📚 Photosynthesis | 光合作用
Photosynthesis is the fundamental process by which green plants, algae and some bacteria capture light energy and convert it into chemical energy stored in glucose. It provides the oxygen we breathe and forms the base of almost all food webs.
光合作用是绿色植物、藻类和某些细菌捕获光能并将其转化为储存在葡萄糖中的化学能的基本过程。它为我们提供了呼吸所需的氧气，并构成了几乎所有食物网的基础。
This revision article covers the key ideas required for Edexcel IGCSE Science, including equations, leaf structure, limiting factors and practical investigations.
本篇复习文章涵盖Edexcel IGCSE科学的主要考点，包括方程式、叶片结构、限制因子及实验探究。
1. The Importance of Photosynthesis | 光合作用的重要性
Photosynthesis maintains atmospheric oxygen and carbon dioxide balance. It also produces organic molecules that all heterotrophs depend on directly or indirectly.
光合作用维持着大气中氧气和二氧化碳的平衡，并产生所有异养生物直接或间接依赖的有机分子。
Energy enters ecosystems: Light energy is converted into chemical energy in glucose, which becomes the energy source for consumers and decomposers.
能量进入生态系统：光能转化为葡萄糖中的化学能，成为消费者和分解者的能量来源。
Oxygen production: Almost all free oxygen in Earth's atmosphere comes from photosynthetic organisms, making aerobic respiration possible.
氧气产生：地球大气中几乎所有的游离氧都来自光合生物，使有氧呼吸成为可能。
Carbon fixation: Photosynthesis removes carbon dioxide from the atmosphere, reducing the greenhouse effect and supplying carbon skeletons for organic compounds.
碳固定：光合作用从大气中移除二氧化碳，减轻温室效应，并为有机化合物提供碳骨架。
2. The Word and Balanced Symbol Equation | 文字方程与配平符号方程
The overall process can be summarised by a word equation that is required at IGCSE level.
整个过程可以用IGCSE阶段要求的文字方程来概括。
carbon dioxide + water → glucose + oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
The balanced symbol equation shows the exact ratio of molecules involved.
配平符号方程显示了参与反应的分子的准确数量关系。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Carbon dioxide is reduced to glucose, while water is oxidised to oxygen. This reaction is endothermic because energy is absorbed from light.
二氧化碳被还原为葡萄糖，水被氧化为氧气。该反应是吸热的，因为能量来自对光的吸收。
3. Raw Materials and Products | 原料与产物
Photosynthesis requires several raw materials obtained from the environment, and it produces materials used by the plant.
光合作用需要从环境中获得的多种原料，并产生植物自身使用的多种产物。
| Raw material / 原料 | Source / 来源 |
| Carbon dioxide (CO₂) | Diffuses into leaves through stomata from the atmosphere or from respiration. |
| Water (H₂O) | Absorbed by root hair cells and transported to leaves via xylem. |
| Light energy | Sunlight or artificial light absorbed by chlorophyll. |
| Chlorophyll | Green pigment in chloroplasts; not consumed but essential. |
Glucose and oxygen are the direct products. Oxygen is diffused out as a waste product, while glucose is either used in respiration or converted into larger molecules.
葡萄糖和氧气是直接产物。氧气作为废物扩散出叶片，葡萄糖则用于呼吸作用或转化为更大的分子。
4. Chlorophyll and the Absorption of Light | 叶绿素与光吸收
Chlorophyll is located in the chloroplasts, mainly in the palisade mesophyll cells. This pigment absorbs light energy and uses it to split water and generate ATP.
叶绿素位于叶绿体中，主要集中在栅栏层叶肉细胞。这种色素吸收光能，并利用光能分解水并产生ATP。
Chlorophyll absorbs most light in the red and blue-violet regions of the spectrum, while green light is reflected. This is why leaves appear green.
叶绿素主要吸收光谱中的红光和蓝紫光区域，而绿光被反射，所以叶片呈现绿色。
In the light-dependent reaction, light energy excites electrons in chlorophyll. These electrons pass along an electron transport chain, leading to the production of ATP and reduced NADP. Water is split in the process, releasing oxygen.
在光依赖反应中，光能激发叶绿体中的电子。这些电子沿电子传递链传递，产生ATP和还原型NADP。同时水被分解并释放氧气。
5. Leaf Structure and Photosynthesis | 叶片结构与光合作用
The leaf is a highly specialised organ for maximising photosynthesis efficiency.
叶片是高度特化的器官，能够最大化光合作用效率。
Cuticle and epidermis: The outer waxy cuticle reduces water loss, while the transparent upper epidermis allows light to pass through.
角质层与表皮：外层角质层减少水分蒸发，透明的上表皮可以让光通过。
Palisade mesophyll: Long, tightly packed cells containing many chloroplasts are found near the upper surface, capturing maximum light.
栅栏层叶肉：长形且排列紧密的细胞含有大量叶绿体，位于靠近上表面处，以捕获最多光照。
Spongy mesophyll: Loosely packed cells create air spaces, allowing carbon dioxide to diffuse rapidly to photosynthesising cells.
海绵层叶肉：排列疏松的细胞形成气室，使二氧化碳快速扩散到进行光合作用的细胞中。
Stomata: Tiny pores, mostly in the lower epidermis, allow gas exchange: CO₂ enters and O₂ leaves.
气孔：主要位于下表皮的小孔，允许气体交换：二氧化碳进入，氧气离开。
Vascular bundles: Xylem supplies water; phloem transports glucose and other products away.
维管束：木质部供应水分，韧皮部将葡萄糖和其他产物运走。
6. Factors Affecting the Rate of Photosynthesis | 影响光合作用速率的因素
The rate of photosynthesis is affected by several environmental variables, especially light intensity, carbon dioxide concentration and temperature.
光合作用速率受多种环境变量影响，尤其是光照强度、二氧化碳浓度和温度。
Light intensity: As light intensity increases, more energy is available to excite chlorophyll, so photosynthesis speeds up. However, a point is reached where another factor becomes limiting.
光照强度：随着光照强度增加，更多能量可用于激发叶绿素，光合作用加快。然而，当达到某一点后，其他因素会成为限制因素。
Carbon dioxide concentration: CO₂ is a raw material needed in the Calvin cycle. Increasing CO₂ concentration raises the rate until the enzymes become saturated.
二氧化碳浓度：二氧化碳是暗反应需要的原料。增加CO₂浓度会提高速率，直到酶达到饱和。
Temperature: Photosynthesis is enzyme-controlled. Higher temperatures increase molecular collisions until the optimum temperature is exceeded, after which enzymes denature and the rate falls sharply.
温度：光合作用由酶控制。温度升高增加分子碰撞频率，直到超过最适温度，之后酶变性失活，速率急剧下降。
7. Limiting Factors and Graphs | 限制因子与曲线图
A limiting factor is any factor that is in shortest supply, which determines the maximum possible rate of photosynthesis at that moment.
限制因子是指供应最短缺的某一因素，它决定了那一刻光合作用能达到的最大速率。
In a typical graph of light intensity against rate of photosynthesis, the curve rises steeply at first, then levels off. At the flat part, light is no longer the limiting factor; a second factor such as CO₂ concentration or temperature is limiting.
在典型的光照强度与光合作用速率关系曲线中，曲线起初迅速上升，然后趋于平缓。在平缓部分，光不再是限制因子；第二个因子如CO₂浓度或温度成为限制因子。
For example, if the CO₂ concentration is doubled, the initial slope remains the same but the plateau rises. This proves that at the plateau, CO₂ was the limiting factor.
例如，如果CO₂浓度加倍，初始斜率不变但平台升高。这说明在平台区，CO₂是以前的限制因子。
8. Light-Dependent and Light-Independent Stages | 光依赖阶段与光非依赖阶段
Although IGCSE does not require detailed biochemistry, an overview helps you understand why light and temperature affect photosynthesis differently.
虽然IGCSE不要求详细生化知识，但概述能帮助你理解为什么光和温度对光合作用的影响不同。
Light-dependent stage (in thylakoids): Light energy splits water (photolysis), producing oxygen, ATP and reduced NADP. This stage stops immediately in darkness.
光依赖阶段（在类囊体中）：光能分解水（光解），产生氧气、ATP和还原型NADP。此阶段在黑暗中立即停止。
Light-independent stage (in stroma): ATP and reduced NADP from the light-dependent stage are used to convert carbon dioxide into glucose. This stage is driven by enzymes and therefore strongly affected by temperature, but it does not need light directly.
光非依赖阶段（在基质中）：利用光依赖阶段产生的ATP和还原型NADP将二氧化碳转化为葡萄糖。此阶段由酶驱动，因此受温度影响很大，但不需要光直接参与。
9. Translocation and Storage of Products | 产物的转运与储存
Glucose produced in photosynthesis is rapidly converted into sucrose for transport through the plant. Sucrose is dissolved in water and moves in the phloem to roots, fruits and growing regions.
光合作用产生的葡萄糖会迅速转化为蔗糖以便运输。蔗糖溶解在水中，通过韧皮部运往根、果实和生长区域。
At the destination, sucrose can be stored as starch in amyloplasts or used in cellular respiration. Starch is insoluble, so it does not affect water potential and is ideal for storage.
在目的地，蔗糖可以淀粉形式储存在淀粉体中，或用于细胞呼吸。淀粉不溶于水，因此不会影响水势，非常适合储存。
10. Uses of Glucose in Plants | 葡萄糖在植物中的用途
Glucose is used by plants in several important ways.
葡萄糖在植物中有多种重要用途。
Respiration: Glucose is broken down to release energy for cellular processes, such as active transport and protein synthesis.
呼吸作用：葡萄糖被分解以释放能量，供主动运输和蛋白质合成等生命过程使用。
Cellulose: Glucose molecules are joined to form cellulose, a structural polysaccharide strengthening plant cell walls.
纤维素：葡萄糖分子连接形成纤维素，这是增强植物细胞壁的结构性多糖。
Starch: Excess glucose is converted to starch for storage in leaves, roots and seeds.
淀粉：多余的葡萄糖转化为淀粉，储存在叶、根和种子中。
Amino acids: Glucose is combined with nitrate ions from the soil to synthesise amino acids, which form proteins.
氨基酸：葡萄糖与从土壤吸收的硝酸根离子结合，合成氨基酸，再形成蛋白质。
Lipids (oils): Glucose can be converted into oils and fats, which store large amounts of energy in seeds.
脂质（油脂）：葡萄糖可转化为油和脂肪，在种子中储存大量能量。
11. Investigating Photosynthesis with Aquatic Plants | 用水生植物探究光合作用
A common Edexcel practical uses an aquatic plant such as Cabomba or Elodea to measure the rate by counting oxygen bubbles.
Edexcel常见的实验是使用金鱼藻（Cabomba）或伊乐藻（Elodea）等水生植物，通过计算氧气气泡来测定速率。
Procedure: Place a cut stem of the plant upside down in a test tube of water with 0.2% sodium hydrogen carbonate solution. Adjust a lamp to a fixed distance, allow the plant to acclimatise for five minutes, then count the number of bubbles produced per minute. Repeat at different light intensities by changing lamp distance.
步骤：将植物切下的茎倒放在装有0.2%碳酸氢钠溶液（提供CO₂）的试管中。将台灯调至固定距离，让植株适应五分钟后，计每分钟产生的气泡数。通过改变灯距，在不同光照强度下重复实验。
Expected result: As the lamp is moved closer, light intensity increases, so the bubble rate increases. The rate may plateau if the lamp is very close because light is no longer limiting.
预期结果：随着灯靠近，光照强度增大，气泡速率增加。当灯非常近时速率可能达到平台，因为光不再是限制因子。
To improve the experiment, use a gas syringe to collect oxygen volume or measure the increase in mass of dried plant material. Remember to control temperature to avoid changing enzyme activity.
为了改进实验，可使用气体注射器收集氧气的体积，或测量干燥植物材料的质量增加。注意控制温度，避免改变酶活性。
12. Common Misconceptions and Exam Tips | 常见误区与考试技巧
Many students lose marks on photosynthesis questions due to a few common misunderstandings.
许多学生在光合作用题目中因为几个常见误解而失分。
Misconception 1: "Plants respire only at night." In fact, plants respire all the time, absorbing O₂ and releasing CO₂ both day and night.
误区1：“植物只在夜间呼吸。”事实上，植物一直进行呼吸作用，白天和夜间都吸收氧气并释放二氧化碳。
Misconception 2: "Photosynthesis only happens in the light-dependent reaction." The light-independent reaction also uses ATP and NADPH made in the light, so it cannot continue for long in darkness.
误区2：“光合作用只发生在光依赖反应中。”光非依赖反应也需要在光中生成的ATP和NADPH，因此在黑暗中不能持续太久。
Misconception 3: "Oxygen is the only product." Glucose is stored or used; oxygen is only a by-product.
误区3：“氧气是唯一产物。”葡萄糖会被储存或利用；氧气只是副产物。
Exam tip: When asked to identify a limiting factor from a graph, compare the distance between curves. If raising one factor raises the plateau, that factor was limiting. Always quote the data in your answer.
考试技巧：当要求根据图像识别限制因子时，比较曲线之间的间距。如果提高某一因子后平台升高，说明该因子以前是限制因子。作答时一定要引用数据。
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