The nitrogen cycle
The nitrogen cycle (Supplement)
- The nitrogen cycle recycles nitrogen for making proteins. Microorganisms do most of the work.
The key steps
- decomposition — decomposers break dead protein into ammonium ions.
- nitrification — bacteria change ammonium ions into nitrate ions.
- nitrogen fixation — lightning and some bacteria turn nitrogen gas into compounds plants can use.
- plants absorb nitrate ions to make amino acids and proteins; animals get them by feeding.
- denitrification — some bacteria turn nitrate ions back into nitrogen gas.
Practice
Nitrogen fixation is when:
Nitrogen fixation converts nitrogen gas into usable nitrogen compounds; denitrification does the reverse.
Practice
Plants take up nitrogen from the soil mainly as:
Plants absorb nitrate ions and use them to build amino acids and proteins.
Practice
Microorganisms carry out most of the steps of the nitrogen cycle.
Bacteria do decomposition, nitrification, nitrogen fixation and denitrification — most of the cycle.
You've got it
Key idea
- the nitrogen cycle recycles nitrogen for proteins, driven by microorganisms (Supplement)
- nitrogen fixation (gas → compounds) and nitrification (ammonium → nitrate) make nitrogen usable
- plants take up nitrate ions; denitrification returns nitrogen gas to the air