📚 Carbon Cycle Key Points for IGCSE WJEC Biology | IGCSE WJEC 生物：碳循环 考点精讲
The carbon cycle describes how carbon atoms are recycled between the atmosphere, living organisms, oceans, and rocks. For IGCSE WJEC Biology, you need to understand the key processes that add carbon dioxide to the atmosphere and those that remove it, as well as the impact of human activities on this delicate balance. This article breaks down every essential concept with clear explanations and exam-focused tips.
碳循环描述了碳原子如何在大气、生物体、海洋和岩石之间循环。在 IGCSE WJEC 生物考试中，你需要理解向大气中添加二氧化碳的关键过程以及移除二氧化碳的过程，同时还要掌握人类活动对这种微妙平衡的影响。本文分解了每一个核心概念，提供清晰的解释和考试技巧。
1. Overview of the Carbon Cycle | 碳循环概述
Carbon is a fundamental element found in all organic molecules, including carbohydrates, proteins, and lipids. It exists in the atmosphere mainly as carbon dioxide (CO₂) and in water as dissolved CO₂ or hydrogen carbonate ions. The carbon cycle ensures that carbon is continuously reused, preventing it from being permanently locked away in one form.
碳是构成所有有机分子的基本元素，包括碳水化合物、蛋白质和脂质。它在大气中主要以二氧化碳 (CO₂) 的形式存在，在水中则以溶解的 CO₂ 或碳酸氢根离子形式存在。碳循环确保碳不断被重复利用，防止碳永久锁定在某种形式中。
2. Key Processes: Photosynthesis | 关键过程：光合作用
Photosynthesis removes CO₂ from the atmosphere. Green plants and algae use light energy to convert carbon dioxide and water into glucose and oxygen. The chemical summary is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This process is the main way carbon enters the biotic part of the cycle, transforming inorganic carbon into organic compounds.
光合作用从大气中移除二氧化碳。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。化学方程式概括为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。这一过程是碳进入生物循环部分的主要途径，将无机碳转化为有机化合物。
3. Key Processes: Respiration | 关键过程：呼吸作用
Respiration returns CO₂ to the atmosphere. All living organisms, including plants and animals, break down glucose to release energy for cellular activities. Aerobic respiration can be summarised as: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy). Decomposers such as bacteria and fungi also respire, releasing CO₂ as they break down dead organic matter.
呼吸作用将二氧化碳返回大气。所有生物体，包括植物和动物，分解葡萄糖以释放能量用于细胞活动。有氧呼吸概括为：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O（+ 能量）。分解者如细菌和真菌也会进行呼吸作用，在分解死亡有机物时释放 CO₂。
4. Key Processes: Combustion | 关键过程：燃烧
Combustion of fossil fuels and organic matter releases stored carbon back into the atmosphere as CO₂. When wood, coal, oil, or natural gas is burned, the carbon that was locked in these materials combines with oxygen. The equation for complete combustion of a hydrocarbon is: hydrocarbon + O₂ → CO₂ + H₂O. Wildfires also contribute to carbon dioxide emissions.
化石燃料和有机物质的燃烧会将储存的碳以 CO₂ 的形式释放回大气中。当木材、煤、石油或天然气燃烧时，被锁在这些物质中的碳与氧气结合。碳氢化合物完全燃烧的方程式为：碳氢化合物 + O₂ → CO₂ + H₂O。野火也会造成二氧化碳排放。
5. Key Processes: Decomposition | 关键过程：分解作用
Decomposition breaks down dead plants and animals, returning nutrients and carbon to the soil and atmosphere. Decomposers (mainly bacteria and fungi) secrete enzymes to digest organic matter externally, then absorb the products. During this process, they respire, releasing CO₂. If decomposition occurs without oxygen (anaerobic conditions), methane (CH₄) may be produced instead, which is another carbon-containing greenhouse gas.
分解作用分解死去的植物和动物，将养分和碳归还到土壤和大气中。分解者（主要是细菌和真菌）分泌酶在体外消化有机物，然后吸收产物。在此过程中，它们进行呼吸作用，释放 CO₂。如果分解在无氧条件下进行（厌氧条件），则可能产生甲烷 (CH₄)，这是另一种含碳的温室气体。
6. Feeding and Carbon Transfer | 摄食与碳转移
Carbon moves through food chains when organisms eat each other. Producers absorb CO₂ during photosynthesis and convert it into organic compounds. Primary consumers eat producers, secondary consumers eat primary consumers, and so on. At each trophic level, carbon is used to build biomass and is then respired or excreted. Only a small proportion (about 10%) is passed on to the next level.
碳通过食物链在生物之间转移。生产者通过光合作用吸收 CO₂ 并将其转化为有机化合物。初级消费者吃生产者，次级消费者吃初级消费者，依此类推。在每个营养级，碳用于构建生物质，随后被呼吸作用消耗或排泄。只有一小部分（约 10%）传递到下一级。
7. Fossilisation and Long-term Storage | 化石形成与长期储存
When dead organisms are buried under sediment and subjected to high pressure and temperature over millions of years, they can form fossil fuels. Plants may turn into coal, while marine plankton may become oil and natural gas. This process locks carbon away from the atmosphere for geological timescales. Limestone (calcium carbonate, CaCO₃) also stores carbon, formed from the shells and skeletons of marine organisms.
当死去的生物体被沉积物掩埋，并在数百万年间承受高压和高温时，它们会形成化石燃料。植物可能变成煤，而海洋浮游生物可能形成石油和天然气。这一过程在地质时间尺度上将碳从大气中锁定。石灰石（碳酸钙，CaCO₃）也能储存碳，它由海洋生物的壳和骨骼形成。
8. The Role of Oceans | 海洋的作用
Oceans act as a major carbon sink. CO₂ dissolves directly in seawater, where it can remain as dissolved gas or react with water to form carbonic acid (H₂CO₃), which dissociates into hydrogen carbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻). This process helps regulate atmospheric CO₂ levels. Marine organisms use carbonate ions to build shells, which eventually become sedimentary rock.
海洋是一个主要的碳汇。CO₂ 直接溶解在海水中，可以以溶解气体形式存在，或与水反应生成碳酸 (H₂CO₃)，碳酸再解离成碳酸氢根离子 (HCO₃⁻) 和碳酸根离子 (CO₃²⁻)。这一过程有助于调节大气中的 CO₂ 浓度。海洋生物利用碳酸根离子来构建外壳，最终形成沉积岩。
9. Impact of Deforestation | 森林砍伐的影响
Deforestation increases the amount of CO₂ in the atmosphere. Forests act as carbon sinks because trees absorb CO₂ for photosynthesis. When forests are cleared and burned, stored carbon is released quickly as CO₂. Even without burning, fewer trees mean less CO₂ is removed from the atmosphere. Additionally, wood removed from the forest often decomposes or is used as fuel, further releasing carbon.
森林砍伐增加了大气中的 CO₂ 含量。森林作为碳汇，因为树木通过光合作用吸收 CO₂。当森林被砍伐并焚烧时，储存的碳迅速以 CO₂ 形式释放。即使不燃烧，树木的减少也意味着从大气中移除的 CO₂ 减少了。此外，从森林中移出的木材经常分解或被用作燃料，进一步释放碳。
10. Impact of Increased Burning of Fossil Fuels | 化石燃料燃烧增加的影响
The burning of fossil fuels releases carbon that has been stored underground for hundreds of millions of years. This massive addition of CO₂ to the atmosphere disrupts the natural carbon cycle, causing the greenhouse effect to intensify. Extra CO₂ traps more heat in the atmosphere, leading to global warming and climate change. This is one of the most significant human impacts on the carbon cycle.
化石燃料的燃烧释放了在地下储存了数亿年的碳。大量排放到大气中的 CO₂ 破坏了自然的碳循环，导致温室效应加剧。额外的 CO₂ 捕获更多热量在大气中，导致全球变暖和气候变化。这是人类对碳循环最显著的影响之一。
11. The Greenhouse Effect and Carbon Cycle | 温室效应与碳循环
CO₂ and methane are greenhouse gases that absorb infrared radiation and re-radiate it back to Earth, warming the surface. The natural greenhouse effect is essential for life, but enhanced greenhouse effect due to human activity leads to rising global temperatures. The carbon cycle normally maintains a balance, but increased emissions and reduced sinks tip this balance, causing more carbon to stay in the atmosphere.
二氧化碳和甲烷是温室气体，它们吸收红外辐射并将其重新辐射回地球，导致地表变暖。天然的温室效应对生命至关重要，但由人类活动导致的增强温室效应引起了全球气温上升。碳循环通常维持平衡，但排放增加和碳汇减少打破了这种平衡，导致更多碳留在大气中。
12. Exam Tips and Common Diagrams | 考试技巧与常见图表
In WJEC IGCSE exams, you may be asked to draw or label a carbon cycle diagram, explain why respiration and photosynthesis are opposite processes, or discuss the consequences of deforestation and increased fossil fuel use. Always use the terms: photosynthesis, respiration, combustion, decomposition, fossilisation, feeding, and ocean absorption. Be precise about the direction of carbon flow. Use arrows in diagrams to show carbon movement, and label sinks and sources correctly.
在 WJEC IGCSE 考试中，你可能被要求绘制或标注碳循环图、解释为什么呼吸作用和光合作用是相反的过程，或讨论森林砍伐和化石燃料使用增加的后果。始终使用这些术语：光合作用、呼吸作用、燃烧、分解、化石形成、摄食和海洋吸收。准确表示碳流动的方向。在图表中用箭头表示碳的移动，并正确标注碳汇和碳源。
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