📚 Photosynthesis | 光合作用
Photosynthesis is the biological process by which green plants, algae, and some bacteria make glucose using carbon dioxide, water, and light energy. It supplies the organic molecules and oxygen that most life on Earth depends on.
光合作用是绿色植物、藻类和部分细菌利用二氧化碳、水和光能制造葡萄糖的生理过程。它为地球上绝大多数生命提供了有机物和氧气，是碳循环和食物链的基础。
1. What Is Photosynthesis? | 什么是光合作用？
Photosynthesis is an endothermic reaction because it transfers light energy into chemical energy stored in glucose. The energy in glucose can later be released by respiration.
光合作用是吸热反应，因为它把光能转化为储存在葡萄糖中的化学能。葡萄糖中的能量之后可通过呼吸作用释放出来，供细胞使用。
It takes place mainly in the leaves of plants, where leaf cells contain large numbers of chloroplasts. Each chloroplast contains green chlorophyll pigment.
光合作用主要发生在植物的叶片中，因为叶肉细胞内含有大量叶绿体。每个叶绿体中都含有绿色的色素——叶绿素。
2. The Word Equation | 文字方程式
You must be able to recall the word equation for photosynthesis exactly:
你必须能够准确记住光合作用的文字方程式：
carbon dioxide + water → glucose + oxygen
The arrow means "give" under the influence of light and chlorophyll. Light energy is absorbed by chlorophyll and drives the reaction.
箭头表示“生成”，条件是在光照和叶绿素作用下。叶绿素吸收光能并提供反应所需的能量。
3. Balanced Symbol Equation | 配平的符号方程式
The symbol equation is also essential for Edexcel IGCSE Science. It must be balanced:
配平的符号方程式同样是Edexcel IGCSE科学考试的重点，必须熟练掌握：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Here, six carbon dioxide molecules react with six water molecules to produce one glucose molecule and six oxygen molecules. Light energy and chlorophyll are required.
在这个方程中，6个二氧化碳分子与6个水分子反应，生成1个葡萄糖分子和6个氧气分子，并且需要光能和叶绿素参与。
4. Requirements | 光合作用的条件
For photosynthesis to occur at a measurable rate, four main inputs must be present:
要使光合作用以可测的速率发生，必须有四种主要原料或条件：
Light: provides energy for making bonds in glucose.
光照：为合成葡萄糖提供能量。
Carbon dioxide: enters leaves through stomata and is the carbon source.
二氧化碳：通过气孔进入叶片，是葡萄糖的碳源。
Water: absorbed by root hairs and transported to leaves via xylem.
水：由根毛吸收，通过木质部运输至叶片。
Chlorophyll: green pigment that absorbs light and transfers energy.
叶绿素：吸收光能并传递能量的绿色色素。
In addition, temperature must be high enough for enzymes to work efficiently. If any of these factors is missing or too low, photosynthesis slows down.
此外，温度需足够高，酶才能高效工作。上述任何一项缺乏或过低，光合作用速率都会下降。
5. The Role of Chlorophyll | 叶绿素的作用
Chlorophyll is found inside chloroplasts. Its main role is to absorb light and pass the energy to the light-dependent reactions of photosynthesis.
叶绿素位于叶绿体内部，主要作用是吸收光线，并把能量传递给光合作用的光反应阶段。
Because chlorophyll absorbs blue and red light strongly but reflects green light, leaves appear green. The pigment is not destroyed in the process, so it acts as a reusable catalyst-like molecule.
叶绿素主要吸收蓝光和红光，反射绿光，所以叶片呈绿色。叶绿素在反应中不会被消耗，因此可反复使用，起到类似催化剂的作用。
6. Limiting Factors | 限制因子
A limiting factor is the substance or condition that is in shortest supply and therefore controls the rate of photosynthesis.
限制因子是指供应最不足、因而控制光合作用速率的某种物质或条件。
| Factor / 因素 | How it limits photosynthesis / 如何限制光合作用 |
| Light intensity 光照强度 | More light increases energy available, until another factor is limiting. 光照越多，可利用的能量越多，直到另一个因子成为限制因素。 |
| CO₂ concentration 二氧化碳浓度 | CO₂ is a raw material; low CO₂ limits carbon supply. 二氧化碳是原料，浓度过低会限制碳源的供应。 |
| Temperature 温度 | Affects enzyme activity; very high temperatures denature enzymes. 影响酶活性；温度过高会使酶变性。 |
If a factor is not limiting, it means it is in sufficient supply. Increasing it will not increase the rate.
如果某个因素“不成为限制因子”，说明它的供应量已经充足，增加它并不会提高光合作用速率。
7. Light Intensity and CO₂ Concentration | 光照强度与二氧化碳浓度
When light intensity rises, the rate of photosynthesis increases almost proportionally at first. This is because light provides energy for the reaction.
当光照强度上升时，光合作用速率最初几乎成比例增加，因为光能为反应提供能量。
Eventually the graph levels off. This plateau shows that light is no longer the limiting factor; carbon dioxide or temperature has become limiting.
最终曲线趋于平缓。这种“平台”表明光照已不再是限制因子，而是二氧化碳浓度或温度成为了限制因子。
Similarly, increasing CO₂ concentration from a low value raises the rate, but only until another factor limits the reaction.
类似地，在较低浓度下增加二氧化碳浓度会提高速率，但这种提高会在另一个因子成为限制条件时停止。
8. Temperature | 温度的影响
As temperature rises from cold conditions, enzymes involved in photosynthesis work faster, so the rate increases. The optimum is usually around 25–35 °C for many plants.
当温度从低温升高时，参与光合作用的酶活性增强，速率加快。对多数植物而言，最适温度通常在25–35 °
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