📚 The Carbon Cycle | 碳循环
The carbon cycle describes the continuous movement of carbon atoms through the atmosphere, living organisms, oceans, and the Earth's crust. For IGCSE Edexcel Science students, this topic bridges biology, chemistry, and environmental studies, and it is essential for understanding how ecosystems function and how human actions influence the planet's climate.
碳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过程。对于 IGCSE Edexcel 科学课程的学生来说，该主题衔接了生物学、化学与环境研究，是理解生态系统如何运作以及人类行为如何影响地球气候的关键。
1. What is the Carbon Cycle? | 什么是碳循环？
The carbon cycle is a biogeochemical process that exchanges carbon among the biosphere, geosphere, hydrosphere, and atmosphere. Carbon exists in several reservoirs: as carbon dioxide (CO₂) in the air, as organic compounds in living tissues, as fossil fuels buried underground, and as carbonate minerals in rocks. The cycle operates through both rapid biological processes, such as photosynthesis, and slow geological processes, including sedimentation and volcanic activity.
碳循环是碳在生物圈、岩石圈、水圈和大气圈之间交换的生物地球化学过程。碳存在于多个储存库中：空气中的二氧化碳 (CO₂)、生物组织内的有机化合物、埋藏地下的化石燃料以及岩石中的碳酸盐矿物。该循环通过光合作用等快速生物过程和沉积、火山活动等缓慢地质过程共同运行。
A carbon sink absorbs more carbon than it releases, while a carbon source releases more than it absorbs. Forests, oceans, and soil are major carbon sinks, whereas burning fossil fuels and volcanic eruptions are significant carbon sources. The balance between sinks and sources determines the net amount of CO₂ in the atmosphere.
碳汇吸收的碳多于其释放的碳，而碳源释放的碳多于其吸收的碳。森林、海洋和土壤是主要的碳汇，而化石燃料燃烧和火山喷发则是重要的碳源。碳汇与碳源之间的平衡决定了大气中 CO₂ 的净含量。
2. Why Carbon is Important | 碳为何重要
Carbon is the fundamental building block of life. It forms the backbone of all four major classes of biological macromolecules: carbohydrates, proteins, lipids, and nucleic acids. Without carbon's unique ability to form four stable covalent bonds, the complex structures necessary for life could not exist.
碳是生命的基本构成单元。它构成四大类生物大分子的骨架：碳水化合物、蛋白质、脂质和核酸。如果没有碳能够形成四个稳定共价键的独特能力，生命所需的复杂结构便无法存在。
Carbon atoms can bond with each other and with other elements to create long chains, rings, and branched molecules. This versatility allows for the immense diversity of organic compounds, from simple sugars to DNA. The carbon cycle ensures that this essential element is constantly recycled, making it available for new generations of organisms.
碳原子可以彼此结合或与其他元素结合，形成长链、环状和分支分子。这种多功能性使得从单糖到 DNA 的有机化合物具有巨大的多样性。碳循环确保这一关键元素被不断循环，为新一代生物体提供可利用的碳。
3. Photosynthesis – Removing Carbon Dioxide | 光合作用——移除二氧化碳
Photosynthesis is the process by which green plants and algae absorb carbon dioxide from the atmosphere and combine it with water using light energy to produce glucose and oxygen. The overall word equation is:
光合作用是绿色植物和藻类吸收大气中的二氧化碳，利用光能将其与水结合生成葡萄糖和氧气的过程。总文字方程如下：
carbon dioxide + water → glucose + oxygen
This reaction removes CO₂ from the atmosphere and locks carbon into solid biomass. The glucose produced can be used immediately for respiration, stored as starch, or converted into other molecules such as cellulose for cell walls. In this way, photosynthesis acts as the primary pathway for carbon to enter the biotic part of the cycle.
该反应将 CO₂ 从大气中移除，并将碳锁定在固态生物量中。生成的葡萄糖可立即用于呼吸作用，以淀粉形式储存，或转化为其他分子（如构成细胞壁的纤维素）。通过这种方式，光合作用是碳进入碳循环生物部分的主要途径。
During the growing season, photosynthesis rates are high, leading to a net uptake of carbon dioxide. In temperate regions, deciduous trees shed their leaves in autumn, reducing photosynthesis and altering the local carbon balance. Understanding seasonal changes helps explain short-term fluctuations in atmospheric CO₂ levels.
在生长季节，光合作用速率高，导致二氧化碳净吸收。在温带地区，落叶树在秋季落叶，降低光合作用，从而改变局部碳平衡。理解季节性变化有助于解释大气 CO₂ 浓度的短期波动。
4. Respiration – Releasing Carbon Dioxide | 呼吸作用——释放二氧化碳
All living organisms, including plants, carry out aerobic respiration to release energy from glucose. The word equation is the reverse of photosynthesis:
所有生物体，包括植物，都进行有氧呼吸以从葡萄糖中释放能量。文字方程是光合作用的逆过程：
glucose + oxygen → carbon dioxide + water
Respiration returns CO₂ to the atmosphere continuously. While plants are net absorbers of CO₂ during daylight due to higher photosynthesis rates, they become net emitters at night when photosynthesis ceases and only respiration continues.
呼吸作用不断将 CO₂ 归还大气。虽然植物在白天因光合作用速率更高而成为 CO₂ 的净吸收者，但在夜间光合作用停止而呼吸作用持续时，它们则成为净排放者。
Animals rely entirely on respiration and the consumption of organic matter to meet their energy needs. Their exhaled CO₂ adds directly to the atmospheric carbon pool. The combined respiration of all organisms makes this process one of the largest natural sources of CO₂.
动物完全依赖呼吸作用和摄入有机物来满足其能量需求。它们呼出的 CO₂ 直接增加大气碳库。所有生物体的呼吸作用总和使得该过程成为 CO₂ 最大的自然来源之一。
5. Feeding and Assimilation | 取食与同化
Carbon moves through the food chain when organisms feed on one another. Primary consumers eat plants and assimilate the carbon compounds into their own tissues. Secondary and tertiary consumers then obtain carbon by eating other animals. Each feeding level transfers carbon-containing molecules such as proteins and fats up the trophic levels.
当生物彼此取食时，碳通过食物链传递。初级消费者摄食植物，将碳化合物同化为自身组织。次级和三级消费者继而通过捕食其他动物获取碳。每个营养级都传递含碳分子，如蛋白质和脂肪。
Not all carbon is transferred efficiently; a large portion is lost as CO₂ through respiration at each step. This is why food chains rarely exceed four or five trophic levels. The carbon stored in animal biomass eventually re-enters the cycle through death and decomposition, or through predation.
并非所有碳都能有效传递；每一步都有大部分碳通过呼吸作用以 CO₂ 形式流失。这就是食物链很少超过四到五个营养级的原因。储存在动物生物量中的碳最终通过死亡和分解或被捕食重新进入循环。
6. Decomposition – Recyclers at Work | 分解作用——回收者工作
Decomposers, mainly bacteria and fungi, break down dead organisms and waste products. They secrete enzymes that digest complex organic molecules externally, then absorb the simpler compounds. Through their own respiration, they release CO₂, water, and mineral ions back into the soil and atmosphere.
分解者，主要是细菌和真菌，分解死去的生物体和废物。它们分泌酶在体外消化复杂的有机分子，然后吸收较简单的化合物。通过自身的呼吸作用，它们将 CO₂、水和矿物质离子释放回土壤和大气。
Decomposition is most rapid in warm, moist, and well-oxygenated conditions. In waterlogged or acidic environments, such as peat bogs, decay slows down, leading to the accumulation of partially decomposed organic matter. This can store carbon for thousands of years, forming fossil fuels under the right geological conditions.
在温暖、潮湿和氧气充足的条件下，分解最为迅速。在渍水或酸性环境（如泥炭沼泽）中，腐烂减缓，导致部分分解的有机物堆积。这可以将碳储存数千年，并在适当的地质条件下形成化石燃料。
Human practices such as composting intentionally speed up decomposition to recycle nutrients and return carbon to the soil. This shows how understanding the carbon cycle can be applied in sustainable agriculture.
堆肥等人类活动有意加速分解，以回收养分并将碳归还土壤。这表明对碳循环的理解可以应用于可持续农业。
7. Combustion – Carbon from Fossil Fuels | 燃烧——来自化石燃料的碳
Fossil fuels contain carbon that was captured by photosynthesis millions of years ago and stored underground. When coal, oil, or natural gas are burned, the carbon rapidly combines with oxygen to form CO₂. The combustion of a simple hydrocarbon can be summarised as:
化石燃料中所含的碳是数百万年前通过光合作用捕获并储存在地下的。当煤、石油或天然气燃烧时，碳迅速与氧结合生成 CO₂。简单碳氢化合物的燃烧可概括为：
hydrocarbon + oxygen → carbon dioxide + water
Since the Industrial Revolution, large-scale combustion of fossil fuels has dramatically increased the concentration of atmospheric CO₂. This release happens much faster than natural carbon sinks can remove it, disrupting the equilibrium of the global carbon cycle.
自工业革命以来，化石燃料的大规模燃烧极大地增加了大气 CO₂ 浓度。这种释放速度远快于自然碳汇的移除速度，破坏了全球碳循环的平衡。
Burning biomass, such as wood or bioethanol, also releases CO₂, but it is often described as carbon-neutral because the plants absorbed an equivalent amount of CO₂ during their growth. However, if forests are not replanted, the net increase in atmospheric carbon remains a concern.
燃烧生物质（如木材或生物乙醇）也会释放 CO₂，但通常被描述为碳中性，因为这些植物在生长期间吸收了等量的 CO₂。然而，如果森林未被重新种植，大气碳的净增加依然值得关注。
8. The Role of Oceans and Rocks | 海洋和岩石的作用
Oceans absorb about a quarter of human-produced CO₂ emissions, acting as a vast carbon sink. Dissolved CO₂ reacts with seawater to form carbonic acid, which can then combine with calcium ions to produce calcium carbonate (CaCO₃). Marine organisms use this to build shells and skeletons.
海洋吸收了约四分之一的人类活动产生的 CO₂ 排放，充当巨大的碳汇。溶解的 CO₂ 与海水反应生成碳酸，进而与钙离子结合生成碳酸钙 (CaCO₃)。海洋生物利用它来构建外壳和骨骼。
The table below shows the approximate sizes of major carbon reservoirs on Earth:
下表显示了地球上主要碳库的大致规模：
| Carbon Reservoir |
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