The Carbon Cycle for A-Level CIE Biology | A-Level CIE 生物：碳循环考点精讲

📚 The Carbon Cycle for A-Level CIE Biology | A-Level CIE 生物：碳循环考点精讲

Carbon is the backbone of all organic molecules, and its movement through the living and non-living world is fundamental to life on Earth. In the A-Level CIE Biology syllabus, the carbon cycle describes how carbon atoms are recycled between the atmosphere, organisms, oceans, and rocks, maintaining a balance that supports ecosystems. Understanding the key processes – photosynthesis, respiration, decomposition, combustion, and sedimentation – is essential for explaining how carbon flows globally and how human activities are disrupting this delicate cycle.

碳是所有有机分子的骨架，它在生物与非生物世界中的流动是地球生命的基础。在 A-Level CIE 生物大纲中，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，维持着支撑生态系统的平衡。理解光合作用、呼吸作用、分解作用、燃烧和沉降等关键过程，对于解释碳如何在全球流动以及人类活动如何扰乱这一精妙的循环至关重要。

1. Overview of the Carbon Cycle | 碳循环概述

The carbon cycle is a biogeochemical cycle that shows how carbon atoms are exchanged among the biosphere, geosphere, hydrosphere, and atmosphere. Most carbon is stored in rocks and sediments, but the relatively small, actively cycling pool in the atmosphere, oceans, and living organisms is what drives the climate system and supports life. In your exam, you need to be able to draw and label a simple diagram of the carbon cycle, showing the major stores (reservoirs) and the processes that transfer carbon between them.

碳循环是一个生物地球化学循环，展示了碳原子如何在生物圈、岩石圈、水圈和大气圈之间交换。大部分碳储存在岩石和沉积物中，但大气、海洋和生物体中相对较小但活跃循环的碳库，驱动着气候系统并支撑生命。在考试中，你需要能够绘制并标注碳循环的简图，展示主要的储存库（碳库）以及碳在它们之间转移的过程。

2. Major Reservoirs of Carbon | 碳的主要储存库

The four main carbon reservoirs you must know are: the atmosphere (as CO₂ and methane), the oceans (dissolved CO₂, bicarbonate ions, and carbonate shells), terrestrial biomass (living plants and animals, plus soil organic matter), and the lithosphere (fossil fuels and carbonate rocks like limestone). The atmosphere is a relatively small sink but is the most dynamic, with rapid exchange. The deep ocean and sedimentary rocks hold carbon for millions of years.

你必须掌握的四个主要碳库是：大气圈（以 CO₂ 和甲烷形式存在）、海洋（溶解的 CO₂、碳酸氢根离子以及碳酸盐贝壳）、陆地生物量（活的动植物和土壤有机质）以及岩石圈（化石燃料和如石灰岩等碳酸盐岩）。大气圈是一个相对较小的碳库，但最为动态，交换迅速。深海和沉积岩中的碳可储存数百万年。

3. Photosynthesis – Fixing Carbon into Organic Matter | 光合作用——将碳固定为有机物

Photosynthesis is the process by which photoautotrophs (mainly plants, algae, and cyanobacteria) use light energy to convert inorganic CO₂ from the atmosphere or water into organic carbon in the form of glucose. The balanced equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This process removes carbon dioxide from the atmosphere and incorporates it into biomass, making it the primary entry point of carbon into the food chain.

光合作用是光能自养生物（主要是植物、藻类和蓝细菌）利用光能将大气或水中的无机 CO₂ 转化为葡萄糖形式的有机碳的过程。其配平方程为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。这一过程从大气中移走二氧化碳并将其嵌入生物量中，是碳进入食物链的主要入口。

4. Feeding and Assimilation – Passing Carbon Through Food Chains | 捕食与同化——碳在食物链中的传递

When primary consumers eat producers, the organic carbon compounds are digested, absorbed, and assimilated into their own tissues. At each trophic level, carbon is transferred. However, not all carbon ingested is assimilated; some is lost in faeces, and much of the assimilated carbon is used in respiration and released as CO₂. Only a small fraction (about 10% on average) is passed on to the next trophic level as new biomass.

当初级消费者取食生产者时，有机碳化合物被消化、吸收并同化到自身组织中。在每个营养级，碳被传递。然而，并非所有摄取的碳都被同化；一部分随粪便流失，而同化的大部分碳用于呼吸作用并以 CO₂ 形式释放。只有一小部分（平均约 10%）作为新的生物量传递到下一个营养级。

5. Respiration – Returning Carbon to the Atmosphere | 呼吸作用——将碳归还大气

All living organisms carry out respiration to release energy from organic molecules. Aerobic respiration completely oxidises glucose to CO₂ and water: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP). This process occurs in all cells, including plants during the night and day, and in decomposers breaking down dead matter. Respiration is the primary route by which organic carbon is returned to the atmosphere as CO₂.

所有生物体都进行呼吸作用，从有机分子中释放能量。有氧呼吸将葡萄糖彻底氧化为 CO₂ 和水：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量（ATP）。这一过程发生在所有细胞中，包括植物在黑夜和白天，以及分解者分解死物时。呼吸作用是生物碳以 CO₂ 形式归还大气的主要途径。

6. Decomposition – The Role of Microorganisms | 分解作用——微生物的角色

Decomposition is the breakdown of dead organic matter by saprobiontic bacteria and fungi. These organisms secrete enzymes onto the dead material (extracellular digestion), absorbing the soluble products. During this process, they respire aerobically if oxygen is present, releasing CO₂, or anaerobically in waterlogged soils, producing methane (CH₄). Decomposition recycles nutrients and returns vast amounts of carbon to the atmosphere and soil. Factors affecting decomposition rate – temperature, oxygen availability, and water content – are common exam questions.

分解是腐生细菌和真菌对死亡有机物质的分解。这些生物向死物分泌酶（胞外消化），吸收可溶性产物。在此过程中，如果有氧，它们进行有氧呼吸释放 CO₂；在积水土壤中则进行无氧呼吸，产生甲烷（CH₄）。分解作用使营养物质再循环，并将大量碳归还到大气和土壤中。影响分解速率的因素——温度、氧气供应和含水量——是常见的考题。

7. Combustion – Fossil Fuels and Wildfires | 燃烧——化石燃料与野火

Combustion occurs when organic materials, particularly fossil fuels (coal, oil, natural gas) and biomass (wood, peat), are burned in the presence of oxygen. The reaction is essentially the same as respiration: organic carbon + O₂ → CO₂ + water. Combustion releases carbon that has been locked away in the lithosphere for millions of years back into the atmosphere over a very short period, dramatically increasing atmospheric CO₂ concentration. Natural wildfires also contribute but human activities have massively accelerated this process since the Industrial Revolution.

当有机物，特别是化石燃料（煤、石油、天然气）和生物质（木材、泥炭）在氧气存在下燃烧时，即发生燃烧作用。反应本质与呼吸作用相同：有机碳 + O₂ → CO₂ + 水。燃烧将储存在岩石圈中数百万年的碳在极短时间内释放回大气，大幅增加大气 CO₂ 浓度。自然野火也有贡献，但自工业革命以来人类活动极大加速了这一过程。

8. Oceanic Carbon Sequestration and the Carbonate Pump | 海洋碳汇与碳酸盐泵

Oceans absorb CO₂ from the atmosphere at the surface. Dissolved CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates into H⁺ and HCO₃⁻ ions. Marine organisms such as coccolithophores and foraminifera use bicarbonate to produce calcium carbonate (CaCO₃) shells and skeletons. When these organisms die, their remains sink to the ocean floor, forming sediments that eventually turn into limestone rock over geological time. This ‘biological pump’ transfers carbon from the surface ocean to the deep ocean and lithosphere, a key long-term sink. CIE often links this to the formation of chalk deposits.

海洋在表层从大气中吸收 CO₂。溶解的 CO₂ 与水反应生成碳酸（H₂CO₃），并解离为 H⁺ 和 HCO₃⁻ 离子。诸如颗石藻和有孔虫等海洋生物利用碳酸氢盐生产碳酸钙（CaCO₃）壳体和骨骼。当这些生物死后，其残骸沉入海底，形成沉积物，最终在地质时间尺度上变成石灰岩。这一“生物泵”将碳从表层海洋转移到深海和岩石圈，是一个关键的长期碳汇。CIE 常将其与白垩沉积的形成联系起来。

9. Formation of Fossil Fuels and Limestone | 化石燃料与石灰岩的形成

Over millions of years, some organic matter that settles in anaerobic conditions (such as ancient swamps and seas) is not fully decomposed. Compression and heat convert it into fossil fuels: peat → coal from terrestrial plants, and plankton → crude oil and natural gas from marine microorganisms. In parallel, calcium carbonate from shells accumulates as sediments that compact and cement into limestone. These processes lock carbon away from the atmosphere for geological timescales. Be prepared to describe the conditions needed – anoxia, pressure, and time – in the CIE exam.

在数百万年间，沉降在厌氧条件下（如远古沼泽和海洋）的部分有机物质未被完全分解。压缩和加热将其转化为化石燃料：泥炭 → 煤（来自陆地植物），浮游生物 → 石油和天然气（来自海洋微生物）。同时，贝壳中的碳酸钙以沉积物形式积累，经压实和胶结作用形成石灰岩。这些过程将碳从大气中封存起来，时间尺度达地质纪年。在 CIE 考试中，要准备好描述所需条件——缺氧、压力和时间。

10. Human Impact – Greenhouse Effect and Climate Change | 人类影响——温室效应与气候变化

Human activities, mainly fossil fuel burning, deforestation, and intensive livestock farming, have significantly increased the concentration of greenhouse gases – CO₂ and methane – in the atmosphere. CO₂ absorbs infrared radiation reflected from the Earth’s surface, trapping heat and raising global temperatures (enhanced greenhouse effect). Deforestation reduces the biosphere’s capacity to absorb CO₂ through photosynthesis. In CIE essays, you might be asked to explain how carbon cycle disruption contributes to global warming and to suggest ways to reduce emissions, linking back to the processes you have learned.

人类活动，主要是化石燃料燃烧、森林砍伐和集约化畜牧业，显著增加了大气中温室气体——CO₂ 和甲烷的浓度。CO₂ 吸收从地表反射的红外辐射，截留热量，升高全球温度（增强的温室效应）。森林砍伐降低了生物圈通过光合作用吸收 CO₂ 的能力。在 CIE 的论述题中，你可能会被要求解释碳循环的扰乱如何导致全球变暖，并提出减少排放的方法，这需要联系到你所学的过程。

11. Key Exam Tips for CIE Carbon Cycle Questions | CIE 碳循环考题关键技巧

Ensure you can draw a labelled diagram from memory, using arrows to show the direction of carbon flow. Use precise terminology: ‘photosynthesis’ not ‘absorbing CO₂’ alone, ‘respiration’ not ‘breathing’. Distinguish between processes that fix carbon (photosynthesis, carbonate deposition) and those that release it (respiration, combustion, volcanic activity). Be ready to compare the rate of carbon turnover in different reservoirs – atmospheric carbon cycles rapidly (years to decades), while lithospheric carbon cycles on a scale of millions of years. Practice explaining how waterlogged conditions slow decomposition and lead to peat formation, a classic CIE question.

确保你能凭记忆绘制标注清晰的示意图，用箭头标明碳流动方向。使用精确术语：“光合作用”而非仅仅“吸收 CO₂”，“呼吸作用”而非“呼吸”。区分固定碳的过程（光合作用、碳酸盐沉积）和释放碳的过程（呼吸作用、燃烧、火山活动）。准备好比较不同碳库中碳周转的速率——大气碳循环快速（数年到数十年），而岩石圈碳循环需要数百万年。练习解释积水条件如何减缓分解并导致泥炭形成，这是 CIE 的经典题目。

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The Carbon Cycle for A-Level CIE Biology | A-Level CIE 生物：碳循环 考点精讲