Topic 4c
4c – Cycles within ecosystems
Objectives
4.10 Describe the stages in the carbon cycle, including respiration,
photosynthesis, decomposition and combustion.
4.11B Describe the stages in the nitrogen cycle, including the roles of
nitrogen fixing bacteria, decomposers, nitrifying bacteria and denitrifying
bacteria (specific names of bacteria are not required).
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4c – Cycles within ecosystems
Recycling
• Have you ever wondered where all
the leaves go when they fall off the
trees?
• They are broken down by bacteria
and fungi.
• The nutrients are recycled back into
the ecosystem.
https://www.youtube.com/watch?v=
cWj3u8voDSg
3
4c – Cycles within ecosystems
Recycling
• There is a constant recycling of the chemicals on the
planet.
• The carbon atoms that are in your body have
probably been part of someone else before!
• Microorganisms such as bacteria, play a key role in
this.
• They break down complex organic molecules and
return the elements to the environment so they can
be used again.
4
4c – Cycles within ecosystems
Biological cycles
• Elements on the planet are recycled between the
living world and the non-living world (air, water, soil
and rocks).
• We will look at two elements and how they are
recycled.
• Carbon is one of these elements and so is nitrogen.
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4c – Cycles within ecosystems
Carbon cycle
• Photosynthesis ‘fixes’ carbon atoms from carbon
dioxide into organic compounds.
• These get passed along a food chain by feeding.
• Respiration releases carbon dioxide back into the
atmosphere.
• Fossilisation can occur and carbon can get stored as
fossil fuels like coal and natural gas.
• Combustion releases carbon dioxide from the fossil
fuels.
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4c – Cycles within ecosystems
Carbon cycle
7
• Page 203
• Question 10
Answer
PAPER 2
PAPER 2
4c – Cycles within ecosystems
Nitrogen cycle
• Nitrogen is a very important element in many
biological compounds.
• It’s found in DNA, proteins, amino acids and
vitamins.
• 78% of the air is nitrogen but it’s very unreactive.
• Therefore we rely on bacteria to convert it.
10
https://www.youtube.com/watch?v=cWj3u8v
oDSg
PAPER 2
PAPER 2
Nitrogen Cycle Summary
Nitrogen
Gas
PAPER 2
78% of the air is nitrogen but
The nitrogen
gas
is cant
converted
This
is done
bacteria
in the
animals
and by
plants
use
it
Nitrogen
also
occurs
to
nitrates
in
ause
process
called
soil
or root
nodules
of nitrates
plants.
infixation
this
form.
The
plants
can
the
The
nitrogen
isnitrogen
then
passed
when
lightning
strikes
the
nitrogen
fixation
They
are
called
to
make
proteins
and
other
Nitrogen returns to the fixing
soil
along the
food
chain
by
ground.
bacteria.
structures.
when
the
animal
excretes
or
When
this
happens
it iscontrol
in the
Decomposing
bacteria
feeding.
Ammonia
cant
be used by the
dies
and
is
decomposed.
form
of
ammonia.
this process by converting
plant
it is converted
further
Theseso
nitrates
can be used
by
proteins or urea from the
intoplants
nitrites
andorthen
nitrates
again
in some
animal into ammonia
by nitrifying bacteria.
circumstances
they are
converted back into nitrogen
gas by denitrifying bacteria
PAPER 2
4c – Cycles within ecosystems
Nitrogen cycle
There are 4 types of
bacteria controlling the
nitrogen cycle.
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PAPER 2
PAPER 2
4c – Cycles within ecosystems
Nitrogen cycle
Type of bacteria
What they do
Nitrogen-fixing (live in the soil or in
Convert nitrogen gas in the air into nitrates
the root nodules of leguminous plants)
(lightning also does this)
Decomposing (Live in soil)
Convert proteins from dead organisms and urea
into ammonia
Nitrifying
Convert this ammonia into nitrites and then
nitrates
Denitrifying
Convert nitrates back into nitrogen gas
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4c – Cycles within ecosystems
CONCEPT CHECK – True or false?
1. Carbon is released into the atmosphere by combustion
2. Respiration uses up carbon dioxide from the atmosphere
3. Bacteria and fungi decompose dead organisms
4. Photosynthesis returns carbon to the atmosphere
5. Carbon passed up the food chain by feeding
6. Nitrogen can be fixed by lightning
7. Denitrifying bacteria return to nitrogen to the soil
8. Nitrites are converted to nitrates by nitrifying bacteria
9. Nitrogen fixing bacteria in the soil fix nitrogen to nitrates
10. Nitrogen is used to make proteins
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4c – Cycles within ecosystems
CONCEPT CHECK – True or false?
1. Carbon is released into the atmosphere by combustion
2. Respiration uses up carbon dioxide from the atmosphere
3. Bacteria and fungi decompose dead organisms
4. Photosynthesis returns carbon to the atmosphere
5. Carbon is passed up the food chain by feeding
6. Nitrogen can be fixed by lightning
7. Denitrifying bacteria return to nitrogen to the soil
8. Nitrites are converted to nitrates by nitrifying bacteria
9. Nitrogen fixing bacteria in the soil fix nitrogen to nitrates
10. Nitrogen is used to make proteins
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4c – Cycles within ecosystems
CONCEPT CHECK – Exam style question
Q: Match up the following bacteria to their descriptions (4)
Decomposing
Converts nitrogen gas to nitrates
Nitrifying
Breaks down dead organisms
Denitrifying
Converts nitrites to nitrates
Pathogenic
Causes disease
Nitrogen-fixing
Converts nitrates back to nitrogen gas
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4c – Cycles within ecosystems
CONCEPT CHECK – Exam style question
Q: Match up the following bacteria to their descriptions (4)
Decomposing
Converts nitrogen gas to nitrates
Nitrifying
Breaks down dead organisms
Denitrifying
Converts nitrites to nitrates
Pathogenic
Causes disease
Nitrogen-fixing
Converts nitrates back to nitrogen gas
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