📚 Carbon Cycle in IB WJEC Biology | IB WJEC 生物：碳循环考点精讲
The carbon cycle is a fundamental biogeochemical cycle that describes the movement of carbon among the atmosphere, oceans, biosphere, and geosphere. For IB and WJEC Biology students, mastering this topic means understanding not only the key reservoirs and fluxes, but also the biochemical processes, human impacts, and quantitative reasoning expected in exam questions. This article provides an in-depth, bilingual breakdown of the carbon cycle, tailored to the IB and WJEC specifications.
碳循环是一个基本的生物地球化学循环，描述了碳在大气圈、海洋、生物圈和岩石圈之间的流动。对于 IB 和 WJEC 生物课程的学生来说，掌握这个主题不仅意味着理解关键碳库和通量，还包括生化过程、人类影响以及考试中可能出现的定量推理。本文提供了针对 IB 和 WJEC 大纲的碳循环深入双语解析。
1. The Global Carbon Reservoirs | 全球碳库概述
Carbon on Earth is stored in several major reservoirs: the atmosphere (as CO₂ and CH₄), the hydrosphere (dissolved CO₂, bicarbonate, and carbonate ions in oceans and freshwater), the lithosphere (fossil fuels, limestone, and sedimentary rocks), and the biosphere (living organisms and soil organic matter). The size of each reservoir and the rate at which carbon moves between them determine the overall balance of the cycle.
地球上的碳储存在几个主要的碳库中：大气圈（以 CO₂ 和 CH₄ 形式存在）、水圈（海洋和淡水中的溶解 CO₂、碳酸氢根和碳酸根离子）、岩石圈（化石燃料、石灰岩和沉积岩）以及生物圈（生物体和土壤有机质）。每个碳库的大小以及碳在它们之间移动的速率决定了整个循环的平衡状态。
In IB Biology, you are expected to know that the atmosphere holds about 750–800 gigatons of carbon, the oceans around 38,000 Gt, fossil fuel reserves approximately 4,000 Gt, and soils and vegetation combined about 2,500 Gt. For WJEC, emphasis is placed on comparing the relative sizes and residence times of carbon in different stores, particularly highlighting how relatively small the atmospheric pool is, yet how sensitive it is to fluxes.
在 IB 生物中，你需要了解大气中储存约 750–800 亿吨碳，海洋约 38000 亿吨，化石燃料储量约 4000 亿吨，土壤和植被合计约 2500 亿吨。WJEC 则强调比较不同碳库的相对大小和碳的停留时间，特别强调大气碳库相对较小，但它对通量变化非常敏感。
2. Photosynthesis: The Primary Fixation Pathway | 光合作用：主要的固碳途径
Photosynthesis is the process by which autotrophs—mainly plants, algae, and cyanobacteria— ₁₂O₆ + 6O₂. This process removes CO₂ from the atmosphere and fixes it into biomass, forming the foundation of nearly all food chains.
光合作用是自养生物（主要是植物、藻类和蓝细菌）将无机碳（CO₂）转化为葡萄糖等有机化合物的过程。总反应式为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。这个过程从大气中吸收 CO₂ 并将其固定为生物质，构成了几乎所有食物链的基础。
In both IB and WJEC specifications, you need to link the Calvin cycle to carbon fixation. In the light-independent reactions, CO₂ is combined with ribulose bisphosphate (RuBP) using the enzyme rubisco, ultimately producing triose phosphate, which can be used to synthesise carbohydrates, lipids, and proteins. WJEC candidates should be able to explain how this fixation rate can be affected by light intensity, temperature, and CO₂ concentration, and how these factors link to seasonal fluctuations in atmospheric CO₂.
在 IB 和 WJEC 大纲中，你需要将卡尔文循环与碳固定联系起来。在暗反应中，CO₂ 在酶 rubisco 的作用下与 1,5-二磷酸核酮糖（RuBP）结合，最终生成磷酸丙糖，后者可用于合成碳水化合物、脂质和蛋白质。WJEC 考生应能解释这种固定速率如何受光照强度、温度和 CO₂ 浓度的影响，以及这些因素如何与大气 CO₂ 的季节性波动相关联。
3. Respiration and Carbon Release | 呼吸作用与碳释放
which living organisms oxidise organic molecules to release energy, producing CO₂ as a by‑ P, returns carbon to the atmosphere. Both autotrophs and heterotrophs respire, and in ecosystems, plant respiration roughly balances photosynthesis over a full year, although gross primary production always exceeds respiration to allow net growth.
呼吸作用是生物体氧化有机分子以释放能量的过程，产生 CO₂ 作为副产物。有氧呼吸总反应为 C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP，将碳返还给大气。自养生物和异养生物都进行呼吸作用，在生态系统中，植物呼吸作用在全年尺度上大致与光合作用平衡，但由于净生长，总初级生产力总是超过呼吸作用。
IB students should differentiate between autotrophic and heterotrophic respiration, and understand the concept of net primary productivity (NPP). WJEC requires linking respiration rates to environmental factors such as temperature and moisture, and explaining how soil microbes contribute significantly to CO₂ efflux via decomposition and respiration of soil organic matter.
IB 学生应区分自养呼吸和异养呼吸，并理解净初级生产力（NPP）的概念。WJEC 要求将呼吸速率与环境因素（如温度和湿度）联系起来，并解释土壤微生物如何通过分解和呼吸土壤有机质对 CO₂ 排放起重要作用。
4. Decomposition and the Role of Microorganisms | 分解作用与微生物的角色
Decomposition is the breakdown of dead organic matter by saprotrophic bacteria and fungi. These organisms secrete enzymes that digest complex organic compounds externally, absorbing the simpler molecules for respiration. This process releases CO₂ back into the atmosphere and returns mineral nutrients to the soil, completing the short‑term carbon cycle.
分解作用是腐生细菌和真菌分解死有机物质的过程。这些生物分泌酶在体外消化复杂的有机化合物，吸收简单的分子用于呼吸。这个过程将 CO₂ 释放回大气，并将矿质养分归还土壤，完成短期碳循环。
WJEC emphasises the role of detritivores (e.g. earthworms, woodlice) in fragmentation, which increases the surface area for microbial action. IB students should be comfortable explaining how temperature, oxygen availability, and water content affect the rate of decomposition, linking to carbon cycling in different biomes such as tropical rainforests (rapid decomposition) and peat bogs (slow, leading to carbon storage).
WJEC 强调食碎屑动物（如蚯蚓、潮虫）在粉碎中的作用，这增加了微生物作用的表面积。IB 学生应能熟练解释温度、氧气可用性和含水量如何影响分解速率，并将其与不同生物群落（如热带雨林快速分解与泥炭沼泽缓慢分解导致碳储存）中的碳循环联系起来。
5. Oceanic Carbon Uptake and Chemistry | 海洋碳吸收与化学过程
Oceans are the largest active carbon reservoir. CO₂ dissolves in seawater and undergoes a series of chemical equilibria: CO₂(g) ⇌ CO₂(aq) + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻. The equilibrium between dissolved CO₂, bicarbonate (HCO₃⁻), and carbonate (CO₃²⁻) ions buffers the ocean’s pH but is sensitive to increasing atmospheric CO₂, which leads to ocean acidification.
海洋是最大的活跃碳库。CO₂ 溶解在海水中并经历一系列化学平衡：CO₂（气）⇌ CO₂（溶）+ H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻。溶解 CO₂、碳酸氢根（HCO₃⁻）和碳酸根（CO₃²⁻）离子之间的平衡缓冲了海洋 pH 值，但对大气 CO₂ 增加很敏感，从而导致海洋酸化。
IB Biology requires understanding that increased H⁺ concentration reduces the availability of carbonate ions, which are essential for calcifying organisms like corals and molluscs to build their CaCO₃ shells. WJEC focuses on the biological pump: phytoplankton fix CO₂ via photosynthesis in the surface waters; when they die, some sink to the deep ocean, locking carbon away for centuries to millennia.
IB 生物要求理解增加 H⁺ 浓度会降低碳酸根离子的可用性，这对珊瑚和软体动物等钙化生物构建它们的 CaCO₃ 壳至关重要。WJEC 关注生物泵：浮游植物通过光合作用在表层水体中固定 CO₂；当它们死亡时，一部分沉入深海，将碳封存数百年至数千年。
6. Sedimentation, Fossilisation, and Long‑term Storage | 沉积、化石作用与长期储存
Over geological timescales, carbon becomes locked in sedimentary rocks and fossil fuels. When marine organisms with calcareous shells die, their remains settle on the seabed and over millions of years form limestone (CaCO₃). Similarly, partially decomposed organic matter in anaerobic conditions, such as ancient swamps, can be transformed into coal, oil, and natural gas (methane) under heat and pressure.
在地质时间尺度上，碳被锁定在沉积岩和化石燃料中。当具有钙质壳的海洋生物死亡后，其遗骸沉入海底，经过数百万年形成石灰岩（CaCO₃）。同样，在厌氧条件下（如古代沼泽）部分分解的有机物在热和压力作用下可以转化为煤、石油和天然气（甲烷）。
IB students should connect these long‑term stores to the concept of carbon sequestration. WJEC asks learners to estimate the timescale of these processes and to contrast the rapid combustion of fossil fuels (releasing carbon in decades) with the extremely slow formation rate. This imbalance is central to understanding human disruption of the carbon cycle.
IB 学生应将这些长期储存库与碳封存的概念联系起来。WJEC 要求学习者估计这些过程的时间尺度，并对比化石燃料的快速燃烧（数十年内释放碳）与极慢的形成速率。这种不平衡是理解人类破坏碳循环的核心。
7. Combustion of Fossil Fuels and Biomass | 化石燃料与生物质燃烧
Combustion is the rapid oxidation of carbon‑based fuels, releasing stored carbon as CO₂. The burning of fossil fuels (coal, oil, gas) is the primary anthropogenic flux adding about 9–10 Gt C per year to the atmosphere. Biomass burning, whether natural (wildfires) or deliberate (deforestation, slash‑and‑burn agriculture), also transfers significant amounts of carbon from the biosphere to the atmosphere.
燃烧是碳基燃料的快速氧化，将储存的碳以 CO₂ 形式释放。化石燃料（煤、石油、天然气）的燃烧是主要的人为通量，每年向大气增加约 9–10 亿吨碳。生物质燃烧，无论是自然的（野火）还是人为的（森林砍伐、刀耕火种），也将大量碳从生物圈转移到大气中。
₂O, and incomplete combustion producing carbon monoxide and soot. IB students should analyse data on global CO₂ emissions by sector (energy, transport, industry, land‑use change) and relate this to the witnessed rise in atmospheric CO₂ from ~280 ppm pre‑industrial to over 420 ppm today.
WJEC 大纲突出了碳氢化合物完全燃烧的方程式，如 CH₄ + 2O₂ → CO₂ + 2H₂O，以及不完全燃烧产生一氧化碳和碳烟。IB 学生应分析各行业（能源、交通、工业、土地利用变化）全球 CO₂ 排放数据，并将其与大气 CO₂ 从工业革命前约 280 ppm 上升到如今超过 420 ppm 联系起来。
8. The Greenhouse Effect and Radiative Forcing | 温室效应与辐射强迫
CO₂ and CH₄ are greenhouse gases that absorb and re‑emit infrared radiation, trapping heat in the atmosphere. The natural greenhouse effect maintains Earth’s average temperature at about 15 °C; without it, the planet would be roughly −18 °C. The enhanced greenhouse effect, driven by increased concentrations of these gases due to human activity, leads to global warming and climate change.
CO₂ 和 CH₄ 是温室气体，能吸收并重新辐射红外辐射，将热量困在大气中。天然的温室效应使地球平均温度维持在约 15 °C；没有它，地球温度将约为 −18 °C。由于人类活动导致这些气体浓度增加而产生的增强温室效应，导致全球变暖和气候变化。
IB Biology candidates must be able to explain the mechanism: short‑wave solar radiation passes through the atmosphere, heats the Earth’s surface, which then emits long‑wave infrared; greenhouse gases absorb this and re‑radiate it in all directions, warming the lower atmosphere. WJEC extends this to quantifying the global warming potential (GWP) of different gases, demonstrating that methane has a GWP of about 28 over 100 years, making it more potent per molecule than CO₂.
IB 生物考生必须能够解释这一机制：短波太阳辐射穿过大气层，加热地球表面，然后地球表面发射长波红外辐射；温室气体吸收这些辐射并向各个方向重新辐射，加热低层大气。WJEC 将其扩展到量化不同气体的全球变暖潜能值（GWP），表明甲烷在 100 年尺度上的 GWP 约为 28，因此每个分子的增温效应比 CO₂ 更强。
9. Feedback Loops in the Carbon Cycle | 碳循环中的反馈循环
Positive feedback amplifies changes, while negative feedback dampens them. In the carbon cycle, warming can trigger permafrost thaw, releasing trapped methane and CO₂, which causes further warming—a positive feedback. Conversely, higher CO₂ can stimulate plant growth (CO₂ fertilisation effect), increasing carbon uptake—a negative feedback, though its long‑term capacity is limited by other factors like nutrient availability.
正反馈会放大变化，而负反馈则会抑制变化。在碳循环中，变暖可能引发永久冻土融化，释放被困的甲烷和 CO₂，导致进一步变暖——这是一个正反馈。相反，较高的 CO₂ 可刺激植物生长（CO₂ 施肥效应），增加碳吸收——一个负反馈，但其长期能力受到养分可用性等其他因素的限制。
IB students should explore the interactions between carbon and water cycles, and how deforestation in the Amazon reduces evapotranspiration, leading to regional drying and forest dieback (another positive feedback). WJEC expects candidates to use systems thinking to identify inputs, outputs, stores, and flows, and to recognise that feedback loops can lead to tipping points where the system shifts to a new state.
IB 学生应探究碳循环与水循环之间的相互作用，以及亚马逊森林砍伐如何减少蒸散作用，导致区域干燥和森林枯死（另一个正反馈）。WJEC 期望考生运用系统思维来识别输入、输出、储存和流动，并认识到反馈循环可能导致临界点，使系统转变到新的状态。
10. Human Impacts: Deforestation and Land‑use Change | 人类影响：毁林与土地利用变化
Deforestation removes carbon sinks, as forests store large quantities of carbon in biomass and soil. When trees are cut and burned, the carbon is rapidly released. Additionally, the loss of trees reduces future CO₂ uptake. Agriculture, urbanisation, and wetland drainage all convert carbon‑rich ecosystems into net emitters, decreasing the Earth’s capacity to absorb anthropogenic emissions.
森林砍伐消除了碳汇，因为森林在生物质和土壤中储存了大量碳。当树木被砍伐和焚烧时，碳迅速释放。此外，树木的丧失减少了未来的 CO₂ 吸收。农业、城市化和湿地排水都将富碳生态系统转变为净排放源，降低了地球吸收人为排放的能力。
WJEC specifically highlights tropical deforestation as a major contributor to global carbon emissions (about 10–15 % of the total). IB students must be prepared to evaluate case studies such as the impact of palm oil plantations in Indonesia and Malaysia, linking ecological concepts with economic and ethical considerations through the Theory of Knowledge (TOK) lens.
WJEC 特别强调热带森林砍伐是全球碳排放的主要贡献者之一（约占总量的 10–15%）。IB 学生必须准备好评估案例研究，如印度尼西亚和马来西亚棕榈油种植园的影响，通过知识论（TOK）视角将生态概念与经济及伦理考量联系起来。
11. Carbon Cycle Quantification and Modelling | 碳循环的量化与建模
Quantitative skills are essential. IB and WJEC assessments may ask you to calculate carbon fluxes from given data, interpret graphs of seasonal CO₂ variation (the Keeling Curve), or compare carbon footprints. Typical units are gigatons (Gt) or petagrams (Pg) of carbon per year. Remember that 1 ppm of atmospheric CO₂ equals approximately 2.13 Gt C.
量化技能至关重要。IB 和 WJEC 评估可能要求你根据给定数据计算碳通量，解释季节 CO₂ 变化图（基林曲线），或比较碳足迹。典型单位是每年亿吨碳（Gt）或拍克（Pg）。记住 1 ppm 大气 CO₂ 约等于 2.13 Gt C。
, 8 Gt C emitted equals 8 × 3.67 ≈ 29.4 Gt CO₂. This conversion frequently appears in data‑based questions.
学生应练习碳质量与 CO₂ 质量之间的换算：将 C 转换为 CO₂，乘以（44/12）或 3.67，因为 CO₂ 的摩尔质量是 44 g mol⁻¹，而 C 是 12 g mol⁻¹。例如，排放 8 Gt C 相当于 8 × 3.67 ≈ 29.4 Gt CO₂。这种换算经常出现在基于数据的问题中。
12. Mitigation Strategies and Carbon Management | 减缓策略与碳管理
Mitigation involves reducing emissions and enhancing sinks. Key strategies include reforestation and afforestation, improved agricultural practices (conservation tillage, biochar), renewable energy transition (solar, wind, nuclear), carbon capture and storage (CCS), and ocean fertilisation. International agreements, such as the Paris Agreement, aim to limit global temperature rise to well below 2 °C.
减缓包括减少排放和增强碳汇。关键策略包括重新造林和植树造林、改进农业实践（保护性耕作、生物炭）、向可再生能源过渡（太阳能、风能、核能）、碳捕获与封存（CCS）以及海洋施肥。国际协议如《巴黎协定》旨在将全球温度上升限制在远低于 2 °C。
IB expects you to evaluate the effectiveness and potential drawbacks of these approaches. For example, afforestation can reduce albedo in high latitudes, offsetting some cooling benefits. WJEC requires knowledge of how peatland restoration and wetland conservation prevent oxidation of stored carbon, keeping it locked in waterlogged, anaerobic conditions where decomposition is extremely slow.
IB 希望你能评估这些方法的有效性和潜在缺点。例如，在高纬度地区植树造林可能降低反照率，抵消部分冷却效益。WJEC 要求了解泥炭地恢复和湿地保护如何防止储存碳的氧化，使其保持在水淹、厌氧条件下，分解速度极慢。
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