📚 IGCSE CIE Biology: Carbon Cycle – Key Points | IGCSE CIE 生物：碳循环 考点精讲
The carbon cycle is a fundamental concept in IGCSE CIE Biology, describing how carbon atoms are recycled through the Earth’s biosphere, atmosphere, oceans and geosphere. Carbon is the backbone of all organic molecules, and understanding the processes that move carbon between different reservoirs is essential for explaining ecosystem dynamics and global climate change. This revision guide focuses on the key exam points you need to master, including photosynthesis, respiration, decomposition, combustion, fossil fuel formation and the role of oceans.
碳循环是 IGCSE CIE 生物学中的基本概念，描述了碳原子如何在地球生物圈、大气、海洋和岩石圈中循环。碳是所有有机分子的骨架，理解碳在不同储库之间转移的过程对于解释生态系统动态和全球气候变化至关重要。本复习指南聚焦于你需要掌握的考点，包括光合作用、呼吸作用、分解作用、燃烧、化石燃料形成以及海洋的作用。
1. Carbon Cycle Overview | 碳循环概述
The carbon cycle is the continuous movement of carbon in various forms between the atmosphere, living organisms, oceans and the Earth’s crust. Carbon exists as carbon dioxide (CO₂) in the air, dissolved carbonates in water, organic compounds in living things and fossil fuels, and as limestone (CaCO₃) in rocks. In IGCSE exams, you are expected to recognise the main processes that add carbon to the atmosphere and those that remove it, as well as interpret carbon cycle diagrams with arrows indicating the direction of carbon flow.
碳循环是指碳以各种形式在大气、生物体、海洋和地壳之间持续移动的过程。碳在大气中以二氧化碳（CO₂）形式存在，在水中以溶解的碳酸盐形式存在，在生物体和化石燃料中以有机化合物形式存在，在岩石中以石灰石（CaCO₃）形式存在。在 IGCSE 考试中，你需要识别向大气中增加碳的主要过程和从大气中移除碳的过程，并能解读带有箭头指示碳流向的碳循环图。
The cycle can be divided into short-term processes occurring over days to decades (e.g. photosynthesis, respiration, decomposition) and long-term processes occurring over millions of years (e.g. formation of fossil fuels and limestone). Both are crucial for maintaining the carbon balance on Earth.
碳循环可分为在数天至数十年内发生的短期过程（如光合作用、呼吸作用、分解作用）和在数百万年内发生的长期过程（如化石燃料和石灰石的形成）。两者对于维持地球的碳平衡都至关重要。
2. Photosynthesis – Carbon Fixation | 光合作用：碳固定
Photosynthesis is the primary process that removes carbon dioxide from the atmosphere. Green plants, algae and some bacteria use light energy to convert CO₂ and water into glucose, releasing oxygen as a by-product. This process converts inorganic carbon into organic carbon, making it available to other organisms through food chains.
光合作用是从大气中去除二氧化碳的主要过程。绿色植物、藻类和一些细菌利用光能将 CO₂ 和水转化为葡萄糖，并释放氧气作为副产物。这一过程将无机碳转化为有机碳，使其通过食物链供给其他生物。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
The glucose produced can be used for respiration, stored as starch, or converted into other organic molecules such as cellulose, proteins and lipids. In carbon cycle diagrams, the arrow from atmospheric CO₂ to plants is usually labelled ‘photosynthesis’. This is the most significant carbon sink in the short-term cycle.
产生的葡萄糖可用于呼吸作用，以淀粉形式储存，或转化为其他有机分子，如纤维素、蛋白质和脂质。在碳循环图中，从大气 CO₂ 指向植物的箭头通常标记为“光合作用”。这是短期循环中最重要的碳汇。
3. Carbon Transfer via Feeding | 捕食与碳传递
When herbivores consume plants, the carbon-containing organic molecules (carbohydrates, fats, proteins) are transferred to the animal’s body. This carbon is then used to build new tissues, provide energy, or is passed further along the food chain when carnivores eat herbivores. Carbon moves from producer to primary consumer, to secondary consumer, and so on.
当食草动物取食植物时，含碳有机分子（碳水化合物、脂肪、蛋白质）被传递到动物体内。这些碳随后被用于构建新组织、提供能量，或当食肉动物捕食食草动物时沿食物链继续传递。碳从生产者转移到初级消费者，再到次级消费者，依此类推。
All organic carbon in consumers ultimately originates from photosynthetic organisms. While animals do not fix CO₂ directly, they play a key role in cycling carbon by releasing it back through respiration and excretion. Decomposers later break down dead organic matter from all trophic levels.
消费者体内的所有有机碳最终都来源于光合生物。虽然动物不能直接固定 CO₂，但通过呼吸和排泄将碳释放回循环，从而起关键作用。分解者随后分解来自各个营养级的死亡有机质。
4. Respiration – Carbon Release | 呼吸作用：碳释放
Respiration is the process by which all living cells break down organic molecules (e.g. glucose) to release energy, producing carbon dioxide and water as waste products. This aerobic respiration occurs continuously in plants, animals and decomposers. It is a major source of atmospheric CO₂ in the short-term cycle.
呼吸作用是所有活细胞分解有机分子（如葡萄糖）以释放能量的过程，产生二氧化碳和水作为废物。这种有氧呼吸在植物、动物和分解者体内不断发生。它是短期循环中大气 CO₂ 的主要来源。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
In plants, photosynthesis dominates during daylight, so they often appear to absorb CO₂ overall. However, plants respire day and night, releasing CO₂ into the atmosphere. At night, when photosynthesis stops, plants become net producers of CO₂. IGCSE questions often ask you to compare the net carbon exchange of a plant during light and dark conditions.
植物在白天光合作用占主导，因此总体上似乎吸收 CO₂。然而植物日夜都在进行呼吸作用，向大气释放 CO₂。夜晚光合作用停止，植物成为 CO₂ 的净生产者。IGCSE 考题常要求比较植物在光照和黑暗条件下的碳净交换。
5. Decomposition | 分解作用
Decomposition is carried out by microorganisms, mainly bacteria and fungi (saprotrophs), which break down dead organic material and waste products. During this process, enzymes are secreted to digest complex organic compounds externally, releasing carbon dioxide back into the atmosphere. Decomposition is essential for recycling carbon and other nutrients.
分解作用由微生物，主要是细菌和真菌（腐生者）完成，它们分解死去的有机物质和废物。此过程中，微生物分泌酶在体外消化复杂的有机化合物，将二氧化碳释放回大气。分解作用对碳和其他营养物质的循环至关重要。
Warm, moist conditions and good oxygen supply speed up decomposition. In waterlogged or acidic environments (e.g. bogs), decomposition is slowed, leading to the accumulation of partially decayed organic matter, which can eventually form fossil fuels such as peat and coal.
温暖、潮湿和充足的氧气供应会加速分解。在积水或酸性环境（如沼泽）中，分解减缓，导致部分腐烂的有机物积累，最终可能形成泥炭和煤等化石燃料。
6. Fossil Fuel Formation | 化石燃料的形成
Fossil fuels (coal, oil, natural gas) represent carbon that has been locked away from the active cycle for millions of years. They form when dead plants or marine organisms are buried under sediments with little or no oxygen, preventing full decomposition. Over geological time, heat and pressure transform the organic material into energy-rich fossil fuels.
化石燃料（煤、石油、天然气）代表了数百万年以来从活跃循环中被锁定的碳。它们形成于死亡植物或海洋生物被埋藏在沉积物下且几乎没有氧气的环境中，由此阻止了完全分解。经过地质时间，热力和压力将有机物质转化为富含能量的化石燃料。
Coal originates mainly from ancient forests and swamp vegetation (terrestrial organic matter), while oil and natural gas are typically derived from microscopic marine plankton. This distinction is important – IGCSE may ask where the carbon in crude oil comes from. Fossil fuels are considered a carbon store or ‘long-term carbon sink’.
煤主要来源于远古森林和沼泽植被（陆地有机质），而石油和天然气通常来源于微型海洋浮游生物。这一区别很重要——IGCSE 可能会问原油中的碳来自何处。化石燃料被视为碳库或“长期碳汇”。
7. Combustion | 燃烧
Combustion (burning) of carbon-based materials rapidly releases large amounts of carbon dioxide into the atmosphere. When fossil fuels such as coal, oil or natural gas are burned in the presence of oxygen, the carbon combines with oxygen to form CO₂. The general simplified equation is:
含碳物质的燃烧会迅速向大气中释放大量二氧化碳。当煤、石油或天然气等化石燃料在有氧气的情况下燃烧时，碳与氧结合形成 CO₂。简化的通用方程式为：
C + O₂ → CO₂
For hydrocarbons like methane (CH₄), the combustion equation is: CH₄ + 2O₂ → CO₂ + 2H₂O. In the carbon cycle, combustion is a natural process (e.g. forest fires) but human industrial activities and vehicle engines have dramatically increased the rate of combustion, disrupting the natural carbon balance.
对于甲烷（CH₄）等碳氢化合物，燃烧方程式为：CH₄ + 2O₂ → CO₂ + 2H₂O。在碳循环中，燃烧是一个自然过程（如森林大火），但人类工业活动和汽车引擎极大地加快了燃烧速率，打破了自然的碳平衡。
8. Oceans and Limestone | 海洋与石灰石
Oceans act as a major carbon sink by absorbing atmospheric CO₂. When CO₂ dissolves in seawater, it forms carbonic acid (H₂CO₃), which can dissociate into hydrogen ions and bicarbonate ions (HCO₃⁻). This dissolved inorganic carbon is used by marine organisms like corals and molluscs to build their shells and skeletons from calcium carbonate (CaCO₃).
海洋通过吸收大气中的 CO₂ 充当主要碳汇。当 CO₂ 溶解在海水中，会形成碳酸（H₂CO₃），碳酸可解离为氢离子和碳酸氢根离子（HCO₃⁻）。这些溶解的无机碳被珊瑚和软体动物等海洋生物用来构建由碳酸钙（CaCO₃）构成的壳和骨骼。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
When these organisms die, their calcium carbonate remains settle on the ocean floor and are gradually compressed into limestone rock. This locks carbon away for millions of years. Weathering and volcanic activity can eventually return this carbon to the cycle, but limestone is a very long-term carbon store.
当这些生物死亡，其碳酸钙残骸沉入海底并逐渐被压缩为石灰石。这可以将碳锁定数百万年。风化和火山活动最终可能将这些碳释放回循环中，但石灰石是一种非常长期的碳储存。
9. Human Impact on the Carbon Cycle | 人类对碳循环的影响
Human activities have greatly perturbed the carbon cycle, primarily by burning fossil fuels and clearing forests. Combustion of coal, oil and gas for energy releases additional CO₂ that had been locked underground for millions of years, significantly raising atmospheric CO₂ concentration. Deforestation reduces the number of trees available to absorb CO₂ through photosynthesis, and often the cut timber is burned, releasing even more carbon.
人类活动极大地扰乱了碳循环，主要是通过燃烧化石燃料和砍伐森林。为获取能量而燃烧煤、石油和天然气会释放额外 CO₂，这些碳已被锁定在地下数百万年，从而显著提高了大气 CO₂ 浓度。砍伐森林减少了能够通过光合作用吸收 CO₂ 的树木数量，而且被砍伐的木材常常被焚烧，释放出更多碳。
Other human contributions include cement production (which releases CO₂ from limestone calcination) and agricultural practices that accelerate soil decomposition. The increased greenhouse effect leads to global warming, climate change and ocean acidification, as excess CO₂ dissolves in the oceans. IGCSE often asks students to relate these impacts to the carbon cycle.
其他人类贡献包括水泥生产（通过石灰石煅烧释放 CO₂）以及加速土壤分解的农业实践。增强的温室效应导致全球变暖、气候变化和海洋酸化，因为过多的 CO₂ 溶解在海洋中。IGCSE 常要求学生将这些影响与碳循环联系起来。
10. Exam Tips and Diagram Analysis | 考试技巧与图表分析
In IGCSE CIE Biology, you will frequently encounter carbon cycle diagrams. Always check the direction of arrows: arrows pointing towards ‘atmospheric CO₂’ indicate processes that release carbon (respiration, decomposition, combustion), while arrows pointing away from ‘atmospheric CO₂’ indicate processes that remove carbon (photosynthesis, dissolving in oceans). Be prepared to label arrows with the correct process name.
在 IGCSE CIE 生物学考试中，你会经常会遇到碳循环图。一定检查箭头的方向：指向“大气 CO₂”的箭头表示释放碳的过程（呼吸作用、分解作用、燃烧），而远离“大气 CO₂”的箭头表示移除碳的过程（光合作用、溶解于海洋）。准备好用正确的过程名称标注箭头。
| Process | Effect on atmospheric CO₂ |
|---|---|
| Photosynthesis | Removes CO₂ |
| Respiration | Adds CO₂ |
| Decomposition | Adds CO₂ |
| Combustion | Adds CO₂ |
| Fossil fuel formation | Removes carbon long-term |
| Ocean uptake / shell formation | Removes CO₂, stores as carbonates |
过程与对大气 CO₂ 的影响表（英文如上）。在考试中，你可能需要列举四个向大气中添加 CO₂ 的过程和四个移除 CO₂ 的过程。记住砍伐森林的影响：光合作用吸收的 CO₂ 减少，加上燃烧树木增加了额外排放。同样，微生物分解者在温暖湿润条件下活动旺盛，在碳循环简答题中经常出现。方程式（光合作用和呼吸作用）必须准确写出，下标
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