📚 The Carbon Cycle | 碳循环
Carbon is the fundamental building block of life on Earth. It is constantly moving between the atmosphere, living organisms, oceans, and the Earth's crust through a series of processes known as the carbon cycle. In the Edexcel IGCSE Science syllabus, understanding this cycle is essential to grasping how ecosystems function and how human activities are altering the natural balance.
碳是地球上生命的基本构建单元。它通过一系列被称为碳循环的过程，在大气、生物体、海洋和地壳之间不断流动。在 Edexcel IGCSE 科学考试大纲中，理解这一循环对于掌握生态系统如何运作以及人类活动如何改变自然平衡至关重要。
1. Introduction to Carbon and Its Importance | 碳及其重要性简介
Carbon atoms form the backbone of all organic molecules, including carbohydrates, proteins, lipids, and nucleic acids. The element cycles through the environment in various forms, primarily as carbon dioxide (CO₂) in the atmosphere, dissolved carbonates in water, and solid carbon in rocks and fossil fuels. Without this cycling, life as we know it could not exist, because carbon would become locked in one form and unavailable for biological processes.
碳原子是所有有机分子（包括碳水化合物、蛋白质、脂质和核酸）的骨架。该元素以多种形式在环境中循环，主要是大气中的二氧化碳（CO₂）、水中的溶解碳酸盐以及岩石和化石燃料中的固态碳。如果没有这种循环，生命将无法存在，因为碳会被锁定在某种形式中，无法被生物过程利用。
In IGCSE Science, you need to recall that carbon is present in all living things and that the carbon cycle describes how carbon is exchanged among the biosphere, geosphere, hydrosphere, and atmosphere. Key processes include photosynthesis, respiration, decomposition, combustion, and sedimentation.
在 IGCSE 科学中，你需要记住碳存在于所有生物中，碳循环描述了碳如何在生物圈、岩石圈、水圈和大气圈之间交换。关键过程包括光合作用、呼吸作用、分解作用、燃烧和沉积作用。
2. Overview of the Carbon Cycle | 碳循环概述
The carbon cycle can be divided into short-term biological processes and long-term geological processes. The short-term cycle involves the rapid exchange of carbon between living organisms and the atmosphere, while the long-term cycle involves the storage of carbon in rocks and fossil fuels over millions of years.
碳循环可分为短期生物过程和长期地质过程。短期循环涉及碳在生物体与大气之间的快速交换，而长期循环涉及碳在岩石和化石燃料中储存数百万年。
A simplified diagram of the cycle shows the main reservoirs: the atmosphere (as CO₂), plants and animals (as organic compounds), soil (as humus and decomposing matter), oceans (as dissolved CO₂ and carbonates), and fossil fuels (coal, oil, natural gas). Arrows represent the fluxes of carbon — for example, CO₂ is removed from the atmosphere by photosynthesis and returned by respiration and combustion.
碳循环的简化图展示了主要储存库：大气（以 CO₂ 形式）、植物和动物（以有机化合物形式）、土壤（以腐殖质和分解物质形式）、海洋（以溶解 CO₂ 和碳酸盐形式）以及化石燃料（煤、石油、天然气）。箭头表示碳的流动——例如，CO₂ 通过光合作用从大气中移除，并通过呼吸作用和燃烧返回。
3. Photosynthesis: Capturing Carbon | 光合作用：固定碳
Photosynthesis is the primary process by which carbon enters the living world from the atmosphere. Green plants and algae use light energy to convert carbon dioxide and water into glucose and oxygen. The balanced word equation is:
光合作用是碳从大气进入生命世界的主要过程。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。其文字方程式为：
Carbon dioxide + Water → Glucose + Oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
In terms of the chemical equation, it is often written as:
其化学方程式通常写作：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
During this process, atmospheric carbon is 'fixed' into organic molecules. This glucose can then be used to build cellulose, starch, and other carbohydrates, or it can be respired to release energy. On a global scale, photosynthesis removes about 120 gigatonnes of carbon from the atmosphere each year, making it a vital carbon sink.
在此过程中，大气中的碳被“固定”为有机分子。这些葡萄糖随后可用于构建纤维素、淀粉和其他碳水化合物，也可以被呼吸分解释放能量。在全球范围内，光合作用每年从大气中移除约 1200 亿吨碳，使其成为至关重要的碳汇。
4. Respiration: Releasing Carbon | 呼吸作用：释放碳
Respiration is the process by which all living cells break down glucose to release energy for metabolic activities. This process returns carbon dioxide to the atmosphere or to the surrounding water. Both plants and animals respire, and in plants respiration occurs continuously, both day and night.
呼吸作用是所有活细胞分解葡萄糖以释放能量供代谢活动使用的过程。该过程将二氧化碳归还到大气或周围水中。植物和动物都进行呼吸作用，在植物中，呼吸作用昼夜不停地进行。
The word equation for aerobic respiration is:
有氧呼吸的文字方程式为：
Glucose + Oxygen → Carbon dioxide + Water (+ energy)
葡萄糖 + 氧气 → 二氧化碳 + 水（+ 能量）
Notice that respiration is essentially the reverse of photosynthesis in terms of carbon flow. While photosynthesis takes in CO₂, respiration releases it. In a balanced ecosystem, the rates of photosynthesis and respiration roughly cancel each other out over a 24-hour period, but human activities have disrupted this balance.
请注意，就碳的流动而言，呼吸作用基本上是光合作用的逆过程。光合作用吸收 CO₂，而呼吸作用则释放 CO₂。在平衡的生态系统中，光合作用和呼吸作用的速率在 24 小时周期内大致相互抵消，但人类活动已破坏了这一平衡。
5. Decomposition by Microorganisms | 微生物分解作用
When organisms die, the carbon locked in their bodies is not immediately released. Decomposers such as bacteria and fungi break down dead organic matter, respiring in the process and releasing carbon dioxide back into the atmosphere. This is a crucial step because it recycles nutrients and carbon, making them available for new plant growth.
当生物体死亡时，其体内的碳并不会立即释放。细菌和真菌等分解者分解死去的有机物，在此过程中进行呼吸，并将二氧化碳释放回大气中。这是一个关键步骤，因为它可回收养分和碳，使其可供新植物生长利用。
Decomposition works fastest in warm, moist, and aerobic conditions. In waterlogged or acidic environments, such as peat bogs, decomposition is very slow, leading to the accumulation of partially decayed organic matter. This material can eventually form peat and, over geological time, coal.
分解作用在温暖、潮湿和有氧的条件下最快。在渍水或酸性环境（如泥炭沼泽）中，分解作用非常缓慢，导致部分腐烂的有机物积累。这些物质最终可形成泥炭，并在漫长的地质时间中形成煤。
In the IGCSE specification, you should be able to describe the role of microorganisms in decay and the importance of carbon recycling. Without decomposers, dead material would pile up and carbon would remain locked away, starving living plants of essential CO₂.
在 IGCSE 考试大纲中，你应该能够描述微生物在腐烂过程中的作用以及碳循环的重要性。没有分解者，死亡物质将堆积如山，碳将被封锁，从而使活植物缺乏必需的 CO₂。
6. Combustion of Fossil Fuels and Organic Matter | 化石燃料和有机物的燃烧
Combustion (burning) is a chemical reaction that rapidly releases energy and carbon dioxide. When fossil fuels — coal, oil, and natural gas — are burned, carbon that has been locked underground for millions of years is suddenly released into the atmosphere as CO₂. This is the main driver of the enhanced greenhouse effect and climate change.
燃烧是一种快速释放能量和二氧化碳的化学反应。当化石燃料——煤、石油和天然气——被燃烧时，在地下封存了数百万年的碳突然以 CO₂ 的形式释放到大气中。这是增强的温室效应和气候变化的主要驱动因素。
The general word equation for the complete combustion of a hydrocarbon fuel is:
碳氢燃料完全燃烧的一般文字方程式为：
Fuel + Oxygen → Carbon dioxide + Water
燃料 + 氧气 → 二氧化碳 + 水
For example, methane (CH₄) burns as follows: CH₄ + 2O₂ → CO₂ + 2H₂O.
例如，甲烷（CH₄）燃烧如下：CH₄ + 2O₂ → CO₂ + 2H₂O。
In addition to industrial combustion, the burning of forests (deforestation by fire) also sends large amounts of CO₂ into the air. This double impact — reducing the number of trees that can absorb CO₂ while releasing stored carbon — significantly accelerates the rise in atmospheric carbon levels.
除工业燃烧外，森林的燃烧（通过火灾砍伐森林）也会向空气中排放大量的 CO₂。这种双重影响——减少能够吸收 CO₂ 的树木数量，同时释放储存的碳——显著加速了大气碳水平的上升。
7. Ocean Uptake and Carbonate Formation | 海洋吸收与碳酸盐形成
Oceans play a huge role in regulating atmospheric CO₂. Carbon dioxide dissolves in seawater, where it reacts with water to form carbonic acid (H₂CO₃). This weak acid can then dissociate to release hydrogen ions and bicarbonate ions (HCO₃⁻). Some of these ions combine with calcium to form calcium carbonate (CaCO₃), which is used by marine organisms to build shells and skeletons.
海洋在调节大气 CO₂ 方面发挥着巨大作用。二氧化碳溶解于海水中，与水反应生成碳酸（H₂CO₃）。这种弱酸随后可解离，释放出氢离子和碳酸氢根离子（HCO₃⁻）。其中一些离子与钙结合，形成碳酸钙（CaCO₃），海洋生物利用碳酸钙来构建外壳和骨骼。
When these organisms die, their shells sink to the ocean floor and over millions of years form sedimentary rocks such as limestone and chalk. This process locks carbon away in the lithosphere for very long periods. The ocean surface currently absorbs about one-quarter of the CO₂ emitted by human activities, but this is causing ocean acidification, which harms coral reefs and shellfish.
当这些生物死亡时，它们的外壳沉入海底，经过数百万年形成沉积岩，如石灰岩和白垩。这一过程将碳锁定在岩石圈中极其漫长的时间。目前海洋表面吸收了人类活动排放的约四分之一的 CO₂，但这正导致海洋酸化，危害珊瑚礁和贝类。
8. Feeding and Food Chains | 进食与食物链
Carbon moves through ecosystems via feeding relationships. Primary consumers (herbivores) eat plants and obtain carbon-containing organic compounds. Secondary consumers (carnivores) then eat the herbivores, and so on along the food chain.
碳通过摄食关系在生态系统中流动。初级消费者（食草动物）吃掉植物，获得含碳有机化合物。次级消费者（食肉动物）随后吃掉食草动物，并沿食物链依次类推。
At each trophic level, some of the carbon is used for growth and repair, but a significant portion is lost through respiration as CO₂. Eventually, all organisms die and the carbon in their bodies is returned to the environment via decomposition. Food chains therefore represent one of the short-term biological loops in the carbon cycle.
在每个营养级，部分碳被用于生长和修复，但相当大一部分通过呼吸作用以 CO₂ 的形式散失。最终，所有生物都会死亡，其体内的碳通过分解作用回归环境。因此，食物链代表了碳循环中的短期生物环路之一。
esis) → herbivore (feeding) → carnivore (feeding) → CO₂ (respiration by all) → atmosphere.
在考题中，你可能会被要求追踪一个碳原子从大气到顶级捕食者再回到大气的路径。这通常是：CO₂ → 植物（光合作用）→ 食草动物（摄食）→ 食肉动物（摄食）→ CO₂（所有生物的呼吸作用）→ 大气。
9. The Role of Fossilisation | 化石化作用
Not all dead organic matter is completely decomposed. In certain conditions — such as a lack of oxygen in swamps or rapid burial under sediment — organic material may be preserved and slowly transformed into fossil fuels or fossils. Over millions of years, heat and pressure convert peat into coal, and marine plankton into oil and natural gas.
并非所有死亡有机物都会被完全分解。在某些条件下——例如沼泽中缺氧或被沉积物快速掩埋——有机物质可能被保存下来，并慢慢转化为化石燃料或化石。经过数百万年，热量和压力将泥炭转化为煤，将海洋浮游生物转化为石油和天然气。
This is the main long-term removal of carbon from the active cycle. Once carbon is stored as fossil fuels, it is effectively removed from the atmosphere for geological timescales, unless humans extract and burn it. The carbon in limestone and other carbonate rocks is similarly stored.
这是碳从活跃循环中长期移除的主要方式。一旦碳以化石燃料的形式储存起来，除非被人类开采并燃烧，它就在地质时间尺度上有效地从大气中移除。石灰岩和其他碳酸盐岩中的碳也是类似地储存的。
10. Human Impact: Deforestation and Increased Emissions | 人类影响：森林砍伐与排放增加
Human activities have dramatically altered the carbon cycle over the past two centuries. The two main disruptions are the burning of fossil fuels and large-scale deforestation. Industrialisation, transport, and energy production release vast quantities of CO₂ that had been locked underground for millions of years.
过去两个世纪，人类活动极大地改变了碳循环。两个主要破坏因素是化石燃料的燃烧和大规模森林砍伐。工业化、交通运输和能源生产释放了大量被封锁在地下数百万年的 CO₂。
Deforestation has a twofold effect: fewer trees mean less CO₂ is absorbed through photosynthesis, and when forests are burned, the carbon stored in wood is rapidly released. In addition, livestock farming produces methane (CH₄), another potent greenhouse gas, which also contains carbon and affects the climate.
森林砍伐具有双重影响：树木减少意味着通过光合作用吸收的 CO₂ 减少；而当森林被燃烧时，储存在木材中的碳会迅速释放。此外，畜牧业会产生甲烷（CH₄），这是另一种强效温室气体，同样含有碳并影响气候。
As a result, atmospheric CO₂ concentrations have risen from about 280 parts per million (ppm) before the Industrial Revolution to over 420 ppm today. This sharp increase is directly linked to global warming and climate change, topics that are heavily examined in Edexcel IGCSE Science.
其结果是，大气 CO₂ 浓度已从工业革命前的约 280 ppm 上升到如今的 420 ppm 以上。这一急剧增长直接与全球变暖和气候变化相关，而这些正是 Edexcel IGCSE 科学中重点考查的主题。
11. Maintaining the Balance | 维持平衡
In a stable ecosystem, the carbon cycle is balanced: the amount of CO₂ removed from the atmosphere by photosynthesis is roughly equal to the amount returned by respiration, decomposition, and natural combustion events like wildfires. This dynamic equilibrium keeps climate and ecosystems functioning.
在稳定的生态系统中，碳循环是平衡的：通过光合作用从大气中移除的 CO₂ 量，与通过呼吸作用、分解作用和野火等自然燃烧过程返回的量大致相等。这种动态平衡维持着气候和生态系统的运作。
Strategies to restore balance include: reducing fossil fuel consumption, switching to renewable energy sources (solar, wind, hydro), reforesting large areas, protecting existing forests, and adopting sustainable agricultural practices. Carbon capture and storage (CCS) technologies are also being developed to trap CO₂ emitted from power plants and store it underground.
恢复平衡的策略包括：减少化石燃料消耗、转向可再生能源（太阳能、风能、水能）、大面积再造林、保护现有森林以及采用可持续农业实践。碳捕集与封存（CCS）技术也正在开发中，用于捕集发电厂排放的 CO₂ 并将其封存于地下。
In the IGCSE exam, you may encounter questions about how these strategies can help mitigate the effects of human impact on the carbon cycle, so be prepared to link actions to their effects on atmospheric CO₂ levels.
在 IGCSE 考试中，你可能会遇到关于这些策略如何帮助缓解人类对碳循环影响的问题，因此要准备好将行动与它们对大气 CO₂ 水平的影响联系起来。
12. Summary and Exam Tips | 总结与考试提示
The carbon cycle is a complex but logical system of fluxes between different reservoirs. To succeed in Edexcel IGCSE Science, ensure you can:
碳循环是一个复杂但合乎逻辑的系统，在不同储存库之间有着各种流动。为了在 Edexcel IGCSE 科学中取得成功，确保你能够：
- Describe the processes of photosynthesis, respiration, decomposition, combustion, and fossilisation, and explain how each affects carbon transfer.
- 描述光合作用、呼吸作用、分解作用、燃烧和化石化作用的过程，并解释每个过程如何影响碳的转移。
- Write word and simple chemical equations for photosynthesis and respiration.
- 写出光合作用和呼吸作用的文字方程式和简单的化学方程式。
- Explain the roles of microorganisms in recycling carbon.
- 解释微生物在碳循环中的作用。
- Discuss how human activities (deforestation, burning fossil fuels) are disrupting the carbon cycle and contributing to climate change.
- 讨论人类活动（森林砍伐、燃烧化石燃料）如何破坏碳循环并导致气候变化。
- Interpret data on atmospheric CO₂ levels and carbon flux diagrams.
- 解读大气 CO₂ 水平的数据和碳流动示意图。
Key terms to remember include: carbon sink, carbon source, greenhouse gas, acidification, and fossilisation. Use diagrams wherever possible in your answers to illustrate the flow of carbon.
需要记住的关键术语包括：碳汇、碳源、温室气体、酸化和化石化作用。在答案中尽可能使用图表来说明碳的流动。
A strong understanding of the carbon cycle will not only help you answer direct questions but will also underpin topics in ecology, chemistry of the atmosphere, and environmental science. Keep the big picture in mind: carbon atoms are constantly on the move, and every breath you take is part of this global cycle.
扎实理解碳循环不仅有助于你回答直接相关的问题，也将为生态学、大气化学和环境科学等主题奠定基础。牢记大图景：碳原子永不停歇地运动，而你的每一次呼吸都是这个全球循环的一部分。
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