#145 Effects of combustion of fossil fuels on CO2 level

Photosynthesis takes CO₂ out of the atmosphere and replaces it with O₂. Respiration and combustion both do the opposite: they use up O₂ and replace it with CO₂.

#144 Nitrogen cycle

Nitrogen is essential for the formation of amino acids to make proteins. The nitrogen cycle describes the ways in which nitrogen is recycled.

#143 Nutrient cycles - Carbon and water cycles

Most of the chemicals that make up living tissue contain carbon. When organisms die the carbon is recycled so that it can be used by future generations. Four main processes are involved: photosynthesis, respiration, decomposition and combustion.