What is the primary difference between the biological and geological carbon cycles?

The biological cycle involves rapid exchanges between living organisms and the atmosphere (e.g., photosynthesis), while the geological cycle involves slow processes like rock formation and fossilisation over millions of years.

How does the uptake of carbon differ between terrestrial plants and aquatic organisms?

Terrestrial plants absorb gaseous $CO_{2}$ directly from the atmosphere through their leaves, whereas aquatic organisms utilize $CO_{2}$ that has dissolved in the surrounding water.

What distinguishes 'fossilisation' from 'decomposition' in the carbon cycle?

Decomposition is the rapid breakdown of organic matter by microbes that releases $CO_{2}$ immediately, while fossilisation occurs when lack of oxygen prevents decay, trapping carbon in the ground for millions of years.

Why is it a mistake to say that plants only remove carbon from the atmosphere?

While plants remove $CO_{2}$ during photosynthesis, they also release it back into the atmosphere through cellular respiration, which they perform constantly to stay alive.

What is the error in assuming that dead animals 'release' carbon dioxide directly into the air?

Dead matter itself does not release gas; rather, it is the respiration of the decomposers (bacteria and fungi) feeding on the dead matter that releases $CO_{2}$.

Why is it incorrect to view carbon as a limitless resource for ecosystems?

Carbon is a finite element on Earth; if it were not recycled from dead organisms and waste, the supply of available carbon for new growth would eventually be exhausted.

Define a 'Carbon Sink' in the context of the global cycle.

A carbon sink is any natural or artificial reservoir that absorbs and stores more carbon from the atmosphere than it releases, such as oceans or growing forests.

What is 'Carbon Fixation'?

Carbon fixation is the process by which inorganic carbon (usually $CO_{2}$) is converted into organic compounds (like sugars) by living organisms, primarily through photosynthesis.

What does the term 'Combustion' refer to in the carbon cycle?

Combustion is the chemical process of burning organic material (biomass or fossil fuels) in the presence of oxygen, which rapidly releases stored carbon as $CO_{2}$.

How do decomposers contribute to the carbon cycle?

Decomposers break down complex organic molecules in dead organisms and waste, using them for energy and releasing $CO_{2}$ back into the atmosphere through their own respiration.

1.6.5 The Carbon Cycle | Cambridge International Examinations AS-Level Environmental Management

Revision Notes

AS-Level

Cambridge International Examinations

Environmental Management

1. Introduction to Environmental Management

1.6.5 The Carbon Cycle

Summary

The carbon cycle is a fundamental biogeochemical process that ensures the continuous recycling of carbon atoms through the Earth's atmosphere, biosphere, hydrosphere, and geosphere. Since carbon is a finite resource and the primary building block of all organic life, its movement between living organisms and the physical environment is essential for sustaining ecosystems and regulating global temperatures.

1. Definition & Core Reservoirs

A diagram of the carbon cycle showing the flow of carbon between the atmosphere, producers, consumers, decomposers, and fossil fuels through processes like photosynthesis, respiration, feeding, and combustion.

2. Biological Fluxes: Photosynthesis & Respiration

3. Trophic Transfer & Decomposition

Feeding and Biomass Transfer: When consumers eat producers or other consumers, carbon is transferred along the food chain. This movement of carbon is essential for the growth and repair of tissues in all heterotrophic organisms.
The Role of Decomposers: Microorganisms like bacteria and fungi break down dead organic matter and waste products. As they consume this material for nutrition, they perform respiration, which returns the trapped carbon to the atmosphere.
Detritivores: Larger organisms that feed on dead matter, such as earthworms, assist decomposers by breaking material into smaller pieces. This increases the surface area for microbial action, accelerating the return of carbon to the cycle.

4. Long-term Storage: Fossilisation & Combustion

Fossilisation Requirements: If organic matter is buried in environments lacking oxygen or decomposers (such as peat bogs or deep ocean sediments), it does not fully decay. Over millions of years, heat and pressure transform this carbon into fossil fuels like coal, oil, and natural gas.
Combustion: The burning of fossil fuels or biomass (like wood) rapidly oxidizes stored carbon. This process releases large volumes of $CO_{2}$ into the atmosphere that had been sequestered for geological timescales, disrupting the modern cycle's equilibrium.
Carbon Sequestration: Natural processes that store carbon for long periods, such as the formation of limestone or the growth of old-growth forests, act as 'sinks' that help mitigate the accumulation of atmospheric greenhouse gases.

5. Key Distinctions

6. Exam Strategy & Tips