Carbon dioxide (C02) molecules are used in photosynthesis. In this process it helps to form energy-rich organic sugar compounds.

Carbon dioxide is returned to the environment through cell respiration.

C02 is returned when the energy is released by the cells.

Some carbon is also returned to the environment by the decomposition of dead organisms.

Oxygen (O2) is also required by many living things.

Oxygen is used to release the energy in their food.

This energy release is done during cell respiration.

Oxygen is released to the environment as a waste product of the process of photosynthesis.

Other compounds, such as nitrogen, are cycled in the environment.

Organisms synthesize proteins from simpler compounds and then return these nitrogen compounds to the environment when they die and decompose.