📚 The Carbon Cycle: Exam Essentials | IB Edexcel 生物：碳循环 考点精讲
Mastering the carbon cycle is fundamental for IB and Edexcel Biology, as it illustrates how a single element circulates between the atmosphere, biosphere, hydrosphere and lithosphere. This revision guide breaks down every process, carbon sink and human impact you need to recall, with clear diagrams and exam-focused explanations.
掌握碳循环是 IB 与 Edexcel 生物的基础，因为它展示了一种元素如何在大气、生物圈、水圈和岩石圈之间循环。这份考点精讲将逐一拆解你需要记忆的每个过程、碳库与人类影响，并配有清晰的图解思路和应试讲解。
1. Overview of the Carbon Cycle | 碳循环总览
The carbon cycle describes the movement of carbon atoms in different molecular forms, principally carbon dioxide (CO₂), organic macromolecules and carbonate minerals. Carbon reservoirs include the atmosphere, living organisms, soil organic matter, oceans and sedimentary rocks. IB and Edexcel examiners expect you to link each flux to a biological, chemical or geological process.
碳循环描述的是碳原子以不同分子形式——主要是二氧化碳（CO₂）、有机大分子和碳酸盐矿物——移动的过程。碳库包括大气、生物体、土壤有机质、海洋和沉积岩。IB 与 Edexcel 考官期望你将每一种通量与生物、化学或地质过程联系起来。
- Atmospheric CO₂ is the most dynamic and examined reservoir.
- 大气中的 CO₂ 是最动态且最常被考查的碳库。
- Carbon flows are driven by photosynthesis, respiration, decomposition, combustion and geological processes like sedimentation.
- 碳流动由光合作用、呼吸作用、分解作用、燃烧以及沉积等地质过程驱动。
2. Photosynthesis: Carbon Fixation | 光合作用：碳固定
Photosynthesis removes CO₂ from the atmosphere and converts it into glucose and other organic compounds. In IB Biology, you must recall the summary equation and the fact that it represents the primary route by which inorganic carbon enters the biosphere. Edexcel A-level also emphasises the role of RUBISCO in the Calvin cycle.
光合作用从大气中移除 CO₂ 并将其转化为葡萄糖及其他有机物。在 IB 生物中，你必须记住总反应式，并明确它是无机碳进入生物圈的主要途径。Edexcel A-level 还强调 RUBISCO 在卡尔文循环中的作用。
6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
- Forests, phytoplankton and cyanobacteria act as major carbon sinks because they are highly productive photosynthesising communities.
- 森林、浮游植物和蓝细菌是主要碳汇，因为它们是高生产力的光合群落。
- Even though CO₂ is only about 0.04% of the atmosphere, it is the crucial substrate for autotrophs.
- 尽管 CO₂ 仅占大气的约 0.04%，它却是自养生物的关键底物。
3. Respiration: Returning Carbon | 呼吸作用：碳的归还
All living organisms carry out respiration, releasing CO₂ back into the atmosphere or water. In aerobic respiration, glucose is oxidised to produce ATP, and CO₂ is a waste product. IB past papers frequently ask you to distinguish between autotrophic and heterotrophic contributions to atmospheric CO₂.
所有生物都进行呼吸作用，将 CO₂ 释放回大气或水体。在有氧呼吸中，葡萄糖被氧化生成 ATP，CO₂ 是废物。IB 历年试题经常要求区分自养生物与异养生物对大气 CO₂ 的贡献。
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
- Plants perform both photosynthesis and respiration; net CO₂ uptake occurs only when photosynthesis exceeds respiration, typically during daylight.
- 植物同时进行光合作用和呼吸作用；只有当光合速率超过呼吸速率时（通常在白天）才净吸收 CO₂。
- Decomposer respiration is equally important, returning carbon locked in dead organic matter.
- 分解者的呼吸作用同样重要，将锁在死有机物中的碳归还出来。
4. Decomposition and Microbial Action | 分解作用与微生物活动
Decomposition is carried out by saprotrophic bacteria and fungi, which secrete extracellular enzymes to break down organic remains. This process releases CO₂ via respiration and also converts organic carbon into humus. Edexcel markschemes expect the term 'saprobiontic nutrition' and an appreciation of the role of temperature and oxygen in decay rates.
分解由腐生细菌和真菌完成，它们分泌胞外酶来分解有机残骸。此过程通过呼吸释放 CO₂，并将有机碳转化为腐殖质。Edexcel 评分标准要求使用术语“腐生营养”，并理解温度和氧气对分解速率的影响。
- In anaerobic conditions, such as waterlogged soils, decomposition is incomplete and leads to the accumulation of peat and eventually fossil fuels.
- 在厌氧条件下（如淹水土壤），分解不完全，导致泥炭积累并最终形成化石燃料。
- Detritivores like earthworms assist by fragmenting organic matter, increasing surface area for microbial attack.
- 蚯蚓等食碎屑动物通过粉碎有机物来增加微生物作用的表面积。
5. Combustion: Natural and Anthropogenic | 燃烧：自然源与人为源
Combustion of biomass and fossil fuels releases stored carbon as CO₂. While wildfires constitute natural combustion, human burning of coal, oil and gas for energy has dramatically altered the carbon balance since the Industrial Revolution. In both IB and Edexcel specifications, you must relate combustion to the enhanced greenhouse effect.
生物质与化石燃料的燃烧将储存的碳以 CO₂ 形式释放。虽然野火属于自然燃烧，但自工业革命以来，人类为获取能源而燃烧煤、石油和天然气，已大幅改变碳平衡。IB 与 Edexcel 考纲均要求你将燃烧与增强的温室效应相联系。
- Fossil fuels are essentially buried, geologically compressed organic matter that escaped decomposition.
- 化石燃料本质上是被埋藏并经地质压缩而免于分解的有机物。
- Complete combustion produces CO₂ and water; incomplete combustion also yields carbon monoxide and soot.
- 完全燃烧产生 CO₂ 和水；不完全燃烧还会产生一氧化碳和炭黑。
6. Feeding, Fossilisation and Sedimentation | 摄食、化石作用与沉积
Carbon moves through food chains when consumers eat producers or other consumers, incorporating organic carbon into their own tissues. Over geological timescales, the remains of marine organisms can form carbonate rocks, such as limestone, locking carbon in the lithosphere. A small fraction of organic matter becomes fossil fuels under high pressure and anoxic conditions.
碳通过食物链移动：消费者取食生产者或其他消费者，将有机碳纳入自身组织。在地质时间尺度上，海洋生物的遗骸可形成碳酸盐岩（如石灰岩），将碳锁在岩石圈中。一小部分有机物在高压和缺氧条件下转化为化石燃料。
- Shelled organisms use dissolved carbonates to build calcium carbonate (CaCO₃) shells; after death these shells sink and contribute to sediment.
- 带壳生物利用溶解碳酸盐构建碳酸钙（CaCO₃）外壳；死后这些外壳沉入海底，参与沉积。
- Edexcel Biology B mentions lithification and diagenesis as processes converting sediment into rock, effectively removing carbon from the active cycle for millions of years.
- Edexcel 生物 B 提及石化和成岩作用是把沉积物变成岩石的过程，有效地将碳从活跃循环中移出数百万年。
7. Oceanic Carbon Sinks | 海洋碳汇
Oceans absorb roughly one-quarter of anthropogenic CO₂ emissions. CO₂ dissolves in seawater and participates in a series of equilibrium reactions, forming carbonic acid, bicarbonate and carbonate ions. This buffering system moderates atmospheric CO₂ but causes ocean acidification, an ecosystem threat covered explicitly in IB Environmental Systems and Societies and Biology.
海洋吸收约四分之一的人为 CO₂ 排放。CO₂ 溶于海水，参与一系列平衡反应，形成碳酸、碳酸氢根和碳酸根离子。这一缓冲系统调节大气 CO₂ 但导致海洋酸化，这在 IB 环境系统与社会以及生物中均有明确涉及。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻
- Phytoplankton at the ocean surface fix CO₂ via photosynthesis; when they die, some carbon sinks to the deep ocean in 'biological pump' action.
- 海洋表层的浮游植物通过光合作用固定 CO₂；它们死亡后，部分碳通过“生物泵”作用沉入深海。
- Colder polar waters absorb more CO₂; warming oceans reduce gas solubility, a positive feedback loop.
- 较冷的极地水域吸收更多 CO₂；海洋变暖会降低气体溶解度，形成正反馈循环。
8. The Greenhouse Effect and Climate Change | 温室效应与气候变化
CO₂ is a greenhouse gas that traps outgoing long-wave radiation, warming the Earth's surface. The natural greenhouse effect is essential for life, but human activities have intensified it. In exams, you are asked to explain the mechanism: short-wave solar radiation penetrates the atmosphere, Earth re-radiates as infrared, and CO₂ absorbs this IR energy, re-emitting it in all directions, including back to the surface.
CO₂ 是一种温室气体，能捕获向外逸出的长波辐射，使地球表面变暖。自然温室效应对生命至关重要，但人类活动使其增强。考试中要求解释其机制：短波太阳辐射穿透大气，地面以红外线形式再辐射，CO₂ 吸收此红外能量并向各个方向重新发射，包括返回地表。
- Major greenhouse gases: CO₂, methane (CH₄), water vapour, nitrous oxide (N₂O). CO₂ is the reference gas with a long atmospheric lifetime.
- 主要温室气体：CO₂、甲烷（CH₄）、水蒸气、一氧化二氮（N₂O）。CO₂ 是大气寿命较长的参考气体。
- Enhanced greenhouse effect correlates strongly with industrialisation, deforestation and agriculture, all of which perturb the carbon cycle.
- 增强温室效应与工业化、森林砍伐和农业密切相关，这些活动都扰乱了碳循环。
9. Human Impact on the Carbon Cycle | 人类对碳循环的影响
Deforestation reduces the biosphere's capacity to absorb CO₂ while simultaneously releasing carbon stored in trees. Agriculture, especially livestock farming, contributes CH₄ and CO₂ from land-use change and respiration. Combustion of fossil fuels remains the dominant anthropogenic source. IB profiles often include analysis of Keeling Curve data, showing the seasonal oscillation and long-term rise in atmospheric CO₂.
森林砍伐降低了生物圈吸收 CO₂ 的能力，同时释放树木中储存的碳。农业，尤其是畜牧业，通过土地利用变化和呼吸作用贡献 CH₄ 和 CO₂。化石燃料燃烧仍是主要的人为排放源。IB 案例常包含基林曲线数据分析，展示大气 CO₂ 的季节性波动与长期上升趋势。
- Carbon footprint: total greenhouse gas emissions caused directly and indirectly by an individual, organisation or product, expressed in CO₂ equivalents.
- 碳足迹：个人、组织或产品直接和间接造成的温室气体排放总量，以 CO₂ 当量表示。
- Mitigation strategies include reforestation, carbon capture and storage (CCS), renewable energy and improved soil management to sequester organic carbon.
- 减缓策略包括植树造林、碳捕获与封存（CCS）、使用可再生能源以及通过改善土壤管理来固定有机碳。
10. Carbon Cycle Diagrams: Exam Tips | 碳循环图表：考试技巧
You must be able to draw and label a simple carbon cycle diagram showing pools (boxes) and fluxes (arrows). IB data-based questions often provide a complicated diagram and ask you to calculate net fluxes or identify the largest carbon reservoirs. Label all arrows with processes and use clear quantifiable units, like gigatonnes per year, if data are available.
你必须能够绘制并标注简单的碳循环图，显示碳库（方框）与通量（箭头）。IB 数据题常提供复杂图表，要求计算净通量或辨别最大碳库。在所有箭头上标注过程，若有数据则使用清晰的量化单位，如每年十亿吨。
| Carbon Reservoir | Carbon 储量（十亿吨碳） |
| Atmosphere | ~ 870 |
| Terrestrial biomass + soil | ~ 2,300 |
| Ocean (dissolved + biomass) | ~ 38,000 |
| Fossil fuels | ~ 4,000 (recoverable) |
| Sedimentary rocks | ~ 100,000,000 |
Always distinguish between fast cycle (biological processes over days to centuries) and slow cycle (geological processes over millions of years). This distinction is a popular extended-response point.
始终区分快速循环（日到百年尺度的生物过程）与慢速循环（百万年尺度的地质过程）。这一区分是常见的论述题要点。
11. Key Terms and Definitions | 关键术语与定义
Precision with terminology earns marks. Here are terms examiners love to see:
术语精确才能得分。以下是考官青睐的术语：
- Carbon sink – a reservoir that absorbs more carbon than it releases, e.g. mature forest, ocean.
- 碳汇 – 吸收碳多于释放碳的库，例如成熟森林、海洋。
- Carbon source – a process or reservoir releasing more carbon than it absorbs, e.g. deforestation, fossil fuel combustion.
- 碳源 – 释放碳多于吸收碳的过程或库，例如森林砍伐、化石燃料燃烧。
- Net primary production (NPP) – gross primary production minus autotrophic respiration; represents carbon fixed into biomass.
- 净初级生产力（NPP） – 总初级生产力减去自养呼吸；代表固定到生物量中的碳。
- Peat – partially decayed organic matter in waterlogged, acidic conditions; precursor to coal.
- 泥炭 – 在淹水、酸性条件下部分腐烂的有机物；煤的前身。
- Carbon sequestration – deliberate capture and long-term storage of atmospheric CO₂.
- 碳封存 – 有意捕获并长期储存大气 CO₂。
12. Common Misconceptions and Exam Pitfalls | 常见误区与考试陷阱
Avoid saying 'plants do not respire' – they respire continuously, and CO₂ is released day and night. Photosynthesis only fixes carbon during daylight; at night, plants are net carbon sources. Another misconception is that decomposition always releases CO₂ instantly; in fact, carbon can be stored in humus for decades. Finally, do not confuse the natural greenhouse effect with the enhanced greenhouse effect – the former is essential, the latter is problematic.
不要说“植物不呼吸”——它们持续呼吸，昼夜都释放 CO₂。光合作用仅在白天固碳；夜间植物是净碳源。另一个误区是认为分解总会立即释放 CO₂；实际上，碳可在腐殖质中储存数十年。最后，不要混淆自然温室效应与增强温室效应——前者不可缺，后者则成问题。
- Misinterpretation of seasons in Keeling Curve: draw the link between spring/summer CO₂ decline and high photosynthesis in the northern hemisphere, where most landmass exists.
- 对基林曲线季节变化的误解：要联系春夏季 CO₂ 下降与北半球（陆地最多处）高光合作用的关系。
- Overlooking the role of marine carbonate sediments as the largest long-term sink: this shows understanding of geological scale.
- 忽视海洋碳酸盐沉积物作为最大长期碳汇的作用：这体现对地质尺度的理解。
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