Carbon Cycle Key Points for IB OCR Biology | IB OCR 生物：碳循环考点精讲

📚 Carbon Cycle Key Points for IB OCR Biology | IB OCR 生物：碳循环考点精讲

Understanding the carbon cycle is essential for IB and OCR Biology. This article summarises the key processes, reservoirs, and the role of carbon in maintaining life, as well as the impact of human activity on this balance. We’ll explore photosynthesis, respiration, decomposition, combustion, ocean exchange, and fossilisation, linking them all into one dynamic biogeochemical cycle.

理解碳循环对于 IB 和 OCR 生物而言至关重要。本文总结了关键过程、碳库以及碳在维持生命中的作用，并讨论了人类活动对这一平衡的影响。我们将深入探讨光合作用、呼吸作用、分解作用、燃烧、海洋交换以及化石形成，将它们串联成一个动态的生物地球化学循环。

1. What is the Carbon Cycle? | 什么是碳循环？

The carbon cycle describes the continuous movement of carbon atoms through the Earth’s biosphere, geosphere, hydrosphere, and atmosphere. Carbon is a fundamental building block of life, found in all organic molecules such as carbohydrates, proteins, lipids, and nucleic acids. The cycle maintains a balance that supports ecosystems, regulating climate and energy flow.

碳循环描述了碳原子在地球生物圈、岩石圈、水圈和大气圈中的持续移动。碳是生命的基本组成部分，存在于碳水化合物、蛋白质、脂质和核酸等所有有机分子中。该循环维持着支持生态系统的平衡，调节气候和能量流动。

Key carbon reservoirs include the atmosphere (as CO₂), oceans (dissolved CO₂, bicarbonates), living organisms (biomass), soil organic matter, and fossil fuels. The total amount of carbon on Earth is fixed, but its distribution changes over time due to both natural processes and human intervention.

关键的碳库包括大气（以 CO₂ 形式）、海洋（溶解的 CO₂、碳酸氢盐）、生物体（生物量）、土壤有机质和化石燃料。地球上的碳总量是固定的，但其分布会因自然过程和人为干预而随时间变化。

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

Photosynthesis is the primary process by which atmospheric carbon dioxide is converted into organic carbon. Autotrophs (plants, algae, and cyanobacteria) use light energy to combine CO₂ and H₂O, producing glucose and oxygen. The simplified equation is: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. This process removes CO₂ from the atmosphere and stores carbon in the form of biomass.

光合作用是将大气中的二氧化碳转化为有机碳的主要过程。自养生物（植物、藻类和蓝细菌）利用光能结合 CO₂ 和 H₂O，产生葡萄糖和氧气。简化的方程式为：6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂。该过程从大气中去除 CO₂，并以生物量的形式储存碳。

In the Calvin cycle, the enzyme RuBisCO fixes CO₂ into a 3-carbon compound (3-phosphoglycerate), which eventually becomes glucose. This glucose can be used for energy, stored as starch, or used to synthesise other organic molecules. On a global scale, photosynthesis absorbs roughly 120 gigatonnes of carbon per year, making it the largest flux in the carbon cycle.

在卡尔文循环中，RuBisCO 酶将 CO₂ 固定为一种三碳化合物（3-磷酸甘油酸），最终生成葡萄糖。这种葡萄糖可用于提供能量、以淀粉形式储存或用于合成其他有机分子。在全球范围内，光合作用每年吸收约 120 亿吨碳，使其成为碳循环中最大的通量。

3. Respiration: Returning Carbon to the Atmosphere | 呼吸作用：将碳返回大气

Respiration, both aerobic and anaerobic, releases carbon back into the atmosphere as CO₂. Aerobic respiration oxidises glucose to produce ATP, with CO₂ and water as by-products: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP. All living organisms, including plants, animals, fungi, and many microorganisms, perform respiration.

呼吸作用，包括有氧呼吸和无氧呼吸，将碳以 CO₂ 的形式释放回大气。有氧呼吸氧化葡萄糖以产生 ATP，并产生 CO₂ 和水作为副产物：C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP。所有生物，包括植物、动物、真菌和许多微生物，都进行呼吸作用。

It is important to note that plants also respire, consuming some of the carbon they fix during photosynthesis. At night, when photosynthesis ceases, plants become net emitters of CO₂. The balance between photosynthesis and respiration determines whether an ecosystem is a carbon sink or source on a daily or seasonal timescale.

需要注意的是，植物也会进行呼吸，消耗一部分它们通过光合作用固定的碳。在夜间，当光合作用停止时，植物成为 CO₂ 的净排放者。光合作用和呼吸作用之间的平衡决定了生态系统在日或季节时间尺度上是碳汇还是碳源。

4. Decomposition: The Role of Microorganisms | 分解作用：微生物的作用

Decomposition is the breakdown of dead organic matter by saprotrophic bacteria and fungi. These decomposers secrete extracellular enzymes that digest complex organic compounds into simpler substances. During this process, the microorganisms respire, releasing CO₂ into the atmosphere and returning mineral nutrients to the soil.

分解作用是腐生细菌和真菌对死亡有机物的分解。这些分解者分泌细胞外酶，将复杂的有机化合物消化为更简单的物质。在此过程中，微生物进行呼吸，将 CO₂ 释放到大气中，并将矿物质营养返回土壤。

The rate of decomposition depends on temperature, moisture, oxygen availability, and the chemical composition of the dead matter. In waterlogged or acidic conditions, decomposition is slowed, leading to the accumulation of partially decomposed organic material as peat. Peatlands are important long-term carbon stores.

分解速率取决于温度、湿度、氧气供应以及死亡物质的化学成分。在浸水或酸性条件下，分解会减慢，导致部分分解的有机物质积累形成泥炭。泥炭地是重要的长期碳库。

5. Combustion: Rapid Carbon Release | 燃烧：快速碳释放

Combustion (burning) is the rapid oxidation of organic material in the presence of oxygen, releasing energy, CO₂, water vapour, and other gases. This includes natural wildfires and anthropogenic burning of biomass and fossil fuels. The general equation for complete combustion of a hydrocarbon is: fuel + O₂ → CO₂ + H₂O.

燃烧是在氧气存在下有机物质的快速氧化，释放能量、CO₂、水蒸气和其他气体。这包括自然野火以及人类燃烧生物质和化石燃料。碳氢化合物完全燃烧的通用方程式为：燃料 + O₂ → CO₂ + H₂O。

Fossil fuel combustion releases carbon that had been stored underground for millions of years, significantly altering the atmospheric CO₂ concentration. Since the Industrial Revolution, CO₂ levels have risen from about 280 ppm to over 420 ppm, contributing to the enhanced greenhouse effect and climate change.

化石燃料的燃烧释放了已经在地下储存了数百万年的碳，显著改变了大气的 CO₂ 浓度。自工业革命以来，CO₂ 水平已从约 280 ppm 上升到超过 420 ppm，加剧了温室效应和气候变化。

6. Ocean Carbon Exchange | 海洋碳交换

The ocean plays a vital role in the carbon cycle by absorbing and releasing CO₂. Carbon dioxide dissolves in surface water according to Henry’s law, and a dynamic equilibrium exists between atmospheric CO₂ and dissolved CO₂. Once dissolved, CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates to bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.

海洋通过吸收和释放 CO₂ 在碳循环中发挥着至关重要的作用。根据亨利定律，二氧化碳溶解在表层水中，大气 CO₂ 与溶解的 CO₂ 之间存在动态平衡。溶解后，CO₂ 与水反应形成碳酸 (H₂CO₃)，碳酸解离为碳酸氢根 (HCO₃⁻) 和碳酸根 (CO₃²⁻) 离子。

These reactions buffer the ocean’s pH but also lead to ocean acidification when excess CO₂ is absorbed. Marine organisms such as phytoplankton also fix carbon through photosynthesis, and many build calcium carbonate (CaCO₃) shells. When these organisms die, their shells sink, contributing to deep-sea sediments and long-term carbon storage.

这些反应缓冲了海洋的 pH 值，但当过量 CO₂ 被吸收时，也会导致海洋酸化。浮游植物等海洋生物也通过光合作用固定碳，许多生物还构建碳酸钙 (CaCO₃) 外壳。当这些生物死亡后，它们的外壳下沉，形成深海沉积物和长期碳储存。

7. Fossilisation and Sedimentation: Long-term Carbon Storage | 化石形成与沉积：长期碳储存

Over geological timescales, carbon can be locked away as fossil fuels and sedimentary rocks. In anaerobic conditions, dead plant and animal matter may be buried and subjected to heat and pressure, transforming into coal, oil, and natural gas. This process sequesters carbon from the active cycle for millions of years.

在地质时间尺度上，碳可以以化石燃料和沉积岩的形式被锁定。在厌氧条件下，死亡的植物和动物物质可能被掩埋，并经受热和压力作用，转化为煤、石油和天然气。这一过程将碳从活跃循环中隔离了数百万年。

In the oceans, the accumulation of calcium carbonate shells from microorganisms forms limestone (CaCO₃). Lithification compacts these sediments into rock. Carbon stored in fossil fuels is released back only through human extraction and combustion, while limestone carbon is released through volcanic outgassing or weathering over extremely long cycles.

在海洋中，微生物的碳酸钙外壳积累形成石灰岩 (CaCO₃)。岩化作用将这些沉积物压实成岩石。储存在化石燃料中的碳只有通过人类的开采和燃烧才能被释放回大气，而石灰岩中的碳则通过火山排气或风化作用在极其漫长的时间尺度上释放。

8. Carbon Fluxes and Pool Sizes | 碳通量与碳库大小

A carbon flux is the rate of carbon transfer between pools, typically measured in petagrams (Pg) per year. The main pools are the atmosphere (~800 Pg C), terrestrial biomass (~550 Pg C), soils and detritus (~1500 Pg C), surface ocean (~900 Pg C), deep ocean (~37,000 Pg C), and fossil fuels (~5000 Pg C) – noting that fossil fuel reserves are potentially much larger but estimates vary.

碳通量是碳库之间碳转移的速率，通常以每年拍克 (Pg) 度量。主要的碳库为：大气（约 800 Pg C）、陆地生物量（约 550 Pg C）、土壤和碎屑（约 1500 Pg C）、表层海洋（约 900 Pg C）、深海（约 37,000 Pg C）以及化石燃料（约 5000 Pg C）——注意化石燃料储量可能大得多，但估计值不一。

Understanding these pools and fluxes helps ecologists model carbon dynamics and predict the impacts of land-use changes. For example, deforestation reduces the terrestrial biomass pool and releases CO₂ both from burning and from rapid decomposition of exposed soil organic matter.

了解这些碳库和通量有助于生态学家模拟碳动态，并预测土地利用变化的影响。例如，森林砍伐减少了陆地生物量碳库，并通过燃烧和暴露土壤有机质的快速分解释放 CO₂。

9. The Role of Methane in the Carbon Cycle | 甲烷在碳循环中的作用

While carbon dioxide is the most discussed carbon compound, methane (CH₄) is also a significant carbon-containing greenhouse gas. Methane is produced by methanogenic archaea in anaerobic environments such as wetlands, rice paddies, and the digestive tracts of ruminants. It can also be released from melting permafrost and during fossil fuel extraction.

虽然二氧化碳是最常被讨论的碳化合物，但甲烷 (CH₄) 也是一种重要的含碳温室气体。甲烷由产甲烷古菌在厌氧环境中产生，例如湿地、稻田和反刍动物的消化道。它也可以从融化的永久冻土和化石燃料开采过程中释放出来。

Methane is part of the carbon cycle: it can be oxidised to CO₂ in the atmosphere by hydroxyl radicals or consumed by methanotrophic bacteria in soils and water. Although its atmospheric concentration is lower, CH₄ has a global warming potential about 25 times greater than CO₂ over a 100-year period, making it critical to climate models.

甲烷是碳循环的一部分：它可以被大气中的羟基自由基氧化为 CO₂，或被土壤和水中的甲烷氧化菌消耗。尽管其大气浓度较低，但 CH₄ 在 100 年时间尺度内的全球变暖潜能值大约是 CO₂ 的 25 倍，这使其在气候模型中至关重要。

10. Human Impact and the Imbalance | 人类影响与失衡

Human activities have greatly disrupted the natural carbon cycle. Deforestation, land-use change, cement production, and the burning of fossil fuels release vast amounts of CO₂ into the atmosphere. This has increased the atmospheric carbon pool at a rate unprecedented in geological history, overwhelming natural sinks like forests and oceans.

人类活动极大地扰乱了自然碳循环。森林砍伐、土地利用变化、水泥生产以及化石燃料的燃烧将大量 CO₂ 释放到大气中。这以地质历史上前所未有的速度增加了大气碳库，使森林和海洋等自然碳汇不堪重负。

The ocean absorbs about one-quarter of anthropogenic CO₂ emissions, leading to ocean acidification. This lowers the saturation state of carbonate minerals, impacting calcifying organisms such as corals, molluscs, and some plankton. The knock-on effects disrupt marine food webs and ecosystem services that humans depend upon.

海洋吸收了约四分之一的人为 CO₂ 排放，导致海洋酸化。这降低了碳酸盐矿物的饱和度，影响了珊瑚、软体动物和部分浮游生物等钙化生物。其连锁效应扰乱了海洋食物网和人类依赖的生态系统服务。

11. The Calvin Cycle and Environmental Factors (OCR & IB Focus) | 卡尔文循环与环境因素 (OCR 和 IB 重点)

For exam success, it is vital to connect the carbon cycle to plant physiology. The Calvin cycle’s key enzyme, RuBisCO, has a dual affinity for CO₂ and O₂. Under high temperature and low CO₂ conditions, photorespiration occurs, reducing photosynthetic efficiency. This links atmospheric CO₂ concentration directly to plant productivity and carbon storage.

要成功应对考试，必须将碳循环与植物生理学联系起来。卡尔文循环的关键酶 RuBisCO 对 CO₂ 和 O₂ 具有双重亲和性。在高温和低 CO₂ 条件下，会发生光呼吸，从而降低光合效率。这直接将大气 CO₂ 浓度与植物生产力和碳储存联系起来。

Some plants, such as C4 and CAM plants, have evolved mechanisms to concentrate CO₂ around RuBisCO, minimising photorespiration. Understanding these adaptations helps explain how different biomes contribute to the carbon cycle under varying climatic conditions. Syllabus questions often ask you to interpret graphs of CO₂ uptake against temperature or light intensity.

一些植物，如 C4 和 CAM 植物，已经进化出在 RuBisCO 周围浓缩 CO₂ 的机制，从而最大限度地减少光呼吸。了解这些适应有助于解释不同生物群落在不同气候条件下如何为碳循环做出贡献。考纲题目经常要求你解读 CO₂ 吸收量与温度或光照强度之间的关系图。

12. Investigating the Carbon Cycle: Experiments and Techniques | 研究碳循环：实验与技术

In the lab, carbon cycling can be investigated using simple indicators like limewater (turns cloudy with CO₂) or hydrogencarbonate indicator (turns yellow as CO₂ concentration rises). For example, respirometers can measure CO₂ production by germinating seeds or small invertebrates, linking respiration rates to carbon flux calculations.

在实验室中，可以使用简单的指示剂研究碳循环，例如石灰水（遇 CO₂ 变浑浊）或碳酸氢盐指示剂（随着 CO₂ 浓度升高而变黄）。例如，呼吸计可以测量发芽种子或小型无脊椎动物产生的 CO₂，将呼吸速率与碳通量计算联系起来。

Field studies often use quadrats and gas-exchange chambers to measure net ecosystem exchange (NEE) of CO₂. Isotopic analysis (using ¹⁴C or stable isotopes like ¹³C) can trace carbon sources and ages, helping scientists determine whether the CO₂ being respired comes from recent photosynthesis or fossil fuel combustion.

野外研究通常使用样方和气体交换箱来测量生态系统的净 CO₂ 交换 (NEE)。同位素分析（使用 ¹⁴C 或像 ¹³C 这样的稳定同位素）可以追踪碳的来源和年龄，帮助科学家确定被呼吸释放的 CO₂ 是来自最近的光合作用还是化石燃料燃烧。

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Carbon Cycle Key Points for IB OCR Biology | IB OCR 生物：碳循环 考点精讲