IGCSE Biology glossary
Nitrogen cycle
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Definition
The biogeochemical cycle in which nitrogen is converted between various chemical forms as it circulates through the atmosphere, soil and organisms. Key processes include nitrogen fixation, nitrification, denitrification and decomposition.
What to include in an exam answer
Know the roles of the bacteria: nitrogen-fixing bacteria (convert N2 to ammonium/NH4+), nitrifying bacteria (convert ammonium to nitrite then nitrate), denitrifying bacteria (convert nitrate back to N2). Plants absorb nitrogen as nitrate ions.
Example in context
Explain the role of decomposers in the nitrogen cycle (2 marks): Decomposers break down proteins and urea from dead organisms and animal waste into ammonium compounds. These ammonium compounds can then be converted to nitrates by nitrifying bacteria for plant uptake.
Related syllabus topic
Organisms & their environment