Complete revision notes for Topic 19 of the CIE IGCSE Biology course: Organisms and their environment. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
State that the Sun is the principal source of energy input to biological systems
Describe the flow of energy through living organisms including light energy from the Sun
and chemical energy in organisms and its eventual transfer to the environment
19.2 Food chains and food webs
Define a food chain as showing the transfer of energy from one organism to the next,
beginning with a producer
The arrows in a food chain show the transfer of energy from one trophic level to the next.
State that energy is transferred between organisms in a food chain by ingestion
Construct simple food chains
Describe how energy is transferred between trophic levels
Define trophic level as the position of an organism in a food chain, food web, pyramid of
numbers or pyramid of 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
Explain why the transfer of energy from one trophic level to another is inefficient
Explain why food chains usually have fewer than five trophic levels
In order for the energy to be passed on, it has to be consumed (eaten).
, However not all of the energy grass plants receive goes into making new cells that can be
eaten. The same goes for the energy the vole gets from the grass, and the energy the barn
owl gets from the vole: only the energy that is made into new cells remains with the
organism to be passed on.
Even then, some of this energy does not get consumed – for example few organisms eat an
entire organism, including roots of plants or bones of animals – but energy is still stored in
these parts and so it does not get passed on.
The majority of the energy an organism receives gets ‘lost’ (or ‘used’) through:
Making waste products e.g. (urine) that get removed from the organism
As movement
As heat (in mammals and birds that maintain a constant body temperature)
As undigested waste (faeces) that is removed from the body and provides food for
decomposers
This inefficient loss of energy at each trophic level explains why food chains are rarely more
than 5 organisms long.
Explain why there is a greater efficiency in supplying plants as human food, and that there is
a relative inefficiency in feeding crop plants to livestock that will be used as food
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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