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It's likely that everyone has some carbon atoms in their body that were once inside Charles Darwin or Marie Curie, or any other individual that is now dead. Atoms exist as part of different compounds and cycle between them through an ecosystem. The materials cycle between the biotic and abiotic components of an ecosystem. The biotic components are the living parts, such as plants, and the abiotic components are the non-living parts, such as the soil.

This cycling is seen in the elements carbon and nitrogen, and in the compound water. Just as rocks can cycle between igneous, sedimentary and metamorphic, carbon and water can exist in different forms at different times.

The nitrogen cycle

Many humans eat protein in the form of meat from other animals. Our bodies break down this protein into its constituent parts, called amino acids. These amino acids are used to make proteins inside our bodies, for example, in order to repair damaged tissue. When we eventually die our proteins are broken down into the amino acid building blocks and often returned to the soil to be used by other living organisms, such as plants.

The importance of microorganisms

Decomposing bacteria and fungi are microorganisms that play an important role in breaking down dead organisms. Microorganisms help return minerals and nutrients back to the environment so that the materials can then be used by other organisms. As the bacteria and fungi decompose dead matter, they also respire and so release carbon dioxide to the environment, contributing to the carbon cycle.