📚 Photosynthesis: Process, Equation and Factors | 光合作用：过程、方程式与影响因素
Photosynthesis is one of the most important biological processes on Earth. It is the way green plants, algae and some bacteria convert light energy into chemical energy, producing glucose and oxygen.
光合作用是地球上最重要的生物过程之一。绿色植物、藻类和某些细菌通过光合作用将光能转化为化学能，并生成葡萄糖和氧气。
1. What Is Photosynthesis? | 什么是光合作用
Photosynthesis is the process by which green plants make carbohydrates from carbon dioxide and water using light energy absorbed by chlorophyll. The energy is used to join carbon dioxide and water together, and oxygen is released as a by-product.
光合作用是绿色植物利用叶绿素吸收光能，将二氧化碳和水合成碳水化合物的过程。该能量将二氧化碳和水结合，并释放出氧气作为副产物。
Photosynthesis takes place mainly in the leaves of plants, inside structures called chloroplasts. Chloroplasts contain the green pigment chlorophyll, which absorbs light energy, mostly in the red and blue regions of the spectrum.
光合作用主要发生在植物的叶片中，具体部位是叶绿体。叶绿体含有绿色色素叶绿素，主要吸收光谱中的红光和蓝紫光。
2. The Word Equation | 文字方程式
The overall process of photosynthesis can be represented by the following word equation:
光合作用的总过程可以用以下文字方程式表示：
carbon dioxide + water → glucose + oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
This equation shows the reactants (carbon dioxide and water) and the products (glucose and oxygen). Light energy is not a substance, so it is written above the arrow, not as a reactant.
该方程式显示了反应物（二氧化碳和水）和产物（葡萄糖和氧气）。光能不是物质，因此写在上方箭头上，而不作为反应物。
3. The Balanced Chemical Equation | 平衡化学方程式
The balanced chemical equation for photosynthesis shows the exact number of molecules involved:
光合作用的平衡化学方程式显示了参与反应的分子的准确数量：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Six carbon dioxide molecules react with six water molecules to produce one glucose molecule and six oxygen molecules. Light energy is required for this reaction and is often shown above the arrow.
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。该反应需要光能，通常标注在箭头上方。
In the Edexcel IGCSE specification, you must be able to write both the word equation and the balanced symbol equation. Remember that the formula for glucose is C₆H₁₂O₆, and oxygen is released as O₂.
在Edexcel IGCSE考试大纲中，你必须会写出文字方程式和平衡符号方程式。记住葡萄糖的分子式是C₆H₁₂O₆，释放的氧气是O₂。
4. The Role of Chlorophyll and Chloroplasts | 叶绿素与叶绿体的作用
Chlorophyll is a green pigment located in the chloroplasts. Its main role is to absorb light energy and transfer it into chemical energy during photosynthesis.
叶绿素是一种位于叶绿体内的绿色色素。它的主要作用是吸收光能，并在光合作用过程中将其转化为化学能。
Chloroplasts also contain membranes and enzymes needed for the light-dependent and light-independent stages. Because chlorophyll is green, it reflects green light rather than absorbing it, which is why most leaves appear green.
叶绿体还含有光反应和暗反应所需的膜结构及酶。由于叶绿素是绿色的，它会反射绿光而不是吸收，因此大多数叶子呈绿色。
5. The Light-Dependent Stage | 光反应阶段
The light-dependent stage takes place in the thylakoid membranes of the chloroplasts. It needs light energy to split water molecules into oxygen gas and hydrogen ions.
光反应阶段发生在叶绿体的类囊体膜上。它需要光能将水分子分解为氧气和氢离子。
H₂O → 2H⁺ + 2e⁻ + ½O₂
H₂O → 2H⁺ + 2e⁻ + ½O₂
The oxygen is released into the atmosphere, while the hydrogen ions are carried by a coenzyme (NADP) to the next stage. Light energy is also used to make ATP from ADP and phosphate.
氧气释放到大气中，而氢离子由辅酶（NADP）携带至下一阶段。光能还可用于将ADP和磷酸转化为ATP。
6. The Light-Independent Stage | 暗反应阶段
The light-independent stage, also called the Calvin cycle, takes place in the stroma of the chloroplast. It does not directly require light, but it uses the ATP and hydrogen ions produced in the light-dependent stage.
暗反应阶段也称为卡尔文循环，发生在叶绿体基质中。它不直接需要光，但使用光反应阶段产生的ATP和氢离子。
Carbon dioxide is fixed to a five-carbon compound, and eventually one molecule of glucose is formed. This stage is affected by the temperature because it involves enzyme-catalysed reactions.
二氧化碳与五碳化合物结合，最终形成一个葡萄糖分子。该阶段受温度影响，因为它涉及酶催化的反应。
7. How Plants Get the Raw Materials | 植物如何获取原料
Carbon dioxide enters the leaf through small pores called stomata. It diffuses from the air into the leaf when the concentration inside the leaf is lower than outside.
二氧化碳通过叶片上的气孔进入。当叶片内二氧化碳浓度低于外界时，它从空气扩散进入叶片。
Water is absorbed by the roots from the soil and transported to the leaves through xylem vessels. The xylem forms part of the plant’s transport system, moving water and mineral ions upward.
水由根从土壤中吸收，并通过木质部导管输送至叶片。木质部是植物运输系统的一部分，负责向上输送水分和矿质离子。
8. The Products of Photosynthesis | 光合作用的产物
Glucose is the main carbohydrate product. It can be used in respiration to release energy, or converted into other substances such as starch for storage, cellulose for cell walls, and sucrose for transport.
葡萄糖是主要的碳水化合物产物。它可用于呼吸作用释放能量，也可以转化为其他物质，如用于储存的淀粉、用于细胞壁的纤维素和用于运输的蔗糖。
Glucose can also be converted into amino acids when nitrogen is available, and into lipids for energy storage in seeds. Oxygen is a waste product of photosynthesis but is essential for nearly all living organisms.
在有氮元素时，葡萄糖还可转化为氨基酸，并转化为脂质储存在种子中。氧气是光合作用的废弃产物，但对几乎所有生物都至关重要。
9. Factors Affecting the Rate of Photosynthesis | 影响光合作用速率的因素
The main factors that limit the rate of photosynthesis are light intensity, carbon dioxide concentration and temperature. An increase in any of these factors can speed up photosynthesis, but only up to a certain point.
限制光合作用速率的主要因素是光照强度、二氧化碳浓度和温度。这些因素中任何一个增加都能加快光合作用，但只能达到一定限度。
The table below summarises the effects of each factor:
下表总结了各因素的影响：
| Factor / 因素 | Effect / 影响 |
| Light intensity / 光照强度 | As light intensity increases, the rate increases until light is no longer the limiting factor. |
| CO₂ concentration / 二氧化碳浓度 | Increasing CO₂ raises the rate until another factor becomes limiting. |
| Temperature / 温度 | Temperature increases the rate up to the optimum, then enzymes denature above a critical point. |
In Edexcel IGCSE, you also need to know that a shortage of water can close stomata and reduce CO₂ uptake, slowing photosynthesis.
在Edexcel IGCSE中，你还需要知道水分不足会导致气孔关闭，减少CO₂吸收，从而降低光合作用速率。
10. Limiting Factors | 限制因子
A limiting factor is any condition that slows down a process when it is in short supply. In photosynthesis, if light is abundant but CO₂ is low, CO₂ is the limiting factor.
限制因子是指供应不足时拖慢过程速率的任何条件。在光合作用中，如果光照充足但CO₂很少，则CO₂成为限制因子。
For example, increasing light intensity when light is already saturating will not increase the rate further. The limiting factor might then be temperature or carbon dioxide concentration.
例如，当光照已饱和时，增加光照强度不会进一步提高速率。此时限制因子可能是温度或二氧化碳浓度。
Farmers can use this knowledge in greenhouses by adding artificial light, enriching CO₂ levels, and using heaters to maintain an optimum temperature.
农民可以在温室中利用这一知识：提供人工光照、增加CO₂浓度，并使用加热器保持最适温度。
11. Measuring the Rate of Photosynthesis | 测量光合作用速率
The rate of photosynthesis can be measured by the volume of oxygen produced, the amount of carbon dioxide absorbed, or the increase in biomass of the plant.
光合作用速率可以通过产生氧气的体积、吸收二氧化碳的量或植物生物量的增加来测量。
A common practical method uses an aquatic plant such as Elodea or Cabomba in a test tube with water. As photosynthesis occurs, oxygen bubbles are released, and the number of bubbles per minute or the volume of gas collected can be counted.
一个常见的实验方法是将水生植物（如金鱼藻或角藻）放入装有水的试管中。光合作用发生时释放氧气气泡，可以计算每分钟气泡数或收集的气体体积。
The distance between the light source and the plant is varied to investigate light intensity. The closer the light, the higher the intensity, which follows the inverse square law.
改变光源与植物之间的距离来研究光照强度。光源越近，光强越高，符合平方反比定律。
12. Core Practical: Investigating Light Intensity | 核心实验：探究光照强度
In the Edexcel IGCSE core practical, you may be asked to investigate the effect of light intensity on the rate of photosynthesis using an aquatic plant.
在Edexcel IGCSE核心实验中，你可能会被要求使用水生植物探究光照强度对光合作用速率的影响。
Method outline:
方法概述：
- Place a piece of aquatic plant (e.g. Cabomba) in a beaker of water with sodium hydrogen carbonate solution to supply CO₂.
- 将一段水生植物（如角藻）放入装有碳酸氢钠溶液的烧杯中，以提供CO₂。
- Position a lamp at a measured distance and count the bubbles produced in one minute.
- 将灯放置在测量好的距离处，计算一分钟内产生的气泡数。
- Repeat at different distances (e.g. 10 cm, 20 cm, 30 cm).
- 在不同的距离（如10 cm、20 cm、30 cm) 重复实验。
- Keep temperature and CO₂ concentration constant throughout.
- 在整个实验过程中保持温度和CO₂浓度恒定。
Results should show that as the light source is moved closer, the bubble rate increases, until light is no longer the limiting factor.
结果应显示，当光源移近时，气泡速率增加，直到光不再是限制因子。
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