This guide covers IGCSE Biology nutrient cycles (carbon and nitrogen). You will revise the carbon cycle (photosynthesis, respiration, feeding, decomposition, fossil fuels, combustion) and the nitrogen cycle (nitrogen fixation, nitrification, denitrification, decomposition, feeding).

Key concepts to revise

• Carbon cycle – CO2 in air; photosynthesis (plants take in CO2); feeding (carbon in food chains); respiration (CO2 released); decomposition (decay releases CO2); combustion (burning fossil fuels releases CO2).
• Nitrogen cycle – Nitrogen in air (N2); nitrogen-fixing bacteria (N2 to ammonia/nitrates); nitrifying bacteria (ammonia to nitrates); plants absorb nitrates; feeding; decomposers (break down dead matter to ammonia); denitrifying bacteria (nitrates to N2).
• Decomposers – Bacteria and fungi; break down dead matter; release nutrients (e.g. carbon, nitrogen) for reuse.

Exam tips and command words

• Describe – The carbon cycle or nitrogen cycle in steps (use arrows: e.g. CO2 to plants by photosynthesis).
• State the role – Of decomposers (break down dead matter; release nutrients); of nitrogen-fixing bacteria (convert N2 to nitrates).
• Explain – Why nutrient cycles are important (nutrients recycled; not lost; ecosystems sustained).
• Label – A diagram of the carbon or nitrogen cycle with the main processes.

Common mistakes

• Confusing nitrification (ammonia to nitrates) with denitrification (nitrates to N2).
• Not stating that decomposers (bacteria, fungi) break down dead matter (not living).
• Saying plants get nitrogen from the air (they get nitrates from the soil; nitrogen fixation happens in soil or root nodules).

Revision checklist

• Describe the carbon cycle (photosynthesis, respiration, feeding, decomposition, combustion).
• Describe the nitrogen cycle (fixation, nitrification, absorption by plants, feeding, decomposition, denitrification).
• State the role of decomposers and of nitrogen-fixing and nitrifying bacteria.
• Explain why nutrient cycles are important for ecosystems.

Next steps

Book a free trial with an IGCSE Biology tutor to practise nutrient cycle questions, or explore Tutopiya’s learning portal for more revision resources.