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GCSE Biology Summary Notes Human Impact
Summary Notes on the GCSE Biology topic Human Impact, specifically curated for the AQA Biology Exam Board.
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Human ImpactDecay
Microorganisms break down plant and animal material and waste to get energy. This
process of decay allows the material contained within the organisms to be released, so that
it can cycle through the ecosystem. Decomposition is vital for the recycling of materials in an
ecosystem.
Decomposition can produce compost which is decomposed organic matter that is used as a
natural fertiliser for crops and garden plants, it can return minerals, improve soil structure
and water retention. Farmers and gardeners try to provide the ideal conditions for quick
decay to make compost, which is full of nutrients released by the decomposers, but it can
take months to years.
Microorganisms such as bacteria and fungi, as well as detritus feeders are responsible for
decomposition:
Temperature: Warmer temperatures make things decompose quicker because they increase
the rate that the enzymes involved in decomposition work at. If it’s too hot, decomposition
slows down or stops because the enzymes are destroyed and the organisms die. Really cold
temperatures slow the rate of decomposition too.
Water: Decay takes place faster in moist environments because the organisms involved in
decay need water to carry out biological processes.
Oxygen: Many organisms need oxygen to respire which they need to do to survive. The
microorganisms involved in anaerobic decay don’t need oxygen. With oxygen the
microorganisms respire aerobically and transfer energy to their surroundings, killing off
weed seeds and speeding up the process.
Organisms: The more microorganisms and detritus feeders there are, the faster
decomposition happens.
Biogas
Biogas is mainly made up of methane, which can be burned as fuel. Lots of different
microorganisms are used to produce biogas. They decay plant and animal waste
anaerobically. This type of decay produces methane gas. Sludge waste from sewage works
or sugar factories is used to make biogas on a large scale. Biogas is made in a simple
fermenter called a digester or generator. Biogas generators need to be kept at a constant
temperature to keep the microorganisms respiring. It can’t be stored as a liquid (too high
pressure needed) so it has to be used straight away for heating, cooking, lighting or to
power a turbine to generate electricity. The bacteria work best at 300 oc so generators are
best in hot climates. The process is exothermic so once heated at the start, if the generator
is insulated, it can work anywhere.
, Batch Generators – make biogas in small batches, they’re manually loaded with waste which
is left to digest and the by-products are cleared away at the end of each session.
Continuous Generators – make biogas all the time, waste is continuously fed in and biogas is
produced at a steady rate, these are more suited for large-scale biogas projects.
Both processes need an inlet for waste material to be put in, an outlet for digested material
to be removed through and an outlet so that the biogas can be piped to where it’s needed.
It’s a naturally occurring process but by carrying it out in generators we can harness the
biogas produced and use it to produce energy for different processes, this process can clean
our sewage and other wastes so that they don’t pollute our water and land.
Decay Practical
This practical looks at how temperature affects the rate of decay. We can investigate decay
by observing the action of the enzyme lipase on a sample of milk that has been made
alkaline. When the lipase breaks the milk down, the pH of the milk decreased. In it, an
indicator dye called phenolphthalein is used, it has a pink colour when the pH is around 10
but becomes colourless when the pH falls below 8.3.
1) Measure 5cm3 of lipase solution and add it to a test tube. Label this tube with an L for
lipase.
2) Measure out 5cm3 of milk and add it to a different test tube.
3) Add 5 drops of phenolphthalein indicator to the tube containing milk.
4) Measure out 7cm3 of sodium carbonate solution and add it to the tube containing milk
and phenolphthalein. This makes the solution in the tube alkaline, so it should turn pink.
5) Put both tubes into a water bath set to 30oc and leave them to reach the temperature of
the water bath. Use a thermometer in the milk tube to check this.
6) Once the tubes have reached 30oc use a calibrated dropping pipette to put 5cm3 of the
lipase solution into the milk tube and start a stopwatch straight away.
7) Stir the contents of the tube with a glass rod, the enzyme will start to decompose the
milk.
8) As soon as the solution loses its pink colour, stop the stopwatch and record how long the
colour change took in a table.
9) Repeat the experiment at a range of different temperatures. Make sure you carry out
the experiment thrice at each, discard anomalies and calculate a mean time.
10) Use results to calculate the rate of decay by doing 1000/ time with the unit s -1 as rate is
given per unit of time.
Biodiversity
Biodiversity is a measure of the variety of all the different species of organisms on Earth, or
within an ecosystem.
High biodiversity is important, it ensures the stability of ecosystems reducing the
dependence of one species on another for food, shelter and the maintenance of the
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