The Carbon Cycle | 碳循环

📚 The Carbon Cycle | 碳循环

Carbon is the backbone of life on Earth. It cycles continuously between the atmosphere, living organisms, oceans and rocks. This movement of carbon atoms, known as the carbon cycle, maintains a balance that makes the planet habitable. For IGCSE Edexcel Science students, understanding how carbon is added to and removed from the atmospheric reservoir is essential for explaining both natural ecosystems and human-driven climate change.

碳是地球上生命的骨架。它在大气、生物体、海洋和岩石之间不断循环。这种碳原子的运动被称为碳循环，维持着使地球适宜居住的平衡。对IGCSE Edexcel 科学课程的学生来说，理解碳如何进入和离开大气库是解释自然生态系统以及人类活动引发的气候变化的关键。

1. Why the Carbon Cycle Matters | 碳循环为何重要

All living things contain carbon compounds such as proteins, carbohydrates and fats. The carbon cycle describes how carbon atoms move from the environment into organisms and back again. Key reservoirs include the atmosphere (as carbon dioxide, CO₂), biomass, oceans, fossil fuels and carbonate rocks. Processes that remove CO₂ from the air are called carbon sinks, while those that add CO₂ are carbon sources.

所有生物都含有碳化合物，如蛋白质、碳水化合物和脂肪。碳循环描述了碳原子如何从环境中进入生物体再返回环境。主要的碳库包括大气（以二氧化碳 CO₂ 形式存在）、生物质、海洋、化石燃料和碳酸盐岩。从空气中移除 CO₂ 的过程称为碳汇，而增加 CO₂ 的过程则称为碳源。

2. Photosynthesis – A Vital Carbon Sink | 光合作用 – 重要的碳汇

Green plants, algae and some bacteria perform photosynthesis. They use light energy to combine carbon dioxide from the air with water, forming glucose and releasing oxygen. This process removes huge quantities of CO₂ from the atmosphere and locks carbon into organic form. The word equation is: carbon dioxide + water → glucose + oxygen. The balanced chemical equation for photosynthesis is:

绿色植物、藻类和一些细菌进行光合作用。它们利用光能将空气中的二氧化碳与水结合，生成葡萄糖并释放氧气。这一过程从大气中移除大量 CO₂，并将碳锁定为有机形式。光合作用的词方程式为：二氧化碳 + 水 → 葡萄糖 + 氧气。光合作用的化学方程式为：

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

3. Feeding – Passing Carbon Along Food Chains | 摄食 – 沿食物链传递碳

When animals eat plants (or other animals), they take in carbon compounds that were originally produced by photosynthesis. The carbon becomes part of the consumer’s body, stored in molecules such as cellulose, starch, glycogen, lipids and proteins. In this way, carbon atoms move through the food web, from producers to primary consumers and on to secondary and tertiary consumers.

动物在食用植物（或其他动物）时，摄入了最初由光合作用产生的碳化合物。这些碳成为消费者身体的一部分，储存在纤维素、淀粉、糖原、脂质和蛋白质等分子中。这样，碳原子通过食物网从生产者流向初级消费者，再流向次级和三级消费者。

4. Respiration – Releasing Carbon Dioxide | 呼吸作用 – 释放二氧化碳

All living organisms carry out respiration to release energy from glucose. Aerobic respiration uses oxygen to break down glucose, producing carbon dioxide and water as waste products. This returns CO₂ to the atmosphere. Even plants respire, although during daylight photosynthesis usually outweighs respiration. The balanced equation for aerobic respiration is essentially the reverse of photosynthesis:

所有生物都进行呼吸作用，以从葡萄糖中释放能量。有氧呼吸利用氧气分解葡萄糖，产生二氧化碳和水作为废物。这将 CO₂ 送回大气。植物也会呼吸，尽管在白天光合作用通常强于呼吸作用。有氧呼吸的化学方程式基本上是光合作用的逆反应：

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (+ energy)

5. Decomposition – Breaking Down Dead Organic Matter | 分解作用 – 分解死去的有机物

When organisms die, decomposers such as bacteria and fungi break down the carbon compounds in their remains. Microorganisms secrete enzymes that digest complex organic materials and release carbon dioxide through their own respiration. In warm, moist, aerobic conditions decomposition is rapid, making this a significant short-term carbon source. In waterlogged or acidic conditions, decomposition may slow down, leading to the accumulation of partly decomposed organic matter, which over millions of years can turn into fossil fuels.

当生物死亡时，细菌和真菌等分解者会分解其遗骸中的碳化合物。微生物分泌酶来消化复杂的有机物，并通过自身的呼吸作用释放二氧化碳。在温暖、潮湿、有氧的条件下，分解速度很快，使其成为一个重要的短期碳源。在浸水或酸性条件下，分解可能减慢，导致部分分解的有机物积累，经过数百万年可能转变为化石燃料。

6. Combustion – Rapid Release of Stored Carbon | 燃烧 – 储存碳的快速释放

Combustion (burning) of carbon-containing materials such as wood and fossil fuels immediately releases large amounts of CO₂ into the atmosphere. The burning of fossil fuels like coal, oil and natural gas has accelerated since the Industrial Revolution, injecting carbon that was locked away in geological reserves for millions of years into the atmosphere within decades. The general equation for the complete combustion of a hydrocarbon is:

燃烧含碳物质（如木材和化石燃料）会立即向大气释放大量 CO₂。自工业革命以来，煤、石油和天然气等化石燃料的燃烧加速，将数百万年来被锁在地质储层中的碳在几十年内注入大气。碳氢化合物完全燃烧的一般方程式为：

CₓHᵧ + O₂ → CO₂ + H₂O

7. Formation of Fossil Fuels | 化石燃料的形成

Fossil fuels are formed from the remains of ancient organisms that died millions of years ago. In the absence of oxygen, under high pressure and temperature, the carbon-rich material gradually transformed into coal (from land plants), crude oil and natural gas (from marine plankton). This process removed carbon from the atmosphere over geological timescales and stored it underground. Today, human extraction and burning of these fuels reverse that ancient carbon sink.

化石燃料是由数百万年前死亡的古代生物遗骸形成的。在缺氧、高压和高温条件下，富含碳的物质逐渐转变为煤（来自陆地植物）、原油和天然气（来自海洋浮游生物）。这一过程在漫长的地质年代中从大气中移除碳并将其储存在地下。如今，人类开采并燃烧这些燃料，正在逆转这个远古的碳汇。

8. The Ocean – A Major Carbon Sink | 海洋 – 一个主要的碳汇

Oceans absorb significant quantities of carbon dioxide from the atmosphere. CO₂ dissolves in seawater and reacts with water to form carbonic acid (H₂CO₃), which can dissociate into hydrogen carbonate ions (HCO₃⁻) and hydrogen ions (H⁺). Some of this dissolved carbon is used by marine photosynthetic organisms, while some carbonates eventually precipitate and form limestone (CaCO₃). This long-term geological sink locks carbon away in carbonate rocks, but increased atmospheric CO₂ also leads to ocean acidification, harming marine life.

海洋从大气中吸收大量的二氧化碳。CO₂ 溶解在海水中，与水反应生成碳酸 (H₂CO₃)，碳酸可解离为碳酸氢根离子 (HCO₃⁻) 和氢离子 (H⁺)。部分溶解的碳被海洋光合生物利用，而一些碳酸盐最终沉淀形成石灰岩 (CaCO₃)。这一长期地质碳汇将碳锁定在碳酸盐岩中，但大气 CO₂ 增加也会导致海洋酸化，危害海洋生物。

9. Human Impact and Disrupted Balance | 人类影响与平衡失调

Human activities are dramatically altering the carbon cycle. Deforestation reduces the number of trees available to photosynthesise, thus shrinking a critical biological carbon sink. At the same time, the burning of fossil fuels for energy and transport releases CO₂ far faster than it can be reabsorbed by natural sinks. The result is a rapid increase in atmospheric CO₂ concentration, which enhances the greenhouse effect and drives global warming. In IGCSE exams, you should be able to explain how these two human actions create a net addition of carbon to the atmosphere.

人类活动正在急剧改变碳循环。砍伐森林减少了能够进行光合作用的树木数量，从而缩小了一个关键的生物碳汇。与此同时，为了获取能源和交通动力而燃烧化石燃料所释放 CO₂ 的速度，远远超过自然碳汇重新吸收的速度。结果是大气 CO₂ 浓度迅速上升，强化了温室效应并推动了全球变暖。在 IGCSE 考试中，你应当能解释这两种人类行为如何造成大气中碳的净增加。

10. Interpreting Carbon Cycle Diagrams | 解读碳循环图

Edexcel IGCSE questions often provide a simplified diagram of the carbon cycle with arrows labelled by processes. You need to identify whether each arrow represents a carbon sink (e.g. photosynthesis, dissolving in oceans, formation of fossil fuels) or a carbon source (e.g. respiration, combustion, decomposition). Typical reservoirs shown are the atmosphere, biomass, oceans, fossil fuels and limestone. Practise counting the number of arrows leaving and entering the atmospheric box – an imbalance suggests a change in atmospheric CO₂ levels.

Edexcel IGCSE 考试经常会给出带标记箭头的简化碳循环图。你需要判断每条箭头代表的是碳汇（例如光合作用、溶解于海洋、化石燃料的形成）还是碳源（例如呼吸作用、燃烧、分解）。图中通常出现的碳库有大气、生物质、海洋、化石燃料和石灰岩。练习数一数离开和进入大气方框的箭头数量——不平衡即暗示大气 CO₂ 浓度发生了变化。

11. Summary Table of Key Processes | 关键过程汇总表

Process	Effect on atmospheric CO₂	Key equation or description
Photosynthesis	Removes CO₂ (sink)	6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Respiration	Adds CO₂ (source)	C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Combustion	Adds CO₂ (source)	Hydrocarbon + O₂ → CO₂ + H₂O
Decomposition	Adds CO₂ (source)	Microbial breakdown of dead organic matter
Ocean uptake	Removes CO₂ (sink)	CO₂ dissolves, forms carbonate ions
Fossil fuel formation	Removes CO₂ over geological time scales	Burial and compression of organic matter

12. Exam Tips for IGCSE Edexcel Science | IGCSE Edexcel 科学考试技巧

Always distinguish clearly between processes that add carbon dioxide to the atmosphere and those that remove it. Learn the balanced chemical equations for photosynthesis and respiration – marks are often awarded for correct formulas and balancing. When describing human impact, link a specific activity (e.g. burning fossil fuels) to a clear effect on the carbon cycle (increased atmospheric CO₂). Use the terms ‘carbon sink’ and ‘carbon source’ accurately. Finally, be prepared to interpret a diagram: label arrows, identify reservoirs and suggest consequences of deforestation or increased combustion.

务必清楚区分向大气中增加二氧化碳的过程和从大气中移除二氧化碳的过程。熟记光合作用与呼吸作用的化学方程式——正确的分子式和配平常能得分。描述人类影响时，要将具体活动（如燃烧化石燃料）与其对碳循环的明确影响（大气 CO₂ 增加）联系起来。准确使用“碳汇”和“碳源”术语。最后，要准备好解读示意图：标记箭头、识别碳库，并能够推论森林砍伐或燃烧增加可能带来的后果。

Published by TutorHao | Science Revision Series | aleveler.com

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