The Carbon Cycle for A-Level Edexcel Biology | A-Level Edexcel 生物：碳循环考点精讲

📚 The Carbon Cycle for A-Level Edexcel Biology | A-Level Edexcel 生物：碳循环考点精讲

The carbon cycle describes the movement of carbon atoms through the Earth’s biosphere, geosphere, hydrosphere and atmosphere. For A-Level Edexcel Biology, you need to understand the major carbon reservoirs, the processes that transfer carbon between them, and the impact of human activities on this delicate balance.

碳循环描述了碳原子在地球生物圈、地质圈、水圈和大气圈中的移动。在 A-Level Edexcel 生物考试中，你需要掌握主要的碳库、碳在不同库之间转移的过程，以及人类活动对这种微妙平衡的影响。

1. Carbon Reservoirs and Pools | 碳储库与碳库

The largest carbon reservoir is sedimentary rocks and fossil fuels, containing about 100 million gigatonnes of carbon. The ocean is the second largest active reservoir, holding around 38,000 GtC, mostly as dissolved inorganic carbon. The atmosphere contains about 750 GtC, mainly as CO₂, while terrestrial biomass (living organisms) stores roughly 560 GtC.

最大的碳储库是沉积岩和化石燃料，约含 1 亿吉吨碳。海洋是第二大活跃碳库，约储存 38,000 GtC，主要以溶解无机碳形式存在。大气中约有 750 GtC，主要是 CO₂；陆地生物量（活生物体）约储存 560 GtC。

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

Photosynthesis is the primary process that removes CO₂ from the atmosphere. Plants, algae and cyanobacteria use light energy to convert CO₂ and water into glucose and oxygen. The overall equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

光合作用是将 CO₂ 从大气中移除的主要过程。植物、藻类和蓝细菌利用光能将 CO₂ 和水转化为葡萄糖和氧气。总反应方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

In the Calvin cycle, the enzyme RuBisCO fixes CO₂ by attaching it to ribulose bisphosphate (RuBP). This carbon fixation step produces two molecules of glycerate 3-phosphate (GP), which are then reduced to triose phosphate using ATP and NADPH from the light-dependent reactions.

在卡尔文循环中，酶 RuBisCO 将 CO₂ 连接到核酮糖二磷酸 (RuBP) 上进行碳固定。该固定步骤产生两个甘油酸-3-磷酸 (GP) 分子，随后利用光反应产生的 ATP 和 NADPH 将其还原为磷酸丙糖。

3. Respiration and Carbon Release | 呼吸作用与碳释放

Aerobic respiration returns CO₂ to the atmosphere by oxidising organic compounds. All living organisms, including plants, carry out respiration. The summary equation is the reverse of photosynthesis:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy

有氧呼吸通过氧化有机物将 CO₂ 释放回大气。所有生物体，包括植物，都进行呼吸作用。总方程式是光合作用的逆反应：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量

Respiration occurs in mitochondria and involves glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation. The Krebs cycle generates CO₂ when acetyl CoA is broken down. Exam questions often ask you to explain how respiring organisms contribute to the short-term carbon cycle.

呼吸作用发生在线粒体中，包括糖酵解、连接反应、三羧酸循环和氧化磷酸化。当乙酰辅酶 A 被分解时，三羧酸循环产生 CO₂。考题常要求解释进行呼吸作用的生物如何参与短期碳循环。

4. Decomposition and Microbial Action | 分解与微生物作用

Saprobionts (decomposers) such as bacteria and fungi secrete enzymes onto dead organic matter, breaking it down by extracellular digestion. They then absorb the soluble products. This process releases CO₂ through respiration and returns mineral ions like nitrates and phosphates to the soil.

腐生生物（分解者），如细菌和真菌，向死亡有机物分泌酶，通过胞外消化将其分解。然后它们吸收可溶性产物。这一过程通过呼吸作用释放 CO₂，并将硝酸盐和磷酸盐等矿质离子归还土壤。

In waterlogged, anaerobic conditions, decomposition is incomplete and leads to the formation of peat and eventually fossil fuels. Methanogenic archaea can produce methane (CH₄) in these environments, which is a potent greenhouse gas.

在淹水厌氧条件下，分解不完全，导致泥炭形成，并最终形成化石燃料。产甲烷古菌在此类环境中可产生甲烷 (CH₄)，这是一种强效温室气体。

5. Combustion and Fossil Fuels | 燃烧与化石燃料

Fossil fuels such as coal, oil and natural gas are formed from the remains of ancient organisms over millions of years, under high pressure and temperature. The carbon in these fuels has been locked away from the active carbon cycle for geological time scales.

煤炭、石油和天然气等化石燃料是由远古生物的遗骸在高压高温下历经数百万年形成的。这些燃料中的碳在地质时间尺度上一直被锁在活跃碳循环之外。

Combustion (burning) of fossil fuels rapidly oxidises carbon compounds to CO₂, releasing energy. The balanced equation for complete combustion of methane is:

CH₄ + 2O₂ → CO₂ + 2H₂O

化石燃料的燃烧将碳化合物迅速氧化为 CO₂，释放能量。甲烷完全燃烧的配平方程式为：

CH₄ + 2O₂ → CO₂ + 2H₂O

Industrialisation has vastly increased the rate of combustion, transferring carbon from a long-term geological reservoir into the atmosphere in a few centuries. This is the main driver of rising atmospheric CO₂ concentrations since the Industrial Revolution.

工业化极大地提高了燃烧速率，使碳从长期地质储库在几个世纪内转移至大气。这是自工业革命以来大气 CO₂ 浓度上升的主要驱动力。

6. The Oceanic Carbon Cycle | 海洋碳循环

Oceans absorb CO₂ from the atmosphere through diffusion. Once dissolved, CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates into bicarbonate ions (HCO₃⁻) and carbonate ions (CO₃²⁻). This equilibrium allows oceans to store 50 times more carbon than the atmosphere.

海洋通过扩散从大气中吸收 CO₂。溶解后，CO₂ 与水反应生成碳酸 (H₂CO₃)，然后解离为碳酸氢根离子 (HCO₃⁻) 和碳酸根离子 (CO₃²⁻)。这一平衡使海洋储存的碳比大气多 50 倍。

Marine organisms such as coccolithophores and foraminifera use carbonate ions to build shells and skeletons of calcium carbonate (CaCO₃). When these organisms die, their shells sink and form chalk and limestone sediments, sequestering carbon for millions of years.

海洋生物如颗石藻和有孔虫利用碳酸根离子构建碳酸钙 (CaCO₃) 的外壳和骨骼。这些生物死亡后，其外壳下沉形成白垩和石灰岩沉积物，将碳封存数百万年。

7. Feeding Relationships and Carbon Transfer | 摄食关系与碳传递

Carbon moves through food chains as consumers eat producers or other consumers. Plants assimilate carbon into carbohydrates, proteins and lipids. Primary consumers obtain carbon by eating plants; secondary and tertiary consumers obtain it by eating other animals. At each trophic level, some carbon is lost through respiration and egestion.

碳通过食物链传递，消费者捕食生产者或其他消费者。植物将碳同化为碳水化合物、蛋白质和脂质。初级消费者通过取食植物获得碳；次级和三级消费者通过捕食其他动物获得碳。在每个营养级，部分碳通过呼吸和排遗而损失。

The inefficiency of energy transfer means that carbon biomass decreases sharply at higher trophic levels. This is why pyramids of biomass typically show a regular decrease. The carbon in faeces and dead organisms becomes available to decomposers.

能量传递效率低下意味着较高营养级的碳生物量急剧减少。这就是为什么生物量金字塔通常呈现规律递减。粪便和死亡生物体中的碳可被分解者利用。

8. Human Impacts: Deforestation and Agriculture | 人类影响：森林砍伐与农业

Deforestation removes carbon sinks. Forests store huge amounts of carbon in biomass and soil. When trees are cut and burned, the stored carbon is rapidly oxidised to CO₂. Even if wood is used for timber, decomposition eventually returns carbon to the atmosphere.

森林砍伐消除了碳汇。森林在生物量和土壤中储存大量碳。树木被砍伐并焚烧时，储存的碳被迅速氧化为 CO₂。即使木材用于建材，分解最终仍会将碳归还大气。

Intensive agriculture often involves ploughing, which exposes soil organic matter to oxygen, speeding up decomposition and CO₂ release. Drainage of peatlands for farming or peat extraction further oxidises carbon stored for millennia, contributing significantly to greenhouse gas emissions.

集约化农业常涉及翻耕，使土壤有机质暴露于氧气，加速分解和 CO₂ 释放。为耕种或开采泥炭而排干泥炭地，会进一步氧化封存了数千年的碳，显著增加温室气体排放。

9. The Greenhouse Effect and Climate Change | 温室效应与气候变化

CO₂, CH₄ and water vapour are greenhouse gases. They absorb infrared radiation reflected from the Earth’s surface and re-emit it in all directions, trapping heat in the atmosphere. This natural greenhouse effect keeps Earth warm enough for life. However, increased greenhouse gas concentrations enhance this effect, leading to global warming.

CO₂、CH₄ 和水蒸气是温室气体。它们吸收从地球表面反射的红外辐射，并将其向各个方向重新发射，从而将热量困在大气中。这种天然的温室效应使地球保持适合生命生存的温度。然而，温室气体浓度增加会增强这种效应，导致全球变暖。

Climate change driven by rising CO₂ includes more frequent extreme weather events, melting polar ice, rising sea levels and disruption to ecosystems. The Edexcel specification expects you to link changes in the carbon cycle to these environmental consequences.

由 CO₂ 升高驱动的气候变化包括更频繁的极端天气事件、极地冰盖融化、海平面上升以及生态系统的破坏。Edexcel 大纲要求你将碳循环的变化与这些环境后果联系起来。

10. Measuring Carbon Fluxes | 碳通量的测量

Scientist measure carbon fluxes using a variety of methods. Direct measurements of atmospheric CO₂ are taken at stations like Mauna Loa in Hawaii. Eddy covariance towers measure CO₂ exchange between ecosystems and the atmosphere. Satellites monitor vegetation cover and estimate primary productivity.

科学家使用多种方法测量碳通量。在夏威夷莫纳罗亚等站点对大气 CO₂ 进行直接测量。涡度相关塔测量生态系统与大气之间的 CO₂ 交换。卫星监测植被覆盖并估算初级生产力。

Carbon dating and ice core data provide historical records of CO₂ concentrations. Air bubbles trapped in polar ice show that CO₂ levels have increased from about 280 ppm in pre-industrial times to over 420 ppm today. These data are crucial for constructing carbon budgets and informing climate models.

碳定年和冰芯数据提供了 CO₂ 浓度的历史记录。被困在极地冰层中的气泡显示，CO₂ 含量已从工业革命前的约 280 ppm 上升到今天的 420 ppm 以上。这些数据对于构建碳收支和为气候模型提供信息至关重要。

11. The Role of Peat Bogs and Wetlands | 泥炭沼泽和湿地的作用

Peat bogs accumulate partially decomposed plant material in waterlogged, acidic and anaerobic conditions. The low oxygen levels inhibit microbial respiration, preventing full decomposition. This locks away carbon, making peatlands one of the most important carbon stores on land.

泥炭沼泽在淹水、酸性和厌氧条件下积累部分分解的植物物质。低氧水平抑制微生物呼吸，阻止完全分解。这锁住了碳，使泥炭地成为陆地上最重要的碳储存库之一。

Destruction of peatlands through drainage, burning for agriculture or peat extraction releases massive amounts of stored carbon as CO₂. Conservation of peatlands is a cost-effective strategy to mitigate climate change. Exam questions may ask you to discuss the environmental impact of peat harvesting for garden compost.

通过排水、农业焚烧或泥炭开采破坏泥炭地，会以 CO₂ 形式释放大量储存的碳。保护泥炭地是缓解气候变化的一种经济有效策略。考题可能要求讨论开采泥炭用于园艺堆肥的环境影响。

12. Balancing the Carbon Cycle: Mitigation Strategies | 平衡碳循环：缓解策略

Reforestation and afforestation increase carbon sequestration by photosynthesis. Protecting existing forests and restoring degraded ecosystems can significantly reduce net CO₂ emissions. Marine conservation also helps, as seagrass meadows and mangroves are highly efficient carbon sinks.

重新造林和植树造林通过光合作用增加碳封存。保护现有森林和恢复退化生态系统可以显著减少净 CO₂ 排放。海洋保护也有帮助，因为海草床和红树林是高效的碳汇。

Technological approaches such as carbon capture and storage (CCS) capture CO₂ from power plants and industrial sources before it reaches the atmosphere, then inject it into underground geological formations. While promising, these methods are still being developed and can be expensive.

碳捕获与封存 (CCS) 等技术方法在 CO₂ 到达大气之前将其从发电厂和工业源捕获，然后注入地下地质构造。虽然前景广阔，但这些方法仍在开发中，且成本高昂。

Global agreements like the Paris Agreement aim to limit temperature rise by reducing greenhouse gas emissions. Understanding the carbon cycle is fundamental to designing effective policies and making informed decisions at both national and individual levels.

《巴黎协定》等全球协议旨在通过减少温室气体排放来限制温度上升。理解碳循环对于制定有效政策以及在国家和个人层面做出明智决策至关重要。

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The Carbon Cycle for A-Level Edexcel Biology | A-Level Edexcel 生物：碳循环 考点精讲