📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy stored in glucose. This is one of the most important biological processes on Earth because it produces the oxygen we breathe and forms the base of almost all food chains.
光合作用是绿色植物、藻类和一些细菌将光能转化为储存在葡萄糖中的化学能的过程。这是地球上最重要的生物过程之一，因为它产生我们呼吸的氧气，并构成几乎所有食物链的基础。
1. The Word Equation | 光合作用的总反应式
In IGCSE Biology, you need to know the word equation for photosynthesis. The reactants are carbon dioxide and water; the products are glucose and oxygen.
在 IGCSE 生物中，你需要知道光合作用的文字方程式。反应物是二氧化碳和水；产物是葡萄糖和氧气。
carbon dioxide + water → glucose + oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
The balanced chemical equation is also important for understanding the mole ratio:
平衡化学方程式对于理解摩尔比也很重要：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Light energy and chlorophyll are required for this reaction to take place, and they must be written above the arrow.
该反应需要光能和叶绿素，它们应写在箭头上方。
2. The Importance of Photosynthesis | 光合作用的重要性
Photosynthesis is vital for three main reasons. First, it produces glucose that can be used as a respiratory substrate by the plant itself. Second, it releases oxygen into the atmosphere, which is needed by most living organisms for aerobic respiration. Third, it is the ultimate energy source for almost all ecosystems, because animals obtain their energy by eating plants or other animals that have eaten plants.
光合作用之所以至关重要，主要有三个原因。第一，它产生的葡萄糖可被植物自身用作呼吸底物。第二，它向大气中释放氧气，而大多数生物在有氧呼吸中都需要氧气。第三，它是几乎所有生态系统的最终能量来源，因为动物通过食用植物或食用植物的动物来获取能量。
Biomass production: Glucose is converted into starch, cellulose and other organic molecules.
生物量生产：葡萄糖可转化为淀粉、纤维素和其他有机分子。
Oxygen release: Maintains atmospheric oxygen levels.
氧气释放：维持大气中的氧气水平。
Energy transfer: Converts light energy into chemical energy.
能量转移：将光能转化为化学能。
3. The Role of Chlorophyll and Light | 叶绿素与光的作用
Chlorophyll is the green pigment located in chloroplasts. It absorbs light energy, mainly in the blue-violet and red regions of the spectrum, and uses that energy to drive the chemical reactions of photosynthesis.
叶绿素是位于叶绿体中的绿色色素。它吸收光能，主要吸收光谱中的蓝紫光和红光区域，并利用该能量驱动光合作用的化学反应。
Light energy is transferred to electrons in the chlorophyll molecules, which become excited. This energy is then used to split water molecules (photolysis) and to generate ATP and reduced NADP, which are used in the later stages.
光能被传递给叶绿素分子中的电子，使电子获得能量而激发。这些能量随后用于分解水分子（光解），并生成 ATP 和还原型 NADP，供后续阶段使用。
H₂O → 2H⁺ + 2e⁻ + ½O₂
This photolysis of water provides protons and electrons for the light-independent reactions, and releases oxygen gas as a by-product.
水的光解为光不依赖反应提供质子和电子，并释放氧气作为副产物。
4. The Two Stages: Light-Dependent and Light-Independent | 光依赖和光不依赖两个阶段
Photosynthesis occurs in two main stages. The light-dependent reactions happen in the thylakoid membranes of the chloroplast and require light. The light-independent reactions (also called the Calvin cycle) happen in the stroma and do not require light directly.
光合作用主要分两个阶段。光依赖反应发生在叶绿体的类囊体膜上，需要光。光不依赖反应（也称为卡尔文循环）发生在基质中，不直接需要光。
In the light-dependent reactions, light energy is absorbed by chlorophyll and used to split water, producing oxygen, ATP and reduced NADP.
在光依赖反应中，光能被叶绿素吸收，用于分解水，产生氧气、ATP 和还原型 NADP。
In the light-independent reactions, carbon dioxide is fixed by the enzyme RuBisCO, and with the help of ATP and reduced NADP, glucose is synthesized. This stage does not require light, but it depends on the products of the light-dependent stage.
在光不依赖反应中，二氧化碳被核酮糖-1,5-二磷酸羧化酶固定，并在 ATP 和还原型 NADP 的帮助下合成葡萄糖。该阶段不需要光，但依赖光依赖阶段的产物。
5. The Internal Structure of the Leaf | 叶片内部结构
The leaf is the main photosynthetic organ of most plants. Its structure is highly adapted to capture light and exchange gases efficiently.
叶片是大多数植物主要的光合作用器官。其结构高度适应于有效捕获光和交换气体。
Epidermis: A transparent protective layer that allows light to pass through.
表皮：透明的保护层，允许光线透过。
Palisade mesophyll: Long, thin cells packed with chloroplasts, located near the upper surface to absorb maximum light.
栅栏组织：长而薄、含有大量叶绿体的细胞，位于靠近上表面处，以吸收最多光线。
Spongy mesophyll: Loosely packed cells with air spaces, allowing CO₂ to diffuse rapidly through the leaf.
海绵组织：细胞排列疏松且有气隙，使 CO₂ 能快速扩散穿过叶片。
Stomata: Pores in the epidermis, mainly on the lower surface, enabling gas exchange.
气孔：表皮上的小孔，主要位于下表面，用于气体交换。
Vascular bundles (xylem and phloem): Transport water and dissolved sugars to and from the leaf.
维管束（木质部和韧皮部）：运输水分和溶解糖进出叶片。
6. Limiting Factors | 限制因素
A limiting factor is a condition that, when in short supply, slows down the rate of a process. In photosynthesis, the main limiting factors are light intensity, carbon dioxide concentration and temperature.
限制因素是当供应不足时会降低过程速率的条件。在光合作用中，主要限制因素是光强度、二氧化碳浓度和温度。
At any given time, the factor in shortest supply will limit the rate. Increasing another factor will have no effect until the limiting factor is raised.
在任何给定时间，供应最不足的因素将限制速率。在限制因素提高之前，增加另一个因素不会产生效果。
| Factor | Effect on rate |
| Light intensity | Increases rate up to a saturation point, then plateau. |
| CO₂ concentration | Increases rate until other factors become limiting. |
| Temperature | Increases rate with enzyme kinetics, but above optimum enzymes denature. |
7. Effects of Light Intensity, CO₂ Concentration and Temperature | 光强度、CO₂ 浓度和温度的影响
When light intensity is low, it is the limiting factor. As light intensity increases, the rate of photosynthesis rises linearly. However, at high intensities, the rate levels off because other factors such as CO₂ or temperature become limiting.
当光强度低时，它是限制因素。随着光强度增加，光合作用速率线性上升。然而，在高强度下，速率趋于平稳，因为其他因素如 CO₂ 或温度成为限制因素。
Similarly, increasing carbon dioxide concentration from a low level will increase the rate of photosynthesis. But beyond a certain concentration, no further increase occurs because light or temperature limits the reaction.
类似地，从低水平增加二氧化碳浓度会增加光合作用速率。但超过一定浓度后，不会进一步增加，因为光或温度限制了反应。
Temperature affects the enzymes involved. Between about 5°C and 35°C, the rate increases with temperature. Above the optimum (usually around 35-40°C), enzymes such as RuBisCO begin to denature, and the rate falls sharply.
温度影响相关的酶。在约 5°C 到 35°C 之间，速率随温度升高而增加。超过最适温度（通常在 35-40°C 左右），如 RuBisCO 等酶开始变性，速率急剧下降。
8. Experiments on Photosynthesis | 光合作用实验
Several common experiments are used to investigate the factors affecting photosynthesis. You should be familiar with the following:
有几种常见实验用于研究影响光合作用的因素。你应该熟悉以下内容：
The effect of light intensity: Use an aquatic plant (e.g. Elodea) and count the bubbles of oxygen produced per minute at different distances from a light source.
光强度的影响：使用水生植物（如黑藻），在不同距离的光源下计算每分钟产生的氧气气泡数。
The effect of CO₂ concentration: Add different concentrations of sodium hydrogencarbonate solution to provide varying CO₂ levels, and measure bubble rate.
CO₂浓度的影响：加入不同浓度的碳酸氢钠溶液以提供不同的 CO₂ 水平，并测量气泡速率。
The effect of temperature: Control the temperature of the water bath around the plant and measure gas production at each temperature.
温度的影响：控制植物周围水浴的温度，并在每个温度下测量产气量。
A more accurate method uses the uptake of CO₂ or the production of oxygen in a gas syringe, or the production of starch tested with iodine solution.
更准确的方法是测量 CO₂ 的吸收或使用气体注射器收集氧气，或使用碘液检测淀粉的产生。
9. The Fate of Glucose | 葡萄糖的去向
Glucose produced during photosynthesis is used in several ways inside the plant:
光合作用产生的葡萄糖在植物体内有几种用途：
Immediately used in respiration to release energy as ATP.
立即用于呼吸作用以释放 ATP 形式的能量。
Converted to starch for short-term storage in leaves, roots and seeds.
转化为淀粉用于叶片、根和种子的短期储存。
Converted to sucrose for transport in the phloem to non-photosynthetic tissues.
转化为蔗糖，通过韧皮部运输到非光合组织。
Converted to cellulose for cell wall synthesis.
转化为纤维素用于细胞壁合成。
Converted to amino acids when nitrogen is available, for protein synthesis.
在有氮的条件下转化为氨基酸，用于蛋白质合成。
Converted to lipids for energy storage in seeds.
转化为脂质用于种子中的能量储存。
10. Photosynthesis and Gas Exchange | 光合作用与气体交换
During photosynthesis, plants take in carbon dioxide and release oxygen. During respiration, they take in oxygen and release carbon dioxide. In daylight, the net exchange is usually a net uptake of CO₂ and a net release of O₂ because photosynthesis is faster than respiration.
在光合作用期间，植物吸收二氧化碳并释放氧气。在呼吸作用期间，植物吸收氧气并释放二氧化碳。在白天，净交换通常是二氧化碳的净吸收和氧气的净释放，因为光合作用快于呼吸作用。
At night, photosynthesis stops, so plants only respire, giving out CO₂. This is why it is often said that plants should not be kept in bedrooms overnight — though the effect is tiny compared to human respiration.
在夜间，光合作用停止，因此植物只进行呼吸，释放 CO₂。这就是为什么常说晚上不应将植物放在卧室——尽管与人类呼吸相比其影响很小。
11. The Rate of Photosynthesis and Plant Productivity | 光合速率与植物生产力
In agriculture, understanding limiting factors helps farmers increase crop yields. Glasshouses can be artificially lit, CO₂ generators can be used, and heating can be controlled to keep conditions at the optimum.
在农业中，了解限制因素有助于农民提高作物产量。温室可提供人工光照，可使用 CO₂ 发生器，并可通过控制加热使条件保持最佳状态。
For example, a tomato grower may add extra CO₂ during the day to boost photosynthesis, and use heating at night to keep the temperature above the minimum for growth. These measures increase the rate of photosynthesis and therefore biomass production.
例如，番茄种植者可能在白天添加额外的 CO₂ 以促进光合作用，并在夜间使用加热使温度保持在生长最低温度以上。这些措施提高了光合作用速率，从而增加生物量产量。
12. Summary of Key Points | 要点总结
Photosynthesis is a multi-step process that converts light energy into chemical energy. The key points to remember are:
光合作用是一个将光能转化为化学能的多步骤过程。需要记住的关键点包括：
Equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light and chlorophyll.
方程式：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂，需要光和叶绿素。
Two stages: light-dependent (thylakoid) and light-independent (stroma).
两个阶段：光依赖（类囊体）和光不依赖（基质）。
Limiting factors: light intensity, CO₂ concentration, temperature.
限制因素：光强度、CO₂ 浓度、温度。
Leaf adaptations: large surface area, thin, palisade cells, stomata.
叶片适应：表面积大、薄、栅栏细胞、气孔。
Glucose is used for respiration, storage, transport, and building materials.
葡萄糖用于呼吸、储存、运输和建造材料。
Understanding these principles is essential for answering exam questions on energy transfer, plant nutrition and crop production.
理解这些原理对于回答有关能量转移、植物营养和作物生产的考试问题至关重要。
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