📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants, algae and some bacteria use sunlight, carbon dioxide and water to produce glucose and oxygen. This fundamental process sustains nearly all life on Earth by forming the base of most food chains and releasing the oxygen we breathe.
光合作用是绿色植物、藻类和某些细菌利用阳光、二氧化碳和水制造葡萄糖和氧气的过程。这一基本过程通过构成大多数食物链的基础并释放我们呼吸的氧气，维持着地球上几乎所有生命。
1. The Importance of Photosynthesis | 光合作用的重要性
Photosynthesis is crucial for life on Earth. It provides food for plants and indirectly for all heterotrophic organisms.
光合作用对地球生命至关重要。它为植物提供食物，并间接为所有异养生物提供食物。
It releases oxygen into the atmosphere, which is essential for aerobic respiration in most living organisms.
它将氧气释放到大气中，氧气对大多数生物的需氧呼吸至关重要。
It helps regulate the carbon cycle by removing carbon dioxide from the air.
它还通过从空气中移除二氧化碳来调节碳循环。
2. Chemical Equation of Photosynthesis | 光合作用的化学方程式
The word equation for photosynthesis can be written as: carbon dioxide + water → glucose + oxygen.
光合作用的文字方程可以写作：二氧化碳 + 水 → 葡萄糖 + 氧气。
The balanced chemical equation is shown below.
平衡的化学方程式如下所示。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
二氧化碳 + 水 → (在光能和叶绿素作用下) 葡萄糖 + 氧气
In this equation, six molecules of carbon dioxide combine with six molecules of water, using light energy trapped by chlorophyll, to produce one molecule of glucose and six molecules of oxygen.
在这个方程中，六个二氧化碳分子与六个水分子在叶绿素捕获的光能作用下，生成一个葡萄糖分子和六个氧分子。
3. Leaf Structure Adapted for Photosynthesis | 叶片结构对光合作用的适应
Leaves are highly specialised organs for photosynthesis. Their broad, flat shape provides a large surface area for capturing sunlight.
叶片是高度特化的光合作用器官。其宽阔扁平的形状提供了大面积来捕获阳光。
The palisade mesophyll layer contains many chloroplasts and is located near the upper surface, maximising light absorption.
栅栏组织层含有大量叶绿体，并位于靠近上表皮的位置，从而最大化光吸收。
Stomata allow carbon dioxide to enter and oxygen to leave the leaf. Guard cells regulate their opening and closing.
气孔允许二氧化碳进入叶片，氧气离开叶片。保卫细胞调节气孔的开放和关闭。
Xylem and phloem in leaf veins transport water to the cells and carry away glucose produced.
叶脉中的木质部和韧皮部将水运输到细胞，并运走生成的葡萄糖。
4. Chlorophyll and Photosynthetic Pigments | 叶绿素和光合色素
Chlorophyll is the green pigment found in chloroplasts. It absorbs light, primarily in the red and blue-violet parts of the spectrum, and reflects green light, which is why leaves appear green.
叶绿素是叶绿体中的绿色色素。它主要吸收红光和蓝紫光，并反射绿光，这就是叶片呈绿色的原因。
Chlorophyll also transfers the captured light energy into chemical energy, which is used to convert carbon dioxide and water into glucose.
叶绿素还将捕获的光能转化为化学能，用于将二氧化碳和水转化为葡萄糖。
Other pigments, such as carotenoids, absorb additional wavelengths of light and pass the energy to chlorophyll, helping plants use a wider range of light.
其他色素，如类胡萝卜素，吸收额外波长的光并将能量传递给叶绿素，帮助植物利用更广泛的光。
5. The Two Stages of Photosynthesis | 光合作用的两个阶段
Photosynthesis occurs in two main stages: the light-dependent stage and the light-independent stage.
光合作用主要分两个阶段：光依赖阶段和光不依赖阶段。
In the light-dependent stage, light energy is absorbed by chlorophyll and splits water into oxygen, hydrogen ions and electrons. This stage produces ATP and reduced NADP, which are used in the next stage. The oxygen is released as a by-product.
在光依赖阶段，叶绿素吸收光能并将水分解为氧气、氢离子和电子。该阶段产生ATP和还原型NADP，用于下一阶段。氧气作为副产品被释放。
In the light-independent stage (also called the Calvin cycle), carbon dioxide is fixed and reduced using the ATP and reduced NADP produced earlier to form glucose. This stage does not directly require light, but stops if light is absent. In IGCSE, students are often taught a simplified version of these stages.
在光不依赖阶段（又称卡尔文循环），利用前一步产生的ATP和还原型NADP固定并还原二氧化碳，形成葡萄糖。该阶段不直接需要光，但缺少光时就会停止。IGCSE课程通常教授这些阶段的简化版本。
6. Factors Affecting Photosynthesis | 影响光合作用的因素
The rate of photosynthesis is limited by three main factors: light intensity, carbon dioxide concentration and temperature.
光合作用速率受到三个主要因素的限制：光强度、二氧化碳浓度和温度。
As light intensity increases, the rate of photosynthesis rises until another factor becomes limiting. The graph plateaus when light is no longer the limiting factor.
随着光强度增加，光合作用速率会上升，直到另一个因素成为限制因素。当光不再是被限制因素时，曲线达到平台。
Similarly, increasing carbon dioxide concentration increases the rate up to a saturation point. Temperature affects the activity of enzymes involved in photosynthesis; above an optimum temperature, enzymes denature and the rate sharply decreases.
类似地，增加二氧化碳浓度可提高速率，直至达到饱和点。温度影响光合作用相关酶的活性；超过最适温度，酶会变性，速率急剧下降。
This relationship can be summarised by the concept of limiting factors: the slowest factor determines the overall rate.
这种关系可以用限制因素的概念来概括：最慢的因素决定总速率。
7. Photosynthesis and Respiration | 光合作用与呼吸作用
Photosynthesis and respiration are complementary processes. Photosynthesis builds glucose using carbon dioxide and water, while respiration breaks down glucose to release energy, producing carbon dioxide and water.
光合作用和呼吸作用是互补的过程。光合作用利用二氧化碳和水制造葡萄糖，而呼吸作用分解葡萄糖释放能量，产生二氧化碳和水。
| Feature | Photosynthesis | Respiration |
| Energy | Stores light energy as chemical energy | Releases chemical energy |
| Reactants | CO₂ and H₂O | Glucose and O₂ |
| Products | Glucose and O₂ | CO₂ and H₂O |
| Occurs in | Chloroplasts (plants, algae) | Mitochondria (all living cells) |
When light is absent, plants perform only respiration. When light is present, they perform both photosynthesis and respiration, with the net exchange of gases depending on the rates of each process.
当没有光照时，植物只进行呼吸作用。有光照时，植物同时进行光合作用和呼吸作用，净气体交换量取决于这两个过程的速率。
8. Applications in Agriculture | 农业中的应用
Farmers and gardeners can increase crop yields by controlling the factors that affect photosynthesis.
农民和园丁可通过控制影响光合作用的因素来提高作物产量。
In greenhouses, supplementary lighting extends the day length and raises light intensity, allowing photosynthesis to continue longer.
在温室中，补充光照可延长日照时长并提高光强度，使光合作用持续更久。
Adding carbon dioxide generators increases the CO₂ concentration, which can enhance the rate of photosynthesis, especially in closed greenhouses.
添加二氧化碳发生器可提高CO₂浓度，尤其在封闭温室中能增强光合作用速率。
Heating systems maintain an optimum temperature for enzyme activity, but too high a temperature can denature enzymes, so thermostats are used to regulate it.
加热系统可维持酶活动的最适温度，但过高的温度会使酶变性，因此使用温控器进行调节。
9. Investigating Photosynthesis | 探究光合作用
A common experiment to demonstrate photosynthesis uses a leaf that has been partly covered with a stencil or black paper. After exposing the plant to light, the leaf is tested for starch using iodine solution.
演示光合作用的常见实验是使用一张部分被模板或黑纸遮住的叶片。将植物光照后，用碘液测试叶片是否含有淀粉。
Steps: place a destarched plant in light for a few hours, then remove the leaf, boil it in water to kill cells, boil it in ethanol to remove chlorophyll, and finally add iodine solution.
步骤：将去除淀粉的植物置于光照下数小时，然后摘取叶片，用热水煮沸以杀死细胞，再用乙醇煮沸去除叶绿素，最后滴加碘液。
Only the parts of the leaf that received light turn blue-black, proving that light and chlorophyll are needed for photosynthesis to produce starch.
只有受光部分变为蓝黑色，证明光合作用需要光和叶绿素才能产生淀粉。
Another simple method uses water weed (Elodea) to count bubbles of oxygen released under different light intensities. A faster bubble rate indicates a higher rate of photosynthesis.
另一种简单方法使用水草（如伊乐藻）在不同光强度下计数释放的氧气气泡。气泡逸出越快，说明光合作用速率越高。
10. Summary | 总结
Photosynthesis is an essential biological process that converts light energy into chemical energy. It occurs in chloroplasts using sunlight, carbon dioxide and water to produce glucose and oxygen.
光合作用是将光能转化为化学能的重要生物过程。它发生在叶绿体中，利用阳光、二氧化碳和水产生葡萄糖和氧气。
The rate of photosynthesis is affected by light intensity, carbon dioxide concentration and temperature. Understanding these limiting factors allows growers to improve crop productivity.
光合作用速率受光强度、二氧化碳浓度和温度影响。理解这些限制因素有助于种植者提高作物生产力。
By testing for starch or measuring oxygen production, scientists can investigate the conditions required for this vital reaction.
通过检验淀粉或测定氧气产生量，科学家可以探究这一重要反应所需的条件。
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