📚 The Carbon Cycle: GCSE OCR Biology Revision | GCSE OCR 生物：碳循环考点精讲
The carbon cycle describes the continuous movement of carbon atoms through Earth's atmosphere, biosphere, hydrosphere and geosphere. In the GCSE OCR Biology specification, you need to understand the key processes – photosynthesis, respiration, feeding, decomposition and combustion – as well as the main carbon stores and how human activities are disturbing the natural balance. This topic is a favourite for exam questions because it links so many core biological and environmental ideas.
碳循环描述了碳原子在地球大气、生物圈、水圈和岩石圈之间持续不断的运动。在 GCSE OCR 生物学考纲中，你需要掌握光合作用、呼吸作用、摄食、分解和燃烧等关键过程，熟悉主要的碳库，并解释人类活动如何扰乱自然的碳平衡。该主题几乎每年必考，因为它将许多核心的生物学和环境概念联系在了一起。
1. Overview of the Carbon Cycle | 碳循环概述
The carbon cycle is a closed biogeochemical system in the sense that the total amount of carbon on Earth remains roughly constant. Carbon simply moves between different "reservoirs" or stores, changing its chemical form along the way. In the atmosphere it exists mainly as carbon dioxide (CO₂); in organisms it becomes carbohydrates, fats and proteins; in the ocean it is found as dissolved CO₂, bicarbonate and carbonate ions; and in rocks it is locked up as calcium carbonate (CaCO₃) and fossil fuels. The cycle is driven by biological, geological and chemical processes, and any disruption can have far-reaching effects on climate and ecosystems.
碳循环是一个封闭的生物地球化学系统，地球上碳的总量基本不变，碳只是在不同“库”之间移动并改变化学形态。在大气中，碳主要以二氧化碳（CO₂）形式存在；在生物体中转化为糖类、脂肪和蛋白质；在海洋里以溶解 CO₂、碳酸氢根和碳酸根离子存在；在岩石中则被锁定为碳酸钙（CaCO₃）以及煤、石油和天然气等化石燃料。循环由生物、地质和化学过程共同驱动，一旦受到干扰，便会对气候和生态系统产生深远影响。
For your OCR exam, it is essential to be able to name all the major stores and processes, and to explain how an increase in atmospheric CO₂ is linked to human actions such as burning fossil fuels and deforestation. You should also be able to interpret carbon cycle diagrams, applying your knowledge to unfamiliar contexts.
在 OCR 考试中，你必须能够说出所有主要碳库和过程的名称，并能解释大气中 CO₂ 含量的升高与燃烧化石燃料、毁林等人类行为的关联。你还需要会解读碳循环示意图，并将所学知识应用到陌生情境中。
2. Carbon Stores (Reservoirs) | 碳库（储存库）
The major reservoirs of carbon vary enormously in size and turnover time. The atmosphere holds carbon as CO₂ and is the most rapidly exchanged pool. Living organisms (the biosphere) contain carbon mainly as organic compounds; this store is relatively small but turns over quickly through growth and death. The oceans represent the largest active carbon pool, containing dissolved CO₂ and vast amounts of carbon in the form of bicarbonate and carbonate ions. Sedimentary rocks, including limestone (CaCO₃) and chalk, hold the largest amount of carbon, but it cycles extremely slowly over geological timescales. Fossil fuels – coal, oil and natural gas – are essentially ancient, compressed organic matter and form a long-term store that humans are now rapidly releasing.
碳的主要储存库在规模和周转时间上差异巨大。大气以 CO₂ 形式储碳，是交换最快的库。生物圈中的碳主要存在于有机化合物中，总量较小但通过生长和死亡迅速周转。海洋是最大的活跃碳库，含有溶解 CO₂ 以及大量碳酸氢根和碳酸根离子。沉积岩（包括石灰石和白垩，主要成分 CaCO₃）储存的碳最多，但其循环极其缓慢，以地质时间尺度进行。化石燃料——煤、石油和天然气——本质上是远古压缩的有机物，构成了一个长期储存库，而人类现在正在迅速将其释放出来。
3. Photosynthesis: Fixing Carbon | 光合作用：固定碳
Photosynthesis is the process by which green plants, algae and some bacteria convert inorganic carbon (CO₂) into organic compounds, mainly glucose. The overall word equation and balanced chemical equation are central to GCSE Biology:
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。总文字方程式和配平的化学方程式是 GCSE 生物学的核心：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This reaction takes place in chloroplasts, using light energy absorbed by chlorophyll. In the carbon cycle, photosynthesis is the main pathway that removes CO₂ from the atmosphere and locks it into biomass. Without it, the concentration of CO₂ would rise dramatically and food chains would collapse.
该反应在叶绿体中进行，利用叶绿素吸收的光能。在碳循环中，光合作用是从大气中移除 CO₂ 并将其锁定在生物量中的主要途径。没有光合作用，大气 CO₂ 浓度会急剧上升，食物链也将崩溃。
4. Respiration: Returning CO₂ | 呼吸作用：释放 CO₂
All living organisms – plants, animals, fungi and most bacteria – carry out respiration to release energy from glucose. Aerobic respiration uses oxygen and produces carbon dioxide and water as waste products:
所有生物——植物、动物、真菌和大多数细菌——都进行呼吸作用以释放葡萄糖中的能量。有氧呼吸利用氧气，产生二氧化碳和水作为废物：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Respiration is essentially the reverse of photosynthesis. It returns carbon to the atmosphere continuously, day and night. In the carbon cycle, respiration by decomposers is particularly important because it breaks down dead organic matter, ensuring that carbon is recycled rather than permanently locked away.
呼吸作用本质上与光合作用相反。它日夜不断地将碳送回大气。在碳循环中，分解者的呼吸尤为重要，因为它们分解死亡的有机物，确保碳被循环利用而非永久封存。
5. Feeding and Food Chains | 摄食与食物链
Carbon moves through ecosystems when organisms feed. Producers (plants) are eaten by primary consumers, which are in turn eaten by secondary and tertiary consumers. At each trophic level, the carbon in the form of carbohydrates, proteins and fats is transferred from one organism to another. Some of this carbon is used for growth and repair, while the rest is lost as CO₂ through respiration. The feeding relationships mean that carbon fixed by photosynthesis passes up the food chain, but the amount of stored carbon decreases at higher trophic levels due to energy losses.
碳通过摄食在生态系统中移动。生产者（植物）被初级消费者吃掉，后者又被次级和三级消费者捕食。在每一个营养级上，以糖类、蛋白质和脂肪形式存在的碳从一个生物体传递到另一个生物体。其中一部分用于生长和修复，其余则通过呼吸作用以 CO₂ 的形式散失。这种摄食关系意味着光合作用固定的碳沿着食物链向上传递，但由于能量损耗，较高营养级的碳储存量逐渐减少。
If a plant or animal is not consumed, its carbon may enter the soil through death and shedding of waste. This detritus pathway is a major route for carbon to reach decomposers, linking back to respiration and soil carbon stores.
若动植物未被取食，其碳便会通过死亡和排泄进入土壤。这条碎屑路径是碳抵达分解者的一条主要途径，将碳重新连接到呼吸作用和土壤碳库。
6. Decomposition and Detritus | 分解与碎屑
Decomposition is the breakdown of dead organic material and waste products by decomposers such as bacteria and fungi. These microorganisms secrete enzymes that digest complex organic molecules, and they then absorb the simpler products. During this process they respire, releasing CO₂ back into the atmosphere. Decomposition is accelerated in warm, moist conditions and in the presence of oxygen, which is why the carbon cycle operates fastest in tropical ecosystems.
分解是腐生细菌和真菌等分解者对死亡有机物和废物的分解过程。这些微生物分泌酶来消化复杂有机分子，然后吸收较简单的产物。在此过程中它们进行呼吸，将 CO₂ 释放回大气。温暖、潮湿且有氧的环境会加速分解，这就是热带生态系统中碳循环运转最快的原因。
Not all carbon is immediately released. Some of the partially decomposed organic matter is converted into humus, a stable organic component of soil. Over millions of years, if buried under sediments without oxygen, this material can be transformed into fossil fuels – coal, oil and natural gas – locking carbon away in the geosphere.
并非所有碳都会立即被释放。部分未完全分解的有机物质会转化为腐殖质，即土壤中稳定的有机组分。如果这些物质被埋藏在无氧的沉积物下，经过数百万年可转变成煤、石油和天然气等化石燃料，从而将碳封存于岩石圈中。
7. Combustion: Burning Fossil Fuels | 燃烧：化石燃料的燃烧
Combustion (burning) is the rapid oxidation of carbon-containing materials. When wood or fossil fuels such as coal, oil and natural gas are burned, the carbon they contain combines with oxygen to form CO₂, which is released into the atmosphere. The simplified equation for the combustion of a hydrocarbon fuel like methane can be written as:
燃烧是含碳物质被快速氧化的过程。当木柴或煤炭、石油、天然气等化石燃料被点燃时，其中的碳与氧气结合生成 CO₂，并释放到大气中。以甲烷为例的碳氢化合物燃烧可简写为：
CH₄ + 2O₂ → CO₂ + 2H₂O
Combustion has become a dominant carbon cycle process because of industrialisation. The vast quantities of CO₂ released in a short time far exceed the rate at which photosynthesis and ocean absorption can remove it. This imbalance is the primary cause of the enhanced greenhouse effect and global warming.
由于工业化，燃烧已成为碳循环中占主导地位的过程。短时间内释放的大量 CO₂ 远超光合作用和海洋吸收所能移除的速率，这种失衡是温室效应加剧和全球变暖的主因。
8. Carbon in Oceans and Sediments | 海洋与沉积物中的碳
The oceans act as a massive carbon sink. At the sea surface, CO₂ dissolves directly and reacts with water to form carbonic acid (H₂CO₃), which dissociates into hydrogen ions and hydrogen carbonate ions (HCO₃⁻). This chemical equilibrium allows the ocean to absorb excess atmospheric CO₂, but it also leads to ocean acidification, threatening marine life with calcium carbonate shells.
海洋是一个巨大的碳汇。在海面，CO₂ 直接溶解并与水反应生成碳酸（H₂CO₃），碳酸解离成氢离子和碳酸氢根离子（HCO₃⁻）。这种化学平衡使海洋能够吸收过量的大气 CO₂，但同时也导致海洋酸化，威胁具有碳酸钙外壳的海洋生物。
Over long geological periods, the carbonate ions combine with calcium ions to form calcium carbonate (CaCO₃), which is used by organisms like corals, mollusks and plankton to build shells and skeletons. When these organisms die, their remains sink and accumulate on the ocean floor, compacting into sedimentary rocks such as limestone and chalk. This process removes carbon from the short-term cycle and locks it into the lithosphere for millions of years.
在漫长的地质时期里，碳酸根离子与钙离子结合形成碳酸钙（CaCO₃），被珊瑚、软体动物和浮游生物用来建造外壳和骨架。这些生物死亡后，遗骸沉入海底并压实，形成石灰石和白垩等沉积岩。这一过程将碳从短期循环中移出，在岩石圈中封存数百万年。
9. Human Impact: Deforestation and More | 人类影响：砍伐森林与更多
Human activities have significantly altered the carbon cycle on a global scale. The two most important disruptions are the burning of fossil fuels and deforestation. When forests are cleared, often by slash and burn, the stored carbon in trees is released as CO₂ immediately through combustion or gradually through decomposition. Moreover, the removal of trees reduces the rate of photosynthesis, so less CO₂ is absorbed from the atmosphere. The combined effect is more CO₂ entering the atmosphere and less being removed, intensifying the greenhouse effect.
人类活动已在全球范围显著改变了碳循环。两种最重要的干扰是化石燃料的燃烧和森林砍伐。当森林被砍伐，常采用刀耕火种的方式，树木中储存的碳会通过燃烧立即释放 CO₂，或通过分解逐渐释放。此外，树木的减少降低了光合速率，因此从大气中吸收的 CO₂ 变少。两者叠加使得更多 CO₂ 排入大气而更少被移除，加剧了温室效应。
Other human actions, such as cement production (which releases CO₂ from limestone) and intensive agriculture (which exposes soil organic carbon to oxidation), also contribute. For GCSE OCR Biology, the key is to recognise that these anthropogenic inputs are pushing the carbon cycle out of its natural equilibrium.
其他人类活动，如水泥生产（从石灰石中释放 CO₂）和集约化农业（使土壤有机碳暴露而氧化），同样有所贡献。对于 GCSE OCR 生物学而言，关键是要认识到这些人为输入正将碳循环推离其自然平衡。
10. Maintaining the Carbon Balance | 维持碳平衡
Nature has mechanisms that regulate the carbon cycle. Forests and oceans absorb CO₂, balancing the emissions from respiration and volcanic activity. This dynamic equilibrium resulted in a relatively stable atmospheric CO₂ concentration for thousands of years. However, the scale of modern fossil fuel use and land-use change has overwhelmed the natural sinks. Maintaining or restoring the carbon balance requires reducing emissions and protecting carbon-absorbing ecosystems such as tropical rainforests and peat bogs.
自然界拥有调节碳循环的机制。森林和海洋吸收 CO₂，与呼吸作用和火山活动排放的 CO₂ 相平衡。这种动态平衡曾使大气 CO₂ 浓度在数千年里保持相对稳定。然而，现代化石燃料使用和土地利用变化的规模已超出天然碳汇的吸纳能力。维持或恢复碳平衡需要减少排放并保护热带雨林和泥炭沼泽等吸收碳的生态系统。
From a biological perspective, encouraging reforestation and sustainable farming practices increases the amount of carbon fixed through photosynthesis and stored in biomass and soil. Every tree planted contributes to drawing CO₂ out of the atmosphere and can help slow the rate of climate change.
从生物学角度来看，鼓励植树造林和可持续农业能增加通过光合作用固定并储存于生物量与土壤中的碳量。每多种一棵树都有助于从大气中吸收 CO₂，使气候变化的速度有所放缓。
11. Importance of the Carbon Cycle | 碳循环的重要性
The carbon cycle is essential for life. Carbon is the backbone of all organic molecules – carbohydrates, lipids, proteins and nucleic acids. By circulating carbon, the cycle ensures a continuous supply of the raw materials needed for building cells. It also plays a crucial role in regulating Earth's temperature; CO₂ in the atmosphere is a greenhouse gas that traps heat, keeping the planet warm enough to sustain life. Too little CO₂ would make the Earth too cold; too much raises global temperatures with severe consequences for biodiversity, food security and sea levels.
碳循环对生命至关重要。碳是所有有机分子——糖类、脂质、蛋白质和
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