📚 IGCSE OCR Biology: Carbon Cycle Exam-Focused Revision | IGCSE OCR 生物：碳循环考点精讲
Understanding the carbon cycle is essential for IGCSE OCR Biology. It explains how carbon atoms move between the atmosphere, living organisms, oceans and rocks, and it underpins topics such as photosynthesis, respiration and climate change. This revision guide breaks down every key process you need to know, with clear explanations and exam-focused insights.
理解碳循环是 IGCSE OCR 生物的重要基础。它解释了碳原子如何在大气、生物体、海洋和岩石之间循环，并支撑着光合作用、呼吸作用和气候变化等主题。本考点精讲分解了每一个你需要掌握的关键过程，提供清晰的解释和针对考试的解析。
1. What is the Carbon Cycle? | 什么是碳循环？
The carbon cycle describes the recycling of carbon atoms through the Earth’s spheres. Carbon is the backbone of organic molecules such as carbohydrates, proteins and fats. It exists in different forms: atmospheric carbon dioxide (CO₂), dissolved carbon in oceans, organic carbon in living things, and stored carbon in fossil fuels and carbonate rocks. The cycle keeps the concentration of CO₂ in the atmosphere relatively balanced when undisturbed.
碳循环描述了碳原子在地球各圈层中的循环往复。碳是碳水化合物、蛋白质和脂肪等有机分子的骨架。它以不同形式存在：大气中的二氧化碳（CO₂）、海洋中溶解的碳、生物体中的有机碳，以及储存在化石燃料和碳酸盐岩石中的碳。在未受干扰的情况下，该循环使大气中 CO₂ 的浓度保持相对平衡。
2. Major Carbon Reservoirs | 主要的碳库
Earth’s carbon is stored in four main reservoirs: the atmosphere, the biosphere (living organisms), the hydrosphere (oceans, lakes), and the lithosphere (fossil fuels, limestone). The largest reservoir is the lithosphere, but carbon there moves very slowly. The atmosphere holds carbon mainly as CO₂ and methane (CH₄). Oceans contain dissolved CO₂, hydrogen carbonate ions and marine organisms.
地球上的碳主要储存在四个库中：大气圈、生物圈（生命体）、水圈（海洋、湖泊）和岩石圈（化石燃料、石灰石）。最大的库是岩石圈，但其中的碳移动非常缓慢。大气中的碳主要以 CO₂ 和甲烷（CH₄）形式存在。海洋中含有溶解的 CO₂、碳酸氢根离子和海洋生物。
3. Photosynthesis: Removing CO₂ from the Air | 光合作用：从空气中移除 CO₂
Photosynthesis is the primary process that removes carbon dioxide from the atmosphere. Green plants, algae and some bacteria use light energy to convert CO₂ and water into glucose and oxygen. The overall equation is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
This fixed carbon becomes part of plant biomass. Plants are thus carbon sinks. The carbon is passed to consumers when they eat plants.
光合作用是从大气中移除二氧化碳的主要过程。绿色植物、藻类和一些细菌利用光能将 CO₂ 和水转化为葡萄糖和氧气。总方程式为：
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
这些固定的碳成为植物生物质的一部分。因此植物是碳汇。当消费者食用植物时，碳被传递给它们。
4. Respiration: Returning CO₂ to the Atmosphere | 呼吸作用：将 CO₂ 归还大气
All living organisms carry out respiration, which releases energy from glucose. Aerobic respiration converts glucose and oxygen back into carbon dioxide and water, returning CO₂ to the atmosphere. The equation is the reverse of photosynthesis:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
Respiration occurs continuously in plants, animals and microorganisms. It is a key part of the short-term carbon cycle.
所有生物都进行呼吸作用，该过程从葡萄糖中释放能量。有氧呼吸将葡萄糖和氧气转化回二氧化碳和水，将 CO₂ 归还大气。方程式是光合作用的逆反应：
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 能量
呼吸作用在植物、动物和微生物中持续进行。它是短期碳循环的关键部分。
5. Feeding and Carbon Transfer | 取食与碳的传递
Carbon moves through food chains when organisms feed. Primary consumers eat producers, incorporating plant carbon into their own tissues. Secondary consumers then obtain carbon by eating primary consumers. At each trophic level, some carbon is respired and released as CO₂, and some is passed along. When organisms produce waste or die, detritus feeders and decomposers break down the remaining organic carbon.
当生物取食时，碳通过食物链传递。初级消费者取食生产者，将植物碳纳入自身组织。次级消费者随后通过捕食初级消费者获取碳。在每个营养级，一部分碳被呼吸并释放为 CO₂，一部分继续传递。当生物排出废物或死亡时，食碎屑者和分解者会分解剩余的有机碳。
6. Decomposition by Microorganisms | 微生物的分解作用
Decomposition is carried out by bacteria and fungi (saprotrophs). They secrete enzymes onto dead organic matter, breaking it down and absorbing the soluble products. During this process, microorganisms respire, releasing CO₂ back into the atmosphere. Decomposition is crucial for recycling carbon locked in dead organisms and waste. Conditions such as warmth, moisture and oxygen availability affect the rate of decay.
分解由细菌和真菌（腐生生物）完成。它们向死亡有机物分泌酶，将其分解并吸收可溶性产物。在此过程中，微生物进行呼吸，将 CO₂ 释放回大气。分解对于再循环封闭在死亡生物和废物中的碳至关重要。温度、湿度和氧气等条件会影响腐烂速率。
7. Combustion: Rapid Carbon Release | 燃烧：碳的快速释放
Combustion (burning) of organic material rapidly releases carbon dioxide. When wood, coal, oil or natural gas is burned in the presence of oxygen, carbon is oxidised to CO₂. This process returns ancient carbon to the atmosphere much faster than natural decomposition. Combustion is used by humans for energy production and in industries, contributing significantly to the rise in atmospheric CO₂.
有机物的燃烧（点燃）会迅速释放二氧化碳。当木材、煤、石油或天然气在有氧条件下燃烧时，碳被氧化成 CO₂。这一过程将远古碳以比自然分解快得多的速度归还大气。燃烧被人类用于能源生产和工业中，显著导致大气 CO₂ 浓度上升。
C + O₂ → CO₂
Burning methane (natural gas) also releases CO₂: CH₄ + 2O₂ → CO₂ + 2H₂O.
C + O₂ → CO₂
燃烧甲烷（天然气）也释放 CO₂：CH₄ + 2O₂ → CO₂ + 2H₂O。
8. Formation of Fossil Fuels | 化石燃料的形成
Fossil fuels – coal, oil and natural gas – are formed over millions of years from the remains of once-living organisms. Coal forms from dead plants that were buried under sediment and subjected to high heat and pressure in the absence of oxygen. Oil and natural gas mainly form from marine plankton and microorganisms deposited on the ocean floor, similarly transformed under layers of rock. This process locks carbon away from the short-term cycle, creating long-term carbon stores.
化石燃料——煤、石油和天然气——是由曾经的生命体遗骸经过数百万年形成的。煤由死亡的植物在沉积物下掩埋，在缺氧和高温高压作用下形成。石油和天然气主要由沉积在海底的海洋浮游生物和微生物，在岩层下经过类似转变而成。这一过程将碳从短期循环中锁住，形成长期碳库。
9. The Role of Oceans | 海洋的作用
Oceans play a major role in storing and exchanging carbon. Carbon dioxide dissolves directly from the atmosphere into the ocean surface. Some is used by marine algae and phytoplankton for photosynthesis. Additionally, CO₂ reacts with water to form carbonic acid (H₂CO₃), which dissociates into hydrogen carbonate ions (HCO₃⁻). These ions eventually contribute to carbonate sediments and shells. Oceans act as a carbon sink, absorbing about a quarter of anthropogenic CO₂ emissions.
海洋在碳的储存和交换中扮演重要角色。二氧化碳从大气直接溶解进入海洋表面。一部分被海洋藻类和浮游植物用于光合作用。此外，CO₂ 与水反应生成碳酸（H₂CO₃），碳酸解离成碳酸氢根离子（HCO₃⁻）。这些离子最终形成碳酸盐沉积物和贝壳。海洋起着碳汇的作用，吸收了大约四分之一的人为 CO₂ 排放。
10. Deforestation and Human Impact | 森林砍伐与人类影响
Deforestation disrupts the carbon cycle in two main ways. First, trees that absorb CO₂ for photosynthesis are removed, reducing the biosphere's capacity to fix carbon. Second, many forests are cleared by burning, which instantly releases stored carbon as CO₂. Additionally, the soil exposed after deforestation often releases carbon through decomposition and erosion. Agriculture and industrial activities further add CO₂ via machinery and fertiliser production.
森林砍伐通过两种主要途径扰乱碳循环。首先，进行光合作用吸收 CO₂ 的树木被移除，降低了生物圈固碳的能力。其次，许多森林通过焚烧被清理，立即将储存的碳以 CO₂ 形式释放。此外，砍伐后裸露的土壤常通过分解和侵蚀释放碳。农业和工业活动通过机械和化肥生产进一步增加 CO₂。
11. The Greenhouse Effect and Climate Change | 温室效应与气候变化
Increased CO₂ and other greenhouse gases (methane, water vapour) trap infrared radiation from the Earth, causing global warming. This enhanced greenhouse effect is predominantly driven by human activities: burning fossil fuels, deforestation and livestock farming. Consequences include rising sea levels, extreme weather and disruption of ecosystems. Understanding the carbon cycle highlights why balancing emissions is critical for limiting climate change.
增加的 CO₂ 和其他温室气体（甲烷、水蒸气）截留来自地球的红外辐射，导致全球变暖。这种增强的温室效应主要由人类活动驱动：燃烧化石燃料、砍伐森林和畜牧业。后果包括海平面上升、极端天气和生态系统破坏。理解碳循环说明为何平衡排放对遏制气候变化至关重要。
12. Exam Tips for Carbon Cycle Questions | 碳循环考题应试技巧
OCR IGCSE exam questions often ask you to: label or draw a simple carbon cycle diagram with arrows; explain the role of photosynthesis and respiration in the cycle; describe how deforestation and burning fossil fuels increase atmospheric CO₂; link decomposition to microorganisms and conditions; and evaluate ways to reduce carbon emissions. Always refer to key terms: carbon sink, combustion, decomposition, respiration, fossil fuel. Use equations to gain marks, and remember that oceans and limestone are long-term stores.
OCR IGCSE 考题经常要求：绘制或标注带箭头的简单碳循环图；解释光合作用和呼吸作用在循环中的作用；描述森林砍伐和燃烧化石燃料如何增加大气中的 CO₂；将分解作用与微生物和环境条件联系起来；并评估减少碳排放的方法。始终引用关键术语：碳汇、燃烧、分解、呼吸作用、化石燃料。运用方程式可得分，并记住海洋和石灰岩是长期碳库。
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