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Cycles In Nature
Cycling maintains homeostasis (balance)
in the environment.
Four cycles to investigate:
1. Oxygen cycle
2. Nitrogen cycle
3. Carbon cycle
4. Water cycle

Introduction:
o This slide presentation briefly discusses the life cycles of various
elements and compounds that are important for the survival of
living organisms within the environment.
o Whilst studying this unit, reflect on the importance of these cycles
and how they inter-relate
o The conditions we call life is sustained because of the interactions
and intra-actions that occur between the biotic and abiotic
components in nature and the built environment

Objectives of the lesson:
o State what the oxygen and nitrogen cycle is
o Name and explain 3 ways oxygen in lost in
the cycle
o State 2 ways nitrogen is converted into
nitrates
o Describe the 2 types of nitrogen-fixing
bacteria
o Use a diagram to explain the oxygen cycle
and the nitrogen cycle

1. Oxygen cycle
Released slowly by weathering
• The oxygen cycle is the movement of
oxygen between the biosphere,
lithosphere and atmosphere
• The main driving – photosynthesis
• The largest reservoir of Oxygen is in
the Lithosphere found on Earths crust
and mantle in the form of oxide
minerals (rocks)

Sources of Oxygen in the cycle:
1. Photosynthesis (main source)
• Takes place in the Biosphere
• Plants and phytoplankton
absorb carbon dioxide and
water to release Oxygen
through the process of
photosynthesis
• Marine plants are
responsible for releasing
majority of the oxygen found
in the atmosphere
2. Photolysis
• Takes place in the
atmosphere
• Sunlight (UV rays) releases
Oxygen when it reacts with
water vapor in the
atmosphere
• Evaporation occurs

Loss of Oxygen in the cycle:
1. Cellular respiration
• Animals,
organisms and
bacteria require
Oxygen to live
• Oxygen is taken in
and used for ATP
and Carbon
dioxide is released
into the
atmosphere
2. Decomposition
• Decomposition is
the breaking down
of dead organic
matter
• Requires Oxygen
and releases CO2
• Carried out by
microbes found in
the soil
3. Combustion
• The burning down
of fossil fuels
requires Oxygen

2. Nitrogen cycle
• Atmospheric nitrogen (N2) makes up nearly 78%-
80% of air.
• Organisms cannot use it in this form.
• Plants can only use nitrogen in the form of
nitrates (NO3).
Amino acid
All amino acids (building blocks
for proteins) contain nitrogen.
Nitrogen is also found in nucleic acids
(DNA & RNA)

grade 10 ecology - nitrogen and oxygen cycle pro.pptx

There are 4 phases of the nitrogen cycle, these are
namely:
Nitrogen fixation – lightning N2O NO3
Decay - organic nitrogen ammonia(NH3)
Nitrification – NH3 NO2 NO3
Denitrification

2. Nitrogen cycle
Nitrogen gas (N2) is
converted into nitrates in two
ways:
1. Electrical fixation by
lightning and rain
2. Nitrogen-fixing bacteria

Nitrogen-fixing bacteria
• Free-living soil bacteria absorb
N2 from the air spaces between
soil particles – convert to
nitrates
• Mutualistic nodule bacteria on
roots of legumes convert N2 to
ammonia for the host plant. In
exchange the plant provides
carbohydrates (food) and a
secure habitat for the bacteria,
e.g. soybeans, peanuts, clover

Nitrogen fixation – conversion of atmospheric
nitrogen (N2) into ammonia which can be used directly by
plants to make organic compounds like amino acids.
N2 ammonia
Animals eat plants to obtain
nitrogen which is used to make
animal proteins and nucleic acids.

2. Nitrogen cycle
• Decomposition of dead plants and animals - proteins in
their bodies are broken down into ammonia by bacteria.
• Animal urine (contains urea) which is also converted to
ammonia by bacteria.
Nitrifying bacteria in the soil convert:
Ammonia Nitrites
Nitrites Nitrates – absorbed by plant roots
Denitrifying bacteria in the soil convert:
Nitrites / Nitrates N2 gas (to complete the cycle!)

Referencing:
o Cornelia, A. (2017). Physical Environment and BioGeoChemical Cycles. Available from �ݺ�ߣshare at:
Physical Environment and BioGeoChemical Cycles (slideshare.net) (accessed 18 August 2022)
o Elearningja. (2015). Integrated Science M4 Cycles in Nature. Available from �ݺ�ߣshare at:
/eLearningJa/int-sc-m4u2l4-cycles-in-nature (accessed 18 August 2022)
o Lekshmyraja. (2015). Nitrogen cycle. Available from �ݺ�ߣshare at: Nitrogen cycle (slideshare.net)
(accessed 18 August 2022)
o Nahian, A. (2017). Biogeochemical cycle. Available from �ݺ�ߣshare at: Biogeochemical cycle
(slideshare.net) (accessed 19 August 2022)
o Price, R. (2015). DP Biology Option C6 Nitrogen & Phosphorus Cycles AHL. Available from �ݺ�ߣshare
at: /scienceteacherprice/dp-biology-option-c6-nitrogen-phosphorus-cycles-ahl
(accessed 18 August 2022)

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grade 10 ecology - nitrogen and oxygen cycle pro.pptx

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grade 10 ecology - nitrogen and oxygen cycle pro.pptx