Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is fundamental to life on Earth, providing both oxygen and organic compounds for almost all living organisms.

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。它是地球生命的基础，为几乎所有生物提供氧气和有机化合物。

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1. The Word Equation | 文字方程式

Photosynthesis can be summarised by the following word equation: carbon dioxide + water → glucose + oxygen. In this reaction, light energy is absorbed by chlorophyll and converted into chemical energy.

光合作用可用以下文字方程式概括：二氧化碳 + 水 → 葡萄糖 + 氧气。在此反应中，光能被叶绿素吸收并转化为化学能。

Carbon dioxide + Water → Glucose + Oxygen

二氧化碳 + 水 → 葡萄糖 + 氧气

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2. The Chemical Equation | 化学方程式

The balanced chemical equation for photosynthesis is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The numbers show that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.

光合作用的平衡化学方程式为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。数字表明，六个二氧化碳分子与六个水分子反应，生成一个葡萄糖分子和六个氧分子。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

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3. Chlorophyll and Light Absorption | 叶绿素与光吸收

Chlorophyll is a green pigment found in chloroplasts, mainly in the mesophyll cells of leaves. It absorbs red and blue light most effectively and reflects green light, which is why leaves appear green.

叶绿素是叶绿体中的绿色色素，主要存在于叶肉细胞中。它最有效地吸收红光和蓝光，并反射绿光，因此叶子呈现绿色。

• Chlorophyll is located in the thylakoid membranes of chloroplasts.

  叶绿素位于叶绿体的类囊体膜上。

• Light energy excites electrons in chlorophyll, starting the light-dependent reactions.

  光能激发叶绿素中的电子，启动光依赖反应。

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4. The Two Stages of Photosynthesis | 光合作用的两个阶段

Photosynthesis occurs in two main stages: the light-dependent reactions and the light-independent reactions (also called the Calvin cycle). The light-dependent reactions require light and produce ATP and reduced NADP, while the light-independent reactions use these products to fix carbon dioxide into glucose.

光合作用主要分为两个阶段：光依赖反应和光不依赖反应（又称卡尔文循环）。光依赖反应需要光，产生ATP和还原型NADP；光不依赖反应利用这些产物将二氧化碳固定为葡萄糖。

Light-dependent stage → Thylakoid membrane

光依赖阶段 → 类囊体膜

Light-independent stage → Stroma

光不依赖阶段 → 基质

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5. Factors Affecting Photosynthesis | 影响光合作用的因素

The rate of photosynthesis is influenced by three main limiting factors: light intensity, carbon dioxide concentration, and temperature. At any time, the factor in shortest supply limits the rate.

光合作用速率受三个主要限制因素影响：光照强度、二氧化碳浓度和温度。在任何时刻，供应最短缺的因素限制着速率。

Factor | 因素	Effect | 作用
Light intensity | 光照强度	As light intensity increases, rate increases until another factor becomes limiting.
CO₂ concentration | 二氧化碳浓度	Higher CO₂ levels increase rate up to saturation point.
Temperature | 温度	Rate increases with temperature until enzymes denature above optimum (~35–40°C).

在IGCSE考试中，经常要求绘制并解释光合作用速率随这些因素变化的曲线图。

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6. The Role of Stomata and Gas Exchange | 气孔与气体交换的作用

Stomata are tiny pores on the leaf surface, mainly on the lower epidermis. They allow carbon dioxide to enter for photosynthesis and oxygen to exit as a waste product. Stomata also regulate water loss through transpiration.

气孔是叶片表面的微小孔隙，主要分布在下表皮。它们允许二氧化碳进入用于光合作用，并让氧气作为废物排出。气孔还通过蒸腾作用调节水分流失。

• Guard cells control the opening and closing of stomata.

  保卫细胞控制气孔的开启和关闭。

• During hot, dry conditions, stomata close to conserve water, which also reduces CO₂ uptake.

  在炎热干燥条件下，气孔关闭以保水，同时也减少了二氧化碳的吸收。

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7. The Products of Photosynthesis | 光合作用的产物

Glucose is the immediate product of photosynthesis. It can be used in respiration to release energy, converted into starch for storage, used to build cellulose for cell walls, or combined with minerals to form amino acids and other compounds.

葡萄糖是光合作用的直接产物。它可用于呼吸作用释放能量，转化为淀粉储存，用于构建细胞壁中的纤维素，或与矿物质结合形成氨基酸和其他化合物。

• Starch is stored in chloroplasts, roots, and stems.

  淀粉储存在叶绿体、根和茎中。

• Oxygen is released into the atmosphere through stomata.

  氧气通过气孔释放到大气中。

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8. Investigating Photosynthesis with a Pondweed | 使用金鱼藻探究光合作用

An Edexcel IGCSE practical involves placing a fresh piece of pondweed (Elodea) in water with sodium hydrogen carbonate solution. A lamp is placed at different distances, and the number of oxygen bubbles produced per minute is counted to measure the rate of photosynthesis.

Edexcel IGCSE的一个实验是将新鲜金鱼藻放入含有碳酸氢钠溶液的水中。将灯放置在不同距离，计算每分钟产生的氧气气泡数，以测量光合作用的速率。

Rate = bubbles per minute = oxygen production rate

速率 = 每分钟气泡数 = 产氧速率

To improve accuracy, repeat the experiment and calculate the mean. The distance from the lamp is inversely proportional to light intensity (inverse square law), but at IGCSE level you only need to describe the qualitative relationship.

为提高准确性，应重复实验并计算平均值。灯的距离与光强度成反比（平方反比定律），但在IGCSE阶段只需描述定性关系。

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9. Limiting Factor Graphs | 限制因素曲线图

Exam questions often show a graph of photosynthesis rate against a variable. For light intensity, the curve rises steeply then plateaus. The plateau indicates that another factor is limiting. For temperature, the curve rises to an optimum then falls sharply as enzymes denature.

考试题目常展示光合速率随某一变量变化的曲线图。对于光照强度，曲线先陡升后平缓。平缓段表明有其他因素限制。对于温度，曲线上升到最适点后急剧下降，因为酶变性。

• At low light intensity, light is the limiting factor.

  在低光照下，光是限制因素。

• At high light intensity, CO₂ or temperature becomes limiting.

  在高光照下，二氧化碳或温度成为限制因素。

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10. Photosynthesis and the Global Carbon Cycle | 光合作用与全球碳循环

Photosynthesis is a key process in the carbon cycle. It removes carbon dioxide from the atmosphere and converts it into organic carbon in plants. When plants respire, decompose, or are burned, carbon dioxide returns to the atmosphere.

光合作用是碳循环中的关键过程。它从大气中移除二氧化碳，并将其转化为植物中的有机碳。当植物呼吸、分解或燃烧时，二氧化碳返回大气。

Understanding photosynthesis helps explain how deforestation increases atmospheric CO₂ and contributes to climate change. Planting trees enhances carbon fixation and helps mitigate the greenhouse effect.

理解光合作用有助于解释滥伐森林如何增加大气中的二氧化碳并导致气候变化。植树能够增强碳固定，有助于缓解温室效应。

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