• Matter moves between the biotic and abiotic environments in a constant cycle.
• Different nutrients contain different vital elements and all are cycled in different ways.
• In the carbon cycle, carbon enters the biotic environment through photosynthesis.
• Other organisms eat the plants for carbon.
• Both plants and animals respire which releases carbon back into the atmosphere as carbon dioxide.
• Organic material is returned to the soil through excretion, death and decay which decomposers break down into humus.
• Detritivores and saprophytes feed on this humus and as they feed they respire the carbon as CO2 back into the atmosphere.
• The nitrogen cycle involves five main processes: assimilation, ammonification, nitrification, dentrification and nitrogen fixation.
• N2 gas found in the atmosphere is fixed into the biotic environment by nitrogen fixing bacteria.
• Ammonification converts organic waste into ammonium ions which is then converted into nitrates and nitrites by nitrification.
• Plants absorb and assimilate nitrates which are passed onto the animals that eat the plants.
• Nitrogen returns to the soil through excretion and death and the nitrites and nitrates are converted back into N2 gas by denitrifying bacteria.
• Carbon dioxide and methane are both greenhouse gases.
• An increase in greenhouse gases has led to global warming which has had a negative impact on plant and animal species.
• Artificial fertilisers leach into the soil which can lead to eutrophication.

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TEST IT!

1.

Deforestation has had effects on the amount of carbon dioxide in the atmosphere.

a) Describe how and why.

b) Name another way in which carbon dioxide has increased in the atmosphere.

2.

Different nutrients contain different vital elements and all are cycled in different ways.

a) Describe how carbon is cycled in the carbon cycle.

b) What is a fossil fuel?

c) How has burning fossil fuels affected the natural balance of the carbon cycle?

3.

The nitrogen cycle can be split up into five main processes.

a) At which part of the nitrogen cycle do the following organisms appear:

i. saprophytes
ii. nitrogen fixing bacteria
iii. chemoautotrophic bacteria

b) Which enzyme fixes nitrogen gas into the biotic environment?

ANSWERS

1.

a) Trees absorb a lot of carbon dioxide and act as a carbon sink. By cutting large numbers of them down this ability is lost and they become instead a carbon source. In addition, any carbon stored in the tree is released back into the carbon cycle.

b) Combustion

2.

a) Carbon enters the biotic environment through photosynthesis in which photosynthetic organisms use sunlight energy to transform carbon dioxide into carbohydrates. When other organisms eat the plants they take up this carbon. Both plants and animals respire which releases carbon back into the atmosphere as carbon dioxide. Combustion also releases carbon dioxide. Organic material is returned to the soil through excretion, death and decay. Decomposers then break this down into humus. Detritivores and saprophytes feed on this humus and as they feed they respire the carbon as carbon dioxide back into the atmosphere.

b) Fossil fuels are fuels created by natural processes like the anaerobic decomposition of buried dead organisms.

c) Naturally, the carbon contained in these fossil fuels would be stored for millions of years. However, through combustion we’ve released this stored carbon as CO2 into the atmosphere.

3.

a)

i. ammonification
ii. nitrogen fixation
iii. nitrification

b) nitrogenase