IGCSE Biology glossary
Nitrogen fixation
Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.
Definition
The conversion of atmospheric nitrogen gas (N2) into ammonium compounds or nitrates that can be used by plants. This is carried out by nitrogen-fixing bacteria, some of which live freely in the soil and others in root nodules of leguminous plants. Lightning can also fix nitrogen.
What to include in an exam answer
Nitrogen-fixing bacteria in root nodules of legumes (e.g. Peas, beans, clover) are a common exam example. The bacteria convert N2 into ammonia/ammonium ions, which the plant uses to make amino acids.
Example in context
Explain the role of nitrogen-fixing bacteria in root nodules (2 marks): Nitrogen-fixing bacteria convert atmospheric nitrogen gas into ammonium compounds. The plant uses these ammonium compounds to make amino acids for protein synthesis.
Related syllabus topic
Organisms & their environment