BRAINCAKEThe Nitrogen Cycle
Nitrogen is needed to make amino acids, proteins, nucleic acids (DNA and RNA) and ATP. The atmosphere is about 78% nitrogen gas (N2), but plants and animals cannot use it directly because the triple bond is very strong. Plants take up nitrogen from the soil as nitrate ions (NO3−) or ammonium ions; animals get it by eating plants or other animals. Bacteria carry out the key steps.
1. Nitrogen fixation: N2 is converted to ammonia (NH3), which forms ammonium ions (NH4+) in the soil. This is done by nitrogen-fixing bacteria, such as Rhizobium in root nodules and free-living Azotobacter. Lightning and the industrial Haber process also fix nitrogen.
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More in Ecosystems
- Ecological Efficiency
- Calculating Efficiency of Biomass Transfer
- Human Activities and Biomass Transfer
- Decomposers
- Nitrogen-Fixing Bacteria
- Nitrification and Denitrification
- The Carbon Cycle
- Succession
All 21 lessons in Ecosystems · All OCR A-level Biology topics