The Carbon Cycle | 碳循环

📚 The Carbon Cycle | 碳循环

The carbon cycle describes the continuous movement of carbon atoms through the Earth’s atmosphere, hydrosphere, lithosphere and biosphere. Understanding this cycle is fundamental to topics in both IGCSE Biology and Chemistry, as it links photosynthesis, respiration, combustion and the formation of fossil fuels into a single global system. This article covers the key processes, reservoirs and the impact of human activities on the carbon cycle, following the Edexcel IGCSE Science specifications.

碳循环描述碳原子在地球大气圈、水圈、岩石圈和生物圈之间持续不断的运动过程。理解这一循环对于 IGCSE 生物和化学课程至关重要，因为它将光合作用、呼吸作用、燃烧以及化石燃料的形成连接成一个完整的全球系统。本文根据爱德思 IGCSE 科学纲的要求，梳理关键过程、碳库以及人类活动对碳循环的影响。

1. The Carbon Atom and Its Reservoirs | 碳原子及其储库

Carbon is the fourth most abundant element in the universe and forms the chemical backbone of all known life. In the Earth system, carbon exists in several major reservoirs: the atmosphere (as CO₂ and CH₄), the oceans (dissolved CO₂, bicarbonate ions and living marine organisms), the terrestrial biosphere (plants, animals and soil organic matter), and the lithosphere (sedimentary rocks, fossil fuels and limestone).

碳是宇宙中第四丰富的元素，也是所有已知生命的化学骨架。在地球系统中，碳主要储存在以下几大储库中：大气（以 CO₂ 和 CH₄ 形式存在）、海洋（溶解的 CO₂、碳酸氢根离子以及海洋生物）、陆地生物圈（植物、动物和土壤有机质）以及岩石圈（沉积岩、化石燃料和石灰岩）。

2. Photosynthesis: Fixing Atmospheric Carbon | 光合作用：固定大气中的碳

Photosynthesis is the process by which green plants, algae and some bacteria convert light energy into chemical energy, using carbon dioxide and water to produce glucose and oxygen. The overall symbol equation is:

光合作用是绿色植物、藻类和某些细菌将光能转化为化学能的过程，利用二氧化碳和水生成葡萄糖和氧气。总符号方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

During photosynthesis, carbon is removed from the atmosphere and incorporated into carbohydrate molecules. This makes photosynthesis the primary pathway through which inorganic carbon enters the biotic component of ecosystems. On a global scale, terrestrial and marine photosynthesis annually fix approximately 120 gigatonnes of carbon.

在光合作用中，碳从大气中被移除并纳入碳水化合物分子中。这使得光合作用成为无机碳进入生态系统中生物组分的主要途径。在全球范围内，陆地和海洋光合作用每年大约固定 1200 亿吨碳。

3. Respiration: Returning Carbon to the Atmosphere | 呼吸作用：将碳归还大气

Respiration is the biochemical process in which living cells break down glucose to release energy for metabolic activities. Aerobic respiration can be summarised as:

呼吸作用是活细胞分解葡萄糖以释放代谢活动所需能量的生化过程。有氧呼吸可概括为：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

All organisms – plants, animals, fungi and many microorganisms – carry out respiration day and night. The carbon dioxide produced is released back into the atmosphere, balancing the drawdown that occurs during photosynthesis. Respiration is therefore a critical link that returns organic carbon to the atmospheric CO₂ pool.

所有生物——植物、动物、真菌以及许多微生物——白天黑夜都在进行呼吸作用。产生的二氧化碳被释放回大气，平衡了光合作用期间发生的碳吸收。因此，呼吸作用是将有机碳返回大气 CO₂ 库的关键环节。

4. Decomposition and the Role of Microorganisms | 分解作用与微生物的角色

When organisms die, their remains contain complex organic carbon compounds. Saprotrophic bacteria and fungi secrete extracellular enzymes that digest these materials externally, converting organic carbon back into CO₂, water and mineral salts. This decomposer activity recycles nutrients and releases carbon dioxide through microbial respiration.

当生物死亡时，其残骸中含有复杂的有机碳化合物。腐生细菌和真菌分泌胞外酶，在外部消化这些物质，将有机碳重新转化为 CO₂、水和无机盐。这种分解者活动通过微生物的呼吸作用释放二氧化碳，同时循环养分。

In waterlogged and oxygen-depleted environments such as peat bogs, decomposition is severely slowed. This can lead to the accumulation of partially decayed organic matter, which, over geological time, may be compressed to form fossil fuels.

在淹水和缺氧环境（如泥炭沼泽）中，分解作用严重减缓。这会导致部分腐烂的有机物堆积，经过漫长的地质年代，这些有机物可能被压缩形成化石燃料。

5. Combustion and the Rapid Release of Carbon | 燃烧与碳的快速释放

Combustion is an exothermic chemical reaction between a fuel and an oxidant, usually oxygen, producing CO₂ and water. When biomass or fossil fuels burn, the carbon that was stored in organic molecules is oxidised rapidly, returning to the atmosphere as carbon dioxide. Incomplete combustion may also produce carbon monoxide (CO) and particulate carbon (soot).

燃烧是燃料与氧化剂（通常是氧气）之间发生的放热化学反应，生成 CO₂ 和水。当生物质或化石燃料燃烧时，储存在有机分子中的碳被快速氧化，以二氧化碳的形式返回大气。不完全燃烧还可能产生一氧化碳（CO）和碳颗粒（烟灰）。

In the natural carbon cycle, wildfires release carbon from vegetation and surface litter. However, human activities, particularly the burning of coal, oil and natural gas, have dramatically increased combustion rates, sending vast quantities of ancient carbon into the modern atmosphere.

在自然碳循环中，野火会将植被和地表凋落物中的碳释放出来。然而，人类活动，尤其是煤、石油和天然气的燃烧，大大加快了燃烧速率，将巨量古代碳释放到现代大气中。

6. Fossil Fuels and the Geological Carbon Store | 化石燃料与地质碳库

Fossil fuels – coal, crude oil and natural gas – formed from the remains of plants and marine organisms that were buried under layers of sediment millions of years ago. High pressure and temperature, combined with anaerobic conditions, transformed this biomass into energy-dense hydrocarbons. These fuels represent a vast, long-term geological store of carbon.

化石燃料——煤、原油和天然气——是由数百万年前被埋藏在沉积层下的植物和海洋生物残骸形成的。高压、高温以及缺氧条件将这些生物质转化为能量密集的碳氢化合物。这些燃料代表着一个巨大的长期地质碳库。

When we extract and combust fossil fuels, we transfer this locked-away carbon into the atmosphere–hydrosphere system, significantly altering the natural balance of the carbon cycle. Understanding the formation and composition of fossil fuels is equally relevant to IGCSE Chemistry topics on hydrocarbons and crude oil.

当我们开采并燃烧化石燃料时，我们便将原本封存的碳转移到了大气–水圈系统中，显著改变了碳循环的自然平衡。理解化石燃料的形成和组成，同样与 IGCSE 化学中关于碳氢化合物和石油的主题密切相关。

7. Ocean-Atmosphere Exchange and the Marine Carbon Pump | 海洋–大气交换与海洋碳泵

The oceans contain about 50 times more carbon than the atmosphere. Carbon dioxide dissolves in seawater and reacts with water molecules to form carbonic acid, which dissociates into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions. This reversible uptake acts as a powerful buffer for atmospheric CO₂ concentrations.

海洋中所含的碳比大气中多出约 50 倍。二氧化碳溶于海水，与水分子反应生成碳酸，再解离为碳酸氢根离子（HCO₃⁻）和碳酸根离子（CO₃²⁻）。这种可逆吸收作用成为了大气 CO₂ 浓度的强大缓冲剂。

In addition, marine phytoplankton photosynthesise near the ocean surface, fixing CO₂ into organic matter. When these organisms die, a portion of their remains sinks to the deep ocean, sequestering carbon for centuries. This biological pump helps regulate global climate over long timescales.

此外，海洋浮游植物在海洋表层进行光合作用，将 CO₂ 固定为有机物。当这些生物死亡时，部分残骸沉入深海，将碳封存数个世纪。这一生物泵有助于在长时间尺度上调节全球气候。

8. Carbon Sequestration and Sinks | 碳的封存与碳汇

A carbon sink is any reservoir that absorbs more carbon than it releases. Major natural sinks include forests, peatlands, soils and the deep ocean. Young, actively growing forests are particularly effective carbon sinks because photosynthesis exceeds total respiration. However, mature forests may become carbon neutral or even carbon sources if disturbed.

碳汇是指吸收的碳多于其释放的碳的任何储库。主要的天然碳汇包括森林、泥炭地、土壤和深海。正在积极生长的年轻森林是特别高效的碳汇，因为它们的光合作用超过了总体呼吸作用。然而，成熟森林可能变为碳中性，甚至当受到干扰时成为碳源。

In recent decades, scientists have explored artificial carbon capture and storage (CCS) technologies that trap CO₂ from industrial emissions and inject it into underground geological formations. These methods aim to reduce net atmospheric carbon additions.

近几十年来，科学家们探索了人工碳捕获与封存（CCS）技术，从工业排放中捕集 CO₂并将其注入地下地质构造中。这些方法旨在减少净大气碳的增加。

9. The Carbon Cycle and Human Interference | 碳循环与人为干扰

Human activities have disrupted the carbon cycle in two major ways: deforestation and fossil fuel combustion. Deforestation reduces the size of terrestrial carbon sinks, while often accelerating decomposition and erosion of soil carbon. The combined effect has pushed atmospheric CO₂ concentrations above 420 ppm, compared with pre-industrial levels of about 280 ppm.

人类活动以两种主要方式扰乱了碳循环：森林砍伐和化石燃料燃烧。森林砍伐缩小了陆地碳汇的规模，同时往往加速了土壤碳的分解和侵蚀。这两种影响的叠加已将大气 CO₂ 浓度推高至 420 ppm 以上，而工业化前水平约为 280 ppm。

The enhanced greenhouse effect, driven by rising CO₂ and methane levels, leads to global warming, ocean acidification and shifts in weather patterns. Edexcel examinations frequently ask students to explain how human actions affect the carbon cycle and why these changes matter for ecosystems and climate.

由 CO₂ 和甲烷浓度上升驱动的增强温室效应，导致全球变暖、海洋酸化以及气象模式的变化。爱德思考试经常要求学生解释人类行为如何影响碳循环，以及这些变化为何对生态系统和气候如此重要。

10. Key Equations and Summary Diagram | 关键方程式与总结图示

To master the carbon cycle, it is essential to recall the chemical equations that underpin its fluxes. The following table summarises the core reactions:

要掌握碳循环，必须熟记支撑其通量的化学方程式。下表总结了核心反应：

Process | 过程	Equation | 方程式
Photosynthesis	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Aerobic respiration	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Combustion of methane	CH₄ + 2O₂ → CO₂ + 2H₂O
Carbonate formation in oceans	CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻

When drawing a carbon cycle diagram, be sure to represent pools as boxes (atmosphere, biomass, soil, fossil fuels, oceans) and processes as arrows labelled with the appropriate mechanism. Accuracy with chemical symbols and balanced equations is often assessed in both Biology and Chemistry papers.

在绘制碳循环图时，要将各个储库（大气、生物质、土壤、化石燃料、海洋）画成方框，将过程画成箭头并标注相应的机制名称。化学符号和配平方程式的准确性在生物和化学试卷中都经常被考查。

---

11. Exam Tips for IGCSE Edexcel Science | IGCSE 爱德思科学考试技巧

When answering questions about the carbon cycle, always connect the process to its chemical equation and state the direction of carbon movement. For example, state that photosynthesis removes CO₂ from the atmosphere, not simply that ‘plants use CO₂’. Use precise terminology such as ‘carbon sink’, ‘sequestration’ and ‘decomposition’ where appropriate.

在回答有关碳循环的问题时，一定要将过程与其化学方程式联系起来，并指明碳的流动方向。例如，要说出光合作用从大气中移除 CO₂，而不只是说“植物使用 CO₂”。要酌情使用精确的术语，例如“碳汇”、“封存”和“分解作用”。

Common pitfalls include confusing respiration with photosynthesis (respiration occurs in all cells, at all times) and forgetting that combustion of fossil fuels adds ‘new’ carbon to the active short-term cycle. Also, be prepared to interpret data graphs showing seasonal fluctuations in atmospheric CO₂ – these reflect the balance between photosynthesis and respiration over the year.

常见的误区包括混淆呼吸作用与光合作用（呼吸作用在所有细胞中、在任何时间发生），以及忘记化石燃料燃烧会向活跃的短期循环中添加“新”碳。此外，还要准备好解释显示大气 CO₂ 季节性波动的数据图表——这些波动反映了全年光合作用与呼吸作用之间的平衡。

---

Published by TutorHao | Science Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）