Photosynthesis | 光合作用

📚 Photosynthesis | 光合作用

Photosynthesis is the fundamental biochemical process by which green plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. This process sustains nearly all life on Earth by producing oxygen and forming the base of most food chains.

光合作用是绿色植物、藻类及某些细菌利用光能，将光能转化为储存在葡萄糖中的化学能的基本生化过程。这一过程产生氧气并构成大多数食物链的基础，几乎维持了地球上所有生命。

1. The Photosynthesis Equation | 光合作用方程式

The overall equation for photosynthesis summarises the conversion of six molecules of carbon dioxide and six molecules of water into one molecule of glucose and six molecules of oxygen, using light energy trapped by chlorophyll.

光合作用的总体方程式概括了在叶绿素捕获的光能作用下，六个二氧化碳分子与六个水分子转化为一个葡萄糖分子和六个氧分子的过程。

6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂

This equation shows that photosynthesis is an endothermic reaction, as energy from sunlight is absorbed and stored in the chemical bonds of glucose.

该方程式表明，光合作用是一个吸热反应，因为来自太阳光的能量被吸收并储存在葡萄糖的化学键中。

The symbol equation must be balanced: atoms of carbon, hydrogen and oxygen are conserved.

符号方程式必须配平：碳、氢、氧原子数量守恒。
Chlorophyll acts as the catalyst and pigment that captures light energy only in the visible spectrum.

叶绿素充当催化剂和色素，仅捕获可见光谱中的光能。

2. Leaf Structure and Adaptations | 叶的结构与适应

Leaves are highly adapted to maximise the rate of photosynthesis. Their broad, thin shape provides a large surface area for light absorption and short diffusion distances for gases.

叶片高度适应以最大化光合作用速率。其宽阔扁平的形状提供了较大的光吸收表面积，并缩短了气体的扩散距离。

Structure / 结构	Adaptation / 适应
Upper epidermis / 上表皮	Transparent to allow light through / 透明以透光
Palisade mesophyll / 栅栏组织	Tightly packed, many chloroplasts / 排列紧密，含大量叶绿体
Spongy mesophyll / 海绵组织	Air spaces for gas exchange / 气隙便于气体交换
Stomata / 气孔	Allow CO₂ in and O₂ out / 允许CO₂进入和O₂排出

Stomata open during the day to let in carbon dioxide, but this also leads to water loss through transpiration. Guard cells regulate this opening.

气孔在白天开放以让二氧化碳进入，但同时也会因蒸腾作用造成水分流失。保卫细胞调节气孔的开闭。

3. Light-Dependent Reactions | 光依赖反应（概述）

These reactions occur in the thylakoid membranes of chloroplasts. Light energy splits water molecules into hydrogen ions, electrons and oxygen gas — a process called photolysis.

这些反应发生在叶绿体的类囊体膜上。光能将水分子分解为氢离子、电子和氧气——这一过程称为光解。

2H₂O → 4H⁺ + 4e⁻ + O₂

The hydrogen ions and electrons are used to reduce the coenzyme NADP, forming NADPH, while ATP is also generated through photophosphorylation. The oxygen is released as a by-product.

氢离子和电子用于还原辅酶NADP，形成NADPH，同时通过光合磷酸化生成ATP。氧气则作为副产品释放。

ADP + Pi → ATP using light energy / 光能将ADP和Pi合成ATP

ADP + Pi → ATP（利用光能）
NADP + H⁺ + 2e⁻ → NADPH / NADP + H⁺ + 2e⁻ → NADPH

NADP + H⁺ + 2e⁻ → NADPH

4. Light-Independent Reactions | 光非依赖反应（卡尔文循环）

These reactions take place in the stroma (the liquid matrix surrounding the thylakoids). They do not require light directly, but they rely on the ATP and NADPH produced in the light-dependent stage.

这些反应发生在基质（类囊体周围的液态基质）中。它们不直接需要光，但依赖光反应阶段产生的ATP和NADPH。

Carbon dioxide is fixed by reacting with ribulose biphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This GP is then reduced to glyceraldehyde 3-phosphate (G3P) using ATP and NADPH. G3P molecules are used to regenerate RuBP and to build glucose.

二氧化碳与二磷酸核酮糖（RuBP）结合被固定，形成两分子三磷酸甘油酸（GP）。随后，GP利用ATP和NADPH被还原为三磷酸甘油醛（G3P）。G3P分子用于再生RuBP并构建葡萄糖。

3CO₂ + 9ATP + 6NADPH → 1 G3P + 9ADP + 8Pi + 6NADP⁺

For every six molecules of G3P produced, five are used to regenerate three RuBP, leaving one net molecule for glucose synthesis.

每产生六分子G3P，其中五分子用于再生三分子RuBP，剩下净一分子用于合成葡萄糖。

5. Factors Affecting Photosynthesis | 影响光合作用的因素

Several environmental factors influence the rate of photosynthesis. The three main factors are light intensity, carbon dioxide concentration and temperature.

多种环境因素影响光合作用速率。三大主要因素为光照强度、二氧化碳浓度和温度。

Light intensity: As light intensity increases, the rate rises until other factors become limiting.

光照强度：光照强度增大，光合速率上升，直至其他因素成为限制因素。
CO₂ concentration: Increasing CO₂ concentration boosts the rate up to a saturation point.

CO₂浓度：提高CO₂浓度可增加速率，直至达到饱和点。
Temperature: Higher temperatures increase molecular collisions, but excessive heat can denature enzymes.

温度：较高的温度增加分子碰撞，但过热会使酶变性。
Water availability: Insufficient water causes stomata to close, reducing CO₂ uptake.

水分供应：水分不足会导致气孔关闭，减少CO₂吸收。

6. Limiting Factors and Graphs | 限制因子与曲线

According to the law of limiting factors, the rate of photosynthesis is limited by the factor furthest from its optimum value. At any moment, one factor will be the bottleneck.

根据限制因子定律，光合作用速率受离最适值最远的因素限制。在任一时刻，总有一个因素成为瓶颈。

For example, if light intensity is low, increasing CO₂ concentration has no effect until light intensity is raised. This is often shown in graphs where the curve flattens at a plateau.

例如，若光照强度低，则增加CO₂浓度无效，直到提高光照强度。这在曲线图中常表现为平台期。

Factor / 因素	Graph shape / 曲线形状	Limiting explanation / 限制解释
Light intensity / 光强度	Increase → plateau / 上升后趋于平缓	At high light, another factor limits / 高光下受其他因素限制
CO₂ concentration / CO₂浓度	Increase → plateau / 上升后趋于平缓	Rubisco saturation / 酶RuBisCO饱和
Temperature / 温度	Optimum peak then drop / 先升后降	Enzyme denaturation above optimum / 超过最适温度酶变性

7. Experiments on Photosynthesis | 光合作用实验

The classic experiment to test photosynthesis involves destarching a plant (keeping it in the dark for 24 hours), then exposing part of a leaf to light while covering another part with a stencil. After several hours, the leaf is tested with iodine.

检验光合作用的经典实验包括：将植物暗处理24小时以耗尽淀粉，然后把叶片一部分遮光，一部分暴露在光下。数小时后用碘液测试叶片。

Detail / 细节	Procedure / 步骤	Result / 结果
Starch test / 淀粉检测	Boil leaf → soak in alcohol → add iodine / 沸水煮叶→酒精脱色→加碘液	Black/blue indicates starch / 变蓝黑色说明有淀粉
Oxygen production / 氧气产生	Use aquatic plant (Elodea), collect gas in a test tube / 用水生植物（伊乐藻），试管收集气体	A glowing splint relights in O₂ / 带火星的木条复燃

Controls are essential: a boiled leaf (without chlorophyll) shows no starch, and a leaf kept in darkness shows no starch either.

对照至关重要：煮过的叶片（无叶绿体）不显示淀粉，置于黑暗中的叶片也不显示淀粉。

8. Importance of Photosynthesis | 光合作用的重要性

Photosynthesis is vital for life because it is the primary source of both organic carbon and atmospheric oxygen. It removes carbon dioxide from the atmosphere and releases oxygen for respiration.

光合作用对生命至关重要，因为它是有机碳和大气中氧气的主要来源。它从大气中去除二氧化碳并释放氧气供呼吸作用使用。

Produces glucose, which can be converted into starch, cellulose and other organic molecules / 产生葡萄糖，并可转化为淀粉、纤维素及其他有机分子

产生葡萄糖，可进一步转化为淀粉、纤维素和其他有机分子。
Provides the energy base for nearly all food webs / 为几乎所有食物网提供能量基础

为几乎所有食物网提供能量基础。
Regulates atmospheric CO₂/O₂ balance, mitigating climate change / 调节大气CO₂/O₂平衡，缓解气候变化

调节大气CO₂/O₂平衡，缓解气候变化。

9. Common Misconceptions | 常见误区

Students often confuse photosynthesis with respiration. Photosynthesis stores energy, while respiration releases it. They occur simultaneously in green plants during daylight.

学生常混淆光合作用与呼吸作用。光合作用储存能量，呼吸作用释放能量。白天绿色植物中两者同时进行。

“Photosynthesis only happens in leaves” — actually any green part with chloroplasts can do it.

“光合作用只在叶片中发生”——实际上任何含叶绿体的绿色部分都能进行。
“Light-independent reactions happen at night” — they require NADPH and ATP, which are only produced in light, so they stop in the dark.

“光非依赖反应在夜间进行”——它们需要NADPH和ATP，而这些只在光下产生，因此黑暗中会停止。
“Plants do not respire” — they respire continuously.

“植物不呼吸”——植物一直在呼吸。

10. Exam Tips | 考试提示

In Edexcel IGCSE Science exams, photosynthesis questions often require you to interpret graphs, design experiments, and explain limiting factors. Use the correct terminology and show balanced equations.

在Edexcel IGCSE科学考试中，光合作用问题常要求解读曲线图、设计实验并解释限制因素。要使用准确的术语并写出配平的方程式。

Always mention oxygen as a by-product of photolysis.

务必提到氧气是光解的副产品。
Remember to state that temperature affects enzyme activity, not the light reaction directly.

记住温度影响酶活性，而非直接作用于光反应。
When drawing graphs, label axes clearly and indicate plateau regions with a reason.

绘图时，清晰标注坐标轴，并标明平台区及其原因。
In practical questions, specify controls and independent/dependent variables.

在实验题中，明确对照组及自变量/因变量。

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Photosynthesis | 光合作用