📚 Photosynthesis | 光合作用
Photosynthesis is the biochemical process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It is arguably the most important biological process on Earth, because it supplies the oxygen we breathe and forms the base of almost every food chain.
光合作用是绿色植物、藻类及某些细菌将光能转化为储存在葡萄糖中的化学能的生化过程。它可以说是地球上最重要的生物过程，因为它提供我们呼吸所需的氧气，并构成了几乎所有食物链的基础。
1. What is Photosynthesis? | 什么是光合作用
Photosynthesis takes place in the chloroplasts of plant cells. Chloroplasts contain chlorophyll, the green pigment that absorbs light energy. This energy is then used to synthesise carbohydrates from carbon dioxide and water.
光合作用发生在植物细胞的叶绿体中。叶绿体含有叶绿素，这是一种能吸收光能的绿色色素。然后利用这种能量将二氧化碳和水合成碳水化合物。
- Location: mainly in the leaves, especially the palisade mesophyll layer.
- 部位：主要发生在叶片中，尤其是栅栏组织层。
- Organelle: chloroplasts.
- 细胞器：叶绿体。
- Overall purpose: to produce glucose, which is used for respiration, growth, and storage.
- 总体目的：生成葡萄糖，用于呼吸、生长和储存。
2. The Chemical Equation | 化学方程式
The overall balanced equation for photosynthesis can be written in words and symbols. Six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen.
光合作用的总平衡方程式可以用文字和符号表示。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
Carbon dioxide is absorbed from the air through stomata. Water is taken up by the roots and transported to the leaves via xylem vessels. Light energy drives the reaction.
二氧化碳通过气孔从空气中吸收。水分由根系吸收，并通过木质部导管输送到叶片。光能驱动反应。
3. Light, Chlorophyll and Pigments | 光、叶绿素与色素
Light is the energy source that powers photosynthesis. Different wavelengths of light are absorbed at different rates. Chlorophyll mainly absorbs red and blue-violet light, and reflects green light, which is why leaves appear green.
光是驱动光合作用的能量来源。不同波长的光被吸收的速率不同。叶绿素主要吸收红光和蓝紫光，并反射绿光，这就是叶子呈现绿色的原因。
- Chlorophyll a: main photosynthetic pigment, absorbs red and blue light.
- 叶绿素a：主要光合色素，吸收红光和蓝光。
- Chlorophyll b: accessory pigment, broadens the absorption spectrum.
- 叶绿素b：辅助色素，拓宽吸收光谱。
- Carotenoids: accessory pigments that protect against excessive light.
- 类胡萝卜素：辅助色素，保护免受过度光照的伤害。
4. The Light-dependent Reactions | 光依赖反应
The light-dependent reactions occur in the thylakoid membranes of the chloroplasts. They require light energy and produce ATP, reduced NADP, and oxygen gas.
光依赖反应发生在叶绿体的类囊体膜上。它们需要光能，并产生ATP、还原型NADP和氧气。
Water + Light = ATP + Reduced NADP + O₂
Light energy splits water into protons, electrons, and oxygen. This process is called photolysis. The oxygen is a waste product released into the atmosphere.
光能分解水，产生质子、电子和氧气。这个过程称为水的光解。氧气是释放到大气中的废物产物。
5. The Calvin Cycle (Light-independent) | 卡尔文循环（光不依赖反应）
The Calvin cycle takes place in the stroma of the chloroplast. It uses the ATP and reduced NADP produced in the light-dependent reactions, along with carbon dioxide, to make glucose.
卡尔文循环发生在叶绿体基质中。它利用光依赖反应中产生的ATP和还原型NADP，加上二氧化碳来制造葡萄糖。
Carbon dioxide is fixed by combining with ribulose bisphosphate (RuBP). The resulting compound is converted into glycerate-3-phosphate, which is then reduced to triose phosphate and finally used to synthesise glucose.
二氧化碳通过与核酮糖二磷酸（RuBP）结合而被固定。生成的化合物转化为3-磷酸甘油酸，然后在还原生成三碳糖磷酸，最终用于合成葡萄糖。
6. Factors Affecting Photosynthesis | 影响光合作用的因素
The rate of photosynthesis is influenced by several environmental factors. A limiting factor is the one that is in shortest supply relative to the plant’s needs.
光合作用速率受多种环境因素影响。限制因素是指相对于植物需求来说供应最不足的那个因素。
| Factor | 因素 | Effect | 影响 |
|---|---|
| Light intensity | 光照强度 | Increasing light intensity raises the rate up to a saturation point. |
| Carbon dioxide concentration | 二氧化碳浓度 | Higher CO₂ concentration increases the rate until the enzyme becomes saturated. |
| Temperature | 温度 | Affects enzyme activity; too high a temperature causes denaturation. |
7. Rate of Photosynthesis and Limiting Factors | 光合作用速率与限制因素
The rate of photosynthesis can be measured by the volume of oxygen produced, the rate of carbon dioxide uptake, or the increase in biomass. A classic experiment uses a submerged aquatic plant to count bubbles of oxygen.
可以通过产生的氧气体积、二氧化碳吸收速率或生物量增加来衡量光合作用速率。经典实验使用浸没在水中的水生植物来计数氧气气泡。
Rate = 1 / Time taken to produce a fixed volume of oxygen
When light intensity is the limiting factor, the rate increases linearly with intensity. However, at high light intensities, temperature or CO₂ concentration may become the new limiting factor.
当光照强度是限制因素时，速率随光强线性增加。然而，在高光强下，温度或CO₂浓度可能成为新的限制因素。
8. Experiments to Investigate Photosynthesis | 探究光合作用的实验
Several standard experiments help demonstrate the requirements and products of photosynthesis. These include starch testing, light testing, and the bubble-counting method.
几个标准实验有助于证明光合作用的条件和产物。这些包括淀粉测试、光照测试和气泡计数法。
- Starch test: destarch a plant, then place it in light. Leaf samples are treated with hot ethanol and iodine solution.
- 淀粉测试：将植物进行脱淀粉处理后置于光下。叶样品用热乙醇和碘液处理。
- Light requirement: cover part of a leaf with black paper; only the exposed area turns blue-black with iodine.
- 光照需求：用黑纸遮住叶片的一部分；只有曝光区域遇碘变蓝黑。
- Bubble counting: use an aquatic plant (e.g., Cabomba) and count bubbles per minute under different light conditions.
- 气泡计数：使用水生植物（如金鱼藻），在不同光照条件下计数每分钟气泡数。
9. Importance of Photosynthesis | 光合作用的重要性
Photosynthesis is essential for life. It produces oxygen, creates biomass, and removes carbon dioxide from the atmosphere. Almost all ecosystems depend on photosynthetic organisms as primary producers.
光合作用对生命至关重要。它产生氧气、创造生物质，并从大气中去除二氧化碳。几乎所有生态系统都依赖光合生物作为初级生产者。
Fossil fuels such as coal, oil, and natural gas were formed over millions of years from the remains of photosynthetic organisms. Therefore photosynthesis is also linked to the carbon cycle and the energy resources we use.
煤、石油和天然气等化石燃料是在数百万年中由光合生物的遗骸形成的。因此，光合作用也与碳循环和我们使用的能源资源相关。
10. Applications in Agriculture | 在农业中的应用
Farmers and gardeners can use knowledge of photosynthesis to improve crop yields. In greenhouses, conditions such as light intensity, temperature, and CO₂ concentration can be manipulated to overcome limiting factors.
农民和园丁可以利用光合作用知识提高作物产量。在温室中，可以调节光照强度、温度和CO₂浓度等条件以克服限制因素。
- Add artificial lighting to extend the effective day length, especially in winter.
- 冬季尤其通过补充人工光照来延长有效日照时间。
- Increase CO₂ concentration using a paraffin burner, which also releases heat.
- 使用石蜡燃烧器提高CO₂浓度，这同时也释放热量。
- Maintain optimum temperature using heaters or ventilation.
- 通过加热器或通风保持最适温度。
However, increased inputs also raise costs and environmental impacts, so farmers must balance productivity with sustainability.
然而，增加投入也会提高成本和环境影响，因此农民必须在生产力与可持续性之间取得平衡。
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