, while the white parts stay a yellowish-brown.

这个结果说明，淀粉只出现在含叶绿素的地方，白色部分没有叶绿素，无法吸收光能，所以不能进行光合作用。叶绿素是光合作用的必要条件，没有它，即使有二氧化碳、水和阳光，反应也不会发生。

This result shows that starch only appears where chlorophyll is present; the white parts have no chlorophyll and cannot absorb light energy, so they cannot photosynthesise. Chlorophyll is a necessary condition for photosynthesis, and without it, the reaction cannot happen even when carbon dioxide, water and sunlight are all available.

16. Mineral Ions: Magnesium, Nitrate and Healthy Plants | 矿质离子：镁、硝酸盐与植物健康

光合作用和植物的矿质营养密切相关。镁是叶绿素分子的中心原子，缺镁的植物无法制造足够的叶绿素，叶子会变黄（失绿），光合速率下降，生长迟缓。

Photosynthesis is closely linked to the mineral nutrition of a plant. Magnesium is the central atom of the chlorophyll molecule; a plant lacking magnesium cannot make enough chlorophyll, so its leaves turn yellow (chlorosis), the rate of photosynthesis drops, and growth is stunted.

硝酸盐则用于制造氨基酸和蛋白质，而蛋白质是细胞生长所必需的。缺硝酸盐的植物长得矮小、叶片发黄。农民和园丁通过施加含镁和含氮的肥料，保证植物既能高效进行光合作用，又有充足的原料来生长。

Nitrate is used to make amino acids and proteins, which are essential for cell growth. Plants lacking nitrate grow small and their leaves turn yellow. Farmers and gardeners apply fertilisers containing magnesium and nitrogen so plants can both photosynthesise efficiently and have enough raw material to grow.

17. Gas Exchange in the Leaf: Stomata and Guard Cells | 叶片气体交换：气孔与保卫细胞

气孔由两个保卫细胞围成。白天，保卫细胞吸水膨胀，弯曲使气孔张开，二氧化碳得以进入；夜晚或缺水时，保卫细胞失水变软，气孔关闭以减少水分蒸发。这就是叶片调节气体交换和水分平衡的方式。

Each stoma is surrounded by two guard cells. During the day the guard cells take in water, swell, and bend so the stoma opens, letting carbon dioxide in; at night or when water is scarce, the guard cells lose water, become flaccid, and the stoma closes to reduce water loss. This is how a leaf balances gas exchange with water conservation.

气体交换靠扩散完成：叶肉细胞进行光合作用消耗二氧化碳、产生氧气，使细胞间隙里的二氧化碳浓度低于空气，于是二氧化碳顺着浓度梯度扩散进来，氧气则扩散出去。扩散不需要能量，只要有浓度差就能进行。

Gas exchange happens by diffusion: as mesophyll cells use carbon dioxide and produce oxygen, the carbon dioxide concentration inside the air spaces falls below that of the outside air, so carbon dioxide diffuses in down its concentration gradient while oxygen diffuses out. Diffusion needs no energy, only a concentration difference.

18. Photosynthesis in Farming: Greenhouses and the Ideal Conditions | 农业中的光合作用：温室与理想条件

温室（大棚）利用光合作用的原理来提高作物产量。农民在温室里控制温度、二氧化碳浓度和光照，把三个限制因素都维持在较高水平，让植物一直以接近最快的速率进行光合作用。

Greenhouses use the principles of photosynthesis to raise crop yields. Farmers control temperature, carbon dioxide concentration and light inside the greenhouse, keeping all three limiting factors at high levels so plants photosynthesise at close to their maximum rate all the time.

具体做法包括：燃烧天然气或丙烷取暖，同时产生二氧化碳作为副产物；用人工照明在冬季和阴天补充光照；用恒温器保持最适温度。但成本也需要权衡，因为加热、照明和补充二氧化碳都要花钱，农民要算清楚增产的收入是否超过这些开支。

Specific techniques include: burning natural gas or propane for heating, which also produces carbon dioxide as a by-product; using artificial lighting to supplement light in winter and on cloudy days; and using thermostats to hold the optimum temperature. But costs must be weighed, because heating, lighting and extra carbon dioxide all cost money, and farmers must calculate whether the extra yield pays for these inputs.

19. Photosynthesis vs. Respiration: Two Opposite Processes | 光合作用与呼吸作用：两个相反的过程

光合作用和呼吸作用经常被混淆，但它们本质上是相反的过程。下面的表格把它们并排比较，帮助你记住关键区别。

Photosynthesis and respiration are often confused, but they are essentially opposite processes. The table below compares them side by side to help you remember the key differences.

比较项 Feature	光合作用 Photosynthesis	呼吸作用 Respiration
发生场所 Where	叶绿体 Chloroplasts	所有细胞的线粒体 Mitochondria of all cells
是否需要光 Light needed?	需要 Yes	不需要 No (day and night)
气体交换 Gases	吸收 CO₂，释放 O₂ Takes in CO₂, releases O₂	吸收 O₂，释放 CO₂ Takes in O₂, releases CO₂
能量 Energy	储存能量 Stores energy	释放能量 Releases energy
葡萄糖 Glucose	制造葡萄糖 Makes glucose	分解葡萄糖 Breaks glucose down

记住一条口诀：光合作用把能量”存进去”，呼吸作用把能量”取出来”。两者都发生在植物体内，植物白天通常净进行光合作用，夜晚则只进行呼吸作用。

Remember one rule: photosynthesis puts energy in, and respiration takes energy out. Both happen inside plants, which are usually net photosynthesising in the day and only respiring at night.

20. Key Terms Glossary and Quick Revision Checklist | 关键术语表与快速复习清单

考前可以对照这份术语表自查：叶绿体是光合作用的场所；叶绿素是吸收光能的绿色色素；气孔是气体进出的孔；栅栏组织是叶绿体最多的叶肉层；限制因素是短缺而拖慢速率的条件；淀粉是葡萄糖的储存形式；失绿是缺镁导致的叶片变黄。

Use this glossary for a final self-check before the exam: chloroplast is the site of photosynthesis; chlorophyll is the green pigment that absorbs light; stoma is the pore for gas exchange; the palisade layer is the mesophyll layer richest in chloroplasts; a limiting factor is the scarce condition that slows the rate; starch is the storage form of glucose; chlorosis is the yellowing of leaves caused by magnesium deficiency.

快速复习清单：能写出文字方程式和平衡方程式；能说出叶片的两到三种适应结构及其功能；能解释三个限制因素如何影响速率曲线；能描述淀粉检验和伊乐藻实验的步骤与安全要点；能说明植物利用葡萄糖的五种方式。这五点覆盖了 IGCSE 生物光合作用一章的主要考点。

The quick revision checklist: write the word and balanced equations; state two or three leaf adaptations and their functions; explain how the three limiting factors shape the rate curve; describe the steps and safety points of the starch test and the pondweed experiment; and list the five ways plants use glucose. These five points cover the main examinable ideas of the IGCSE Biology photosynthesis topic.

Summary | 总结

光合作用是绿色植物利用光能，在叶绿素帮助下把二氧化碳和水合成为葡萄糖和氧气的过程，平衡方程式为 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。叶片和叶绿体的结构都高度适应这一过程，气孔、栅栏组织和叶脉各司其职。光合速率受光照强度、二氧化碳浓度和温度三个限制因素控制，可以用淀粉检验和伊乐藻实验来探究。葡萄糖用于呼吸、生长和储存，光合作用也因此成为碳循环和整个食物链的基础。

Photosynthesis is the process by which green plants use light energy, with the help of chlorophyll, to combine carbon dioxide and water into glucose and oxygen, following the balanced equation 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The structure of the leaf and chloroplast is highly adapted to this process, with stomata, the palisade layer and veins each playing their own role. The rate is controlled by three limiting factors, light intensity, carbon dioxide concentration and temperature, and can be investigated with the starch test and the pondweed experiment. Glucose is used for respiration, growth and storage, which is why photosynthesis underpins the carbon cycle and every food chain.

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