📚 The Carbon Cycle | 碳循环
The carbon cycle is a fundamental biogeochemical cycle that describes how carbon atoms move continuously between the atmosphere, living organisms, oceans, and the Earth's crust. For IGCSE Edexcel Science, you need to understand the processes that add carbon to the atmosphere and those that remove it, and how human activities disrupt this natural balance.
碳循环是一个重要的生物地球化学循环，描述了碳原子在大气、生物体、海洋和地壳之间持续移动的路径。在IGCSE Edexcel科学课程中，你需要理解使碳进入大气的过程以及从大气中移除碳的过程，并认识人类活动如何打破这种自然平衡。
1. Overview of the Carbon Cycle | 碳循环概述
The carbon cycle is the movement of carbon, in its various chemical forms, through different parts of the Earth system. It involves both biological and geological processes that operate over timescales ranging from days to millions of years. Carbon is essential for all organic molecules, including carbohydrates, proteins, lipids, and nucleic acids, and its cycling is crucial for maintaining life on Earth.
碳循环是指碳元素以各种化学形态在地球系统不同部分中的移动。它涉及生物和地质过程，时间跨度从几天到数百万年不等。碳是所有有机分子（包括碳水化合物、蛋白质、脂质和核酸）的关键组成成分，碳的循环对于维持地球生命至关重要。
2. Key Carbon Reservoirs | 主要的碳储库
Carbon is stored in several major reservoirs. Each reservoir holds carbon in different chemical forms and for varying lengths of time. The table below summarises the main reservoirs you need to know for your IGCSE exam.
碳储存在几个主要的储库中。每个储库中的碳以不同的化学形态存在，停留的时间也各不相同。下表总结了你IGCSE考试需要掌握的主要储库。
| Reservoir / 储库 | Form of Carbon / 碳的形态 | Examples / 示例 |
|---|---|---|
| Atmosphere / 大气圈 | Carbon dioxide (CO₂), methane (CH₄) | Air, volcanic gases |
| Biosphere / 生物圈 | Organic carbon in living organisms and dead organic matter | Plants, animals, microorganisms, humus |
| Hydrosphere / 水圈 (Oceans) | Dissolved CO₂, bicarbonate ions (HCO₃⁻), carbonate ions (CO₃²⁻) | Seawater, shells of marine organisms (CaCO₃) |
| Lithosphere / 岩石圈 | Fossil fuels (coal, oil, natural gas), limestone (CaCO₃), sedimentary rocks | Coal deposits, crude oil, chalk, marble |
3. Photosynthesis: Carbon Enters Living Organisms | 光合作用：碳进入生物体
Photosynthesis is the process by which green plants, algae, and some bacteria convert inorganic carbon (CO₂) into organic compounds, primarily glucose. This process removes carbon dioxide from the atmosphere and fixes it as chemical energy in carbohydrates. Photosynthesis is the main entry point of carbon into the living component of the biosphere.
光合作用是绿色植物、藻类和某些细菌将无机碳（CO₂）转化为有机化合物（主要是葡萄糖）的过程。这一过程从大气中移除二氧化碳，并将其以化学能形式固定在碳水化合物中。光合作用是碳进入生物圈生命组分的主要入口。
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Photosynthesis equation / 光合作用方程式
4. Feeding and Food Chains | 摄食与食物链
When primary consumers (herbivores) eat plants, the carbon stored in plant tissues is transferred to their own bodies. This carbon is then passed along food chains as carnivores eat herbivores and top predators eat smaller carnivores. The carbon is used to build new organic molecules such as proteins, lipids, and nucleic acids in the consumer's body.
当初级消费者（食草动物）取食植物时，植物组织中的碳便转移到它们自己体内。随后，随着食肉动物捕食食草动物、顶级捕食者捕食小型食肉动物，碳沿食物链传递。碳用来在消费者体内构建新的有机分子，如蛋白质、脂质和核酸。
5. Respiration: Returning Carbon to the Atmosphere | 呼吸作用：碳返回大气
All living organisms carry out respiration, a process that breaks down organic molecules such as glucose to release energy for cellular activities. This releases carbon dioxide back into the atmosphere or into the surrounding water. Aerobic respiration is the most common form, but some microorganisms also perform anaerobic respiration, which can release methane (CH₄) in addition to CO₂.
所有生物都进行呼吸作用，该过程分解葡萄糖等有机分子，为细胞活动释放能量。这样便将二氧化碳释放回大气或周围水体中。有氧呼吸是最常见的形式，但某些微生物也进行厌氧呼吸，可能额外释放甲烷（CH₄）。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)
Aerobic respiration / 有氧呼吸
6. Decomposition and the Role of Microorganisms | 分解作用与微生物的作用
When organisms die, decomposers such as bacteria and fungi break down the dead organic matter. During decomposition, the carbon in complex organic molecules is released as CO₂ through the respiration of these decomposers. This process recycles nutrients and returns carbon to the atmosphere. In some oxygen-poor environments (e.g. waterlogged soils), decomposition may be slow and lead to the accumulation of partially decomposed organic matter, which can eventually form fossil fuels.
生物死亡后，细菌和真菌等分解者分解死亡有机质。在分解过程中，复杂有机分子中的碳通过分解者的呼吸作用以CO₂的形式释放出来。这一过程循环了养分，并将碳返回大气。在一些缺氧环境中（如浸水的土壤），分解可能非常缓慢，导致部分分解的有机质积累，最终可能形成化石燃料。
7. Combustion of Organic Material | 有机物的燃烧
Combustion, or burning, is a chemical process in which organic substances (including fossil fuels and wood) react rapidly with oxygen, releasing carbon dioxide and water vapour. Combustion is a major natural and human-driven source of atmospheric CO₂. Wildfires can release carbon stored in forests, while the burning of fossil fuels for energy is the dominant anthropogenic source.
燃烧是含碳有机物（包括化石燃料和木材）与氧气快速反应、释放二氧化碳和水蒸气的化学过程。燃烧是大气CO₂的主要自然和人为排放源之一。野火释放了森林中储存的碳，而燃烧化石燃料则是人为排放的主要来源。
CH₄ + 2O₂ → CO₂ + 2H₂O
Example: combustion of methane / 甲烷燃烧示例
8. Fossilisation and Formation of Fossil Fuels | 化石化与化石燃料的形成
When dead organisms are buried in sediments without being fully decomposed, their carbon-containing remains can be converted into fossil fuels over millions of years under high pressure and temperature. Plant remains in swamps may form coal, while marine plankton may be transformed into crude oil and natural gas. This process locks carbon away from the atmosphere in the lithosphere for geological timescales until it is released by combustion or weathering.
当死亡生物被埋藏在沉积物中而未完全分解时，其含碳遗骸在高压和高温下经过数百万年可以转化为化石燃料。沼泽中的植物遗骸可能形成煤炭；海洋浮游生物则可能转变为原油和天然气。这一过程将碳锁定在岩石圈中，跨越地质时间尺度，直到通过燃烧或风化释放出来。
9. Marine Carbon Processes | 海洋中的碳过程
Oceans play a critical role in the carbon cycle. Atmospheric CO₂ dissolves in seawater, where it forms carbonic acid and dissociates into bicarbonate (HCO₃⁻) and carbonate ions (CO₃²⁻). Marine organisms such as molluscs and corals use these carbonate ions to build their calcium carbonate (CaCO₃) shells and skeletons. When these organisms die, their shells may accumulate on the ocean floor and eventually form limestone rock. This is another way carbon is stored over long periods.
海洋在碳循环中起关键作用。大气中的CO₂溶解在海水中，形成碳酸并离解为碳酸氢根离子（HCO₃⁻）和碳酸根离子（CO₃²⁻）。软体动物和珊瑚等海洋生物利用这些碳酸根离子构建其碳酸钙（CaCO₃）外壳和骨架。这些生物死亡后，外壳可能堆积在海底，最终形成石灰岩。这是碳长期储存的另一种途径。
10. Human Impact and Climate Change | 人类影响与气候变化
Human activities have significantly altered the carbon cycle. The burning of fossil fuels for transport, industry, and electricity generation releases huge amounts of CO₂ into the atmosphere. Deforestation reduces the number of trees available to absorb CO₂ through photosynthesis. Additionally, agricultural practices and landfills produce methane, a potent greenhouse gas. The enhanced greenhouse effect is driving global warming and climate change, making it vital to rebalance the carbon cycle through renewable energy and reforestation.
人类活动已显著改变了碳循环。交通运输、工业和电力生产大量燃烧化石燃料，向大气释放了巨量CO₂。森林砍伐减少了能够通过光合作用吸收CO₂的树木数量。此外，农业活动和垃圾填埋产生了强效温室气体甲烷。增强的温室效应正推动全球变暖和气候变化，因此通过可再生能源和植树造林来恢复碳循环平衡至关重要。
11. Importance of Maintaining the Carbon Cycle | 维持碳循环的重要性
A balanced carbon cycle ensures that the concentration of carbon dioxide in the atmosphere remains relatively stable, supporting a climate suitable for life. Carbon is also the backbone of biological molecules; without the continuous cycling of carbon, life could not be sustained. Understanding the carbon cycle helps scientists and policymakers design strategies to mitigate climate change and protect ecosystems.
均衡的碳循环确保大气中二氧化碳浓度保持相对稳定，支持适宜生命生存的气候。碳也是生物分子的骨架；没有碳的持续循环，生命便无法维系。理解碳循环有助于科学家和决策者设计减缓气候变化、保护生态系统的策略。
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