📚 IGCSE Edexcel Biology: The Carbon Cycle | IGCSE Edexcel 生物：碳循环考点精讲
The carbon cycle describes the continuous movement of carbon atoms between the atmosphere, living organisms, oceans, and the Earth’s crust. Carbon is the backbone of all organic molecules, making this cycle essential for life. In IGCSE Edexcel Biology, you need to understand the main processes that transfer carbon – photosynthesis, respiration, decomposition, combustion, and fossilisation – and how human activities disrupt this balance, leading to climate change.
碳循环描述了碳原子在大气、生物体、海洋和地壳之间的持续运动。碳是所有有机分子的骨架，使该循环对生命至关重要。在IGCSE Edexcel 生物中，你需要理解转移碳的主要过程——光合作用、呼吸作用、分解、燃烧和化石形成——以及人类活动如何破坏这种平衡，导致气候变化。
1. What is the Carbon Cycle? | 什么是碳循环？
The carbon cycle is a biogeochemical cycle in which carbon is exchanged between the biosphere, geosphere, hydrosphere, and atmosphere. Carbon exists in various forms, such as carbon dioxide (CO₂) in the air, dissolved carbonates in the ocean, organic matter in living organisms, and fossil fuels underground. The cycle consists of both rapid biological processes and slow geological processes.
碳循环是一个生物地球化学循环，碳在生物圈、地圈、水圈和大气圈之间进行交换。碳以多种形式存在，例如空气中的二氧化碳（CO₂）、海洋中溶解的碳酸盐、生物体中的有机物以及地下的化石燃料。该循环包括快速的生物过程和缓慢的地质过程。
2. Carbon in Living Organisms | 生物体中的碳
All biological macromolecules – carbohydrates, proteins, lipids, and nucleic acids – contain carbon. Producers (plants and algae) fix atmospheric CO₂ into glucose through photosynthesis. Consumers then obtain carbon by eating other organisms. Thus, carbon flows through food chains and webs.
所有生物大分子——碳水化合物、蛋白质、脂质和核酸——都含有碳。生产者（植物和藻类）通过光合作用将大气中的CO₂固定为葡萄糖。消费者随后通过捕食其他生物获取碳。因此，碳沿着食物链和食物网流动。
3. Photosynthesis: Carbon Fixation | 光合作用：碳的固定
Photosynthesis is the primary process that removes CO₂ from the atmosphere. Green plants and algae use light energy to convert carbon dioxide and water into glucose and oxygen. The overall word equation is:
光合作用是从大气中去除CO₂的主要过程。绿色植物和藻类利用光能将二氧化碳和水转化为葡萄糖和氧气。总文字方程式为：
Carbon dioxide + Water → Glucose + Oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
The balanced chemical equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This process stores light energy as chemical energy in glucose and incorporates carbon into organic compounds.
配平的化学方程式为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。该过程将光能以化学能的形式储存在葡萄糖中，并将碳纳入有机化合物。
4. Respiration: Returning Carbon to the Atmosphere | 呼吸作用：碳返回大气
All living organisms carry out respiration to release energy from glucose. Aerobic respiration uses oxygen to break down glucose, producing CO₂ and water as waste products. This returns carbon to the atmosphere. The word equation is:
所有生物体都进行呼吸作用以从葡萄糖中释放能量。有氧呼吸利用氧气分解葡萄糖，产生CO₂和水作为废物。这将碳返回大气。文字方程式为：
Glucose + Oxygen → Carbon dioxide + Water (+ energy)
葡萄糖 + 氧气 → 二氧化碳 + 水 (+ 能量)
Even plants respire at night when photosynthesis stops, releasing CO₂. Respiration occurs in cells’ mitochondria and is a continuous process in all aerobic organisms.
植物在夜间光合作用停止时也会呼吸，释放CO₂。呼吸作用发生在线粒体中，是所有需氧生物持续进行的过程。
5. Feeding and Carbon Transfer | 取食与碳传递
Carbon moves from producers to consumers when herbivores eat plants, and then to carnivores when they eat herbivores. At each trophic level, the carbon-containing molecules are digested and assimilated to build new biomass or are respired. This is how carbon compounds pass through ecosystems.
当草食动物食用植物时，碳从生产者转移到消费者；当肉食动物捕食草食动物时，碳继续传递。在每个营养级，含碳分子被消化和同化，用于构建新的生物质，或被呼吸消耗。这就是碳化合物在生态系统中传递的方式。
6. Decomposition and the Role of Decomposers | 分解与分解者的作用
When organisms die or produce waste, decomposers such as bacteria and fungi break down the organic matter. They secrete enzymes that digest complex carbon compounds externally, then absorb the simpler molecules. As decomposers respire, they release CO₂ back into the atmosphere. Without decomposition, carbon would remain locked in dead biomass.
当生物死亡或产生废物时，细菌和真菌等分解者会分解有机物。它们分泌酶在体外消化复杂的碳化合物，然后吸收较简单的分子。分解者呼吸时，将CO₂释放回大气。如果没有分解，碳将被锁在死亡的生物质中。
Decomposition is faster in warm, moist, and oxygen-rich conditions, which favour microbial activity. This is an essential part of the carbon cycle in soil and water.
分解在温暖、潮湿和富氧的条件下更快，这些条件有利于微生物活动。这是土壤和水中碳循环的重要部分。
7. Fossilisation: Formation of Fossil Fuels | 化石形成：化石燃料的形成
When dead organisms – especially plankton, plants, and animals – are buried under layers of sediment in oxygen-poor conditions, their decay is incomplete. Over millions of years, heat and pressure convert the remains into fossil fuels: coal (from plant material), crude oil and natural gas (from marine organisms). This process locks carbon underground for geological time scales.
当死亡的生物——尤其是浮游生物、植物和动物——在缺氧条件下被沉积物层掩埋后，其腐烂过程不完全。经过数百万年，热量和压力将这些遗骸转化为化石燃料：煤（来自植物物质）、原油和天然气（来自海洋生物）。这个过程将碳锁定在地下，时间尺度以地质年代计。
8. Combustion: Burning Fossil Fuels | 燃烧：化石燃料的燃烧
Combustion (burning) of fossil fuels releases energy and returns carbon to the atmosphere as CO₂. In a power station, vehicle engine, or domestic fire, hydrocarbons react with oxygen. For example, methane burns: CH₄ + 2O₂ → CO₂ + 2H₂O. This process rapidly releases carbon that was stored underground for millions of years, contributing to the increased atmospheric CO₂ concentration.
化石燃料的燃烧释放能量，并以CO₂的形式将碳返回大气。在发电站、机动车发动机或家庭火炉中，碳氢化合物与氧气反应。例如，甲烷燃烧：CH₄ + 2O₂ → CO₂ + 2H₂O。这个过程迅速释放了在地下储存了数百万年的碳，导致大气CO₂浓度增加。
Combustion also includes natural events such as forest fires, which release stored carbon quickly. However, human combustion of fossil fuels is the main driver of rising CO₂ levels.
燃烧还包括自然事件，如森林火灾，会迅速释放储存的碳。然而，人类燃烧化石燃料是CO₂水平升高的主要驱动因素。
9. Ocean Carbon Sink and Limestone Formation | 海洋碳汇与石灰岩形成
The oceans are a major carbon sink, absorbing CO₂ directly from the atmosphere. Dissolved CO₂ forms carbonic acid, which dissociates into hydrogen carbonate ions (HCO₃⁻). Marine organisms such as corals, molluscs, and plankton use these ions to build their shells and skeletons from calcium carbonate (CaCO₃). When these organisms die, their remains settle on the ocean floor and over time form limestone rock. This locks carbon away for millions of years.
海洋是一个重要的碳汇，直接从大气中吸收CO₂。溶解的CO₂形成碳酸，后者解离为碳酸氢根离子（HCO₃⁻）。珊瑚、软体动物和浮游生物等海洋生物利用这些离子以碳酸钙（CaCO₃）构建外壳和骨骼。当这些生物死亡时，其遗骸沉入海底，久而久之形成石灰岩。这将碳锁定了数百万年。
10. Deforestation and the Carbon Cycle | 森林砍伐与碳循环
Forests are significant carbon sinks because trees remove CO₂ via photosynthesis and store carbon in wood. Deforestation – the large-scale cutting down of forests – reduces the number of trees that can absorb CO₂. Moreover, when trees are burned or left to rot, the stored carbon is released back into the atmosphere as CO₂. This dual effect increases atmospheric CO₂ levels.
森林是重要的碳汇，因为树木通过光合作用移除CO₂并将碳储存在木材中。森林砍伐——大规模砍伐森林——减少了能吸收CO₂的树木数量。此外，当树木被燃烧或腐烂时，储存的碳以CO₂的形式释放回大气。这种双重效应增加了大气CO₂水平。
11. Enhanced Greenhouse Effect and Global Warming | 增强的温室效应与全球变暖
The greenhouse effect is a natural process in which greenhouse gases (CO₂, methane, water vapour) trap heat in the atmosphere, keeping the Earth warm enough for life. However, human activities – burning fossil fuels, deforestation, agriculture – have significantly increased CO₂ and methane concentrations. This leads to an enhanced greenhouse effect, causing global warming, climate change, rising sea levels, and more extreme weather events.
温室效应是一个自然过程，温室气体（CO₂、甲烷、水蒸气）将热量困在大气中，使地球保持足够温暖以维持生命。然而，人类活动——燃烧化石燃料、砍伐森林、农业——显著增加了CO₂和甲烷的浓度。这导致了增强的温室效应，引发全球变暖、气候变化、海平面上升和更多极端天气事件。
12. Maintaining the Balance and Summary | 平衡维持与总结
In a stable ecosystem, the carbon cycle is balanced: the amount of CO₂ removed by photosynthesis roughly equals the amount returned by respiration and decomposition. Human activities have tipped this balance, causing a net increase in atmospheric CO₂. Sustainable practices – reforestation, using alternative energy sources, reducing fossil fuel use – can help restore balance. Key processes to remember: photosynthesis (fixes carbon), respiration (releases carbon), decomposition (returns carbon), combustion (releases carbon rapidly), fossilisation (locks carbon). Understanding these processes is essential for the IGCSE exam.
在稳定的生态系统中，碳循环是平衡的：光合作用移除的CO₂量大致等于呼吸和分解返回的量。人类活动打破了这种平衡，导致大气CO₂净增加。可持续实践——重新造林、使用替代能源、减少化石燃料使用——可以帮助恢复平衡。要记住的关键过程：光合作用（固定碳），呼吸作用（释放碳），分解（返回碳），燃烧（快速释放碳），化石形成（锁定碳）。理解这些过程对IGCSE考试至关重要。
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