Carbon in living organisms

Plants take in carbon dioxide (

\ce{CO2}

) from the atmosphere during photosynthesis. They use its carbon to make organic substances, which become part of the plant. When an animal eats a plant, some of this carbon passes to the animal. Feeding can then transfer carbon to other organisms.

Plants, animals and decomposers all release carbon dioxide into the atmosphere through respiration. Carbon taken into living organisms can therefore return to the atmosphere while those organisms are alive.

Decomposition and fossil fuels

When organisms die or produce waste, decomposers break down the organic material and take in some of its carbon. Decomposers release carbon dioxide through respiration, returning carbon to the atmosphere.

Some remains are buried rather than fully decomposed. Over millions of years, some of this buried material may form fossil fuels, storing its carbon. When fossil fuels burn, their stored carbon is released into the atmosphere as carbon dioxide. This process is called combustion.

Nitrogen cycle

Nitrogen moves between the atmosphere, soil and living organisms in a cycle. It changes chemical form along the way: for example, nitrogen gas in the air can become nitrogen-containing ions in the soil, then form part of proteins in living organisms.

Nitrogen entering the soil

Most plants cannot use nitrogen gas, $\ce{N2}$ , directly from the atmosphere. Nitrogen-fixing bacteria convert atmospheric nitrogen into ammonium compounds. Lightning also causes atmospheric nitrogen to form compounds that enter the soil as nitrates.

Nitrifying bacteria convert ammonium ions, $\ce{NH4+}$ , into nitrite ions, $\ce{NO2-}$ , and then into nitrate ions, $\ce{NO3-}$ . These conversions make nitrogen available to plants as nitrate.

Nitrogen in living organisms

Plants absorb nitrate ions from the soil and use the nitrogen to make amino acids, which they use to build proteins. When animals feed on plants or other animals, proteins are transferred in their food. Digestion breaks dietary proteins down into amino acids, which animals can use to make their own proteins.

Nitrogen returning to the soil and atmosphere

When organisms die or produce waste, decomposer microorganisms break down proteins in the dead material and waste. This releases ammonium ions into the soil, where nitrifying bacteria can convert them into nitrites and then nitrates again.

Animals also remove the nitrogen-containing part of excess amino acids in a process called deamination. This produces nitrogen-containing waste that can return to the environment.

Denitrifying bacteria convert nitrate ions in the soil into nitrogen gas. The gas returns to the atmosphere, where nitrogen can enter the cycle again.