19.3 Nutrient cycles
- Syllabus
- 0610–2026–2027
- Topic
- 19.3
- Level
- —
Learning objectives
19.3.1Carbon cycle• Describe the carbon cycle, limited to: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion19.3.2Nitrogen cycle with reference to: •• Describe the nitrogen cycle with reference to: • decomposition of plant and animal protein to ammonium ions • nitrification • nitrogen fixation by lightning and bacteria • absorption of nitrate ions by plants • production of amino acids and proteins • feeding and digestion of proteins • deamination • denitrification19.3.3Roles of microorganisms• State the roles of microorganisms in the nitrogen cycle, limited to: decomposition, nitrification, nitrogen fixation and denitrification (generic names of individual bacteria, e.g. Rhizobium, are not required)
Trace carbon through six cycle processes
Photosynthesis removes carbon dioxide from the atmosphere and incorporates its carbon into organic molecules in plants.
Feeding transfers carbon-containing compounds from plants to animals. Respiration by plants, animals and decomposers releases carbon dioxide back to the atmosphere.
After death and waste production, decomposition returns carbon through decomposers and their respiration. Some dead organic material forms fossil fuels over very long periods; combustion of these fuels releases carbon dioxide to the atmosphere.
Carbon cycles between stores, but the six required processes have specific directions: photosynthesis removes atmospheric carbon dioxide, while respiration and combustion add it.
Trace nitrogen through its chemical forms
Atmospheric nitrogen gas becomes usable nitrogen compounds through nitrogen fixation by lightning or bacteria. Nitrification converts ammonium ions through nitrite ions into nitrate ions.
Plant roots absorb nitrate ions. Plants use the nitrogen to produce amino acids and then proteins. Feeding transfers protein to animals; digestion breaks dietary proteins into amino acids that can be used to make animal proteins.
Decomposition converts nitrogen in dead plant and animal protein to ammonium ions. In animals, deamination removes the amino group from excess amino acids; nitrogen-containing waste can return through decomposition. Denitrification converts nitrate ions back to nitrogen gas in the atmosphere.
Key direction: nitrogen gas → fixed nitrogen compounds → ammonium/nitrate ions → plant amino acids and proteins → animal proteins → ammonium ions → nitrate ions → nitrogen gas.
Plants absorb nitrate ions, not atmospheric nitrogen gas or ready-made protein. Nitrification forms nitrates; denitrification removes nitrates and returns nitrogen gas.
Assign four microbial roles in the nitrogen cycle
Microorganisms drive four required conversions in the nitrogen cycle; individual bacterial genus names are not required.
| Microbial role | Starting material | Product or effect |
|---|---|---|
| decomposition | protein in dead organisms and waste | ammonium ions |
| nitrification | ammonium ions, through nitrite ions | nitrate ions |
| nitrogen fixation | nitrogen gas | nitrogen compounds available to enter the cycle |
| denitrification | nitrate ions | nitrogen gas returned to the atmosphere |
Identify the role from arrow direction and chemical form: towards nitrate is nitrification; from nitrate to atmospheric nitrogen is denitrification; from atmospheric nitrogen into compounds is fixation.
Do not reverse fixation and denitrification. Decomposition returns organic nitrogen to ammonium ions; it is not the same conversion as nitrification.