📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants, algae and some bacteria use light energy to convert carbon dioxide and water into glucose and oxygen. This process is fundamental to life on Earth because it produces food and releases oxygen.
光合作用是绿色植物、藻类和某些细菌利用光能将二氧化碳和水转化为葡萄糖和氧气的过程。这一过程对地球生命至关重要，因为它既制造了有机物，也释放了氧气。
1. What Is Photosynthesis? | 什么是光合作用？
Photosynthesis is an endothermic reaction in which light energy is absorbed by chlorophyll and converted into chemical energy stored in glucose. In plants, this occurs mainly in the leaves, where the palisade cells contain large numbers of chloroplasts. Chlorophyll is the green pigment that captures light energy.
光合作用是一个吸热反应过程，叶绿素吸收光能并将其转化为储存在葡萄糖中的化学能。在植物中，这一过程主要在叶片中发生，栅栏组织细胞含有大量叶绿体。叶绿素是捕获光能的绿色色素。
2. The Word Equation | 文字方程式
The word equation for photosynthesis is:
carbon dioxide + water → glucose + oxygen
This equation shows the overall reactants and products. Students must be able to state it exactly and use it to explain the requirements of the process. Light energy and chlorophyll are not written in the word equation, but they are essential conditions.
光合作用的文字方程式为：
二氧化碳 + 水 → 葡萄糖 + 氧气
该方程式表示反应物和产物。学生必须能够准确写出它，并用它解释该过程的条件。光能和叶绿素虽不写在文字方程式中，但它们是必需的条件。
3. The Balanced Chemical Equation | 平衡化学方程式
The balanced chemical equation is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This equation is balanced because there are six carbon atoms, twelve hydrogen atoms and eighteen oxygen atoms on each side. Glucose (C₆H₁₂O₆) is a hexose sugar, and the oxygen released comes from the splitting of water in the light-dependent stage.
平衡化学方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
该方程式是平衡的，因为两边各有6个碳原子、12个氢原子和18个氧原子。葡萄糖（C₆H₁₂O₆）是一种六碳糖，释放的氧气来自光反应阶段水分子被分解。
4. Where Does Photosynthesis Occur? | 光合作用发生在哪里？
Photosynthesis takes place in the chloroplasts of plant cells. Chloroplasts contain chlorophyll, which is located in the thylakoid membranes. The surrounding fluid, called the stroma, contains enzymes needed for the Calvin cycle. In leaves, the palisade mesophyll cells are the main site of photosynthesis because they are densely packed with chloroplasts.
光合作用发生在植物细胞的叶绿体中。叶绿体含有叶绿素，位于类囊体膜上。周围基质中含有卡尔文循环所需的酶。在叶片中，栅栏组织细胞是光合作用的主要场所，因为其细胞质中紧密排列着大量叶绿体。
5. Light-Dependent Reactions | 光依赖反应
Light-dependent reactions occur in the thylakoid membranes. Light energy is absorbed by chlorophyll, which raises electrons to a higher energy level. This energy is used to form ATP and reduced NADP. Water is split during photolysis, producing oxygen gas, protons and electrons. The oxygen is released as a waste product.
光依赖反应发生在类囊体膜上。叶绿素吸收光能后，电子被激发到较高的能级。这部分能量用于生成ATP和还原型NADP。在光解过程中，水分子被分解，产生氧气、质子和电子。氧气作为废物被释放。
6. Light-Independent Reactions | 光不依赖反应（卡尔文循环）
The light-independent reactions, also called the Calvin cycle, take place in the stroma. Carbon dioxide is fixed by combining with ribulose bisphosphate (RuBP). The resulting compound is reduced using ATP and reduced NADP to form glucose. These reactions do not require light directly, but they depend on the products of the light-dependent reactions.
光不依赖反应又称卡尔文循环，发生在叶绿体基质中。二氧化碳与核酮糖二磷酸（RuBP）结合而被固定，生成的化合物利用ATP和还原型NADP被还原形成葡萄糖。这些反应不直接需要光，但依赖于光依赖反应的产物。
7. Limiting Factors | 限制因素
A limiting factor is something that slows down the rate of a reaction when it is in short supply. The main limiting factors of photosynthesis are light intensity, carbon dioxide concentration and temperature. At any moment, the factor which is closest to its minimum value is the one that limits the rate. Increasing that factor will speed up photosynthesis until another factor becomes limiting.
限制因素是指当某种条件供应不足时，会使反应速率下降的因素。光合作用的主要限制因素是光强度、二氧化碳浓度和温度。在任何时刻，最接近最小值的那个因素就是限制速率的主要因素。提高该因素会加快光合作用，直到另一个因素成为新的限制因素。
8. The Effect of Light Intensity | 光强度的影响
As light intensity increases, the rate of photosynthesis increases proportionally, because more light energy means more photolysis and ATP production. However, at high light intensities, the rate reaches a plateau. This happens when light is no longer the limiting factor and carbon dioxide or temperature becomes limiting. In experiments, light intensity can be varied by changing the distance between a lamp and a plant.
随着光强度增大，光合作用速率按比例上升，因为更多的光能意味着更快的光解和ATP生成。然而，当光强度很高时，速率会达到平台期。这是因为光不再是限制因素，而二氧化碳或温度成为限制因素。在实验中，可以通过改变光源与植物之间的距离来调节光强度。
9. The Effect of Carbon Dioxide Concentration | 二氧化碳浓度的影响
Carbon dioxide is one of the raw materials for photosynthesis. Increasing its concentration usually increases the rate of photosynthesis, up to a saturation point. Beyond that point, further increases do not raise the rate, because the enzymes in the Calvin cycle are working at maximum speed or another factor is limiting. In greenhouse farming, carbon dioxide can be enriched to improve crop yields.
二氧化碳是光合作用的原料之一。提高其浓度通常能提高光合作用速率，直到达到饱和点。超过该点后，继续增加浓度不会再提高速率，因为卡尔文循环中的酶已经以最大速度工作，或者另一种因素成为限制。在温室农业中，可以通过增施二氧化碳来提高作物产量。
10. The Effect of Temperature | 温度的影响
Photosynthesis is controlled by enzymes, so temperature has a significant effect on its rate. As temperature rises, the kinetic energy of particles increases and enzyme-substrate complexes form more rapidly. The rate reaches an optimum at about 25–35 °C for most plants. Above the optimum, enzymes begin to denature, and the rate drops sharply. Temperature can be controlled in experiments using a water bath around the plant.
光合作用受酶控制，因此温度对其速率有显著影响。随着温度升高，粒子动能增大，酶底物复合物形成更快。对大多数植物而言，最适温度约为25–35 °C。超过最适温度后，酶开始变性，速率急剧下降。实验中可以用水浴来控制温度。
11. Investigating Photosynthesis | 探究光合作用
A common practical test is the floating leaf disc experiment. Leaf discs are placed in a solution with sodium hydrogencarbonate, which provides carbon dioxide. When exposed to light, the discs produce oxygen and begin to float. The time taken for discs to float can be used to estimate the rate of photosynthesis. Alternatively, oxygen production can be measured by counting bubbles released from pondweed.
一个常见实验是叶圆盘浮起实验。将叶圆盘放入含碳酸氢钠的溶液中，碳酸氢钠可提供二氧化碳。在光照下，圆盘释放氧气并逐渐上浮。记录圆盘浮起所需时间可以用来估算光合作用速率。另一种方法是通过计数伊乐藻（pondweed）释放的气泡来测量氧气产生速率。
12. The Importance of Photosynthesis | 光合作用的重要性
Photosynthesis is the primary source of food for almost all living organisms. It produces glucose, which is used for respiration, growth and storage. It also releases oxygen into the atmosphere, maintaining the concentration of oxygen needed for aerobic respiration. In addition, photosynthesis removes carbon dioxide, helping to regulate the carbon cycle and reduce the greenhouse effect.
光合作用几乎是所有生物的食物来源。它产生葡萄糖，用于呼吸作用、生长和储存。它还向大气释放氧气，维持有氧呼吸所需的氧气浓度。此外，光合作用会移除二氧化碳，有助于调节碳循环并减缓温室效应。
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