Concept preview

What you will be able to do

1. 01Explain why nutrients cycle through ecosystems while energy does not.
2. 02Describe the carbon cycle and link it to combustion, fossil fuels and climate change.
3. 03Describe the nitrogen cycle: fixation, nitrification, assimilation and denitrification.
4. 04Outline the water and phosphorus cycles and explain how humans disrupt these cycles.

The previous topic ended with a puzzle. Energy pours into an ecosystem from the Sun and is steadily lost as heat, so it must be constantly resupplied — but the chemical matter that living things are built from is never resupplied from space. The same atoms of carbon, nitrogen, oxygen, hydrogen and phosphorus that made the first cells billions of years ago are still here today, which means they must be used, released and used again in vast, endless loops. These loops are the biogeochemical cycles — "bio" for the living organisms, "geo" for the rocks, air and water, and "chemical" for the elements that pass between them. Each cycle moves an element out of a physical reservoir (the atmosphere, the oceans, the soil or the rocks), through living bodies, and eventually back to a reservoir again, keeping the supply of raw materials for life topped up forever.