A-Level OCR Biology: Carbon Cycle Key Points | A-Level OCR 生物：碳循环 考点精讲

📚 A-Level OCR Biology: Carbon Cycle Key Points | A-Level OCR 生物：碳循环 考点精讲

The carbon cycle is a fundamental biogeochemical cycle that describes the movement of carbon atoms through the biosphere, lithosphere, hydrosphere and atmosphere. In A-Level OCR Biology, understanding the carbon cycle involves grasping how carbon is fixed, released, stored and transferred between organisms and their environment, as well as the role of microorganisms and the impact of human activities on the global carbon balance.

碳循环是描述碳原子在生物圈、岩石圈、水圈和大气圈中流动的基本生物地球化学循环。在 A-Level OCR 生物课程中，理解碳循环需要掌握碳如何被固定、释放、储存以及在生物体与环境之间转移，同时还要了解微生物的作用以及人类活动对全球碳平衡的影响。

1. Overview of the Carbon Cycle | 碳循环概述

The carbon cycle is a closed system on a global scale, with carbon moving between four major reservoirs: the atmosphere (as CO₂ and CH₄), the oceans (dissolved CO₂, carbonates), terrestrial biomass (organic compounds) and sediments/fossil fuels (long-term stores). Most biological molecules—carbohydrates, proteins, lipids and nucleic acids—contain carbon, making the cycle essential for life.

碳循环在全球尺度上是一个闭路系统，碳在四个主要库之间移动：大气（以 CO₂ 和 CH₄ 形式）、海洋（溶解的 CO₂、碳酸盐）、陆地生物质（有机化合物）以及沉积物和化石燃料（长期储存）。大多数生物分子——碳水化合物、蛋白质、脂质和核酸——都含有碳，因此碳循环对生命至关重要。

2. Photosynthesis and Carbon Fixation | 光合作用与碳的固定

Photosynthesis is the primary process that removes CO₂ from the atmosphere and fixes carbon into organic molecules. In the Calvin cycle, the enzyme RuBisCO catalyses the carboxylation of ribulose bisphosphate (RuBP), producing two molecules of glycerate 3-phosphate (GP). These are then reduced to triose phosphate, which can be used to synthesise glucose, starch, cellulose and other carbohydrates.

光合作用是从大气中去除 CO₂ 并将碳固定到有机分子中的主要过程。在卡尔文循环中，酶 RuBisCO 催化核酮糖二磷酸（RuBP）的羧化反应，产生两分子甘油酸 3-磷酸（GP）。这些分子随后被还原为磷酸丙糖，可用于合成葡萄糖、淀粉、纤维素等碳水化合物。

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Plants, algae and cyanobacteria are the main primary producers on Earth. In OCR questions, you may be asked to explain how carbon is incorporated into biomass and how this biomass passes to consumers through feeding relationships.

植物、藻类和蓝细菌是地球上主要的初级生产者。在 OCR 考题中，可能会要求解释碳如何进入生物质，以及这些生物质如何通过摄食关系传递给消费者。

3. Respiration and Decomposition | 呼吸作用与分解

All living organisms release CO₂ back into the atmosphere through respiration. Aerobic respiration completely oxidises glucose, releasing CO₂ and water. Anaerobic respiration in some microorganisms produces CO₂ as well, but in other pathways may yield compounds like ethanol or methane.

所有生物都通过呼吸作用将 CO₂ 释放回大气。有氧呼吸将葡萄糖完全氧化，释放出 CO₂ 和水。某些微生物的无氧呼吸也产生 CO₂，但在其他途径中可能产生乙醇或甲烷等化合物。

Decomposition is the breakdown of dead organic matter by saprobiontic bacteria and fungi. These organisms secrete extracellular enzymes that digest complex organic molecules, then absorb the soluble products. During this process, they respire aerobically, releasing CO₂. A warm, moist and oxygen-rich environment speeds up decomposition.

分解是指死去的有机物质被腐生细菌和真菌分解。这些生物分泌胞外酶来消化复杂的有机分子，然后吸收可溶性产物。在此过程中，它们进行有氧呼吸，释放 CO₂。温暖、潮湿、富氧的环境会加速分解。

4. Combustion of Fossil Fuels and Biomass | 化石燃料与生物质的燃烧

Combustion of organic materials—whether fossil fuels (coal, oil, natural gas) or biomass (wood, peat)—returns carbon that has been stored for millions of years or decades back into the atmosphere as CO₂. The equation for complete combustion of a hydrocarbon is:

有机物质的燃烧——无论是化石燃料（煤、石油、天然气）还是生物质（木材、泥炭）——都将储存了数百万年或数十年的碳以 CO₂ 的形式返回大气。碳氢化合物完全燃烧的方程式为：

CₓHᵧ + (x + y/4)O₂ → xCO₂ + (y/2)H₂O

Human industrialisation has massively increased the rate of combustion, leading to a rise in atmospheric CO₂ concentration from about 280 ppm in pre-industrial times to over 420 ppm today. This is the main driver of enhanced greenhouse effect and global warming.

人类工业化极大地加快了燃烧速率，导致大气 CO₂ 浓度从工业革命前约 280 ppm 上升到当今超过 420 ppm。这是加剧温室效应和全球变暖的主要驱动因素。

5. Formation of Fossil Fuels and Sedimentary Rocks | 化石燃料和沉积岩的形成

When dead organisms decompose in anaerobic conditions—such as in waterlogged soils, bogs or deep ocean sediments—their carbon may not be fully respired and can be turned into fossil fuels over geological timescales. Peat forms from partially decayed plant material in acidic, waterlogged conditions. If buried and subjected to heat and pressure, peat transforms into coal. Marine plankton remains can form oil and natural gas.

当死去的生物在缺氧条件下（例如淹水土壤、沼泽或深海沉积物）分解时，其中的碳可能未被完全呼吸消耗，并在地质时间尺度上转化为化石燃料。泥炭由酸性淹水条件下部分腐烂的植物材料形成。泥炭若被埋藏并经受热和压力，会转变为煤。海洋浮游生物遗骸可形成石油和天然气。

Additionally, carbon is locked away in limestone (CaCO₃) and chalk, which are formed from the shells and skeletons of marine organisms. These carbonates represent the largest carbon reservoir on Earth. Weathering of these rocks releases carbon very slowly, while subduction into the Earth’s mantle removes carbon from the surface cycle for millions of years.

此外，碳还被锁在石灰石和白垩（CaCO₃）中，它们由海洋生物的贝壳和骨骼形成。这些碳酸盐是地球上最大的碳库。岩石风化释放碳的速度非常缓慢，而俯冲进入地幔则会将碳从表层循环中移出数百万年。

6. Oceanic Carbon Sinks | 海洋碳汇

The oceans absorb about one-quarter of anthropogenic CO₂ emissions each year. CO₂ dissolves in seawater and reacts with water to form carbonic acid (H₂CO₃), which dissociates into hydrogen carbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻). This equilibrium is crucial in buffering atmospheric CO₂ levels.

每年海洋吸收约四分之一的人为 CO₂ 排放。CO₂ 溶于海水并与水反应生成碳酸（H₂CO₃），碳酸解离为碳酸氢根离子（HCO₃⁻）和碳酸根离子（CO₃²⁻）。这一平衡对缓冲大气 CO₂ 水平至关重要。

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻

However, increased CO₂ absorption leads to ocean acidification, lowering the pH and affecting the ability of calcifying organisms, like corals and molluscs, to build their calcium carbonate shells. This is a key example of how disrupting the carbon cycle has knock-on ecological effects.

然而，CO₂ 吸收增加会导致海洋酸化，降低 pH 值，影响珊瑚和软体动物等钙化生物构建其碳酸钙外壳的能力。这是碳循环失调如何产生连锁生态效应的一个关键示例。

7. Role of Microorganisms in Carbon Cycling | 微生物在碳循环中的作用

Saprobiontic fungi and bacteria are the primary decomposers that recycle carbon from dead organic matter. They release CO₂ through respiration and make mineral nutrients available for plants. Mycorrhizal fungi form mutualistic associations with plant roots, enhancing water and nutrient (especially phosphate) uptake; in return they receive carbohydrates—a direct transfer of recently fixed carbon from plant to fungus.

腐生真菌和细菌是主要的分解者，能够从死有机质中回收碳。它们通过呼吸作用释放 CO₂，并为植物提供矿质营养。菌根真菌与植物根系形成互利共生关系，增强了水分和养分（尤其是磷酸盐）的吸收；作为回报，它们获得碳水化合物——这是新固定的碳从植物向真菌的直接转移。

Some bacteria are also involved in methanogenesis, producing methane (CH₄) from organic matter in strictly anaerobic environments such as waterlogged soils, landfill sites and the digestive tracts of ruminants. Methane is a potent greenhouse gas with a global warming potential about 28 times that of CO₂ over 100 years.

一些细菌还参与产甲烷过程，在严格厌氧环境（如淹水土壤、垃圾填埋场和反刍动物消化道）中将有机质转化为甲烷（CH₄）。甲烷是一种强效温室气体，其 100 年全球变暖潜势约为 CO₂ 的 28 倍。

8. Carbon Fluxes and Seasonal Variation | 碳通量与季节变化

Carbon flux refers to the rate of carbon exchange between reservoirs. These fluxes can be measured in gigatonnes of carbon per year. Key fluxes include photosynthesis, respiration, ocean-atmosphere exchange, and volcanic emissions. Data from monitoring stations such as Mauna Loa show clear seasonal oscillations in atmospheric CO₂: levels fall in spring and summer as Northern Hemisphere plants photosynthesise, and rise in autumn and winter when decomposition dominates.

碳通量是指碳在库与库之间的交换速率，通常以每年数十亿吨碳为单位计量。关键通量包括光合作用、呼吸作用、海洋-大气交换和火山排放。来自莫纳罗亚等监测站的数据显示，大气 CO₂ 浓度有明显的季节性波动：春夏季因北半球植物进行光合作用而下降，秋冬季因分解占主导而上升。

In OCR exams, you may be asked to interpret graphs of atmospheric CO₂ trends and explain the reasons behind seasonal patterns. Emphasise that Northern Hemisphere land masses dominate global terrestrial photosynthesis, driving the sawtooth pattern.

在 OCR 考试中，可能要求解读大气 CO₂ 趋势图，并解释季节性模式背后的原因。需要强调北半球的陆地面积主导了全球陆地光合作用，从而驱动了锯齿状的波动模式。

9. Human Impact and the Enhanced Greenhouse Effect | 人类影响与增强的温室效应

Human activities are severely disrupting the natural carbon cycle. The main disruptions are: burning of fossil fuels for energy and transport, deforestation (which reduces carbon fixation and often releases CO₂ if forest is burned), agricultural practices (ploughing releases soil carbon; ruminant livestock produce methane), and cement production (calcination of limestone releases CO₂).

人类活动严重扰乱了自然碳循环。主要干扰因素有：为获取能源和交通运输而燃烧化石燃料；森林砍伐（减少了碳固定，若森林被焚烧则往往释放 CO₂）；农业实践（耕作释放土壤中的碳；反刍家畜产生甲烷）；以及水泥生产（石灰石煅烧释放 CO₂）。

These activities have increased atmospheric greenhouse gases—CO₂, CH₄—leading to more infrared radiation being trapped in the atmosphere. This enhanced greenhouse effect causes global temperature rise, climate change, melting ice caps, rising sea levels, and shifts in ecosystems. The carbon cycle is now in a state of imbalance, with annual anthropogenic emissions exceeding the rate at which natural sinks can absorb carbon.

这些活动增加了大气中的温室气体——CO₂、CH₄——导致更多红外辐射被截留在大气中。这种增强的温室效应引起全球气温上升、气候变化、冰盖融化、海平面上升及生态系统变化。目前碳循环处于不平衡状态，人为年排放量超过了自然碳汇吸收碳的速率。

10. Strategies to Mitigate Carbon Imbalance | 缓解碳失衡的策略

To restore balance to the carbon cycle, a combination of approaches is required. Afforestation and reforestation increase the biomass carbon sink. Improved agricultural practices, such as no-till farming and cover cropping, can increase soil organic carbon. Restoration of peatlands prevents oxidation of stored carbon. Transitioning to renewable energy sources reduces combustion emissions. Carbon capture and storage (CCS) technologies aim to trap CO₂ from industrial processes and store it underground.

要恢复碳循环的平衡，需要多管齐下。造林和再造林可增加生物质碳汇。改进的农业实践，如免耕耕作和覆盖作物种植，可以增加土壤有机碳。恢复泥炭地可防止储存碳的氧化。转向可再生能源可减少燃烧排放。碳捕集与封存（CCS）技术旨在从工业过程中捕集 CO₂ 并将其储存在地下。

At the international level, agreements like the Paris Agreement set targets for reducing greenhouse gas emissions. The OCR specification expects students to discuss these socio-scientific issues, applying biological knowledge to evaluate evidence and propose sustainable solutions.

在国际层面，诸如《巴黎协定》等协议为减少温室气体排放设定了目标。OCR 教学大纲要求学生讨论这些社会科学议题，运用生物学知识来评估证据并提出可持续的解决方案。

11. Key Terminology and Exam Tips | 关键术语与考试技巧

Be precise with terminology: ‘respiration’ (not ‘breathing’) is the cellular process that releases CO₂; ‘combustion’ is burning; ‘decomposition’ is breakdown by microbes; ‘carbon sink’ is a reservoir that absorbs more carbon than it releases; ‘carbon source’ releases more than it absorbs. Use terms like ‘carbon fixation’, ‘calvin cycle’, ‘RuBisCO’, ‘saprobiont’, ‘methanogenesis’ correctly.

术语要准确：“呼吸作用”（不是“呼吸”）是释放 CO₂ 的细胞过程；“燃烧”是焚烧；“分解”是微生物的降解；“碳汇”是吸收碳多于释放碳的库；“碳源”是释放多于吸收。正确使用“碳固定”、“卡尔文循环”、“RuBisCO”、“腐生生物”、“产甲烷作用”等术语。

In extended answers, always link the processes together to show how carbon moves from one reservoir to another. Use diagrams to illustrate the cycle, labelling arrows with the appropriate process names. When discussing data, refer to trends over time and distinguish between correlation and causation.

在扩展性作答中，始终要将各个过程联系起来，展示碳如何从一个库转移到另一个库。使用示意图来说明循环，并在箭头上标注相应的过程名称。讨论数据时，要指出随时间变化的趋势，并区分相关性和因果关系。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）