📚 IB CCEA Biology: Carbon Cycle Key Points | IB CCEA 生物：碳循环 考点精讲
The carbon cycle is a fundamental biogeochemical process that describes how carbon atoms move between the atmosphere, living organisms, oceans, soil and geological formations. For CCEA examinations, understanding the major reservoirs, fluxes and the impact of human activities is essential. This article breaks down every key concept you need to master.
碳循环是一个基础的生物地球化学过程，描述了碳原子在大气、生物体、海洋、土壤和地质构造之间的迁移。对 CCEA 考试而言，掌握主要碳库、碳通量以及人类活动的影响至关重要。本文将逐一拆解你需掌握的所有核心概念。
1. Introduction to the Carbon Cycle | 碳循环简介
The carbon cycle consists of reservoirs (where carbon is stored) and fluxes (the movement of carbon between reservoirs). The cycle operates on both short-term (seconds to decades) and long-term (thousands to millions of years) timescales, linking biological, geological and chemical processes. The carbon atom is constantly recycled through photosynthesis, respiration, decomposition, combustion and sedimentation.
碳循环由碳库（碳储存的场所）和碳通量（碳在库之间移动的过程）构成。碳循环在短期（数秒到数十年）和长期（数千年到数百万年）两种时间尺度上运行，将生物、地质和化学过程联系在一起。碳原子通过光合作用、呼吸作用、分解作用、燃烧和沉积等过程不断循环。
In CCEA Biology, you should be able to identify the main stores of carbon and explain how carbon enters and leaves each store. The cycle is often depicted as a diagram with boxes representing reservoirs and arrows representing fluxes, sometimes quantified in gigatonnes of carbon per year.
在 CCEA 生物学中，你应能识别碳的主要储存场所，并解释碳是如何进出每一储存库的。碳循环常以示意图呈现，图中方框代表碳库，箭头代表碳通量，通量有时以每年吉吨碳（GtC）为单位量化。
2. Major Carbon Reservoirs | 主要碳库
The largest carbon reservoir is the lithosphere (Earth's crust), where carbon is stored as sedimentary rocks such as limestone (CaCO₃) and as fossil fuels (coal, oil and natural gas). This reservoir contains more than 99% of the planet's carbon but turns over very slowly.
最大的碳库是岩石圈（地壳），碳主要以沉积岩的形式（例如石灰岩 CaCO₃）以及化石燃料（煤、石油和天然气）储存在此。这一碳库储存了地球上超过 99% 的碳，但其周转极其缓慢。
The second largest active reservoir is the deep ocean, containing dissolved inorganic carbon (bicarbonate and carbonate ions). The atmosphere, although relatively small in total carbon mass, plays a disproportionately important role in regulating climate because it links directly to the greenhouse effect.
第二大的活跃碳库是深海，其中储存着溶解的无机碳（碳酸氢根和碳酸根离子）。大气碳库的总碳量虽然相对较小，但由于与温室效应直接关联，在气候调节中起着举足轻重的作用。
Other key reservoirs include soil organic matter, terrestrial biomass (forests, grasslands), the surface ocean and marine organisms. The soil pool contains carbon from humus and decomposing organisms, roughly double the amount in the atmosphere. Peatlands and permafrost are also significant stores of organic carbon.
其他关键碳库包括土壤有机质、陆地生物量（森林、草原）、表层海洋和海洋生物。土壤碳库包含腐殖质和分解生物体中的碳，其碳量约为大气碳库的两倍。泥炭地和永久冻土也是重要的有机碳储存库。
3. Photosynthesis and Carbon Fixation | 光合作用与碳固定
Photosynthesis is the primary process that removes carbon dioxide from the atmosphere and converts it into organic compounds. In the light-independent reactions (Calvin cycle), the enzyme RuBisCO catalyses the fixation of CO₂ into glycerate-3-phosphate. The overall equation is:
光合作用是从大气中移除二氧化碳并将其转化为有机化合物的主要过程。在光反应之后的暗反应（卡尔文循环）中，RuBisCO 酶催化 CO₂ 固定生成 3-磷酸甘油酸。总反应方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Plants, algae and cyanobacteria are autotrophs that perform oxygenic photosynthesis. The glucose produced can be used in respiration, stored as starch, or used to synthesise cellulose, lipids and proteins. This process represents the major carbon flux from the atmosphere into the biosphere, moving approximately 123 GtC per year.
植物、藻类和蓝细菌是执行产氧光合作用的自养生物。生成的葡萄糖可用于呼吸作用、以淀粉形式储存，或用于合成纤维素、脂质和蛋白质。这一过程代表着碳从大气进入生物圈的主要通量，每年约有 123 吉吨碳通过光合作用被固定。
In marine environments, phytoplankton are responsible for roughly half of global photosynthetic carbon fixation. The rate of photosynthesis is influenced by light intensity, CO₂ concentration, temperature and nutrient availability – factors that are themselves altered by climate change.
在海洋环境中，浮游植物承担了全球约一半的光合固碳量。光合速率受光照强度、CO₂ 浓度、温度和营养盐供应的影响——这些因素本身也因气候变化而改变。
4. Respiration and Carbon Release | 呼吸作用与碳释放
Respiration returns fixed organic carbon back to the atmosphere as CO₂. Aerobic respiration in mitochondria breaks down glucose in the presence of oxygen, producing CO₂ and water. The summarised equation is the reverse of photosynthesis:
呼吸作用将固定的有机碳以 CO₂ 的形式返还到大气中。线粒体中的有氧呼吸在氧气存在的条件下分解葡萄糖，生成 CO₂ 和水。总结方程式与光合作用相反：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy as ATP)
All living organisms – plants, animals, fungi and most bacteria – respire. Plant respiration occurs during both day and night, although during daylight hours photosynthesis typically exceeds respiration, resulting in a net uptake of CO₂. At night, plants are net emitters of CO₂.
所有生物体——植物、动物、真菌和大多数细菌——都进行呼吸作用。植物的呼吸作用在白天和夜间均发生，但在白天光合速率通常超过呼吸速率，因此净效应为吸收 CO₂。夜间植物则为 CO₂ 的净排放源。
Soil respiration, resulting from the metabolic activity of plant roots, microorganisms and soil fauna, is a major flux that moves about 60-80 GtC per year from soil back to the atmosphere. Decomposers drive a significant portion of this flux.
土壤呼吸由植物根系、微生物和土壤动物的代谢活动引起，是一个重要通量，每年将约 60-80 吉吨的碳从土壤释放回大气。分解者驱动了其中相当大的一部分通量。
5. Decomposition and Organic Matter Breakdown | 分解作用与有机物分解
Decomposition is the breakdown of dead organic matter (detritus) by saprotrophic bacteria and fungi. These decomposers secrete extracellular enzymes that digest cellulose, lignin and other complex molecules, releasing CO₂ through their own respiration. This process recycles nutrients and returns carbon to the atmosphere and soil.
分解作用是腐生的细菌和真菌对死亡有机质（碎屑）的分解过程。这些分解者分泌胞外酶消化纤维素、木质素和其他复杂分子，并通过自身的呼吸作用释放 CO₂。这一过程使营养物质得以再循环，并将碳返还到大气和土壤中。
The rate of decomposition depends on temperature, moisture, oxygen availability and the chemical composition of the litter. Waterlogged, acidic and anaerobic conditions slow decomposition, leading to the accumulation of partially decomposed organic matter as peat. This is why peatlands are such significant long-term carbon sinks.
分解速率取决于温度、湿度、氧气供应状况和凋落物的化学组成。积水、酸性和厌氧条件会减慢分解速率，导致部分分解的有机质以泥炭的形式积累。这正是泥炭地成为重要的长期碳汇的原因。
In CCEA questions, you may be asked to explain how saprotrophs contribute to nutrient cycling. Emphasise extracellular digestion and the release of inorganic substances, particularly CO₂ and ammonium ions.
在 CCEA 考题中，可能要求你解释腐生生物如何促进养分循环。要强调胞外消化和无机物的释放，尤其是 CO₂ 和铵根离子。
6. Combustion and Fossil Fuel Emission | 燃烧与化石燃料排放
Combustion, whether natural (wildfires) or anthropogenic (burning of fossil fuels and biomass), oxidises organic carbon to CO₂. The burning of fossil fuels – coal, oil and natural gas – releases carbon that had been locked in the lithosphere for millions of years into the atmosphere in just decades.
燃烧——无论天然火灾还是人为燃烧化石燃料和生物质——都会将有机碳氧化为 CO₂。煤、石油和天然气的燃烧将原本被封存在岩石圈中数百万年的碳在短短几十年内释放到大气中。
The current annual emission from fossil fuel combustion and cement production is approximately 10 GtC. This flux is not fully balanced by natural sinks, causing atmospheric CO₂ concentrations to rise sharply. The equation for complete combustion of methane (natural gas) exemplifies this:
目前，来自化石燃料燃烧和水泥生产的年排放量约为 10 吉吨碳。这一通量未能完全被自然碳汇所抵消，导致大气 CO₂ 浓度急剧上升。甲烷（天然气）完全燃烧的方程式清晰地展示了这一过程：
CH₄ + 2O₂ → CO₂ + 2H₂O
Incomplete combustion also produces carbon monoxide (CO) and soot (particulate carbon), which have harmful health effects but are less significant for the long-term carbon budget. Deforestation combined with burning releases both the carbon stored in trees and reduces the capacity for future CO₂ uptake.
不完全燃烧还会产生一氧化碳（CO）和烟灰（颗粒碳），对健康有害，但对长期碳收支的影响相对较小。砍伐森林并燃烧既会释放树木中储存的碳，又会降低未来吸收 CO₂ 的能力。
7. Oceanic Carbon Exchange | 海洋碳交换
Oceans absorb CO₂ from the atmosphere through physical and biological processes. At the air-sea interface, CO₂ dissolves according to Henry's Law and reacts with water to form carbonic acid (H₂CO₃), which then dissociates into bicarbonate (HCO₃⁻) and carbonate ions (CO₃²⁻).
海洋通过物理和生物过程从大气中吸收 CO₂。在海-气界面上，CO₂ 根据亨利定律溶解，与水反应生成碳酸 (H₂CO₃)，随后解离为碳酸氢根 (HCO₃⁻) 和碳酸根离子 (CO₃²⁻)。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻
This equilibrium makes the ocean the largest net sink of anthropogenic CO₂, absorbing roughly 25-30% of human emissions. However, increased CO₂ uptake leads to ocean acidification: the additional H⁺ ions lower pH, reducing the availability of carbonate ions that marine calcifiers need to build shells and skeletons.
这一化学平衡使海洋成为最大的人为 CO₂ 净汇，吸收了约 25-30% 的人为排放。然而，CO₂ 吸收量的增加会导致海洋酸化：额外的 H⁺ 降低了 pH 值，减少了海洋钙化生物构建外壳和骨骼所需的碳酸根离子。
The biological pump further moves carbon to the deep ocean: phytoplankton fix CO₂ into organic matter, which sinks as marine snow. Some of this carbon reaches the deep sea and may remain sequestered for centuries. The carbonate pump forms calcium carbonate shells that settle as sediments.
生物泵进一步将碳输送到深海：浮游植物将 CO₂ 固定为有机质，有机质以海洋雪的形式沉降。其中一部分碳抵达深海并可能被封存数个世纪。碳酸盐泵则形成碳酸钙外壳并沉降为沉积物。
8. Sedimentation and Long-Term Storage | 沉积与长期储存
Over geological timescales, the shells and skeletons of marine organisms accumulate on the sea floor and are compressed into limestone and chalk (CaCO₃). This lithification process removes carbon from the active short-term cycle and transfers it to the lithosphere reservoir, where it may stay for millions of years.
在地质时间尺度上，海洋生物的外壳和骨骼在海底堆积并被压实，形成石灰岩和白垩（CaCO₃）。这一成岩过程将碳从活跃的短期循环中移除，并转移至岩石圈碳库，可能在那里留存数百万年。
Organic matter that accumulates in anoxic sediments can be transformed by heat and pressure into fossil fuels: coal from terrestrial plant material, and oil and natural gas from marine microorganisms. This process is part of the slow carbon cycle and is responsible for locking away vast amounts of organic carbon.
缺氧沉积物中积累的有机质可在热力和压力作用下转变为化石燃料：陆生植物物质形成煤，海洋微生物形成石油和天然气。这一过程属于缓慢碳循环，封存了巨量的有机碳。
Weathering of silicate rocks and volcanic eruptions also exchange carbon between the lithosphere and atmosphere, but these fluxes are extremely slow compared with biological processes. For exam purposes, students should recognise sediment formation and fossilisation as long-term sinks.
硅酸盐岩的风化和火山喷发也在岩石圈与大气之间交换碳，但这些通量与生物过程相比极其缓慢。就考试而言，学生应认识到沉积物形成和石化是长期的碳汇。
9. Human Impacts on the Carbon Cycle | 人类活动对碳循环的影响
Human activities have dramatically altered the carbon cycle since the Industrial Revolution. The two principal disruptors are the combustion of fossil fuels and land-use changes, especially deforestation. Together they have increased atmospheric CO₂ concentration from about 280 ppm in pre-industrial times to over 420 ppm today.
自工业革命以来，人类活动极大地改变了碳循环。两个主要的干扰因素是化石燃料的燃烧和土地利用变化，尤其是森林砍伐。两者共同使大气 CO₂ 浓度从工业化前的约 280 ppm 上升到今天的 420 ppm 以上。
Deforestation reduces the size of the terrestrial biomass carbon sink and often involves slash-and-burn techniques that release CO₂ directly. The loss of tropical rainforests is particularly significant because these ecosystems cycle huge amounts of carbon and support high biodiversity.
砍伐森林缩减了陆地生物量碳汇的规模，并且常涉及刀耕火种，直接释放 CO₂。热带雨林的消失尤为显著，因为这些生态系统循环着巨量碳并维持着极高的生物多样性。
Agriculture contributes through methane emissions from livestock and rice paddies, as well as carbon losses from ploughing and soil degradation. Cement production releases CO₂ when limestone is heated. The net effect is a disrupted balance: carbon sinks can no longer absorb all the extra emissions.
农业通过牲畜和水稻的甲烷排放、翻耕和土壤退化造成的碳流失也贡献了温室气体排放。水泥生产过程中加热石灰岩同样会释放 CO₂。最终效应是碳收支失衡：碳汇已无法完全吸收额外的排放。
10. The Greenhouse Effect and Climate Change | 温室效应与气候变化
Carbon dioxide and methane (CH₄) are greenhouse gases that trap outgoing infrared radiation in the atmosphere, warming the planet. The enhanced greenhouse effect, driven by elevated concentrations of these gases from human activities, is the primary cause of modern climate change.
二氧化碳和甲烷 (CH₄) 是温室气体，能在大气中捕获溢出的红外辐射，使地球变暖。由人类活动导致这些气体浓度升高而引起的增强温室效应，是现代气候变化的主要推手。
Methane is produced by anaerobic decomposition in wetlands, landfills and the digestive tracts of ruminants. Although its atmospheric concentration is far lower than that of CO₂, CH₄ has a global warming potential about 28-34 times greater over a 100-year period. Melting permafrost poses a feedback risk by releasing long-stored CH₄.
甲烷由湿地、垃圾填埋场和反刍动物消化道中的厌氧分解产生。尽管其大气浓度远低于 CO₂，但在 100 年时间尺度上，CH₄ 的全球增温潜势约是 CO₂ 的 28-34 倍。永久冻土融化释放长期封存的 CH₄，构成了一种危险的正反馈。
Feedback loops amplify warming: higher temperatures reduce CO₂ solubility in seawater, decrease the efficiency of plant carbon fixation under heat stress, and increase the rate of soil respiration. Understanding these connections is vital for CCEA essays on climate change and ecology.
反馈循环会放大变暖效应：温度升高会降低 CO₂ 在海水的溶解度，在热胁迫下降低植物固碳效率，并加快土壤呼吸速率。理解这些联系对于 CCEA 气候变化和生态学相关的论述题至关重要。
11. Measuring Carbon Fluxes | 碳通量测量
Scientists quantify carbon fluxes using several methods. Eddy covariance towers measure CO₂ exchange between an ecosystem and the atmosphere in real time. Ice core data provide a historical record of atmospheric CO₂ and CH₄ concentrations going back 800,000 years, showing sharp recent increases.
科学家使用多种方法量化碳通量。涡度协方差塔可实时测量生态系统与大气之间的 CO₂ 交换。冰芯数据提供了过去 80 万年大气 CO₂ 和 CH₄ 浓度的历史记录，清晰地显示出近期的急剧上升。
Satellite remote sensing, such as NASA's OCO-2 mission, monitors column CO₂ concentrations globally and can track seasonal changes due to photosynthesis. Direct chemical analysis of air samples at monitoring stations like Mauna Loa gives the most famous continuous record, the Keeling Curve.
卫星遥感，例如 NASA 的 OCO-2 任务，可全球监测柱 CO₂ 浓度，并能追踪光合作用引起的季节性变化。在莫纳罗亚等监测站对空气样本进行直接化学分析，提供了最著名的连续记录——基林曲线。
For terrestrial ecosystems, carbon budget studies combine estimates of net primary production (NPP), heterotrophic respiration and disturbances to determine whether an area is a net carbon source or sink. Students should be able to interpret graphs showing seasonal oscillations in CO₂ concentration.
对于陆地生态系统，碳收支研究结合净初级生产力 (NPP)、异养呼吸和干扰的估算，以确定某一区域是净碳源还是净碳汇。学生应能解读展示 CO₂ 浓度季节性波动的图表。
12. Exam Tips and Common Misconceptions | 考试技巧与常见误区
A common mistake is to confuse carbon cycling with nitrogen or water cycles – always specify the form of carbon (e.g. CO₂, glucose, carbonate). Another error is stating that animals do not produce CO₂; all organisms respire and release CO₂. Plants are net absorbers over 24 hours, not permanent sinks during every moment.
一个常见错误是将碳循环与氮循环或水循环混淆——务必指明碳的形式（例如 CO₂、葡萄糖、碳酸盐）。另一个误区是声称动物不产生 CO₂；所有生物都进行呼吸并释放 CO₂。植物在全天 24 小时中是净吸收者，但并非每时每刻都是碳汇。
Students often overlook the role of the ocean as a carbon sink and neglect to mention acidification. Ensure you can link increased CO₂ to changes in ocean chemistry, including the decrease in carbonate ions and the impact on coral and mollusc calcification. Be precise with chemical equations and arrows.
学生常忽视海洋作为碳汇的作用，且忘记提及酸化。确保能将 CO₂ 升高与海洋化学变化相联系，包括碳酸根离子减少及对珊瑚和软体动物钙化的影响。化学方程式和箭头的使用要准确。
When drawing diagrams, always label reservoirs and fluxes with appropriate units (e.g. GtC or GtC/yr). Practice explaining how human activities create an imbalance in the carbon cycle. Use specific vocabulary such as sequestration, anthropogenic, reservoir and flux to demonstrate depth of understanding.
在绘制示意图时，务必注明碳库和通量的适当单位（例如 GtC 或 GtC/yr）。练习解释人类活动如何使碳循环失衡。使用碳封存、人为的、碳库和碳通量等专业词汇，以展现深入的理解。
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