19.3 Nutrient cycles
IGCSE Biology
Name………………………………….Class…………..
Nutrient cycles
The element carbon is present in all living organisms. It is recycled through various processes,
which are described in the carbon cycle.
Nitrogen is essential for the formation of amino acids to make proteins. The nitrogen cycle
describes the ways in which nitrogen is recycled.
Carbon cycle
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. The model that describes the
processes involved is called the carbon cycle.
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19.3 Nutrient cycles
IGCSE Biology
1. Carbon enters the atmosphere as carbon dioxide from respiration and combustion.
2. Carbon dioxide is absorbed by producers to make carbohydrates in photosynthesis.
3. Animals feed on the plant passing the carbon compounds along the food chain. Most of
the carbon they consume is exhaled as carbon dioxide formed during respiration. The
animals and plants eventually die.
4. The dead organisms are eaten by decomposers and the carbon in their bodies is
returned to the atmosphere as carbon dioxide. In some conditions decomposition is
blocked. The plant and animal material may then be available as fossil fuel in the future
for combustion.
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19.3 Nutrient cycles
IGCSE Biology
The effects of the combustion of fossil fuels and the cutting down of
forests on the carbon dioxide concentrations in the atmosphere
Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In
2013, CO2 accounted for about 82% of all U.S. greenhouse gas emissions from human
activities. Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon
cycle (the natural circulation of carbon among the atmosphere, oceans, soil, plants, and
animals). Human activities are altering the carbon cycle—both by adding more CO2 to the
atmosphere and by influencing the ability of natural sinks, like forests, to remove CO2 from
the atmosphere. While CO2 emissions come from a variety of natural sources, human-related
emissions are responsible for the increase that has occurred in the atmosphere since the
industrial revolution.
The main human activity that emits CO2 is the combustion of fossil fuels (coal, natural gas, and
oil) for energy and transportation, although certain industrial processes and land-use changes
also emit CO2. The main sources of CO2 emissions in the United States are described below.
Electricity. Electricity is a significant source of energy in the United States and is used to power
homes, business, and industry. The combustion of fossil fuels to generate electricity is the
largest single source of CO2 emissions in the nation. The type of fossil fuel used to generate
electricity will emit different amounts of CO2. To produce a given amount of electricity,
burning coal will produce more CO2 than oil or natural gas.
Transportation. The combustion of fossil fuels such as
gasoline and diesel to transport people and goods is the
second largest source of CO2 emissions.
This category includes transportation sources such as
highway vehicles, air travel, marine transportation, and rail.
Industry. Many industrial processes emit CO2 through
fossil fuel combustion. Several processes also produce
CO2 emissions through chemical reactions that do not
involve combustion, for example, the production and consumption of mineral products such as
cement, the production of metals such as iron and steel, and the production of chemicals.
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19.3 Nutrient cycles
IGCSE Biology
Deforestation is the conversion of forested areas to non-forest land use See Picture 1.1). It
can be the result of the deliberate removal
development, or it can be a consequence
of forest cover for
agriculture or urban
of
grazing animals, wild or domesticated.
Deforestation has thus many causes. Population
pressures, profits, and
internal social and
political forces can also all push up the rate of
forest loss. Generally the removal or destruction
of significant areas of forest cover has resulted in
a degraded environment with
reduced biodiversity.
Moreover, deforestation also results in a higher
level of carbon dioxide in the atmosphere. This is due to the fact that when trees are cut down,
the excess carbon dioxide in the atmosphere are not absorbed by tree during the process of
photosynthesis.
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Nutrient cycles
Water cycle
The water cycle is also known as the hydrological cycle. It describes how water moves on,
above, or just below the surface of our planet.
Water molecules move between various locations - such as rivers, oceans and the atmosphere by specific processes. Water can change state.
Evaporation
Energy from the Sun heats the Earth’s surface and water evaporates from oceans, rivers and
lakes. The warm air rises, carrying water vapour with it.
Transpiration
Transpiration from plants releases water vapour into the air.
Condensation
The moist air cools down as it rises. Water vapour condenses back into liquid water, and this
condensation process produces clouds.
Precipitation
As the water droplets in the cloud get bigger and heavier, they begin to fall as rain, snow and
sleet. This is called precipitation (it is not the same as precipitation in Chemistry).
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Nutrient cycles
The nitrogen cycle
(*s)
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Nutrient cycles
Nitrogen compounds found in cells include proteins. Nitrogen from the air is converted into
soluble ions that plant roots can absorb. It forms part of nitrogen compounds in the plants,
and is then passed from one organism to the next. It is returned to the atmosphere as nitrogen
gas. This is the nitrogen cycle.
Stage one - Nitrogen fixation
About 78 per cent of the air is nitrogen gas. However, nitrogen is too unreactive to be used
directly by plants to make protein. It must be converted into soluble ions, such as nitrates.
Nitrogen-fixing bacteria in root nodules are able to do this. Lightning can also convert
nitrogen gas into nitrates.
Legumes such as bean and clover have root nodules containing nitrogen-fixing bacteria
The Haber process converts nitrogen gas into ammonia for use in fertilisers. Nitrifying
bacteria in the soil can convert ammonium ions into nitrates.
Stage two -Nitrification absorption into roots and incorporation into plants
Nitrifying bacteria convert ammonium ions into nitrates. Nitrates are converted into amino
acids by plants which are used to make protein.
Stage three – Feeding and digestion of protein: moving along food chains and excretion
When an animal eats a plant, nitrogen from the plant’s proteins becomes proteins in the
animal. Nitrogen is also passed from one animal to another by feeding. When proteins are
digested, excess amino acids are converted to urea. This process is known deamination. The
removal of urea is known as excretion.
Stage four – Detritrification: release of nitrogen into atmosphere
Denitrifying bacteria in the soil break down nitrates and return nitrogen gas to the air.
Stage five – Decomposition: death of animals and plants
Decomposers/ detritivores break down urea, egested material (eg faeces) and dead bodies.
This results in nitrogen being returned to the soil as ammonium ions, which nitrifying
bacteria can convert into nitrates for plants to absorb.
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Nutrient cycles
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Nutrient cycles
Past Paper Question
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Nutrient cycles
(a)
(b)
(c)
(d)
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