Carbon Cycle

OCR

GCSE

The carbon cycle describes the continuous biogeochemical movement of carbon between the atmosphere, biosphere, geosphere, and hydrosphere. Carbon dioxide is sequestered from the atmosphere by producers via photosynthesis to synthesize complex organic molecules such as glucose, starch, and proteins. This carbon is returned to the atmosphere through aerobic respiration in plants, animals, and decomposers, and via the combustion of fossil fuels. The cycle relies heavily on microorganisms (bacteria and fungi) for decomposition, ensuring that essential materials are recycled to support new life.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating that photosynthesis is the only process that removes carbon dioxide from the atmosphere to produce carbon compounds (glucose)
Credit responses that identify respiration in plants, animals, and decomposers as the mechanism returning carbon dioxide to the atmosphere
Award 1 mark for explaining that decomposers (bacteria and fungi) secrete enzymes to break down dead organic matter, releasing carbon dioxide via respiration
Candidates must link the combustion of fossil fuels to the release of carbon that was previously 'locked up' in biomass
Award 1 mark for describing the formation of fossil fuels from the remains of dead plants and animals over millions of years

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified photosynthesis, but remember to specify that it produces 'carbon compounds' or 'glucose', not just 'energy'"
"Avoid using 'breathing' in this context; the correct biological term for the release of CO2 from cells is 'respiration'"
"You mentioned decay, but to access higher marks, you must explain that microorganisms respire while breaking down the material"
"Good use of the diagram. Now, try to evaluate how deforestation would change the size of the carbon reservoirs shown"

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating that photosynthesis is the only process that removes carbon dioxide from the atmosphere to produce carbon compounds (glucose)
Credit responses that identify respiration in plants, animals, and decomposers as the mechanism returning carbon dioxide to the atmosphere
Award 1 mark for explaining that decomposers (bacteria and fungi) secrete enzymes to break down dead organic matter, releasing carbon dioxide via respiration
Candidates must link the combustion of fossil fuels to the release of carbon that was previously 'locked up' in biomass
Award 1 mark for describing the formation of fossil fuels from the remains of dead plants and animals over millions of years

Examiner Tips

Expert advice for maximising your marks

💡In cycle diagrams, identify the arrow pointing FROM the atmosphere first; this is always photosynthesis
💡When explaining decomposition, you must link the breakdown of material to the respiration of the microorganisms involved to gain full credit
💡For 6-mark 'Level of Response' questions on climate change, structure your answer by comparing the rate of carbon removal vs. the rate of carbon addition
💡Ensure you distinguish between 'carbon' (the element moving through the cycle) and 'carbon dioxide' (the gas in the atmosphere)

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that plants only photosynthesize and do not respire, or implying they only respire at night
Using the term 'breathing' instead of 'respiration' when describing how carbon dioxide is released by animals
Failing to explicitly mention 'microorganisms', 'bacteria', or 'fungi' when describing decomposition, vaguely stating that dead matter 'rots'
Confusing the greenhouse effect with the ozone layer hole when discussing the consequences of increased atmospheric carbon dioxide

Key Terminology

Essential terms to know

Photosynthesis and carbon fixation

Aerobic respiration and carbon release

Decomposition by saprobionts

Combustion of fossil fuels

Biotic and abiotic carbon reservoirs

Likely Command Words

How questions on this topic are typically asked

State

Describe

Explain

Calculate

Evaluate

Predict

Practical Links

Related required practicals

•{"code":"PAG 4","title":"Investigation into the rate of decay","relevance":"Demonstrates the activity of decomposers which is central to carbon recycling"}

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