📚 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 the foundation of almost all food chains and supplies the oxygen in the atmosphere.
光合作用是绿色植物、藻类和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。它几乎是所有食物链的基础，并为大气提供氧气。
1. What is Photosynthesis? | 什么是光合作用？
Photosynthesis occurs mainly in the leaves of plants, in organelles called chloroplasts. The green pigment chlorophyll absorbs light energy, which is then used to combine carbon dioxide and water to produce glucose and oxygen.
光合作用主要发生在植物的叶片中，具体在称为叶绿体的细胞器内。绿色色素叶绿素吸收光能，然后利用该能量将二氧化碳和水合成为葡萄糖和氧气。
This process is essential for the plant's survival because the glucose produced is used for respiration, growth, storage and the synthesis of other biological molecules.
该过程对植物的生存至关重要，因为产生的葡萄糖用于呼吸、生长、储存以及合成其他生物分子。
2. The Word Equation and Chemical Equation | 文字方程与化学方程
The word equation summarises photosynthesis clearly:
文字方程清晰地概括了光合作用：
Carbon dioxide + Water → Glucose + Oxygen (in the presence of light and chlorophyll)
The balanced chemical equation shows the exact numbers of atoms involved:
平衡化学方程显示了所涉及原子的精确数目：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This equation is a key exam requirement for IGCSE Science. You need to be able to recall both the word equation and the chemical equation without a calculator.
这个方程是 IGCSE 科学考试的重要考点。你需要能够不看资料回忆起文字方程和化学方程。
3. The Structure of a Leaf | 叶片的结构
A leaf is an organ specially adapted for photosynthesis. Its structure maximises light absorption, gas exchange and water transport.
叶片是专门适应光合作用的器官。其结构最大限度地提高光吸收、气体交换和水分运输的效率。
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Waxy cuticle: prevents water loss; it is transparent to let light through.
蜡质角质层：防止水分流失；透明，能让光线透过。
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Upper epidermis: a thin protective layer that allows light to reach the palisade cells below.
上表皮：一层薄而具有保护作用的组织，让光线到达下方的栅栏细胞。
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Palisade mesophyll: tall, closely packed cells containing many chloroplasts; this is the main site of photosynthesis.
栅栏叶肉：排列紧密的长形细胞，含有大量叶绿体；这是光合作用的主要场所。
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Spongy mesophyll: loosely arranged cells with large air spaces for efficient gas diffusion (CO₂ in, O₂ out).
海绵叶肉：细胞排列疏松，具有大细胞间隙，有利于气体扩散（CO₂进入，O₂排出）。
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Stomata and guard cells: tiny pores on the lower epidermis that control gas exchange and water loss.
气孔和保卫细胞：下表皮上的微小孔洞，控制气体交换和水分流失。
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Vascular bundles (xylem and phloem): transport water and minerals to the leaf and distribute glucose away from it.
维管束（木质部和韧皮部）：将水分和矿物质运输到叶片，并把葡萄糖运走。
4. Factors Affecting Photosynthesis | 影响光合作用的因素
The rate of photosynthesis depends on several environmental factors. If any of these factors is below the optimum level, it will slow down the process.
光合作用速率取决于若干环境因素。如果任何因素低于最适水平，都会减慢该过程。
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Light intensity: more light provides more energy, but only up to a saturation point.
光照强度：更多光提供更多能量，但仅到饱和点为止。
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Carbon dioxide concentration: CO₂ is a raw material; increasing its concentration generally speeds up photosynthesis until another factor becomes limiting.
二氧化碳浓度：CO₂是原料；增加其浓度通常会加速光合作用，直到其他因素成为限制因素。
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Temperature: enzymes control photosynthesis; rate increases with temperature until the optimum, then decreases as enzymes denature.
温度：酶控制光合作用；速率随温度升高而增加，达到最适温度后下降，因为酶会变性。
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Water availability: water is a reactant; insufficient water also closes stomata, reducing CO₂ uptake.
水分可用性：水是反应物；水分不足还会导致气孔关闭，减少CO₂的吸收。
5. Limiting Factors | 限制因素
A limiting factor is a factor that is in short supply and therefore prevents the rate of photosynthesis from increasing further. In exam questions, you may be given a graph and asked to identify the limiting factor.
限制因素是一种供应不足的因素，因此它阻止光合作用速率进一步提高。在考试问题中，你可能会得到一张图并要求识别限制因素。
For example, if you increase light intensity but the rate does not rise, either carbon dioxide concentration or temperature is now the limiting factor.
例如，如果增加光照强度但速率不上升，那么要么是二氧化碳浓度，要么是温度现在成了限制因素。
6. How Light Intensity Affects Rate | 光照强度如何影响速率
As light intensity increases, the rate of photosynthesis increases proportionally at first. This is because more light energy excites more chlorophyll molecules, producing more ATP and reduced NADP for the Calvin cycle.
随着光照强度增加，光合作用速率起初按比例增加。这是因为更多光能激发更多叶绿素分子，产生更多ATP和还原型NADP供卡尔文循环使用。
However, at high light intensities the rate plateaus. The graph becomes horizontal because light is no longer the limiting factor; another factor such as CO₂ concentration or temperature cannot keep up.
然而，在高光照强度下，速率达到平台期。曲线变得水平，因为光不再是限制因素；其他因素如CO₂浓度或温度无法跟上。
7. How Carbon Dioxide Concentration Affects Rate | 二氧化碳浓度如何影响速率
Carbon dioxide is one of the raw materials of photosynthesis. Increasing CO₂ concentration from very low levels causes the rate to increase rapidly, because more substrate is available for the Calvin cycle.
二氧化碳是光合作用的原料之一。从很低水平增加CO₂浓度会导致速率快速增加，因为更多的底物可供卡尔文循环使用。
As with light, the effect levels off at high CO₂ concentrations. The plateau indicates that CO₂ is no longer limiting and the rate is now limited by light intensity or temperature.
与光类似，在高CO₂浓度下效果趋于平缓。平台期表明CO₂不再是限制因素，速率现在受光照强度或温度限制。
8. How Temperature Affects Rate | 温度如何影响速率
Photosynthesis involves many enzyme-controlled reactions. As temperature rises, molecules move faster and collide more frequently, so the rate increases. The optimum temperature for most plant enzymes is around 25–35 °C.
光合作用涉及许多酶促反应。随着温度升高，分子运动加快，碰撞更频繁，因此速率增加。大多数植物酶的最适温度约为25–35 °C。
Above the optimum, enzymes begin to denature. The active site changes shape and the enzyme can no longer bind to its substrate. This causes a rapid decrease in the rate of photosynthesis.
超过最适温度后，酶开始变性。活性位点形状改变，酶无法再与其底物结合。这导致光合作用速率迅速下降。
9. The Importance of Photosynthesis | 光合作用的重要性
Photosynthesis has global significance beyond the plant itself.
光合作用在植物本身之外还具有全球性意义。
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It produces oxygen, which is required for aerobic respiration by nearly all organisms.
它产生氧气，几乎所有生物进行有氧呼吸都需要氧气。
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It removes carbon dioxide from the atmosphere, helping to regulate the greenhouse effect.
它从大气中移除二氧化碳，有助于调节温室效应。
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It is the ultimate source of food for almost all living things, either directly or through food chains.
它是几乎所有生物的食物最终来源，无论是直接还是通过食物链。
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It has produced fossil fuels over millions of years, which are used as energy sources today.
经过数百万年，它形成了化石燃料，这些燃料今天被用作能源。
10. Experiments to Measure Photosynthesis | 测量光合作用的实验
In the IGCSE laboratory, photosynthesis is commonly measured using a submerged aquatic plant such as Elodea or Cabomba.
在 IGCSE 实验室中，通常使用沉水水生植物如伊乐藻或金鱼藻来测量光合作用。
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Count the number of oxygen bubbles produced per minute.
计数每分钟产生的氧气气泡数量。
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Collect the gas in a boiling tube and measure its volume over a fixed time.
将气体收集在试管中，并测量固定时间内的体积。
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Vary the distance of a lamp from the plant to change light intensity. Remember: intensity is inversely proportional to the square of the distance.
改变灯与植物的距离以改变光照强度。记住：强度与距离的平方成反比。
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Keep temperature constant by using a water bath, and keep the plant in a sodium hydrogen carbonate solution to provide a constant CO₂ concentration.
通过使用水浴保持温度恒定，并将植物置于碳酸氢钠溶液中以提供恒定的CO₂浓度。
Alternatively, the uptake of carbon dioxide can be measured using a pH indicator such as hydrogencarbonate indicator, which changes colour based on CO₂ concentration.
或者，可以通过 pH 指示剂（如碳酸氢盐指示剂）测量二氧化碳的吸收情况，该指示剂会根据CO₂浓度改变颜色。
11. Uses of Glucose from Photosynthesis | 光合作用产生葡萄糖的用途
Glucose produced by photosynthesis is used in several different ways inside the plant:
光合作用产生的葡萄糖在植物体内有多种不同的用途：
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Immediately respired to release energy for cellular activities.
立即被呼吸作用分解以释放能量供细胞活动使用。
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Converted to starch for storage in roots, stems and seeds. Starch is insoluble, so it does not affect water balance.
转化为淀粉储存在根、茎和种子中。淀粉不溶于水，因此不会影响水分平衡。
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Used to make cellulose, a structural component of plant cell walls.
用于制造纤维素，纤维素是植物细胞壁的结构成分。
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Combined with nitrate ions (absorbed from the soil) to form amino acids, which are then assembled into proteins.
与从土壤中吸收的硝酸根离子结合形成氨基酸，再组装成蛋白质。
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Converted into fats and oils for storage, especially in seeds.
转化为脂肪和油类储存，尤其是在种子中。
12. Exam Tips | 考试技巧
Here are some practical tips for answering photosynthesis questions in Edexcel IGCSE Science papers.
以下是一些在 Edexcel IGCSE 科学试卷中回答光合作用问题的实用技巧。
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Learn both the word equation and the balanced symbol equation exactly, including the conditions (light and chlorophyll).
准确记住文字方程和平衡符号方程，包括条件（光和叶绿素）。
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When drawing graphs of rate against a factor, always label the axes and describe the trend: an initial steep rise followed by a plateau.
在绘制速率随因素变化的曲线时，务必标注坐标轴并描述趋势：先急剧上升后趋于平缓。
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If the graph plateaus, state that the factor you are increasing is no longer the limiting factor.
如果曲线达到平台期，说明你所增加的因素不再是限制因素。
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Remember that enzymes denature at high temperature; do not say 'the plant dies' unless the temperature is extreme.
记住酶在高温下会变性；不要说“植物死亡”，除非温度极端。
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In experiments, mention the variables you must control: temperature, CO₂ concentration, and the number of leaves (if using a whole plant).
在实验中，提到必须控制的变量：温度、CO₂浓度，以及叶片数量（如果使用整株植物）。
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