📚 The Carbon Cycle | 碳循环
Carbon is the backbone of life on Earth. It is present in all living organisms, in the atmosphere as carbon dioxide (CO₂), dissolved in the oceans, and locked away in rocks and fossil fuels. The carbon cycle describes how carbon atoms move between these different stores, known as reservoirs, through a series of biological, chemical, geological, and physical processes. For IGCSE Edexcel Science students, understanding this cycle is essential not only as a standalone topic but also because it links core concepts in biology, chemistry, and environmental science. In this revision guide, we will break down every major pathway in the carbon cycle, examine human influences, and explore why maintaining a balanced carbon cycle is critical for life and climate stability.
碳是地球生命的基石。它存在于所有生物体内、大气中（以二氧化碳 CO₂ 的形式）、溶解在海洋里，并被封存于岩石和化石燃料中。碳循环描述了碳原子如何通过一系列生物、化学、地质和物理过程在这些不同的储存库（即碳库）之间移动。对于 IGCSE 爱德思科学课程的学生来说，理解这一循环至关重要，它不仅是一个独立主题，还连接了生物学、化学和环境科学的核心概念。在本复习指南中，我们将分解碳循环中的每一条主要路径，考察人类的影响，并探讨为何维持碳平衡对生命和气候稳定不可或缺。
1. What Is the Carbon Cycle? | 什么是碳循环？
The carbon cycle is a natural system that recycles carbon atoms across the biosphere, geosphere, hydrosphere, and atmosphere. Carbon rarely stays in one place; it is constantly on the move. For example, a carbon atom in a molecule of atmospheric CO₂ can be absorbed by a plant during photosynthesis, become part of a carbohydrate, then be eaten by an animal, released back into the air through respiration, and later dissolve in the ocean to eventually form a carbonate rock. Understanding these transformations helps us appreciate how Earth functions as a closed system for carbon, where the total amount of carbon remains relatively constant over long timescales, although its distribution among reservoirs can shift dramatically.
碳循环是一个自然系统，它在生物圈、岩石圈、水圈和大气圈之间回收碳原子。碳很少停留在一个地方；它总是在不停地移动。例如，大气中 CO₂ 分子里的一个碳原子可以被植物通过光合作用吸收，变成碳水化合物的一部分，然后被动物吃掉，再通过呼吸作用释放回空气中，之后溶解在海洋里，最终形成碳酸盐岩。理解这些转化有助于我们认识到地球对于碳来说是一个封闭系统，碳的总量在长时间尺度上保持相对恒定，但其在不同碳库中的分布可以发生剧烈变化。
2. Major Carbon Reservoirs | 主要碳库
Carbon is stored in several major reservoirs. The atmosphere contains carbon mainly as carbon dioxide (CO₂) and a smaller amount as methane (CH₄). The biosphere stores carbon in living organisms, including forests, grasslands, and soil organic matter. The oceans hold a massive amount of dissolved carbon in the form of hydrogen carbonate ions (HCO₃⁻), carbonate ions (CO₃²⁻), and dissolved CO₂. The lithosphere (Earth’s crust) stores carbon in sedimentary rocks such as limestone (CaCO₃) and in fossil fuels like coal, oil, and natural gas. Each reservoir has a typical residence time — the average length of time a carbon atom spends there. For instance, carbon can remain locked in limestone for millions of years, while its stay in a living leaf may be just a single growing season.
碳储存在几个主要碳库中。大气中的碳主要以二氧化碳 (CO₂) 和少量甲烷 (CH₄) 的形式存在。生物圈在活的有机体（包括森林、草原和土壤有机质）中储存碳。海洋以碳酸氢根离子 (HCO₃⁻)、碳酸根离子 (CO₃²⁻) 和溶解态 CO₂ 的形式容纳了大量溶解碳。岩石圈（地壳）在沉积岩（如石灰岩 CaCO₃）以及煤、石油和天然气等化石燃料中储存碳。每个碳库都有典型的停留时间——碳原子在那里度过的平均时间。例如，碳可以被锁在石灰岩中数百万年，而它在活的叶片中可能只停留一个生长季。
3. Photosynthesis – Carbon Fixation | 光合作用——碳固定
Photosynthesis is the primary process that removes CO₂ from the atmosphere and incorporates carbon into the biosphere. Green plants, algae, and cyanobacteria use light energy to convert carbon dioxide and water into glucose and oxygen. The overall word equation is:
光合作用是将 CO₂ 从大气中移除并将碳纳入生物圈的主要过程。绿色植物、藻类和蓝细菌利用光能将二氧化碳和水转化为葡萄糖和氧气。总的文字方程式为：
carbon dioxide + water → glucose + oxygen
二氧化碳 + 水 → 葡萄糖 + 氧气
In chemical terms, we write:
用化学式表示：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Photosynthesis is described as an endothermic reaction because it absorbs energy from sunlight. The glucose produced can be used in respiration or converted into other organic compounds like starch, cellulose, proteins, and lipids, thus building plant biomass. On a global scale, photosynthesis acts as a giant carbon sink, particularly in forests and ocean phytoplankton.
光合作用被描述为吸热反应，因为它吸收太阳光中的能量。产生的葡萄糖可用于呼吸作用，或转化为其他有机化合物，如淀粉、纤维素、蛋白质和脂质，从而构建植物生物量。在全球范围内，光合作用充当了一个巨大的碳汇，尤其是在森林和海洋浮游植物中。
4. Respiration – Carbon Release | 呼吸作用——碳释放
Respiration is the process by which living cells break down glucose to release energy. All organisms — plants, animals, fungi, and most bacteria — perform respiration continuously. Aerobic respiration uses oxygen and gives off carbon dioxide and water, returning carbon from organic molecules back to the atmosphere. The word equation mirrors photosynthesis in reverse:
呼吸作用是活细胞分解葡萄糖以释放能量的过程。所有生物——植物、动物、真菌和大多数细菌——都持续进行呼吸作用。有氧呼吸利用氧气并释放出二氧化碳和水，将有机分子中的碳送回到大气中。其文字方程式与光合作用正好相反：
glucose + oxygen → carbon dioxide + water (+ energy)
葡萄糖 + 氧气 → 二氧化碳 + 水 (+ 能量)
This release of CO₂ by living organisms is one of the main natural sources of atmospheric carbon. Respiration happens during both day and night, whereas photosynthesis only occurs in the light. That is why CO₂ levels in a forest drop during the day (net uptake) and rise at night (net release).
生物体通过呼吸作用释放 CO₂ 是大气碳的主要自然来源之一。呼吸作用在白天和夜晚都会发生，而光合作用只在光下进行。正因如此，森林中的 CO₂ 水平在白天下降（净吸收），而在夜间上升（净释放）。
5. Decomposition | 分解作用
When plants and animals die, their bodies contain complex carbon compounds. Decomposers — microorganisms such as bacteria and fungi, along with detritus feeders like earthworms — break down this dead organic matter. Through their respiration, decomposers release CO₂ back into the atmosphere. At the same time, some carbon is converted into humus, a stable organic component of soil. Decomposition thus recycles nutrients and closes the loop between dead material and atmospheric CO₂. Factors that speed up decomposition include warmth, moisture, and oxygen availability, which is why tropical forests decompose litter much faster than temperate or arctic soils.
当植物和动物死亡后，它们的尸体中含有复杂的碳化合物。分解者——细菌、真菌等微生物以及蚯蚓等腐食者——将这些死亡的有机物质分解。通过自身的呼吸作用，分解者将 CO₂ 释放回大气。与此同时，一部分碳被转化为腐殖质，即土壤中稳定的有机成分。分解作用因此实现了养分循环，并将死亡物质与大气中的 CO₂ 连接起来。加速分解的因素包括温度、湿度和氧气供应，这就是热带森林中凋落物的分解速度远比温带或极地土壤快的原因。
6. Combustion of Fossil Fuels | 化石燃料的燃烧
Fossil fuels — coal, oil, and natural gas — formed from the remains of ancient organisms buried under high pressure and temperature over millions of years. They store enormous quantities of carbon that had been removed from the atmosphere long ago. When these fuels are burned in the presence of oxygen, combustion occurs, rapidly releasing energy and producing carbon dioxide and water vapour. For example, the combustion of methane (the main component of natural gas) is:
化石燃料——煤、石油和天然气——是由远古生物的遗骸在高压和高温下埋藏数百万年形成的。它们储存了大量很久以前从大气中移除的碳。当这些燃料在氧气中燃烧时，就会发生燃烧反应，迅速释放能量并生成二氧化碳和水蒸气。例如，甲烷（天然气的主要成分）的燃烧反应为：
CH₄ + 2O₂ → CO₂ + 2H₂O
Human use of fossil fuels for transport, electricity generation, and industry has massively accelerated carbon transfer from the lithosphere to the atmosphere. This is the dominant cause of the recent rise in atmospheric CO₂ concentration, which has increased from around 280 parts per million (ppm) in pre-industrial times to over 420 ppm today.
人类将化石燃料用于交通运输、发电和工业，极大地加快了碳从岩石圈向大气圈的转移。这是近期大气 CO₂ 浓度上升的主要原因，工业化前浓度约为 280 ppm（百万分之一），如今已超过 420 ppm。
7. Ocean Uptake and Carbonate Rocks | 海洋吸收与碳酸盐岩
Oceans are a vital carbon sink. Carbon dioxide dissolves directly into the surface water, where it reacts with water molecules to form carbonic acid (H₂CO₃), which then dissociates into hydrogen carbonate ions (HCO₃⁻) and hydrogen ions (H⁺). This sequence makes the ocean slightly alkaline and allows it to hold far more carbon than the atmosphere. Marine organisms such as plankton, corals, and shellfish use dissolved calcium ions and carbonate ions to build their calcium carbonate (CaCO₃) shells and skeletons. When these organisms die, their remains sink to the ocean floor and over geological time are compressed into sedimentary rocks like limestone and chalk. This process locks up carbon for millions of years, forming the largest long-term carbon reservoir on Earth.
海洋是一个重要的碳汇。二氧化碳直接溶解进表层海水，在其中与水分子反应生成碳酸 (H₂CO₃)，碳酸再解离为碳酸氢根离子 (HCO₃⁻) 和氢离子 (H⁺)。这一系列反应使海水呈弱碱性，并使得海洋能够容纳比大气多得多的碳。浮游生物、珊瑚和贝类等海洋生物利用溶解的钙离子和碳酸根离子来构建它们的碳酸钙 (CaCO₃) 外壳和骨骼。当这些生物死亡后，它们的遗骸沉入海底，在漫长的地质年代中被压实成为石灰岩和白垩等沉积岩。这一过程将碳封存数百万年，形成了地球上最大的长期碳库。
8. Feeding and Food Chains | 摄食与食物链
Carbon moves through the living parts of the cycle via feeding. Plants (producers) fix carbon into organic compounds. When herbivores eat plants, the carbon-containing molecules are digested, absorbed, and used to build the consumers’ own tissues or are respired for energy. Carnivores then eat herbivores, passing carbon further up the food chain. At each trophic level, a significant portion of the carbon is released back to the atmosphere through respiration. Only about 10 % of the carbon (and energy) is transferred from one trophic level to the next; the rest is lost as heat and in waste products. This inefficient transfer explains why food chains rarely extend beyond four or five levels and why plant biomass is so important for carbon storage.
碳通过摄食在循环的活体部分中移动。植物（生产者）将碳固定为有机化合物。食草动物吃掉植物后，含碳的分子被消化、吸收，并用于构建消费者自身的组织，或者在呼吸作用中被消耗以提供能量。然后，食肉动物吃掉食草动物，使碳沿着食物链继续传递。在每一个营养级上，很大一部分碳都会通过呼吸作用释放回大气中。只有大约 10% 的碳（及能量）从一个营养级转移到下一个营养级；其余的部分以热量和废物的形式损失掉。这种低效的传递解释了什么食物链很少超过四到五个层级，以及为什么植物生物量对碳储存如此重要。
9. Human Impact: Deforestation and Fossil Fuels | 人类影响：森林砍伐与化石燃料
Human activities have disrupted the natural carbon cycle in two main ways: burning fossil fuels and deforestation. Burning fossil fuels releases ‘locked’ geological carbon into the atmosphere at a rate far exceeding natural volcanic or weathering processes. Deforestation, especially the clearing of tropical rainforests, reduces the planet’s capacity to remove CO₂ through photosynthesis. Moreover, when forests are burned or left to decompose, the carbon stored in their trunks and soil is rapidly released. Land-use changes thus not only reduce a crucial carbon sink but also turn forests into carbon sources. For IGCSE exams, you need to be able to explain why these activities lead to an increase in atmospheric CO₂ and contribute to the enhanced greenhouse effect.
人类活动主要通过两种方式扰乱自然碳循环：燃烧化石燃料和砍伐森林。燃烧化石燃料以远快于天然火山或风化过程的速率将“锁定的”地质碳释放到大气中。砍伐森林，特别是热带雨林的清除，削弱了地球通过光合作用移除 CO₂ 的能力。此外，当森林被焚烧或任由其腐烂时，储存在树干和土壤中的碳会迅速释放出来。因此，土地利用的改变不仅缩减了一个关键的碳汇，还可能将森林变为碳源。对于 IGCSE 考试，你需要能够解释为什么这些活动会导致大气 CO₂ 升高，并加剧温室效应。
10. The Greenhouse Effect and Global Warming | 温室效应与全球变暖
A certain level of greenhouse gases, including CO₂, CH₄, and water vapour, is essential for keeping Earth warm enough to sustain life — this is the natural greenhouse effect. These gases trap infrared radiation emitted from Earth’s surface, preventing it from escaping directly into space. However, human-driven increases in CO₂ and methane amplify this effect, causing more heat to be retained and leading to global warming. Consequences include rising sea levels from melting ice caps and thermal expansion, more frequent extreme weather events, disruption of food supplies, and shifts in ecosystems. In the carbon cycle context, a warmer climate can further accelerate decomposition and permafrost thawing, releasing even more CO₂ and methane — a dangerous positive feedback loop.
一定水平的温室气体，包括 CO₂、CH₄ 和水蒸气，对于保持地球足够温暖以维持生命是必不可少的——这即天然的温室效应。这些气体会捕获地球表面发出的红外辐射，阻止其直接逃逸到太空中。然而，由人类活动引起的二氧化碳和甲烷的增加会放大这一效应，导致更多热量被保留，从而引发全球变暖。其后果包括由冰盖融化和热膨胀引起的海平面上升、更频繁的极端天气事件、粮食供应中断以及生态系统的迁移。在碳循环的背景下，更暖的气候会进一步加速分解作用和永久冻土的融化，释放出更多的 CO₂ 和甲烷——一个危险的正反馈循环。
11. Maintaining the Carbon Balance | 维持碳平衡
In the pre-industrial world, natural carbon fluxes were roughly balanced: the CO₂ released by respiration, decomposition, and volcanic activity was approximately equal to that removed by photosynthesis and ocean uptake. Today, human emissions have added an extra ~10 billion tonnes of carbon per year, overwhelming the natural sinks. Strategies to restore balance include reforestation and afforestation, improvements in agricultural soil management to boost soil carbon storage, switching to renewable energy sources, and developing carbon capture and storage (CCS) technologies. On a personal level, reducing energy consumption, minimizing waste, and supporting sustainable products all help lower one’s carbon footprint.
在工业化前，自然碳通量大致是平衡的：由呼吸作用、分解作用和火山活动释放的 CO₂ 与被光合作用和海洋吸收移除的量大致相等。今天，人类排放每年额外增加了约 100 亿吨碳，超出了自然碳汇的容纳能力。恢复平衡的策略包括重新造林和植树造林、改进农业土壤管理以增加土壤碳储存、转向可再生能源，以及发展碳捕集与封存 (CCS) 技术。在个人层面上，减少能源消耗、减少浪费以及支持可持续产品都有助于降低个人碳足迹。
12. Key Exam Points and Common Mistakes | 关键考点与常见错误
For Edexcel IGCSE Science, you are expected to be able to: identify the main carbon reservoirs; describe processes that transfer carbon between them; interpret diagrams of the carbon cycle; explain how human activities are altering the cycle; and link the carbon cycle to the greenhouse effect. Common mistakes include confusing photosynthesis and respiration equations, forgetting that plants respire as well, and stating that combustion is the only source of CO₂. Remember that before the industrial era, decomposition and respiration were the primary natural inputs. Also, do not write that oceans directly absorb all extra CO₂ — the process is equilibration limited, and acidification from excess CO₂ harms marine life. In calculations or data interpretation, always check units (ppm, gigatonnes, percentage).
对于爱德思 IGCSE 科学，你应能做到：识别主要碳库；描述在它们之间转移碳的过程；解读碳循环图解；解释人类活动如何改变这一循环；并将碳循环与温室效应联系起来。常见的错误包括：混淆光合作用和呼吸作用的方程式，忘记植物也会进行呼吸作用，以及声称燃烧是 CO₂ 的唯一来源。请记住，在工业时代之前，分解作用和呼吸作用是主要的自然碳源。同时，不要写海洋会直接吸收所有额外的 CO₂——这一过程受平衡限制，并且过量 CO₂ 引起的酸化会危害海洋生物。在计算或数据解读中，务必检查单位（ppm，十亿吨，百分比）。
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