Cycles within EcosystemsEdexcel IGCSE Biology: Revision notes

The carbon cycle describes how carbon atoms move between living organisms, the atmosphere, and non-living stores.

• Photosynthesis: green plants and algae remove carbon dioxide from the air and convert it into carbohydrates (glucose), locking carbon into living tissue
• Respiration: all living organisms (plants, animals, decomposers) release carbon dioxide back into the atmosphere as they break down glucose for energy
• Feeding: carbon compounds pass from producers to consumers along food chains as organisms eat each other
• Decomposition: when organisms die, decomposers (bacteria and fungi) break down the carbon compounds in their bodies, releasing carbon dioxide through their own respiration
• Combustion: burning fossil fuels (formed from the remains of dead organisms over millions of years) and burning wood releases carbon dioxide that had been locked away

It helps to sort the carbon cycle processes into two groups.

Over long timescales these processes balance, keeping atmospheric carbon dioxide roughly constant. Human combustion of fossil fuels has upset this balance (see Human Influences on the Environment).

Nitrogen makes up 78% of the atmosphere as unreactive nitrogen gas (N2), but most organisms cannot use it directly. The nitrogen cycle describes how nitrogen is converted into usable forms and cycled through ecosystems.

• Plants need nitrogen (as nitrate ions) to build amino acids, proteins and DNA, but cannot use nitrogen gas directly
• Animals obtain nitrogen compounds by eating plants or other animals
• Nitrogen compounds are returned to the soil when organisms die and excrete waste

Four types of bacteria drive the nitrogen cycle:

1. Nitrogen-fixing bacteria — convert nitrogen gas from the air directly into nitrogen compounds that plants can use; found freely in soil and in root nodules of legume plants
2. Decomposers — break down proteins and urea in dead organisms and waste into ammonia/ammonium compounds
3. Nitrifying bacteria — convert ammonium compounds into nitrites, then into nitrates, which plant roots absorb
4. Denitrifying bacteria — convert nitrates back into nitrogen gas, releasing it into the atmosphere, especially in waterlogged soils

Decomposers (mainly bacteria and fungi) are central to both cycles. They break down dead organisms and waste products, and in doing so:

• Release carbon dioxide into the atmosphere through their own respiration (carbon cycle)
• Release ammonia/ammonium compounds from proteins and urea, which nitrifying bacteria then convert to nitrates (nitrogen cycle)

Without decomposers, nutrients would remain locked in dead matter and could not be recycled for new growth.