Relationships of organisms with one another
and with the environment - 1
CAMBRIDGE GCE OL BIOLOGY 2025
PLATINUM BUSINESS ACADEMY
DR SHAKEEL JALEEL
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19.1 Energy flow
1 Understand that the Sun is the principal source of energy input to most biological systems
2 Explain why most forms of life are completely dependent on photosynthesis
3 Describe the flow of energy through food chains and webs including energy from light and energy in living organisms and its eventual transfer to
the environment
4 Construct and interpret simple food chains
5 Understand the terms producer, consumer, herbivore, carnivore and decomposer
6 Describe food webs as networks of interconnected food chains and construct and interpret them
7 Explain why the transfer of energy from one trophic level to another is inefficient
8 Explain why food chains usually have fewer than five trophic levels
9 Explain why it is more energy efficient for humans to eat crop plants than to eat livestock that have been fed on crop plants
10 Construct and interpret pyramids of numbers, biomass and energy
Nutrient cycles
1 Describe the carbon cycle, limited to: photosynthesis, respiration, feeding, decomposition, formation of fossil fuels and combustion
2 Outline the nitrogen cycle in making nitrogen available for plant and animal protein, limited to:
(a) decomposition of plant and animal protein to ammonium ions
(b) nitrification
(c) nitrogen fixation by lightning and bacteria
(d) absorption of nitrate ions by plants
(e) production of amino acids and protein
(f) feeding and digestion of proteins
(g) denitrification
(the names of individual bacteria are not required)
3 Outline the role of fungi and bacteria in decomposition
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Describe a population as a group of organisms of one species, living in the same area, at the same time
2 Describe a community as all of the populations of different species in an ecosystem
3 Describe an ecosystem as a unit containing the community of organisms and their environment, interacting
together
4 Describe biodiversity as the number of different species that live in an area
5 Identify and state the factors affecting the rate of population growth for a population of an organism, limited to:
food supply, competition, predation and disease
6 Understand that the growth of the human population is increasing the demand for global resources
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Ecosystem and its components
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An ecosystem is a unit containing the community of organisms and their environment,
interacting together
Whatever their size, ecosystems usually have the same components:
• producers - plants which photosynthesise to produce food
• consumers - animals that eat plants or other animals
• decomposers - organisms that break down dead material and help to recycle nutrients
• The living components of an ecosystem are called the biotic components. The non-living
(physical) components are the abiotic components (compare these with biotic and abiotic
factors , below).
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An ecosystem contains a variety of habitats.
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A habitat is the place where an organism lives. For example, habitats in a pond ecosystem
include the open water, the mud at the bottom of the pond, and the surface water.
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A population is a group of organisms of one species, living in the same area, at the same
time. eg population of elephants in the Sinharaja forest, population of leopards in the
siharaja forest
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All the immature frogs (tadpoles) swimming in a pond are a population of tadpoles; all the
water lily plants growing in the pond make up a population of water lilies.
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A community is all of the populations of different species in an ecosystem.
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Biodiversity is the number of different species that live in an area
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Interactions in an ecosystem ● The organisms in an ecosystem are continually interacting with each other and with
their physical environment.
Interactions include the following ●
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Feeding among the organisms - the plants, animals and decomposers are continually
recycling the same nutrients through the ecosystem.
Competition among the organisms - animals compete for food, shelter, mates,
nesting sites; plants compete for carbon dioxide, mineral ions, light and water.
Interactions between organisms and the environment –
plants absorb mineral ions, carbon dioxide and water from the environment;
plants also give off water vapour and oxygen into the environment;
animals use materials from the environment to build shelters;
The temperature of the environment can affect processes occurring in the
organisms;
processes occurring in organisms can affect the temperature of the environment (all
organisms produce some heat}.
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Abiotic and biotic factors –
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There are many factors that influence the numbers and distribution of organisms in an
ecosystem. There are two types of factor - biotic and abiotic.
Biotic factors are biological. Many {but not all} involve feeding relationships. They include:
availability of food and competition for food resources
predation
parasitism
•disease
Abiotic factors are physical or chemical factors. They include:
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climate, such as light intensity, temperature and water availability
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hours of daylight
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soil conditions, such as clay content, nitrate level, particle size, water content and pH
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other factors specific to a particular habitat, such as salinity (salt content) in an estuary, flow
rate in a river, or oxygen concentration in a lake
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pollution.
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Food Chain & Web Definitions
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Measuring populations using quadrats
● When an ecologist wants to know how many
organisms there are in a particular habitat, it
would not be possible for him to count them
all. Instead, he is forced to count a smaller
representative part of the population, called a
sample.
● Sampling of plants, or animals that do not
move much (such as snails), can be done using
a sampling square called a quadrat.
● A quadrat is usually made from metal, wood or
plastic. The size of quadrat you use depends
on the size of the organisms being sampled.
For example, to count plants growing on a
school field, you could use a quadrat with sides
0.5 or 1 metre in length (Figure 14.4).
● It is important that sampling in an area is
carried out at random, to avoid bias. For
example, if you were sampling from a school
field, but for convenience only placed your
quadrats next to a path, this probably wouldn't
give you a sample that was representative of
the whole field! It would be a biased sample.
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Explain which field has the greater biodiversity.
Ans.- Although A and B have same three species, B is more biodiverse as it
has more even numbers of the same species.
Explain how a shortage of one name mineral could affect the size of plants
in the fields
Ans.1. Nitrates are essential to make proteins needed for growth.
2. Magnesium is essential for chlorophyll production which is needed for
photosynthesis, which produces glucose that is used in respiration to
produce energy needed for growth of plants.
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Ans.- Grid the field into coordinates and let a computer program, assign
random coordinates to be sampled.
(i)Calculate the mean number of dandelions per quadrat in B
(ii) Calculate the number of dandelions per m2 in field B
(iii) Describe the differences in species distribution in field A and field B
Ans. –
1. There is a higher biodiversity in field A as there is more species evenness.
2. There are more plants per quadrat in field A
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• A food chain shows the transfer of energy from one organism to the next, starting with a
producer
• The source of all energy in a food chain is light energy from the Sun
• This energy is converted into the chemical energy of glucose through photosynthesis
• The simplest way of showing feeding relationships within an ecosystem is a food chain.
• In any food chain, the arrow(-->) means 'is eaten by'. In the food chain illustrated, the grass is
the producer. It is a plant so it can photosynthesise and produce food materials.
• The grasshopper is the primary consumer. It is an animal which eats the producer and is also a
herbivore.
• The lizard is the secondary consumer. It eats the primary consumer and is also a carnivore.
• The different stages in a food chain (producer, primary consumer and secondary consumer) are
called trophic levels.
• Many food chains have more than three links in them. Here are two examples of longer food
chains:
filamentous algae → mayfly nymph → caddis fly larvae → salmon
In this freshwater food chain, the extra link in the chain makes the salmon a tertiary consumer.
plankton → crustacean → fish → ringed seal →polar bear
In this marine (sea) food chain, the fifth link makes the polar bear a quaternary consumer.
Because nothing eats the polar bear, it is also called the top carnivore.
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Food web
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Food chains are a convenient way of showing the feeding relationships between a
few organisms in an ecosystem, but they oversimplify the situation
A food web is a network of interconnected food chains
Food webs are more realistic ways of showing connections between organisms
within an ecosystem as animals rarely exist on just one type of food source
Food webs give us a lot more information about the transfer of energy in an
ecosystem
They also show interdependence – how the change in one population can affect
others within the food web
For example, in the food web above, if the population of earthworms decreased:
1. The population of grass plants would increase as there are now fewer
species feeding off them
2. The populations of frogs and mice would decrease significantly as
earthworms are their only food source
3. The population of sparrows would decrease slightly as they eat
earthworms but also have another food source to rely on (caterpillars)
A food web shows the interdependence of
organisms
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Most of the changes in populations of animals and plants happen as a result of human
impact – either by overharvesting of food species or by the introduction of foreign species
to a habitat
Due to interdependence, these can have long-lasting knock-on effects to organisms
throughout a food chain or web
Do not say an animal or plant would ‘die out’ as this is unlikely to happen – stick to using
the words decrease or increase. If in doubt, always give your reason for the increase or
decrease in population.
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Trophic Levels
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Trophic levels describe the position of an organism in a food chain, web or pyramid of
numbers or biomass.
Animals (known as consumers) can be at different trophic levels within the same food web
as they may eat both primary, secondary and / or tertiary consumers
Energy flows from the sun to the first trophic level (producers) in the form of light
Producers convert light energy into chemical energy and it flows in this form from one
consumer to the next
Eventually all energy is transferred to the environment – energy is passed on from one
level to the next with some being used and lost at each stage
Energy flow is a non-cyclical process – once the energy gets to the top of the food chain or
web, it is not recycled but ‘lost’ to the environment
This is in direct contrast to the chemical elements that organisms are made out of, which
are repeatedly recycled
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Transfer of Energy is non cyclical • When energy in a food chain flows in a noncyclical way. How? Let us see
1. Photosynthesis converts sunlight energy into chemical energy in glucose.
2. This is then consumed by herbivores/primary consumers who are consumed by carnivores/secondary
consumers.
3. However, the energy transfer between trophic levels is not efficient. It is lost as it passed from one
trophic level to another.
4. When a rabbit eats grass, not all of the materials in the grass plant end up as rabbit! There are losses:
• some parts of the grass are not eaten (the roots for example)
• some parts are not digested and so are not absorbed - even though rabbits have a very efficient
digestive system
• some of the materials absorbed form excretory products
• many of the materials are respired to release energy, with the loss of carbon dioxide and water.
• energy is also lost as heat
In fact, only a small fraction of the materials in the grass ends up in new cells in the rabbit. Similar
losses are repeated at each stage in the food chain, so smaller and smaller amounts of biomass are
available for growth at successive trophic levels. The shape of pyramids of biomass reflects this.
Feeding is a way of transferring energy between organisms. Another way of modelling ecosystems looks
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at the energy flow between the various troph
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As you can see, only about 10% of the
energy entering a trophic level is passed
on to the next trophic level. This explains
why not many food chains have more than
five trophic levels. Think of the food chain:
A→B→C→D→E
If we use the idea that only about 10% of
the energy entering a trophic level is
passed on to the next level, then, of the
original 100% reaching A (a producer),
10% passes to B ,1% (10% of 10%)passes
to C, 0.1% passes to D and only 0.001 %
passes to E.
There just isn't enough energy left for
another trophic level. In certain parts of
the world, some marine food chains have
six trophic levels because of the huge
amount of light energy reaching the
surface waters.
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Energy Transfer in Human Food Chains
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Humans are omnivores, obtaining energy from both plants and animals, and this gives us a
choice of what we eat
These choices, however, have an impact on what we grow and how we use ecosystems
Think of the following food chains both involving humans:
wheat → cow → human
wheat → human
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Given what we know about energy transfer in food chains, it is clear that if humans eat the
wheat there is much more energy available to them than if they eat the cows that eat the
wheat
This is because energy is lost from the cows, so there is less available to pass on to humans
Therefore, it is more energy efficient for humans to eat herbivores than carnivores.
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Ans –
Producers such as wheat produce
more food than consumers like
sheep. The same area of land can
produce 7.5 tons of wheat but only
0.3 tons of lamb. It’s more efficient
for humans to directly eat producers
than sheep which are primary
consumers. Less energy is lost
between the transfer from producer
to human, than the transfer from
producer to primary consume to
human. Eating producers driectly
means energy is not lost by
movement, mby defecation of
excretion.
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Pyramids of Number
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A pyramid of numbers shows how many
organisms we are talking about at each
level of a food chain.
The width of the box indicates the number
of organisms at that trophic level
For example, consider the following food
chain:
Ask yourself the following questions:
○ Is it likely that there would be more voles in an area than grass plants?
○ How many voles might one barn owl need to eat per day? If it’s more
than one, is it likely that there are more barn owls in an area than voles?
So, a pyramid of numbers for this food chain would look like this:
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Despite the name (and the example above), a
pyramid of numbers doesn’t always have to be
pyramid-shaped, for example:
This is because the size of the organism is also
important – one large organism, like the oak tree
in the pyramid above, contains enough energy to
support many smaller organisms (the insects)
Pyramids of numbers are not always pyramidshaped
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Rules to remember when drawing a pyramid of numbers:
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You cannot change the trophic level of the organisms – they must stay in the same order
as in the food chain with producers on the bottom, followed by primary consumers, then
secondary consumers, then tertiary consumers
Generally, the larger an individual organism is, the less of them there are.
Pyramids of Biomass
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A pyramid of biomass shows how much mass the creatures at each level would have
without including all the water that is in the organisms (their ‘dry mass’)
Pyramids of biomass are ALWAYS pyramid-shaped, regardless of what the pyramid of
numbers for that food chain looks like
This is because the mass of organisms has to decrease as you go up a food chain – if we
take our first food chain as an example, it’s impossible to have 10kg of grass feeding 50kg of
voles feeding 100 kg of barn owls
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A pyramid of biomass
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Pyramids of biomass provide a much
better idea of the quantity of the plant or
animal material at each level of a food
chain and therefore are a better way of
representing interdependence within the
food chain
Remember that pyramids of biomass are
ALWAYS pyramid-shaped, so they are
simple to draw, but pyramids of number
can be any shape – so make sure you learn
the rules for drawing a pyramid of
numbers.
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Pyramid of energy
● pyramid of energy is a graphical
representation of the amount of energy
at each trophic level of a food chain
● They are expressed in units of energy
per area per time (e.g. kJ m–2 year–1)
● Pyramids of energy will never appear
inverted as some of the energy stored in
one source is always lost upon transfer
● Each level should be roughly one
tenth of the size of the preceding level
(as energy transformations are ~10%
efficient)
● The bottom level will always represent
the producers, with subsequent levels
representing consumers (primary,
secondary, etc.)
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Invasive species
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An invasive species is commonly defined as any living organism not native to
an area that causes economic or environmental harm, or is damaging to
human health
They effect ecosystems by eating native species and spreading disease
The invasive feral pigs (Sus scrofa) found on the United States' Hawaiian
islands are thought to be descended from domesticated pigs (Sus domesticus)
that early Polynesian settlers brought with them for food. Feral pigs dig up
large areas of vegetation, spread invasive plant species, and contribute to soil
erosion.
Other times, invasive species were introduced as ill-advised attempts
at biocontrol. For example, the cane toad (Bufo marinus), native to South and
Central America, was introduced to Australia in the 1930s as a means of
controlling pests in sugar cane plantations. Unfortunately, the plan backfired,
and the cane toads became a pest themselves. The oversized toad is famously
toxic and can be deadly to predators who try to eat it.
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Nutrient Cycles
The Carbon Cycle
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Nutrients such as carbon and nitrogen are not endless resources
There is a finite amount of each element on the planet and as such, they need to be
recycled in order to allow new organisms to be made and grow
Carbon is taken out of the atmosphere in the form of carbon dioxide by plants to be used
for photosynthesis
It is passed on to animals (and microorganisms) by feeding
It is returned to the atmosphere in the form of carbon dioxide by plants, animals and
microorganisms as a result of respiration
If animals and plants die in conditions where decomposing microorganisms are not present
the carbon in their bodies can be converted, over millions of years and significant pressure,
into fossil fuels
When fossil fuels are burned (the process is known as combustion), the carbon combines
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with oxygen and carbon dioxide is released into the atmosphere
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Increased use of fossil fuels is contributing to an
increase in the carbon dioxide content of the
atmosphere
In addition, mass deforestation is reducing the
amount of producers available to take carbon dioxide
out of the atmosphere by photosynthesis
This problem is exacerbated by the fact that in many
areas of the world, deforestation is taking place for
land rather than for the trees themselves, and as such
they are burnt down, releasing yet more carbon
dioxide into the atmosphere
The Carbon Cycle
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Processes that use Carbon
Processes that release carbon
Photosynthesis in producers –
Carbon in CO2 is absorbed from the
atmosphere and is converted into glucose.
Respiration in plants, animal,
microorganims including decomposers
uses glucose releases carbon into the
atmosphere
Fossilization – if decay does not occur rapidly, Combustion of fossil fuels. The carbon
the carbon in bodies of dead animals and
in the fuel combines with oxygen to
plants gets compressed over time, forming
form CO2.
fossil fuels.
Decomposition is breakdown of dead
and decaying organims and waste by
decomposers. When they do this, the
use the nutrients absorbed in
respiration releasing carbon as CO2.
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Decomposers and decomposition
What are decomposers?
• Decomposition is the process by which bacteria and fungi break dead
organisms into their simple compounds.
• Decomposing bacteria and fungi are described as saprophytic because of
the way they break down dead organic matter.
Decomposition occurs this way –
1. Bacteria/fungi secreting enzymes out of their cells into the soil or dead
organism.
2. The enzymes digest the organic material. This is known as extracellular
digestion as it happens outside the cells.
3. The products of digestion are absorbed by the bacteria/fungi.
• The rate of decay is the speed at which dead matter is broken down
by decomposers
• When decomposers decompose, they release ammonia into the soil which
is converted into nitrates. These nitrates are absorbed by plants to make
plant proteins.
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Decomposition happens quickly when:
1. The temperature is warm
2. There is enough moisture
3. The decomposing organism has a large surface area
4. There should be enough oxygen. Anaerobic conditions cause the
rate of decomposition to decrease.
If no bacteria and fungus
• Dead bodies will never decay
-Decomposition can also be bad
• They decompose our food
• Produce toxic chemicals in food
• Eating such food can cause food poisoning
• Preservatives are used to stop the growth of bacteria and fungus, which
allows the food to be preserved for longer period of time
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• Temperature
At colder temperatures decomposing organisms will be less active, thus the rate of
decomposition remains low. This is why we keep food in a fridge. As the temperature
increases, decomposers become more active as the enzymes will begin to catalyze
reactions at higher rates and the rate increases. At extremely high temperatures
decomposers will be killed and decomposition will stop.
• Water
With little or no water there is less decomposition because decomposers cannot survive as
water is needed for activation of enzymes. As the volume of available water increases, the
rate of decomposition also increases. Many decomposers secrete enzymes onto decaying
matter and then absorb any dissolved molecules. Without water these reactions cannot
occur.
• Oxygen
Similar to water, decomposers need oxygen to survive and without it there is little or no
decomposition. Oxygen is needed for many decomposers to respire, to enable them to
grow and multiply. This is why we often seal food in bags or cling film before putting it in
the fridge. As the volume of available oxygen increases, the rate of decomposition also
increases. Some decomposers can survive without oxygen.
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The Water Cycle
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Water molecules move between various locations – such as rivers, oceans and the
atmosphere – by specific processes
This is possible because water changes state at a relatively low temperature
Animals lose water via evaporation, urination and defecation and gain water from
their food and drink
Plants gain water via absorption from the soil and lose it through transpiration
Water enters the atmosphere as water vapour in the following ways:
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Evaporation from oceans, rivers, lakes.
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Transpiration from plants releases water vapour into the air
The warmer air of the lower atmosphere rises, taking the water vapour with it
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The moist air cools down as it rises
Water vapour condenses back into liquid water, forming clouds
The water cycle
Water returns to Earth in the form of precipitation
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As the water droplets in the cloud get bigger and heavier, they begin to fall as rain,
snow and sleet
This is called precipitation
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The Nitrogen Cycle
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Nitrogen as an element is required to make proteins
Neither plants nor animals can absorb it from the air as N2 gas is very stable and the bonds
holding the nitrogen atoms together would need massive amounts of energy to break (the
two nitrogen atoms in a nitrogen molecule are held together by a triple covalent bond)
However, there are two ways it can be taken out of the air and converted into something
easier to absorb:
○ Nitrogen fixing bacteria found ‘free living’ in soil and also in the root nodules of
certain plants (peas, beans, clover – we call them leguminous plants) take N2 gas and
change it into nitrates in the soil
○ Lightning can ‘fix’ N2 gas, splitting the bond between the two atoms and turning them
into nitrous oxides like N2O and NO2 that dissolve in rainwater and ‘leach’ into the soil
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Plants absorb the nitrates they find in the soil and use the nitrogen in them to make proteins
Animals eat the plants (or other animals) and get the nitrogen they need from the proteins in the
plant or animal
Waste (urine and faeces) from animals sends nitrogen back into the soil as ammonium
compounds (the urea in urine contains nitrogen)
When the animals and plants die, they decay and all the proteins inside them are broken down
into ammonium compounds and put back into the soil by decomposers
The plants can’t absorb ammonium compounds though, so a second type of soil bacteria,
nitrifying bacteria, convert the ammonium compounds to nitrites and then to nitrates, which
can then be absorbed by plants – and so the cycle goes on
Finally, there is a third, unhelpful type of (anaerobic) bacteria called denitrifying bacteria found
in poorly aerated soil (ie not much oxygen)
These bacteria take the nitrates out of the soil by using them as a source of energy and convert
them back into N2 gas
Farmers can help reduce the amount of these unhelpful bacteria by ploughing and turning over
soil
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The Nitrogen Cycle
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Organisms in a food web can be classified into different trophic levels
based on their feeding relationships.
1) Explain the feeding relationships of named organisms are different
trophic levels in this food web.
Ans. – the producer in the food chain is grass. Grass produces glucose
during photosynthesis, using energy from sunlight. The primary
consumers our rabbit mouse grasshopper which feed on grass. The
secondary consumers are the snake and a lizard, which feed on the
mouse grasshopper and rabbit. The Hawk is a secondary consumer and
tertiary consumer (5)
2) Describe how energy flows into and through a food web. Explain
how this will determine the biomass of organisms at different trophic
levels.
Ans.- Energy flows from the Sun. Sunlight is absorbed by the producer
and converted to chemical energy through photosynthesis. Energy is
lost as it flows from one trophic level to another. For example, it is lost
as heat through movement, due to respiration and undigested waste.
Energy flowis not cyclical as it does not return to the Sun. Biomass
decreases as you go up the food chain.
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• Explain why only a small proportion of energy in insects passes to the
birds?
Ans.- not all insects and not all parts of the insects are eaten by the
birds. The insects also release energy through respiration and use
energy for movement and heat production. therefore energy remains
in the insect without being passed on to the birds.
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b) (i) the energy from the Sun
may not hit the leaves and may
be reflected off the leaves. Also
not all of the energy from the
Sun may be used for
photosynthesis.
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Ans. -The energy could be used by the song, word for flight and singing. It
may be lost as faeces. Some energy may be used to produce heat to
maintain body temperature. Not all the food is eaten and energy is lost to
decomposition once the songbird dies.
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