Nutrient cycles Revision Notes

Plants absorb carbon dioxide from the atmosphere during photosynthesis and convert it into organic molecules like glucose.

Both plants and animals release carbon dioxide back into the atmosphere through respiration, breaking down organic molecules for energy.

• Photosynthesis removes CO2 from air and stores carbon in plants.
• Respiration releases CO2 back into the atmosphere.

Animals obtain carbon by eating plants or other animals, transferring carbon through food chains.

When organisms die, decomposers break down their bodies, returning carbon to the soil and atmosphere.

• Feeding transfers carbon through trophic levels.
• Decomposition recycles carbon from dead matter.

Over millions of years, some carbon from dead organisms forms fossil fuels underground.

Burning fossil fuels during combustion releases stored carbon dioxide back into the atmosphere, completing the cycle.

• Fossil fuels store carbon long-term.
• Combustion releases CO2, increasing atmospheric carbon.

When plants and animals die or produce waste, their proteins break down into ammonium ions.

Nitrifying bacteria convert ammonium ions into nitrites and then nitrates in a process called nitrification, making nitrogen available to plants.

• Proteins decompose into ammonium ions.
• Nitrification converts ammonium to nitrites and nitrates.

Nitrogen fixation converts nitrogen gas from the atmosphere into nitrates, occurring naturally via lightning and nitrogen-fixing bacteria in soil.

Plants absorb nitrate ions through their roots and use them to make amino acids and proteins.

• Nitrogen fixation by lightning and bacteria produces nitrates.
• Plants take up nitrates to build proteins.

Animals obtain nitrogen by eating plants or other animals and digest proteins into amino acids.

Excess amino acids undergo deamination, producing ammonium ions that return to the soil.

Denitrifying bacteria convert nitrates back into nitrogen gas, releasing it into the atmosphere and completing the cycle.

• Animals digest proteins into amino acids.
• Deamination produces ammonium ions from excess amino acids.
• Denitrification returns nitrogen gas to the atmosphere.

Microorganisms break down dead plants and animals, releasing nitrogen as ammonium compounds into the soil. This process recycles nitrogen from organic matter back into forms usable by other organisms.

• Decomposers release ammonium from dead organic matter.
• This makes nitrogen available for further processing.

Other microorganisms convert ammonium first into nitrites and then into nitrates. Nitrates are the form of nitrogen that plants can absorb and use for growth.

• Convert ammonium to nitrites and nitrates.
• Nitrates are absorbed by plants.

Certain microorganisms convert nitrogen gas from the atmosphere into ammonia. This process makes atmospheric nitrogen available to living organisms, which cannot use nitrogen gas directly.

• Convert nitrogen gas into ammonia.
• Provide usable nitrogen to ecosystems.

Microorganisms convert nitrates back into nitrogen gas, releasing it into the atmosphere. This completes the nitrogen cycle and maintains nitrogen balance in the environment.

• Convert nitrates to nitrogen gas.
• Release nitrogen gas into the atmosphere.