Carbon Cycle, Nitrogen Cycle, Water Cycle and Decomposition
IGCSE AQA GCSE Biology Edexcel
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Biology
Doc Brown's
school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
their importance for ecosystems and a section on a rate of decay investigation, factors
affecting rate of decay, biogas digester, making compost and preserving food
This page will help you answer questions
such as ... Be able to describe and understand the
steps in the carbon cycle? Be able to describe and understand the
steps in the nitrogen cycle? Be able to describe the water cycle? What factors affect the rate of decomposition
of organic material?
Sub-index for this
page
(a)
Introduction to cycles
(b)
The Carbon Cycle
* (c)
The Nitrogen Cycle * (d)
The Water Cycle
(e)
Decomposition - factors affecting the rate of
decay of biological material
(f)
A simple
experiment to investigate effect of temperature on the rate of decay of milk
(g)
Biogas from waste - the process, generator designs
and rate factors
(h)
Making compost -
a natural organic fertiliser * (i)
Food preservation
(j)
Some learning objectives based on the above notes
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Biology Notes
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(a)
Introduction
to cycles
An ecosystem is all the living organisms in a particular area
including non-living conditions (abiotic factors) such as temperature, soil quality, water
sources.
Within an ecosystem materials are
continually recycled in many complex ways involving
biotic
(living) and abiotic (non-living) components in ecosystems.
For more on biotic/abiotic factors see
Ecosystems
and interactions between organisms
gcse biology notes
All living organisms are composed of the chemical
elements they obtain from the environment in some way.
Plants will take in compounds of carbon (e.g.
carbon dioxide from air), hydrogen (e.g. water through roots) and nitrogen (e.g. nitrates
from soil) and the element oxygen from air.
Some plants can metabolise nitrogen from the
air directly (e.g. legumes).
These elements and simple compounds are converted
into all the complex biological molecules you find in plants such as
carbohydrates, fats and proteins.
These are then consumed by plant eating animals,
creating biomass that gets passed on up the food chain.
All of these elements and compounds must be
recycled by some means back into the environment.
All the nutrients in our environment are being
constantly recycled with many inputs balancing the many outputs.
The material might be recycled back into the soil
or into the air.
This recycled material may come from animal
waste products e.g. faeces or emitted gases!
When plants or animas die, their remains will
slowly decay
Decomposer organisms like
bacteria and fungi play are a very important part of
ecosystems.
To decompose any dead plant or
animal material, they secrete enzymes that break the dead organic
material down into smaller soluble food molecules which can
diffuse into the microorganisms i.e. absorption of nutrients.
Therefore decayed material in the soil is
recycled by microorganisms and new plant growth and so re-enters the many complex food
chains in an ecosystem.
All dead organisms decay because they are
broken down by decomposers (often microorganisms) which
digest the once living material replace lost nutrients like
mineral ions back into the soil.
The decomposition by microorganisms is
favoured by warm, moist and aerobic conditions.
A stable community
If the materials that plants take out of the
soil and air to create biomass is balanced by recycled
decayed material the community is described as stable - the
natural cycle repeats itself continuously.
Poor land management and overuse of
agrichemicals leads to an imbalance with a negative impact on
the habitats of many plants and animals.
TOP OF PAGE and
sub-index on cycles and decay
(b)
The
Carbon Cycle
See also
FOSSIL FUELS, coal, oil & natural gas and the Carbon Cycle
gcse chemistry revision notes
and
Limestone and lime
- their chemistry and uses
gcse chemistry revision notes
TOP OF PAGE and
sub-index on cycles and decay
(c)
The
Nitrogen Cycle
- Nitrogen is an extremely
important element for all plant or animal life!
- It is found in
important molecules such as amino acids, which are combined to
form proteins.
- Protein is used everywhere in living organisms from muscle
structure in animals to enzymes in plants/animals.
- Therefore nitrogen containing molecules are
passed along food chains.
- Nitrogen in air cannot be absorbed by plants
until it is converted into ions such as nitrite (NO2-)
or nitrate (NO3-).
- Nitrogen from the
atmosphere - nitrogen fixation processes:
- Air contains ~78% nitrogen gas.
- N2 is a very stable
molecule and the word fixation means to convert the nitrogen in air
into some chemical form that is soluble in water and plants can use
- this can be synthetically in the
Haber process or by
nitrogen-fixing bacteria.
- Action of nitrifying bacteria,
(nitrogen-fixing bacteria) e.g. they function in the root nodules of certain plants like peas/beans (the
legumes), can directly convert atmospheric nitrogen into nitrogen
compounds in plants.
- e.g. nitrogen ==> ammonia ==> nitrates
which plants can absorb.
- This is a naturally occurring nitrogen
fixation process.
- Some nitrogen-fixing bacteria live in soil
whilst others live in the swelled nodules in the roots of legumes.
- When legume plants die and decompose the
nitrogen is returned to the soil.
- Nitrogen ions can leak out of the modules
during plant growth.
- The nitrogen-fixing bacteria have a
mutualistic relationship with the plants - the bacteria get
sugary food from the plant and the plant gets nitrogen ions from the
bacteria (to make amino acids and proteins).
- However, most plants cannot do
this conversion from nitrogen => ammonia, though they can all
absorb nitrates, so the 'conversion' or 'fixing' ability might be introduced into other
plant species by genetic
engineering.
- The nitrogen from air, combining with hydrogen, is
converted into ammonia (NH3) in the chemical industry, and from this
artificial fertilisers are manufactured to add to nutrient
deficient soils.
- This done in the Haber process - a synthetic
fixation process.
- However, some of the fertiliser is washed out of
the soil and can cause pollution -
eutrophication.
- The energy of lightning
is so high it
causes nitrogen and oxygen to combine and form nitrogen oxides which
dissolve in rain that falls on the soil adding to its nitrogen
content. This is also described as a natural nitrogen fixation process -
atmospheric elemental nitrogen converted to a compound that enters
the soil for plants to use.
- N2(g) + O2(g)
==> 2NO(g)
- 2NO(g)
+ O2(g) ==> 2NO2(g)
- NO2(g) + water ==> NO2-(aq)
or NO3-(aq)
nitrates/nitrites in rain/soil
- Incidentally, reactions 1.
and 2. can also happen in a car engine, and NO2 is
acidic and adds to the polluting acidity of rain as well as
providing nutrients for plants! See
air pollution and acid
rain
- Note that the
Haber synthesis of ammonia
is a synthetic method of nitrogen fixation.
- Nitrogen recycling
apart from the atmosphere:
- Nitrogen compounds, e.g.
protein formed in plants or animals, are consumed by animals higher
up the food chain.
- Then bacterial and fungal decomposers
break down animal waste and dead plants/animals to release nitrogen
nutrient compounds into the soil (e.g. in manure/compost)
which can then be re-taken up by plants.
- The bacteria and fungi break down proteins in
rotting plants and animals and urea in animal waste and convert the
nitrogen into ammonia, which is oxidised to nitrite or nitrate ions
- so returning the nitrogen to the soil in a form that can be
absorbed by plants.
- Nitrogen returned to the
atmosphere:
- However, the action denitrifying
bacteria will break down proteins completely and release
nitrogen gas (N2) into the atmosphere.

- More 'biological detail' of the
NITROGEN
CYCLE with reference to the above diagram so you can show an understanding of how nitrogen is recycled.
- Nitrogen gas
in the air (78%, ~4/5th) cannot be used directly by
most plants and all animals.
- No animals and only a few specialised plants
can directly use the very unreactive nitrogen from air, but all
plants nitrogen in some form to synthesise amino acids and proteins for
growth and maintenance and for DNA in cell reproduction.
- However, nitrogen can be changed into
nitrogen compounds like nitrates which the plants can use.
- Animals rely on plants or other animals in
the food chain for their source of nitrogen compounds e.g. protein in grass,
crops or other animals.
- The action of
lightning can convert nitrogen gas into nitrates.
- The very high electrical energy discharges
from lightning activates nitrogen and oxygen molecules to react and form
nitrogen oxides. These dissolve in rain to form nitrates which end up in the
soil when rainwater trickles into the soil.
- There are four types of bacteria involved in the
nitrogen cycle
- (1) Action of Nitrogen-fixing
bacteria (nitrifying bacteria) living in root nodules of plants or in the soil, their function is
to fix nitrogen gas directly from the atmosphere into a chemical form the plant can
metabolise.
- Members of the leguminous family contain
symbiotic bacteria called rhizobia within the root nodules,
producing nitrogen compounds that help the plant to grow and compete
with other plants. When the plant dies, the fixed nitrogen is released,
making it available to other plants and this helps to fertilize the
soil.
- So, leguminous plants like peas, lentils, clover
and beans can absorb nitrogen from the air via their root nodules (swellings
on the root surface) which contain bacterial enzymes capable of converting ('fixing')
atmospheric nitrogen into soluble ammonia/ammonium ions, these are oxidised
to the nitrate ion - a nutrient essential for synthesising amino
acids, proteins for plant growth.
- Legumes and their root nodule bacteria are
an example of mutualism, because the plant root supplies
the bacteria with carbohydrate food (sugars) and minerals and the bacteria supplies
the plant with nitrogen in the form of the nitrite or nitrate ion -
different species of nitrifying bacteria produce different ions.
- This mutual relationship benefits both the
plant and its associated bacteria.
- The process of converting nitrogen in air
into nitrogen compounds is sometimes called 'nitrogen fixation'.
- When these leguminous plants decompose the
nitrogen compounds in the tissues and root nodules is returned to the
soil - a good composting action.
- Ions containing nitrogen can diffuse out of the
nodules during plant growth.
- Leguminous plants have a mutualistic
relationship with bacteria.
- The bacteria get food like sugars from the plant
and the plants get nitrogen ions from the bacteria to use in making
proteins.
- (2) Action of decomposers - soil bacteria:
- The
function of decomposers, bacteria, fungi and worms in the soil, is to break down
the remains of dead
animals and plants converting proteins and urea (from animal waste) into
soluble ammonia
or ammonium ions - so can be absorbed by the roots of plants.
- This decomposition (decay) returns nitrogen to the soil.
- This process is sometimes called putrefaction
by putrefying bacteria.
- The ammonium
ions are oxidised to nitrate ions (see below), which plants can absorb through their
roots to complete the recycling process.
- (3) Action of nitrifying bacteria:
- Nitrifying bacteria oxidise ammonia/ammonium
ions from the decayed material to form nitrites, which are further oxidised
to nitrate ions that be
absorbed by plants through their root systems.
- (4) Action of denitrifying bacteria:
- Denitrifying bacteria convert nitrates back into
nitrogen gas which is returned to the atmosphere.
- Therefore they are of no benefit to any living
organism.
- These particular bacterial organisms can remove
the oxygen from nitrate compounds to form the element nitrogen gas.
- The action of denitrifying bacteria is of no use to
any living organism!
- The function of denitrifying bacteria is the opposite
of the
nitrogen-fixing bacteria (1).
- Denitrifying bacteria are most often found in
waterlogged soils, conditions unsuitable for most plant or animal life.
- These denitrifying bacteria live in
anaerobic conditions like waterlogged soils and use the nitrate ion to
respire.
- These denitrifying bacteria use the oxygen rich nitrate ion (NO3-) as an
oxidant instead of oxygen gas.
- Plants absorb nitrates
from the soil.
- Plants get their nitrogen from soil.
- Plants absorb nitrates (soluble in water) in
the moisture that the roots absorb from the surrounding soil.
- Plants can use the nitrate ion in forming
amino acids from which the plant can make its proteins.
- Nitrogen compounds are then passed along the food
chains e.g. animals eat plants (herbivores) and animals eat animals
(carnivores).
- Nitrates are needed by plants to make proteins for growth.
- Nitrates are an essential nutrient for
plants to synthesis amino acids and hence proteins.
- Nitrogen compounds pass along a food chain or web of food chains.
- All food chains involve the passing of
carbon compounds e.g. sugars, carbohydrates, fats and proteins up to
the next trophic level i.e. the consecutive eating along a food chain (and
waste produced on the way).
- e.g. grass ==> cow ==> human
- Plants make their own protein from nitrates,
but animals must obtain it from plants or other animals. In fact the protein
is broken down in digestion to amino acids and each animal makes its own
proteins from these amino acid residues.
-
Ways
in which farmers can increase the nitrate content of soil
- As plants grow they will use up the available
nitrates in soil, which can become deficient in essential nutrients.
- When the crops are harvested, the nitrate content of
the soil is now reduced - its ended up as protein in the grain.
- Unless the nitrate is replaced, the nitrate content
in the soil will decrease with each crop grown.
- To avoid the soil becoming infertile leading to poor
plant growth and deficiency disease, the nitrate must be replaced.
- There are three ways of doing this.
- Spreading an organic fertilisers like
animal manure or some composted plant material which is decomposed by
bacteria/fungi to release nitrogen compounds into the soil. This is a good method
because it is essentially recycling organic substances from animal waste
or plant material, both returning nitrogen compounds to the soil on
decomposition.
-
Spreading
synthetic fertilisers (e.g. NPK products) made from ammonium
and nitrate compounds.
Can you name them? However, the use of artificial fertilisers can
create pollution problems like eutrophication if overused. See the notes
on ...
Biodiversity, land management, waste management,
maintaining ecosystems - conservation
-
Crop rotation avoids growing
the same crop in the same field over and over again. Several different
crops are grown each year in a seasonal cycle. The cycle should
include a leguminous plant (nitrogen-fixing crop) like beans or peas that will naturally
return nitrates to the soil for another crop the following year -
the plant itself can also be ploughed into the field.
TOP OF PAGE and
sub-index on cycles and decay
(d)
The
Water Cycle and its importance to life
What happens to water on the Earth's Surface?
The water on the Earth's surface is continually
being re-cycled.
Water is the most abundant substance on the
surface of our planet and is essential for all life.
Water in rivers,
lakes and the oceans is evaporated by the heat energy of the Sun's
radiation (liquid ==> gas/vapour, an endothermic
process).
Water also evaporates directly from leaves of
plants in the process called transpiration.
The water
vapour formed rises high in warm air convection currents into the atmosphere, cools and forms clouds of
condensed water (gas/vapour ==> liquid/solid, an exothermic process).
Eventually the water falls as rain, hail or snow,
collectively called
'precipitation'.
If the precipitation falls on land, and any
excess rain/melt water runs off into rivers, lakes, seas and oceans
and eventually re-evaporates by the sun's energy to repeat the cycle.
Some water will percolate into soil or porous
rocks, most becoming ground water, but this will eventually flow into
lakes and rivers. Some water will accumulate in artesian wells.
This is known as the water cycle and is
essential for all life based on land.
Water is essential to maintain all habitats.
All organisms need water for cellular fluids, and
require it for transport systems.
Water is needed for some chemical reactions in all
organisms, including photosynthesis in plants.
When the precipitation of water falls on land it
(usually) provides clean fresh water for plants and animals.
This flow of fresh water via the water cycle
transports nutrients from location to another i.e. from one ecosystem to
another.
Water is an important raw material and has
many uses See
GCSE Chemistry
Notes on Water
The
importance of water to life
If it wasn't for the water cycle raining down
freshwater, life as we know it would not exist.
Soil absorbs fresh water and plants take it in
through their roots for photosynthesis and transporting dissolved
substances like sugars around the plant.
This means some of the water is
incorporated into food chains.
Water in animals is returned to the soil and
atmosphere by excretion, respiration and sweating.
Most land or aquatic plants and animals need
freshwater, apart from organisms adapted to live in salty water.
Salty water is potentially toxic to many organisms
if ingested.
All living things on our planet need water to
survive and perform all their biological functions - in fact most
biochemical reactions need water as a reaction medium.
Therefore a lack of water makes it difficult
for many organisms to exist.
Estimating the percentage mass transfer of a plant
A large volume of water can pass through a
large tree in just one day.
The water is absorbed through the roots, but
most is lost from the leaves by transpiration.
If you know the mass of water absorbed by the
roots and the mass evaporated from the leaves, you can calculate the
% mass transfer of water e.g.
Suppose an oak tree absorbs 3000 kg of
water through its roots and loses 2500 kg through transpiration.
Calculate the % mass transfer of
water.
% mass transfer = (2700/3000) x
100 =
90%
We need good sources of fresh water
We, and most land based animals, need
freshwater from precipitation in the water cycle.
In times of drought e.g. in parts of Africa,
animals suffer and die from lack of water - uncertainties in weather
patterns, maybe worse due to global warming, make the situation
worse.
In 'cooler' developed countries, reservoirs
provide a constant supply of potable water - fit for domestic use.
In 'hot' dry countries that can afford it,
desalination is an option.
Desalination is the process of obtaining
pure water from salty water e.g. removing all the mineral salt
ions from sea water.
All desalination processes are costly - they use lots of energy.
Method 1.
Desalination by distillation
The simplest method is distillation
in which you boil off pure water in a large heated flask/tank.
The thermometer monitors the boiling temperature of the liquid.
From the flask/tank the steam is
cooled in the condenser so the water vapour condenses and the
water runs into a
clean tank/flask. All the much higher boiling mineral salts are
left behind as a solid residue.
This process is sometimes called
thermal desalination.
A
simple laboratory
demonstration is illustrated in the left diagram
(from GCSE chemistry revision notes).
Method
2. Desalination using reverse osmosis
The other principal method used is
reverse osmosis which forces water to go in the opposite
direction to concentration gradient rule.
Normally, water will pass through a
semi-permeable membrane (right diagram of a partially permeable
membrane) from a less concentrated solution to a more
concentrated solution i.e. from a higher concentration of water
to a lower concentration of water.
Think of the purple circles as the
salt ion particles.
In
reverse osmosis, the water molecules are forced to go in
the opposite direction (left diagram).
(i) Salt water is first treated by
filtration to remove any
insoluble solid debris like sand or twigs. The soluble salts are
obviously left in solution.
(ii) The water is then pumped under very
high
pressure into a vessel partitioned by a semi-permeable membrane.
(iii) The high pressure forces the water to go
in the opposite direction to 'normal' osmosis i.e. from a higher
salt concentration to lower salt concentration (from a lower
concentration of water to a higher concentration).
(iv) Therefore the salt particles concentrate
on the input side of the partially-permeable membrane and pure
water on the output side - so the salts are removed to give
potable water.
The more concentrated salt water produced
is steadily replaced by 'fresh' salt water and the purified
water drained off to a clean storage tank.
Research is going on to find more
effective membranes to increase efficiency and reduce costs e.g.
fabricated graphene sheets are being tested for making
semi-permeable membranes.
For more see
Water
Treatment, pollution and producing potable water (gcse chemistry
revision notes)
TOP OF PAGE and
sub-index on cycles and decay
(e)
Decomposition - factors affecting the rate of decay
of biological material
As already described, waste materials from plants and animals
must be recycled to maintain healthy ecosystems and avoid a build-up of too
much waste, including dead plants and animals.
The decay processes ensure the constant recycling of
the 'elements of life' to provide nutrients for new growth in plants and ultimately providing
the minerals, protein and carbohydrate food for animals.
The waste products and dead animals or plants are
broken down by decomposers.
Decomposers break down the smaller bits of
dead material, in doing so they release waste carbon dioxide, water
and energy (from their respiration), and, most importantly,
nutrients that plants can use.
The three main types of decomposers are bacteria
and fungi microorganisms and detritus feeders.
The latter include
millipedes, springtails, woodlice, dung flies and other insects,
maggots, slugs and worms - they
obtain nutrients by digesting the dead plant and animal material - they
are all well equipped with enzymes which they secrete to break
the molecules down!
The smaller the pieces of dead material are,
the greater the surface area exposed to enzyme action, so
helping the decay chemistry to be optimised.
Decomposers decay waste in compost heaps (e.g. in
garden) and in sewage works (as part of the purification process to make
potable water).
Just as in GCSE chemistry, where you study the
factors that control the rates of chemical reactions, we can look
at what factors control the rate of decomposition of this organic material.
The rate of decomposition of organic material from
dead organisms or their waste if living, is affected by various
environmental factors.
Many of the factors you will come across in
your chemistry "Factors
affecting the rates of chemical reactions"
A
note on mouldy food!
The following discussion of the four
factors affecting the rate of decay also apply to the rotting of
food.
Generally speaking the quantity of mould
(microscopic fungi) growing on a portion of food increases
exponentially (curve upwards!), but the decay takes days, weeks or
even months!
From the graph the average rate of mould
growth over the 10 days:
= 17 / 10 =
1.7 arbitrary units of mould per day
For the initial 4 days the average rate is:
2.5 / 4 =
0.63 arbitrary units of mould per day.
See section on
Methods of
preserving food
(1) The
availability of oxygen
Many decomposers require oxygen for aerobic
respiration.
Therefore the rate of decomposition increases with
increase in the ambient oxygen concentration.
This will produce more carbon dioxide, rather
than methane gas (biogas - which you may wish to make).
This is the situation with a garden compost
heap, where the decay is mainly via aerobic bacteria.
DO NOT confuse this situation with the
production of biogas using anaerobic bacteria, where oxygen levels
need to be minimised.
If the oxygen levels are low the rate of
decomposition is reduced - you get this in the anaerobic conditions
in water-logged soils and biogas digesters to make methane fuel.
However, some decomposer microorganisms can respire
anaerobically (not needing oxygen) but this is transfers less energy
(less exothermic) and these decomposers work more slowly.
Anaerobic decomposition-digestion produces more
methane (biogas), rather than carbon dioxide.
See
Respiration - aerobic and anaerobic in
plants and animals. gcse
biology revision notes
and
biogas
production (on this page)
(2)
The temperature
conditions
Most decomposers work most efficiently in warm
conditions, but not at too high a temperature.
(Remember that
enzymes are denatured at high temperatures and these
enzymes control the digestion breakdown of organic material at the
molecular level.)
All
chemical reactions speed up with increase in temperature, but enzymes
work best (fastest) within an optimum temperature range, often around
40oC.
The diagram on the right is for an enzyme with an optimum temperature
around 35oC.
At higher temperatures, reactant molecules have a greater average
kinetic energy to overcome the activation energy barrier and so
reactions can proceed faster.
The enzymes secreted the decomposer organisms that digest the
dead or waste material, can therefore work faster, more efficiently, as
the temperature is increased - but only so far!
However, at temperatures above ~50oC,
the protein structure of the enzymes begins to breakdown (denatured) and the
decomposition rapidly decreases with further increase in temperature.
The disrupted structure of the protein enzymes
means the substrate molecules can't 'dock in' to the active site
where the chemical transformation takes place.
The resulting graph shows the result of the two
competing effects - showing the optimum temperature and at high
temperatures the enzyme controlled digestion reactions stop.
(3)
The water content
of the soil and rotting material
All decomposers (like all of life)
require water to survive, no matter where their location.
The vast majority of chemical reactions in
living organisms require the medium of water.
Decomposition increases in moist conditions
compared to dry conditions.
The vast majority of chemical processes in
living organisms need the medium of water.
Waste material to make biogas methane is mixed
with water - all organisms need water to carry out their biological
processes e.g. enzymes work better mixed with water - better contact
with the organic waste being digested-decomposed.
However, there are situations where there is
little oxygen and too much water!
A good example is water-logged soil, where it
is difficult for air to permeate into the 'mud'.
As a consequence, the water contains little
dissolved oxygen, but decomposers do need it to respire - but can do so
anaerobically producing some methane - but this is a slower rate of
respiration.
Consequently, in water-logged soils, the rate
of decomposition is considerably slowed down.
(4) The
concentration of the 'decay' organisms
The more digesting organisms
(microorganisms/detritus feeders) in contact with a given amount of
waste plant/animal material, the faster the rate of
decomposition-digestion.
The bacteria and fungi that live on dead
material secrete their digestive enzymes onto the food to digest it
into soluble substances (smaller molecules) that can be absorbed.
This is called extracellular digestion
because it happens outside the cells of the microorganisms.
The more microbes the better, because there
are more enzymes in greater concentration!
TOP OF PAGE and
sub-index on cycles and decay
(f)
Simple experiment to
investigate effect of temperature on the rate of decay of milk
Introduction
Milk naturally contains an enzyme called lipase and this
breaks down fats into glycerol and fatty acids.
In the experiment you add extra lipase to speed up the decay
process in milk.
You make the milk alkaline (pH >7) so that the fatty acids formed
are neutralised raising the pH to >7 and this changes the colour of
an indicator - this colour change is a visual marker for the
reaction time.
In other words, as the milk breaks down (decays-decomposes) the
pH of the milk decreases.
The experiment measures the relative rate of decay of fats in
milk at different temperatures but you could adapt the
experiment to keep the temperature constant and vary the
concentration of lipase.
I'm not giving precise details of concentrations, and volumes
quoted are just typical values.
What I do describe are the principles of the experiment and how
to do it.
For an exam you need to appreciate all aspects of the experiment
- design, what is needed (apparatus and chemicals), how to do it and
how to process the results and draw conclusions.
Apparatus and chemicals needed
Thermostated water bath, test tubes, thermometer, 10 cm3
measuring cylinder
Milk, lipase solution, sodium carbonate solution,
phenolphthalein indicator solution
Investigation
method (with added explanation)
For the experiment mixtures:
choose a constant volume and constant
concentration of the lipase, sodium carbonate and
phenolphthalein solutions,
so, for a fair test, the total
volume in each experiment is the same and the only thing that
varies is the temperature of the thermostated bath (see
diagram).
Procedure:
Set the required temperature for the water
bath and check it is constant with a thermometer.
Measure a volume of the lipase solution
into a test tube, enough for several experiments.
Measure out a volume of milk in another
2nd test tube and add a few drops of phenolphthalein indicator
to the milk - then add a measure of the sodium carbonate
solution to this mixture.
NOTE: The solution should turn pink
because the solution is alkaline and phenolphthalein turns
pink above pH 10, but becomes colourless below pH 8.
Both test tubes are placed in the water bath
and left to reach the ambient set temperature of the water bath.
When ready, using a calibrated dropping
pipette, you measure 1 cm3 of the lipase into the
milk mixture, shake gently to mix thoroughly (or stir with clean
glass rod) and start the stopwatch.
The enzyme will immediately start to
decompose the milk producing an acidic product (a fatty acid).
Stop the watch and measure the time taken
for the pink colour of the indicator to become colourless as the
alkaline sodium carbonate is neutralised by the fatty acid
formed.
Repeat the experiment several times for
each temperature and repeat the whole experiment at different
temperatures e.g. 10, 15, 20, 25, 30, 35, 40, 45, 50oC
recording everything in a neat clear table - all the values
should be recorded and the average time for each temperature
too.
You can use ice cubes to cool the water
bath to temperatures below room temperature, but its tricky to
keep the temperature constant.
Results
The reciprocal of the time gives you a measure
of the rate of the decay reaction.
e.g. if the reaction time was 40 seconds, the
rate is 1/40 = 0.025 s-1.
Using the average times and rates for each
temperature, plot a graph of the rate versus temperature.
The rate of reaction is basically a measure of
a fixed quantity of decay (unit of fatty acid formed) per
unit time.
Conclusion
You should find the rate:
(i) increases at first (normal rate of
reaction rule from chemistry),
(ii) goes through a maximum at the optimum
temperature (typical of an enzyme)
(iii) and the rate falls away at higher
temperatures as enzyme lipase protein becomes denatured.
For more details of enzyme theory see
Enzymes - structure, functions, optimum conditions,
investigation experiments, digestion gcse
biology revision notes
Extension to investigation
You can adapt the experiment to keep the temperature constant and
vary the concentration of lipase.
You can choose a constant temperature
close to the optimum e.g. 30oC for the thermostated bath.
For
the experiment mixtures:
choose a constant volume of lipase
solution, BUT using different concentrations,
choose a constant concentration AND
volume of sodium carbonate and phenolphthalein solution,
so, for a fair test, the total
volume in each experiment is the same and the only thing that
varies is the lipase concentration.
All the apparatus, chemicals and method are
the same for the temperature varying experiment previously
described.
Your results should look something like the
graph above-right.
Initially the rate of milk decay should be
proportional to the enzyme concentration - as long as everything
else is kept constant.
The background chemistry to this investigation
lipases
Lipids, like many organic molecules, only
contain the elements carbon, hydrogen and oxygen.
Lipase enzymes break down lipids like
natural fats and oils (triglyceride esters) into glycerol and long
chain fatty acids. Lipids are NOT polymers because they are not very
long chain molecules.
Enzyme reaction word equation:
lipid == lipase enzymes ==> glycerol + long chain
fatty acids
The sort of molecular change that takes place - details you do not
need to know for GCSE level biology.
However in GCSE chemistry you would be expected to recognise the
acidic carboxylic acid group -COOH, an important 'molecular
feature' in understanding this decay experiment.
See also
Enzymes - structure, functions, optimum conditions,
investigation experiments, digestion gcse
biology revision
See also
Enzymes and Biotechnology
(gcse chemistry revision notes)
TOP OF PAGE and
sub-index on cycles and decay
(g)
Biogas from waste - the process and generator/digester/fermenter designs
See also
Biofuels & alternative fuels,
hydrogen, biogas, biodiesel GCSE chemistry revision notes
Introduction to biogas
Biogas (mainly methane CH4) is
produced naturally in marshes, septic tanks and sewers - oxygen
deficient places anywhere anaerobic bacteria thrive!
Compost consists of decomposed organic
material including plant waste from the garden or food waste from
the kitchen.
When enclosed in a compost bin it
gradually decomposes into a rich organic material that is a
really good natural fertiliser - a good example of partially
recycling the biomass from photosynthesis.
Air is admitted to the compost 'heap' and
little methane gas is made, in fact aerobic bacteria decomposers
are producing carbon dioxide.
Its no use for producing useful
quantities of biogas.
As an alternative, using specific
microorganisms, by enclosing the organic waste in a tank, you
can produce biogas.
Sludge waste from a sewage works (animal
waste!) or a sugar factory (plant waste) can be used to make gas
on a larger scale.
So, microorganisms (anaerobic bacteria) can be used to break down organic waste under
anaerobic conditions to produce biogas, which is mainly the
hydrocarbon methane gas, CH4.
Biogas can be produced from variety of
waste raw 'organic' materials such as agricultural waste,
manure, municipal waste, plant material, sewage, green waste or
food waste.
Biogas is produced by anaerobic digestion
with methanogen or anaerobic microorganisms, which digest
biodegradable materials.
Specially grown crops of maize are used in
some large-scale biogas digesters.
The residue from the digestion process can
be used as rich source of fertiliser.
Advantages of using biogas-methane
(a) The biogas can
be burned like any other fuel to produce heat.
Biogas is a relatively cheap fuel for
cooking, heating, vehicle fuels and small scale production of
electricity.
The heat can be
used to generate steam to drive a turbine and electrical
generator.
This is quite handy for small scale electricity
production in remote areas far from a national grid supply. It
could also power road vehicles too.
(b) Theoretically it is
eco-friendly, its a renewable resource and carbon neutral.
The decomposed
plants are replaced by new crops, and, with the animal waste
from eating plant material, the carbon is recycled by carbon
dioxide formation on burning.
The growth of new crops removes
and balances
the same carbon dioxide by the process of photosynthesis in
plant leaves.
(c) The raw materials for biogas
are relatively cheap and readily available, mainly from
agricultural sources.
(d) Burning biogas is also
eco-friendly because it is a relatively
clean fuel, although it produces carbon dioxide and water on
combustion it does not produce much pollutant gases such
as sulfur dioxide, oxides of
nitrogen or carbon or hydrocarbon particulates.
(e) The leftover waste after digestion can
be used as a fertiliser. (f)
In developing countries biogas generation advantages include
(i) reduces soil and water pollution, (ii) its a simple and low-cost
technology that encourages a green recycling economy and (iii) a
healthier and less polluting cooking alternative.
Animal dung makes a great source of biogas
on a small scale for cooking.
Disadvantages of using biogas-methane
(a) At the moment biogas cannot be
produced on a huge scale.
(b) An unfortunate disadvantage is
that the systems used in the production of biogas are not
efficient.
The biogas contains Impurities and even
after refinement and compression, it still contains impurities.
(c) There have been few recent
technological advancements.
(d) Biogas is less suitable for dense
Metropolitan Areas.
(e) Unfortunately, by its nature,
biogas cannot be readily stored as a liquid - you need a
very high pressure and a very low temperature to liquefy it
(boiling point of methane is -161oC at normal
atmospheric pressure!).
Therefore the biogas from the digester
must be used immediately for cooking, heating, lighting or using
the heat from combustion to make steam to drive a
turbine-generator to make electricity.
The composition of biogas
Typical values are quoted below, but a wide
variation depending on the source of the organic material.
Component of biogas |
% in biogas |
Comments |
methane, CH4 |
50 to 80% |
the fuel gas |
carbon dioxide, CO2 |
15 to 50% |
|
water vapour, H2O |
variable |
|
traces of other gases |
< 5% |
small amounts of H2S, N2,
H2, CO |
The design of a biogas generator - also
called a digester or fermenter
All biogas generators have the same basic
design and are based on a tank of varying size.
The tank has to hold sufficient amount of
rotting organic matter to ensure a steady production of biogas.
The starting organic waste material can be
animal dung, farm waste like slurry or garden waste.
The waste material is digested in a tank to
which the microorganisms are be added.
You need an input pipe to inject the waste
organic material into the tank.
You also need an output pipe to extract the
residue (waste slurry), which can be used as a fertiliser.
Batch biogas generators
You manually fill the biogas generator
with a relatively small amount of waste material, the batch to be
digested.
The batch is left to digest and when no
more gas is produced you have to extract the residue (by-product
for fertiliser).
Then the biogas generator is then completely
cleaned out and fresh lot of organic waste put in and the process
repeated to make the next batch of biogas.
This means you cannot have a continuous
stream of biogas.
Continuous biogas generators (the diagram
above is more like a continuous process)
With a continuous biogas generator, the
organic waste is continually fed in through the inlet pipe and, at
the same time, the residue of digested waste is
continually removed to be used as fertiliser.
This system allows for a continuous supply
of biogas, a big advantage over the batch process system and a
better design for larger scale production.
The best optimum reaction conditions for
producing methane
The factors affecting the rate of decay were
discussed in detail in
section (f), so just a brief summary is
repeated here.
To keep the microorganisms continuously
anaerobically respiring away as efficiently as possible!
1. Warm conditions e.g. a constant
temperature of around 35oC to 45oC - an optimum temperature
for many enzymes, which actually carry out the biochemical processes
of digestion.
A typical enzyme graph of rate of reaction
versus temperature.
(i) Initially rate increases with
increase in temperature, molecules have more kinetic energy,
more forceful fruitful collisions to break bonds and form
new products e.g. CH4.
(ii) However, above ~50oC,
the enzyme starts to be become denatured, the protein
structure of the active site is altered and cannot function
properly, and the rate dramatically falls with further
increase in temperature.
2. Exclusion of oxygen, so anaerobic
decomposition takes place via anaerobic respiration.
3. Mix the waste with water to make a
sort of slurry to give a better reaction medium.
4. A high concentration of decomposer
microorganisms - some will be already present in the
animal/plant waste, but you can add more to increase their
concentration.
Initially the rate of decay to produce biogas is
proportional to the enzyme concentration, which in turns depends on the
concentration of anaerobic bacteria.
See also
Biofuels & alternative fuels,
hydrogen, biogas, biodiesel GCSE chemistry revision notes
and
factors affecting the rates of
chemical reactions GCSE chemistry revision notes
TOP OF PAGE and
sub-index on cycles and decay
(h)
Making
compost
- a natural organic fertiliser
Compost is all about recycling organic kitchen or garden waste
to make a natural organic fertiliser and return useful nutrients to the soil
for crops or garden plants - we are usually talking about a relatively small
scale operation.
The 'ideal conditions' for
decomposers like bacteria and fungi to do their 'stuff'!
Some rates of reaction ideas from
chemistry come in handy as well as the decay factors described above.
1. Its best if the compost
material is shredded to increase the surface area for the decomposers
to 'attack'.
2. The compost bin sides
should have holes/mesh to allow air in (provides oxygen) and circulate
- many microorganisms need oxygen for aerobic respiration.
Some microorganisms do not need
oxygen for respiration (anaerobic), but they work more slowly.
For more details
see
Aerobic and anaerobic respiration
3. Warm conditions
favour faster decay, hopefully the bulk of the composting material warms
up from the heat released by the decomposition reactions to further
speed up the decay.
Every 10o rise in temperature
roughly doubles the rate of decay - but not too high or the
microbial enzymes are denatured above 50oC and the
organisms die.
Some compost bins are
thermally insulated
to keep heat in, but they must still be able to allow air to enter
and circulate e.g. a mesh intake or some regular holes in the side
of the compost bin.
4. The compost should contain some
moisture, but not to dry or too damp - water is produced by the
aerobic respiration of the decomposer microorganisms, but more is needed
for the decomposer organisms to carry out their chemistry - water is
both a reactant and a solvent for many biochemical processes.
5. You can add compost makers (decay accelerators) to speed
up the process,
TOP OF PAGE and
sub-index on cycles and decay
(i)
Food preservation methods
Food preservation techniques are important to
Increase the shelf life of food
products and ensure we don't suffer from food
poisoning due to contamination from pathogens.
Shelf life is the length of time that a commodity
may be stored without becoming unfit for use, consumption, or sale. It
might refer to whether a commodity should no longer be on a pantry shelf
and unfit for use, or just no longer on a supermarket shelf (unfit for
sale, but not yet unfit for use).
Here, to make the food product as safe as you can,
you want to
minimise the risk of contamination, and slow down the rate of decay as much as
possible.
Therefore we need to apply our knowledge of the
'rate of decay' factors described in a previous section and the
technicalities of food packaging e.g.
you can slow down the decomposition of food by
reducing the temperature, minimising water content and keeping out air
(oxygen) and potential pathogens.
Methods involved
1. Keeping the food at a low temperature to
slow down the rate of decay e.g. storing food products appropriately in
a refrigerator (~2-4oC ideally, NOT 0oC or
food freezes!)) or a freezer - the latter is so low in
temperature (e.g. -18oC) it stops decay all together!
The lower the temperature the lower the rate
the microorganisms can degrade the food and therefore reproduce -
less organisms - less decay- slower rate of decomposition.
This is all about slowing the rate of the
enzyme decay chemical reactions.
2. If it is possible, and appropriate, to
reduce the water content in food, this removes the medium the
microorganisms need to survive and reproduce.
You can also add salt and/or sugar to food causes
microorganisms to lose water by osmosis, denaturing the
microbial cells
and killing them - but note that modern dietary views advise us to
minimise our intake of salt and sugar!
3. Storing the food in airtight conditions
ensures neither oxygen or pathogens from air can get in and contaminate
the food.
Food cans can be filled and sterilised at
high temperatures to kill pathogens and the cans sealed
under high pressure.
4. Many products are airtight sealed in
plastic packages from which all the air (including oxygen) is removed
- this is vacuum sealing and completely inhibits any
microorganism from respiring aerobically.
TOP OF PAGE and
sub-index on cycles and decay
(j) Some learning objectives based on the above notes
- Be able to demonstrate an understanding of how recycling can reduce the demand for
resources and the problem of waste disposal, including ....
- Paper from wood - recycling paper reduces
the number of trees to be cut down (e.g. deforestation) and both transport
and energy costs are reduced. Recycled paper has become quite acceptable for
many paper based products.
- Plastics from limited oil reserves - oil is
becoming increasingly expensive and the reserves will not last forever, so
recycling plastics makes the oil go a bit further and reduces waste that is
often not biodegradable or take a very long time to degrade and decompose.
- Metals from limited mineral ore deposits -
high grade ores are being used up and less economic lower grade ores are
increasingly exploited using even more energy, often from burning fossil
fuels.
- We are using up lots of non-renewable
resources e.g. like fossil fuels and metal ores.
- However, in the case of metal ores, we can
recycle metals to reduce costs, including energy bills, and make the
original ore source go further.
- BUT note that recycling isn't without its
costs and inconveniences. Recycling involves collection of waste, sorting
into different material categories, purifying each material and then dealing
with the residual waste.
- Sorting can take time and some materials are
difficult to separate efficiently e.g. plastics, whereas iron objects can be
readily separated with a magnet.
- Sorting equipment can be expensive and some
sorting is done by hand.
- Recycled material is often not as good as
the original material and cannot be recycled forever. Its easy to recycle
metals like iron, steel, aluminium and copper many times, though each time
useful metal is lost, but plastics and paper can only be recycled a few
times.
- All living things are made of elements like
carbon, nitrogen, hydrogen and oxygen which are all obtained from the
environment they live in e.g. from the air, soil or water.
- By one means or other these elements are
returned to the environment as e.g. carbon dioxide in air, water or nitrogen
in air or nitrogen compounds in soil.
- If this did not happen, new life could not
be formed from the living feeding on pre-existing food (alive or dead).
- Food chains and decomposers play important
roles in this recycling as exemplified by the carbon cycle and nitrogen
cycle, both of which are illustrated and described below.
- The function of bacteria, decomposers, food
chains etc. is all explained.
- Be able to show an understanding of how carbon is recycled
(CARBON CYCLE diagram above):
- a) during
photosynthesis plants remove carbon dioxide from the atmosphere
-
carbon dioxide + water == light
energy/chlorophyll ==> glucose + oxygen
-
This
is the process by which plants make food, for themselves, and for most
animal life, including us too!
-
Note that the only way carbon
dioxide is removed from the air is photosynthesis in green land based plants
or marine organisms like phytoplankton (this point ignores long term
formation of carbonate rocks like limestone).
- b) carbon compounds pass along a food chain
- All food chains involve the passing of
carbon compounds e.g. sugars, carbohydrates, fats and proteins up to the
next trophic level i.e. the consecutive eating along a food chain (and waste
produced on the way).
- e.g. grass ==> cow ==> human
- c) during plant or animal aerobic respiration organisms release carbon dioxide into the
atmosphere
- sugars e.g. glucose + oxygen ==> carbon dioxide + water (+
energy)
- this is the main aerobic energy releasing
process in most living organisms.
- d) decomposers release carbon dioxide into the atmosphere
- slow aerobic respiration
- Microorganisms like bacteria and fungi in
the soil feed off decaying plant material and animal droppings or remains.
- Most dead plant matter consists of cellulose
which most animals can't digest, but bacteria and fungi, do have the enzymes
to break it down and without their help there would be no carbon cycle.
- Most of these bacteria and fungi respire
aerobically so they need a good supply of oxygen to produce the carbon
dioxide essential to keeping the carbon cycle going.
- e) combustion of fossil fuels releases carbon dioxide into the
atmosphere
-
Coal, formed
millions of years from the remains of tropical plant material, mainly
consists of carbon, Burning coal produces a lot of pollution as
the greenhouse gas carbon dioxide.
-
Natural gas (mainly
methane) and petrol molecules like octane (and lots of other
molecules) from oil and gas reserves.
- HT only: Be able to show an understanding of how nitrogen is recycled
(NITROGEN CYCLE diagram above):
- a) Nitrogen gas
in the air (78%, ~4/5th) cannot be used directly by
most plants and all animals.
- No animals and only a few specialised plants
can directly use the very unreactive nitrogen from air, but all
plants nitrogen in some form to synthesise amino acids and proteins for
growth and maintenance and for DNA in cell reproduction.
- However, nitrogen can be changed into
nitrogen compounds like nitrates which the plants can use.
- Animals rely on plants or other animals in
the food chain for their source of nitrogen compounds e.g. protein in grass,
crops or other animals.
- b) Nitrogen-fixing
bacteria living in root nodules of plants or in the soil can fix nitrogen gas.
- Leguminous plants like peas, lentils, clover
and beans can absorb nitrogen from the air via their root nodules (swellings
on the root surface) which contain enzymes capable of converting ('fixing')
atmospheric nitrogen into soluble nitrate - a nutrient essential for amino
acids, proteins and therefore plant growth.
- Legumes and their root nodule bacteria are
an example of mutualism (see section 3.19 b) because the plant root supplies
the bacteria with carbohydrate food and minerals and the bacteria supplies
the plant in the form of the nitrate ion.
- The process of converting nitrogen in air
into nitrogen compounds is sometimes called 'nitrogen fixation'.
- c) The action of
lightning can convert nitrogen gas into nitrates.
- The very high electrical energy discharges
from lightning activates nitrogen and oxygen molecules to react and form
nitrogen oxides. These dissolve in rain to form nitrates which end up in the
soil when rainwater trickles into the soil.
- d) Decomposers break down dead
animals and plants
- Decomposers, e.g. various organisms like
bacteria, fungi or worms can break down dead animals or plants. They break
down proteins to amino acids.
- e) Soil bacteria convert proteins and urea into ammonia
or ammonium ions.
- Decomposer bacteria in the soil can change
proteins from dead plants/animals and urea in animal urine/droppings into
ammonia/ammonium ion compounds.
- d) plus e) is sometimes called putrefaction
by putrefying bacteria.
- f)
Nitrifying bacteria convert this ammonia to nitrates - the process of
nitrification
- Nitrifying bacteria oxidise ammonia/ammonium
ions from the decayed material to form nitrates, the nitrate ion can be
absorbed by plants through their root systems.
- g) Plants absorb nitrates
from the soil.
- Plants absorb nitrates (soluble in water) in
the moisture that the roots absorb from the surrounding soil.
- Plants can use the nitrate ion in forming
amino acids from which the plant can make its proteins.
- h) Nitrates are needed by plants to make proteins for growth.
- Nitrates are an essential nutrient for
plants to synthesis amino acids and hence proteins.
- i)
Nitrogen compounds pass along a food chain or web of food chains.
- All food chains involve the passing of
carbon compounds e.g. sugars, carbohydrates, fats and proteins up to
the next trophic level i.e. the consecutive eating along a food chain (and
waste produced on the way).
- e.g. grass ==> cow ==> human
- Plants make their own protein from nitrates,
but animals must obtain it from plants or other animals. In fact the protein
is broken down in digestion to amino acids and each animal makes its own
proteins from these amino acid residues.
- j) Denitrifying bacteria
convert nitrates to nitrogen gas.
- Particular bacterial organisms can remove
the oxygen from nitrate compounds to form the element nitrogen gas.
- These denitrifying bacteria live in
anaerobic conditions like waterlogged soils and use the nitrate ion to
respire.
- This is the opposite function of the
nitrogen-fixing bacteria (b).
-
Know that many trees shed their leaves
each year and most animals produce droppings at least once a day.
-
All plants and animals
eventually die and know that microorganisms play an important part in decomposing this
material so that it can be used again by plants.
-
Appreciate that the same material is recycled
over and over again and can lead to stable communities.
-
You are expected to
use your skills, knowledge and understanding to evaluate the necessity
and effectiveness of schemes for recycling organic kitchen or garden waste.
-
Like using a compost bin!, to
which you can add garden waste and kitchen waste. Its best if the compost
material is shredded and the compost bin sides have holes/mesh to allow air
in and circulate. You can add compost makers (decay accelerators) to speed
up the process, but hopefully the bulk of the composting material warms up
by heat released by the decomposition reactions to further speed up the
decay.
-
a) Appreciate that living things remove
materials from the environment for growth and other processes.
-
Plants need carbon as carbon
dioxide from air, hydrogen as water, oxygen from air/water and nitrogen from
air/soil or as nitrates from the soil, plus other minerals via water through
the roots.
-
From these elements and
compounds, plants can make carbohydrates, fats and proteins as well as a
source of absorbed minerals.
-
Therefore, when animals eat
plants they digest the carbohydrates, fats, proteins and minerals, and then
convert these materials into their own fats and proteins.
-
Know that these
materials are returned to the environment either in waste materials or when
living things die and decay.
-
b) Know and understand materials decay because they
are broken down (digested) by microorganisms.
-
c) The decay process releases
substances that plants need to grow.
-
d) Know and understand that in a stable community, the processes
that remove materials (plant/animal growth) are balanced by processes that return materials
(microorganism decay) and the
materials are constantly cycled (in a way 'recycled').
-
Revise and practical
work-investigations that helped develop skills and
understanding which may have included the following (which should also
be revised, helps in understanding 'how science works' and context
examination questions):
-
designing and carrying out an investigation
to measure the rate of decay of bread by, for example, exposing cubes of
bread to air before placing them in sealed Petri dishes at different
temperatures and/or different moisture levels,
-
investigating the rates of
decay using containers (eg thermos flasks) full of grass clippings, one with
disinfectant, one with dry grass, one with wet grass and one with a
composting agent.
-
potato decay competition, using fresh potatoes
- you decide on the
environmental conditions and the rate of decay is measured over a 2 week
period,
-
using a sensor and data logger to
investigate carbon dioxide levels during the decay process.
-
Biogas (mostly methane CH4
and some carbon dioxide), is formed by the anaerobic
decomposition-fermentation of organic waste by microorganisms (bacteria with
right enzymes to decompose organic compounds).
-
Organic waste eg plant or animal
from domestic refuse (waste food), farm waste (usually from animals), sewage
sludge waste, factory waste (from sugar factories) sources etc. is broken
down by microorganism to the simplest organic compound, namely methane.
-
It can be carried in quite
simple biogas fermenters, sometimes called biogas digesters or biogas
generators.
-
The anaerobic fermentation
should carried out in the absence of air with the right bacteria and at a
constant temperature appropriate to the optimum rate of catalysis of the
enzymes in the bacteria e.g. 30-40oC.
-
The biogas is easily stored
because it isn't easily liquified and is an explosive flammable gas.
-
The gas can be used directly for
heating, lighting and cooking.
-
The gas can also be burned to
provide heat to make steam to drive a turbine and electrical generator.
-
Biogas generation can be done on
a small domestic scale or large scale and the residue (what's left after
tapping off the biogas) can be used as fertiliser.
-
All this waste will rot
naturally, often under aerobic conditions, so its worth noting, that,
disposing of organic waste in a biogas generator in this way, is better than
letting the methane diffuse into the atmosphere where its acts as a powerful
greenhouse gas.
-
All biogas generators will have
features in common ...
-
an inlet for the waste organic
material to be fermented,
-
a valve controlled outlet for
the biogas formed,
-
an outlet for the waste material
left over after the digestion has finished.
-
Biogas generators (biogas
digesters or biogas fermenters) are designed to operate in one of two ways
...
-
In a batch process, the
biogas is made in small amounts or batches i.e. the biogas generator is
filled up and left to ferment, the biogas is continually tapped off once the
anaerobic fermentation starts and then when gas production falls
significantly the residue is cleaned out. The generator is then re-filled
new organic waste material and the whole process repeated.
-
Cheaper batch process technology
than continuous process because of simpler digester design.
-
Batch processes are not as
efficient because after each batch has fermented the process must be stopped
and the generator stopped, cleaned and re-filled to restart.
-
In a continuous process,
the waste organic material is continually fed/pumped into the generator, the
gas continually drawn off and the residue continually removed/pumped out.
-
The continuous process generator
employs more costly technology because of the extra pumps needed.
-
A continuous process is more
efficient and economic than a batch process, no stopping, cleaning out and
re-filling required.
-
Either production method
requires good temperature control of 30-40oC, so biogas
generators may need insulation if too cold or a heating source if too cold.
-
Ideally the biogas generator
should be sited near the source of organic waste e.g. a small scale batch
process on a farm or a large scale continuous process at a sewage farm.
-
The use of biofuels has both
environmental and economic benefits.
-
Biofuels should be 'carbon
neutral', that is the carbon dioxide they release on burning is re-absorbed
by the plants the carbon originally came from.
-
On burning they do not release
sulfur oxides that cause acid rain.
-
Biogas digesters are a good way
of using potentially harmful and polluting organic waste (contain
pathogens), rather than just dumping it in the ground, where, it will still
break down and release methane - a powerful greenhouse gas contributing to
global warming.
-
In poor rural areas biogas from
animal dung is a convenient way of providing heat for cooking, especially if
wood is scarce and in general the raw waste organic material is cheap and
readily available.
Practical work you may have done
-
building a simple biogas generator to collect methane and demonstrating how the
methane can be burned as a fuel
-
investigating and designing a way of measuring the gas output of a biogas generator
and compare the amount of
gas produced by different materials.
See also
Ecosystems - biotic & abiotic factors - interactions between organisms
- interdependency gcse biology
Food chains, food webs and biomass
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of decomposition OCR GCSE
9-1 Gateway
biology science
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affecting rates of decomposition WJEC gcse science CCEA/CEA gcse science
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