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The Carbon Cycle

Follow carbon through photosynthesis, respiration, decay and combustion.

  • Define and explain The Carbon Cycle in your own words
  • Use key terms such as ecosystem accurately
  • Apply what you have learned to new examples and questions
  • Avoid the common mistakes learners make with this topic

This lesson focuses on The Carbon Cycle: follow carbon through photosynthesis, respiration, decay and combustion.

Definition: The Carbon Cycle

Follow carbon through photosynthesis, respiration, decay and combustion.

Key ideas

The carbon cycle

Carbon dioxide is removed from the air by photosynthesis and locked into biomass. It returns through respiration by plants, animals and decomposers, through combustion of wood and fossil fuels, and through decay. Burning fossil fuels releases carbon stored millions of years ago, raising atmospheric carbon dioxide faster than natural processes can reabsorb it.

Pyramids of biomass and energy loss

A pyramid of biomass shows the dry mass at each trophic level, and it is almost always pyramid-shaped because biomass is lost between levels. Only about 10% of energy is transferred: the rest is lost through respiration and movement, in uneaten parts, and in waste such as faeces and urine. This is why food chains rarely have more than five trophic levels — there is too little energy left.

Key term — ecosystem: A community of organisms together with their physical environment, interacting as a system.

Worked example: The Carbon Cycle

Describe two processes that return carbon dioxide to the atmosphere.

Any two of: respiration by plants, animals and decomposers; combustion of fossil fuels or wood; decay releasing carbon dioxide.

Answer: Any two of: respiration by plants, animals and decomposers; combustion of fossil fuels or wood; decay releasing carbon dioxide.

Common mistakes
  • Confusing the carbon cycle with the water cycle The carbon cycle moves carbon between the air, living things and fossil stores; the water cycle moves water. Photosynthesis, respiration and combustion belong to the carbon cycle.
  • Saying decomposers are not needed Without decomposers, nutrients would stay locked in dead bodies and waste. Decomposers recycle them into the soil for producers to reuse.

Practice

Give two human activities that increase atmospheric carbon dioxide and one way to reduce them.
Think about burning and trees.

Burning fossil fuels for energy and transport, and deforestation (which also removes photosynthesis). Reduction: any one of using renewable energy, improving energy efficiency, or reforestation.

Give three ways energy is lost between trophic levels.
Think about what the animal does and leaves behind.

Any three of: respiration (released as heat); movement; uneaten parts such as roots or bones; waste products such as faeces and urine.

In the food chain grass → rabbit → fox → flea, identify the producer, a primary consumer and a secondary consumer.
Count the trophic levels from the producer.

Producer: grass. Primary consumer: rabbit. Secondary consumer: fox (the flea is a tertiary consumer).

Explain why a food web gives a better picture than a single food chain.
How many food sources does a fox have?

Most organisms eat several different species and are eaten by several predators, so a web shows the true feeding relationships and how species depend on one another.

Quick check

The Carbon Cycle — quick check

Which of these best defines "ecosystem"?

A community of organisms together with their physical environment, interacting as a system.

Why do food chains rarely have more than five trophic levels?

Only about 10% of energy transfers at each level, so after several transfers too little energy remains to support another trophic level.
Key takeaways
  • The Carbon Cycle: follow carbon through photosynthesis, respiration, decay and combustion.
  • The carbon cycle: Carbon dioxide is removed from the air by photosynthesis and locked into biomass.
  • trophic level: A feeding level in a food chain or web, such as producer, primary consumer or secondary consumer.
  • Watch out for: confusing the carbon cycle with the water cycle
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