📚 Photosynthesis | 光合作用
Photosynthesis is the process by which green plants, algae and some bacteria use sunlight to convert carbon dioxide and water into glucose and oxygen. It is the foundation of almost all food chains on Earth and produces the oxygen we breathe.
光合作用是绿色植物、藻类和某些细菌利用阳光将二氧化碳和水转化为葡萄糖和氧气的过程。它是地球上几乎所有食物链的基础，并产生我们呼吸的氧气。
1. The Word Equation | 文字方程式
The overall process of photosynthesis can be summarised in a simple word equation. Carbon dioxide and water are the raw materials, while glucose and oxygen are the products. Light energy is absorbed by chlorophyll to drive the reaction.
光合作用的整体过程可以用一个简单的文字方程式来概括。二氧化碳和水是原料，葡萄糖和氧气是产物。光能被叶绿素吸收以驱动反应。
Carbon dioxide + Water → Glucose + Oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Note that light energy is not a substance but an energy input.
用符号表示即为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。注意光能不是物质，而是能量输入。
2. The Balanced Chemical Equation | 配平的化学方程式
The balanced symbol equation shows that six molecules of carbon dioxide react with six molecules of water to produce one molecule of glucose and six molecules of oxygen. The equation must be balanced so that the number of atoms of each element is the same on both sides.
平衡的符号方程式显示，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。方程式必须配平，使每种元素的原子数目在两边相同。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
In the Edexcel IGCSE syllabus, you are expected to recall this equation and understand that chlorophyll absorbs light energy. Water is split in the light-dependent stage, releasing oxygen.
在 Edexcel IGCSE 大纲中，你需要记住这个方程式并理解叶绿素吸收光能。水在光依赖阶段被分解，释放出氧气。
3. The Structure of the Leaf | 叶片的结构
The leaf is the main photosynthetic organ. Its structure is adapted to capture light, exchange gases and prevent water loss. The upper epidermis is transparent to let light through, while the palisade mesophyll contains many chloroplasts.
叶片是主要的光合作用器官。它的结构适应于捕获光、进行气体交换和防止水分流失。上表皮透明，让光通过，而栅栏组织含有许多叶绿体。
- Cuticle | 角质层: A waxy layer that reduces water loss. / 一层蜡质，减少水分流失。
- Upper epidermis | 上表皮: Transparent layer that allows light to enter. / 透明的层，允许光进入。
- Palisade mesophyll | 栅栏组织: Column-shaped cells packed with chloroplasts, the main site of photosynthesis. / 柱状细胞，充满叶绿体，是光合作用的主要场所。
- Spongy mesophyll | 海绵组织: Loosely packed cells with air spaces for gas exchange. / 疏松排列的细胞，有气隙用于气体交换。
- Stomata | 气孔: Pores, usually on the lower epidermis, for CO₂ entry and O₂ exit. / 通常在下表皮的气孔，用于二氧化碳进入和氧气排出。
Chloroplasts contain chlorophyll, which gives leaves their green colour and absorbs light mainly in the red and blue-violet regions of the spectrum.
叶绿体含有叶绿素，使叶子呈绿色，并主要吸收光谱中的红光和蓝紫光。
4. The Role of Chlorophyll | 叶绿素的作用
Chlorophyll is a green pigment located in the thylakoid membranes of chloroplasts. Its main role is to absorb light energy and transfer it to electrons, which then drive the light-dependent reactions.
叶绿素是一种绿色色素，位于叶绿体的类囊体膜中。它的主要作用是吸收光能并将其传递给电子，然后驱动光依赖反应。
Without chlorophyll, light energy cannot be captured. Other pigments, such as carotenoids, act as accessory pigments and transfer absorbed energy to chlorophyll.
没有叶绿素，光能就无法被捕获。其他色素如类胡萝卜素作为辅助色素，将吸收的能量传递给叶绿素。
In autumn, chlorophyll breaks down, revealing the orange and yellow carotenoids underneath. This explains why leaves change colour.
在秋天，叶绿素分解，露出下面的橙色和黄色类胡萝卜素。这解释了为什么叶子会变色。
5. Light-Dependent Reactions | 光依赖反应
These reactions take place in the thylakoid membranes and require light. Light energy splits water molecules into hydrogen and oxygen in a process called photolysis. The oxygen is released as a by-product.
这些反应发生在类囊体膜上，需要光。光能在一种称为光解（photolysis）的过程中将水分子分解为氢和氧。氧气作为副产物释放出来。
2H₂O → 4H⁺ + 4e⁻ + O₂
The hydrogen atoms are used to reduce the coenzyme NADP, forming reduced NADP. ATP is also synthesised from ADP and phosphate using energy from excited electrons.
氢原子用于还原辅酶 NADP，形成还原型 NADP。同时，利用激发电子的能量，从 ADP 和磷酸合成 ATP。
These are summary concepts for IGCSE: you need to know that light energy converts ADP into ATP and produces reduced NADP for the next stage.
这些是 IGCSE 的概要概念：你需要知道光能将 ADP 转化为 ATP，并为下一阶段产生还原型 NADP。
6. Light-Independent Reactions (Calvin Cycle) | 光不依赖反应（卡尔文循环）
The light-independent reactions occur in the stroma of the chloroplast. They use the ATP and reduced NADP produced in the light-dependent stage to convert carbon dioxide into glucose. These reactions do not directly require light, but they depend on the products of the light reactions.
光不依赖反应发生在叶绿体的基质中。它们利用光依赖阶段产生的 ATP 和还原型 NADP，将二氧化碳转化为葡萄糖。这些反应不直接需要光，但它们依赖光反应的产物。
Carbon dioxide is fixed by combining with a five-carbon compound called ribulose bisphosphate (RuBP). The resulting six-carbon compound immediately splits into two molecules of glycerate 3-phosphate (GP).
二氧化碳通过结合一种称为核酮糖二磷酸（RuBP）的五碳化合物而被固定。生成的六碳化合物立即分裂成两个甘油酸-3-磷酸（GP）分子。
GP is then reduced to glycerol aldehyde 3-phosphate (GALP) using ATP and reduced NADP. Most of the GALP is recycled to regenerate RuBP, while some is used to make glucose and other carbohydrates.
然后 GP 利用 ATP 和还原型 NADP 被还原为甘油醛-3-磷酸（GALP）。大部分 GALP 被回收以再生 RuBP，而一部分用于制造葡萄糖和其他碳水化合物。
For Edexcel IGCSE, you are not required to recall the names of intermediates, but you should understand that CO₂ is fixed and glucose is synthesised using chemical energy.
对于 Edexcel IGCSE，不要求你记住中间产物的名称，但你应该理解二氧化碳被固定，并使用化学能合成葡萄糖。
7. Factors Affecting Photosynthesis | 影响光合作用的因素
Photosynthesis is affected by several environmental factors. The most important are light intensity, carbon dioxide concentration and temperature. If any of these is in short supply, it can limit the rate of photosynthesis.
光合作用受多种环境因素影响。最重要的是光强度、二氧化碳浓度和温度。如果其中任一项供应不足，就会限制光合作用的速率。
| Factor | 因素 | Effect | 影响 |
|---|---|
| Light intensity | 光强度 | As light intensity increases, the rate increases until another factor becomes limiting. / 光强度增加，速率增加，直到另一个因素成为限制因子。 |
| CO₂ concentration | 二氧化碳浓度 | Higher CO₂ levels generally increase the rate up to a saturation point. / 较高二氧化碳浓度通常提高速率，直到饱和点。 |
| Temperature | 温度 | Rate increases with temperature up to an optimum, then decreases as enzymes denature. / 速率随温度上升至最适点，然后因酶变性而下降。 |
Water is also essential, but it is rarely the immediate limiting factor in most environments unless there is a drought. Mineral availability can also affect chlorophyll production (e.g. magnesium is needed for chlorophyll).
水也是必需的，但在大多数环境中除非干旱，水很少成为直接的限制因子。矿物质可用性也会影响叶绿素的产生（例如，镁是叶绿素所需的）。
8. Limiting Factors | 限制因子
A limiting factor is a condition that, when increased, causes an increase in the rate of a process, provided that all other conditions remain constant. For example, if a plant receives plenty of light and warmth, but the CO₂ concentration is low, then CO₂ is the limiting factor.
限制因子是一种条件，当它增加时，若其他条件保持不变，会导致过程速率增加。例如，如果植物获得充足的光和温暖，但二氧化碳浓度很低，那么二氧化碳就是限制因子。
In a greenhouse, growers can artificially increase the concentration of CO₂, raise temperature, and provide extra lighting to overcome limiting factors and maximise crop yield. However, they must consider the cost of these methods against the increased profit.
在温室中，种植者可以人为提高二氧化碳浓度、升高温度并提供额外照明，以克服限制因子并最大化作物产量。然而，他们必须权衡这些方法的成本与增加的利润。
At night, photosynthesis stops because light is absent, even if other factors are ideal.
在夜间，由于没有光，光合作用停止，即使其他因素理想。
9. Measuring the Rate of Photosynthesis | 测量光合作用速率
The rate of photosynthesis can be measured by monitoring the uptake of carbon dioxide, the production of oxygen, or the production of glucose. A common practical method uses an aquatic plant such as Elodea and counts the number of oxygen bubbles released per minute.
光合作用速率可以通过监测二氧化碳的吸收、氧气的产生或葡萄糖的生成来测量。一个常见的实验方法使用水生植物如伊乐藻（Elodea），并计算每分钟释放的氧气气泡数量。
To investigate the effect of light intensity, the light source is moved closer to or farther from the plant. The distance is inversely proportional to the light intensity (inverse square law). The rate can be calculated as the reciprocal of the time taken for the plant to produce a fixed number of bubbles.
为了研究光强度的影响，将光源移近或移远植物。距离与光强度成反比（平方反比定律）。速率可以计算为植物产生固定数量气泡所需时间的倒数。
Important controls include keeping temperature and CO₂ concentration constant, and waiting a few minutes before counting to allow the plant to acclimatise to each new light intensity.
重要的对照包括保持温度和二氧化碳浓度恒定，并在计数前等待几分钟让植物适应每个新的光强度。
10. Uses of Photosynthesis Products | 光合作用产物的用途
The glucose produced in photosynthesis is used by the plant in several ways. It can be converted into starch for storage, used in respiration to release energy, or transformed into cellulose for cell walls. It also provides the carbon skeleton for amino acids and lipids.
光合作用产生的葡萄糖在植物中有多种用途。它可以转化为淀粉储存、用于呼吸作用释放能量、或转化为纤维素构成细胞壁。它还为氨基酸和脂质提供碳骨架。
- Respiration | 呼吸作用: Glucose is broken down to release energy for cellular processes. / 葡萄糖被分解以释放能量供细胞过程使用。
- Storage | 储存: Stored as starch in leaves, roots (e.g. potatoes) and seeds. / 以淀粉形式储存在叶片、根（如马铃薯）和种子中。
- Structural material | 结构物质: Converted to cellulose for cell walls and to lignin for support. / 转化为纤维素以构成细胞壁，转化为木质素以提供支持。
- Other compounds | 其他化合物: Combined with nitrogen and other minerals to make proteins, and used to synthesise lipids. / 与氮等矿物质结合制造蛋白质，并用于合成脂质。
In agriculture, understanding these uses helps farmers balance nitrogen fertilisers with carbon inputs to ensure healthy crop growth. For animals and humans, photosynthesis ultimately provides all the organic molecules they need, either directly from plants or indirectly through food chains.
在农业中，理解这些用途有助于农民平衡氮肥与碳输入以保证健康作物生长。对于动物和人类，光合作用最终提供了他们所需的所有有机分子，无论是直接来自植物还是通过食物链间接获得。
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