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Chapter 6 – Nutrient Cycles
Where does carbon enter the living component of an ecosystem?

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Carbon dioxide is an inorganic molecule taken in by photosynthetic producers and
used to produce organic macromolecules
When this producer is consumed by a primary consumer, the carbon containing
organic molecules are incorporated into the consumer
This continues so forth with each subsequent consumer in a food chain
When an organism dies, saprobiotic organisms decompose the body and release the
carbon back in its basic form
Where does carbon enter the non-living component of an ecosystem?

Carbon is present in the atmosphere, after being given out as carbon dioxide in respiration
by living organisms
...
This dissolved carbon dioxide is then used by aquatic photosynthetic
organisms
...
Over
millions of years carbon containing rocks build up from these organisms
...

What role is played by saprobiotic organisms in the carbon cycle?

Saprobiotic organisms are decomposers – they break down dead organic matter by
secreting enzymes onto the surface of the organism, which breaks the complex organic
molecules into smaller, soluble molecules which can be absorbed by diffusion
...


What is the greenhouse effect?

The greenhouse effect is the product of the Earth’s atmosphere
...
Of the energy passing through, some is absorbed by clouds and
reflected back towards earth
...

Which are the major greenhouse gases and where do they come from? Why is the production of greenhouse
gases increasing?

The three main greenhouse gases in the atmosphere are:
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Carbon dioxide – released by the combustion of fossil fuels/deforestation
Methane – released by decomposers/micro-organisms in the intestines of primary
consumers
Water vapour – vapour trails left by planes flying at very high altitudes/cooling plants
at power stations

A growing population has increased the levels of greenhouse gases through a rise in
intensive farming of primary consumers (e
...
cattle) as a food source – releasing methane
...

How do greenhouse gases contribute to global warming? What are the consequences of global warming?

Greenhouse gases absorb a greater proportion of the reflected infra-red, causing their
bonds to vibrate more frequently and release this energy as kinetic energy in the form of
heat
...
Plants may also bloom
before insects are available to pollinate them, meaning they cannot reproduce
...
Changes to the
environment disrupt each niche of an organism, which is adapted to certain conditions
within a certain range
...

Where does nitrogen enter the living/non-living components of the eco-system?

Nitrogen is assimilated by plants (using active transport) as nitrate ions (NO3-) to form
nitrogen-containing compounds such as DNA/RNA and amino acids to form proteins
...

Alternatively, nitrogen can enter soil/plants with nitrogen fixation
...
This can either be utilised by the plants (in mutualistic bacteria) or
released into the soil (free bacteria)
...

How is nitrogen recycled in ecosystems? What is the role of saprobiotic organisms in this recycling?

Nitrogen contained in organic compounds in living organisms can be released back into the
soil when the organisms die
...

Nitrification of these ammonium ions is carried out by nitrifying bacteria in two stages to
oxidise the ammonium ions into nitrates
...

−
𝑁𝑂3 → 𝑁2 (Nitrogen gas)

The lower the oxygen content of the soil, the greater the quantity of anaerobic denitrifying
bacteria and the lower the quantity of aerobic nitrifying/nitrogen fixing bacteria
...

What do you understand by the terms ammonification, nitrification, nitrogen fixation and denitrification?

Ammonification – the conversion of organic nitrogen-containing compounds into
ammonium ions by saprobiotic organisms
Nitrification – the oxidation of ammonium ions into nitrites and then nitrates by nitrifying
bacteria
Nitrogen fixation – the conversion of gaseous nitrogen into nitrogen containing compounds
by nitrogen-fixing bacteria
Denitrification – the reduction of nitrates into gaseous nitrogen by denitrifying bacteria
Why are fertilisers needed in agricultural ecosystems?

Agricultural ecosystems often involve intensive farming which involves the extraction of
large quantities of nutrients from the same area of land repeatedly over years
...
With low levels of nutrients, productivity will decrease
...

How do natural and artificial fertilisers differ?

Natural fertilisers are made up using dead and decaying animals/plants, as well as faeces
such as manure
...

Artificial fertilisers are made up using mined deposits of minerals such as nitrogen and
phosphorus and blended to give the desired mineral blend
...

How do fertilisers increase productivity?

Plants require nutrients for growth; these fertilisers ensure the nutrients are in constant
supply
...
This allows for greater growth, leading to increased leaf surface area,
leading to greater rates of photosynthesis and therefore higher net productivity of the crop
...
Water soluble nutrients are
often washed into water sources such as rivers and lakes by rain/irrigation
...
Whilst this is a
natural process, it is increasingly caused by the use of nitrogen containing fertilisers:
1
...
This algal bloom coats the surface layer of the water, blocking light from reaching
the photosynthetic plants below
3
...
Saprobiotic organisms break down the dead plant matter aerobically, rapidly
increasing their oxygen intake and thus the BOD – releasing nitrates
5
...
Without the presence of aerobic organisms, the anaerobic organism populations rise
exponentially to further decompose the dead material – releasing toxic compounds
into the water so it becomes putrid



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