Biology unit 4 module 4.3 revision

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Module 4.3
The integration of respiration and photosynthesis into ecology; energy is transferred through
ecosystems and transfer efficiency can be measured.
Know photosynthesis is the main route by which energy enters ecosystems,
Appreciate energy is transferred through trophic levels in food chains and webs and is
dissipated as heat.
Use numerical data to find the efficiency of transfer between trophic levels.
Understand that poor efficiency of transfer leads to pyramids of number, biomass and energy.
Compare efficiency as net productivity (gross productivity ­ respiratory loss) between natural
ecosystems and modern intensive farming.
Appreciate the use of natural and artificial fertilisers, chemical pesticides, biological agents,
integrated systems of pest control and intensive rearing of livestock increase efficiency of
energy conversion and thus productivity.
Evaluate economic and environmental and ethical issues involved with farming practice.
Key words:
Trophic level
Producer: an organism that synthesises organic molecules from simple inorganic ones such as carbon
dioxide and water. Most producers are photosynthetic and form the first trophic level in a food chain.
Primary/secondary consumer: any organism that obtains energy by `eating' another. Organisms
feeding on plants are known as primary consumers and organisms feeding on primary consumers are
known as secondary consumers.

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Pyramids of numbers:
Length of bar is proportionate to the number of
BUT because it doesn't take the size of the individual into account ­ we get differing pyramids of
Therefore pyramids of biomass (gm-2)
Can weigh wet mass BUT it can vary due to the changes of water retention within the body.
Dry mass however
o Heat below 100C
o Weigh it before heating
o Weigh it after heating
o Continue and weigh until there is a constant mass.…read more

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Energy transfer between trophic levels:
The sun is the source of energy for ecosystems.
It is lost in food chains
o Plants normally convert 1-3% of the suns available energy to them into organic
o Over 90% of the suns energy is reflected back into space by clouds and dust or
absorbed by the atmosphere.
o Not all wavelengths of light can be absorbed for photosynthesis.…read more

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Comparison of natural and agricultural ecosystems;
Natural ecosystems Agricultural ecosystems
Solar energy only ­ no additional energy Solar energy plus energy from food (labour)
input. and fossil fuels (machinery and transport)
Lower productivity Higher productivity
More species diversity Less species diversity
More genetic diversity within a species Less genetic diversity within a species
Nutrients are recycles naturally within the Natural recycling is more limited and
ecosystem with little addition from outside. supplemented by the addition of artificial
fertilisers.…read more

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Once introduced, the control organism Must be reapplied at intervals, making them
reproduces itself very expensive
Pests do not become resistant Pests develop genetic resistance, and new
pesticides have to be developed.…read more


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