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Biology
Unit Four

Chapter One

Chapter One

Definitions
• Species: a group of similar, living organisms that are able to reproduce together
to produce a fertile offspring
...

• Community: a group of interdependent plants or animals growing or living
together in natural conditions or occupying a specified habitat
...

• Niche: a role taken by a type of organism within its community
...

• Biosphere: the life supporting layer of land, air and water that surrounds Earth
...
The flow of energy through the system
...
The cycling of elements within the system
...


•
•

Only small samples are taken due to the time consuming nature
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•
•

Factors to consider when using quadrats 
Size of quadrat – larger species need larger quadrats – if a species occurs in a series
of groups then a large number of small quadrats is used
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Position of quadrat – statistically significant results are obtained by random
sampling
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Lay out two long tape measures at right angles along two sides of the study area
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Obtain a series of coordinates by using random numbers taken from a computer
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Place a quadrat at the intersection of each pair of coordinates and record the
species within it
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• Belt transect – two parallel lines of string/tape are used – species within the
belt (between the lines) are recorded
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Frequency – likelihood of a particular species occurring in a quadrat – quick
idea of species present but doesn’t provide density or distribution of species
...
Percentage cover – estimate of the area within a quadrat – useful when
species are particularly abundant – rapid data collection but it overlaps
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Investigating Populations
• Mark-release-capture method 
• Due to animals being mobile, this method is used
...

• The technique relies on assumptions 
• Proportion of marked to unmarked individuals in the second sample is the same
as the population as a whole
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• Population has definite boundary so no immigration or emigration occurs
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• Marking is not toxic or makes the animal more liable to predation
...


Investigating Populations
• Population Equation 
(total number of individuals in first sample X individuals in second sample)
______________________________________________________________
number of marked individuals recaptured

Variation in Population Size

• Three phases 
1
...

2
...

3
...


Variation in Population Size
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Limiting factors 
Mineral ions
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Temperature
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Food
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Light – ultimate source of energy
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Water – in low water areas, only species well adapted to dry conditions survive
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Competition
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Intraspecific competition 
Between the same species
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Greater the resources available, the larger the population
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Compete for food, light and water
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• The population of the stronger species will gradually increase while the other
diminishes eventually to be removed – competitive exclusion principle
...


Predation
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Relationship between prey and predator 
Predators eat their prey – prey population reduces
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Predator population is reduced – fewer prey eaten
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More prey – predator population increases
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•

Population crashes create selection pressure where survival of the fittest occurs –
survivors will reproduce
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•

Human Populations
• Explosion in human population due to 
• The development of agriculture
...

• War, disease and famine have only been temporary reversals in the upward
trend
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Death rate
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Emigration – individuals leave a population
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Religious – some religions oppose birth control
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Birth control – contraception isn’t always available
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Death rate factors 
Age profile – the more elderly people, the higher the death rate
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Food supply – need a balanced diet to live long
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Medical care availability
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War
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Human Populations
• Increasing population 
• High birth rate (shows wider base) and fewer old people (narrow apex)
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Human Populations
• Survival rates 
• Plotting a survival curve allows life expectancy to be calculated
...


Chapter Two

Definitions
• Hydrolysis: breaking down of large molecules into smaller ones by the addition
of water
...

• Adenosine Triphosphate (ATP): an activated nucleotide found in all living cells
that acts as an energy carrier
...

• Oxidative phosphorylation: in the mitochondria of plant and animal cells
during electron transport
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ATP
• All living organisms require energy to survive
...

Energy cannot be created nor destroyed
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It can change from one form to another
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Movement – for in and out of the body itself
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• Repair and division
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• Maintenance of body temperature – mammals and birds are endothermic and
need energy to replace that lost as heat to the environment
...

• The chemical energy from photosynthesis, in the form of organic molecules, is
converted into ATP during respiration
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• Storing ATP 
• ATP has three phosphate groups which have unstable bonds, thus a low
activation energy – easily broken
...

• The reaction is a hydrolysis reaction
...

• It occurs in three ways 

 Photophosphorylation – in chlorophyll-containing plant cells during
photosynthesis
...

 Substrate-level phosphorylation – in plant and animal cells – when phosphate
groups are transferred from donor molecules to ADP to form ATP
...


ATP
• Roles of ATP 
• Immediate energy source - its energy store is not long lasting due to instability
of the phosphate bonds
...

• It is rapidly regenerated from ADP and an inorganic phosphate
...

• Hydrolysis of ATP is a single reaction
...

Movement – provides energy for contraction
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Secretion – forms lysosomes needed for secretion
...


Chapter Three

Definitions
• Electron carrier molecule: a chain of carrier molecules along which electrons
pass, releasing energy in the form of ATP
...

• Thylakoids: series of flattened membranous sacs in a chloroplast that contain
chlorophyll and the associated molecules needed for the light-dependent
reaction
...

• Stomata: pores surrounded by guard cells that allow gas diffusion
...

Leaves minimise overlapping – no shadowing
...

Transparent cuticle and epidermis let light through to photosynthetic mesophyll
cells
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• Air spaces allow diffusion
...

• Photosynthesis 

Photosynthesis
• Light-dependent reaction  light is converted into chemical energy, an
electron flow is created and causes water to split (photolysis) into protons,
electrons and oxygen
...

• Light-independent reaction  protons are used to reduce carbon dioxide
to produce sugars and other organic molecules
...

• Structure of chloroplast 
• Grana formed from thylakoids house the light-dependent stage
...

• Stroma is a fluid filled matrix where the light-independent reaction occurs
...

• Photolysis occurs in the thylakoid space – this splits water into oxygen,
protons and electrons
...

• The electrons are taken up by photosystem II – these electrons are excited
by the light photons and reach a higher energy level
...

These electrons carries form a transfer chain in the membrane
...

• The energy released is used to combine ADP and an inorganic phosphate to
produce ATP in ATP synthase
...

• NADPH goes on into the light-independent reaction
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• Takes place in the stroma
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Stages 
Carbon dioxide diffuses through the stomata and into the stroma
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Glycerate 3-Phosphate (GP) is formed, two molecules per one combination
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NADP is reformed and goes back to the light-dependent reaction
...

Most TP is used to regenerate RuBP using ATP from the light-dependent
reaction
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1C
3C

5C

3C

5C
-di

3C

3C

3C

Glucose

6C

Factors Affecting Photosynthesis
•

Slowest reaction determines the overall rate
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As light intensity increases, the volume of oxygen produced and carbon dioxide
absorbed will increase until its balanced – compensation point (no net exchange of
gases into or out of the plant)
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•
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Carbon dioxide 
0
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Carbon dioxide concentration affects enzyme activity
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25˚c is the optimum temperature, after this, the rate decreases due to enzymes
becoming denatured
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•

•

Chapter Four

Respiration
• Glucose cannot be used directly by cells as an energy source, so they use ATP
...


• Aerobic respiration  requires oxygen and produces carbon dioxide, water and
lots of ATP
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• Aerobic respiration steps 
1
...

2
...

3
...

4
...


Glycolysis
•

Occurs in the cytoplasm of all living cells
...
Activation of glucose by phosphorylation – glucose is made more reactive by
adding two phosphate molecules – these come from the hydrolysis of two ATP
molecules in a stage known as energy investment
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2
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3
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4
...


Glycolysis

Link Reaction
• Pyruvate produced in the cytoplasm during glycolysis is actively transported
into the matrix of the mitochondria
...

• This occurs in the mitochondria
...
Hydrogen is removed from each pyruvate and accepted by each NAD to form
NADH (reduced NAD)
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The two-carbon molecule (acetate) is formed and then combines with
coenzyme A (CoA) to produce acetylcoenzyme A
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A carbon dioxide molecule is formed from each pyruvate
...

• Decarboxylation – carbon dioxide is removed which converts pyruvate into
acetate
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• Stages 
1
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2
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3
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4
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5
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6
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7
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Products 
Reduce coenzymes – NAD and FAD have the potential to produce ATP molecules
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Three molecules of carbon dioxide
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 It produces hydrogen atoms carried by NAD to the electron
transport chain for oxidative phosphorylation
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 It regenerates the four-carbon molecule that combines with
acetylcoenzyme A
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Krebs Cycle

Electron Transport Chain
• Occurs in the mitochondria
...

• This releases the protons from the hydrogen atoms and these are
actively transported across the inner membrane
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• The protons accumulate in the space between the mitochondrial
membrane before they diffuse back into the matrix through protein
channels – due to a concentration gradient
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• Oxygen is the final acceptor of hydrogens
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• Without it, the final acceptor is missing
...


• NAD will not be regenerated as it will have to re-accept the hydrogens
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• Glycolysis can continue but it will soon run out of NAD
...


Without oxygen, the Krebs Cycle and the Electron Transport Chain cannot
occur
...
For it to continue, its products
of pyruvate and hydrogen must be constantly removed
...
If it wasn’t converted, no NAD could take up the newly
produced hydrogen from glycolysis
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Anaerobic Respiration
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Bacteria, fungi & plants 
Produces ethanol
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Yeast is grown in anaerobic conditions to produce ethanol for brewing
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Occurs commonly in muscles due to strenuous exercise
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Each pyruvate molecule produced takes up to two hydrogen atoms from NADH in
glycolysis to form lactate
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It can then be further oxidised to release energy or be converted into glycogen
...


Chapter Five

Food Chains & Food Webs
• Producers  photosynthetic organisms
...

• Those that eat producers are primary consumers
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• This releases valuable minerals and elements
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• Detritivores  certain animals can carry out the same work but to a lesser
extent
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• Food web  in reality, most animals do not rely upon a single food source –
food chains are linked to form a food web
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Most of the sun’s energy is not converted by photosynthesis because 
Over 90% is reflected back into space or absorbed by the atmosphere
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Light may not fall onto a chlorophyll molecule
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• Gross production  total quantity of energy that plants convert
...

• Net production  rate at which energy is stored
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Secondary/tertiary consumers transfer about 20% of energy available from their
prey to their own bodies
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Some parts not digested – lost in faeces
...
g
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Some energy losses occur as heat from respiration
...

Total mass of organisms in a particular place (biomass) is less at higher trophic
levels
...


•

Energy transfer = (Energy available after/Energy available before) X 100

Ecological Pyramids
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Used to know the number, mass or amount of energy stored by organisms at each
trophic level
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Pyramids of number 
No account is taken of the size of the organism  one giant tree is the same as one
tiny aphid
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The number of individuals can be so great that it is impossible to represent them
accurately on the same scale
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Biomass  total mass of plants and/or animals in a particular place
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Dry mass  organisms must be killed – only a small sample – may not be
representative
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Only organisms present at a particular time are shown
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Data collected in a given area for a set period of time
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• Usually measured in KJ/m2/year
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• This channels the energy flowing through a food web into the human food
chain – increases productivity of human food chain
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• Net productivity in agricultural ecosystems is affected by 
• Efficiency of crop photosynthesis – improved if necessary conditions are
supplied
...


• Energy input 
• To maintain an agricultural ecosystem, climax communities are prevented
from developing
...


Agricultural Ecosystems
• To remove/suppress unwanted species and to maximise growth requires
additional energy input 
• Food – farmers expend energy as they work – comes from the food they
eat
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• Natural ecosystems have low productivity – additional energy increases
productivity in agricultural ecosystems – reduces effect of limiting factors
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• Ground is covered almost exclusively by crop
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Control of Agricultural Pests
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Pests  an organism that competes with humans for food and space, or a danger to health
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An effective pesticide should 
Be specific – only toxic to the directed organism – harmless to humans and others
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Be cost-effective – development costs are high and new pesticides are useful for a limited time –
pests can develop a new genetic resistance
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Aim is to control the pest, not eradicate it
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Biological control advantages 
Very specific
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No resistance
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Control organism may itself become a pest
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Integrated control involves 
Choosing animal or plant varieties that suit the local area and are as pest-resistant as
possible
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Regularly monitoring crops for signs of pests
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Using biological agents if necessary and available
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Pests reduce productivity in agricultural ecosystems
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Pests of domesticated animals may cause disease
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Two different interests involved 
Farmers – must satisfy our demand for cheap food
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Intensive Rearing of Domestic Livestock
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Intensive rearing is about converting the smallest possible amount of food energy
into the greatest quantity of animal mass – minimises energy losses
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Energy conservation can be made more efficient by ensuring that as much energy
from respiration as possible goes into growth
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•

This increases energy-conversion rate because 
Movement is restricted and so less energy is used in muscle contraction
...

Feeding can be controlled so that the animals receive the optimum amount and
type of food for maximum growth with no wastage
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Other means of improving the energy-conversion rate 
Selective breeding – more efficient at converting food into body mass
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Chapter Six

Carbon Cycle
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Nutrient cycle sequence 
Nutrient taken up by producers as simple, inorganic molecules
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Producer is eaten – consumer takes in nutrient
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Producers and consumers die – complex molecules are broken down by
saprobiotic microorganisms (decomposers)
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• Increase in carbon dioxide levels 
• Combustion of fossil fuels – coal, oil and peat
...


Carbon Cycle

Greenhouse Effect & Global Warming
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Greenhouse effect 
Some radiation reaching the surface is reflected back as heat and lost in space
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The gases trap heat close to the surface
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50-70% of global warming is due to carbon dioxide
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This occurs when 
Decomposers break down dead remains
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Global warming 
Mean global temperature increased by 0
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Concentration of carbon dioxide has risen from 270ppm before the industrial revolution to 370ppm
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Rise in sea levels due to thermal expansion of ocean could flood low-lying land
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Greater rainfall and intense storms would occur in some areas due to disturbance of climate
patterns
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Greenhouse Effect & Global Warming

Nitrogen Cycle
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Ammonification 
Production of ammonia from organic ammonium-containing compounds – urea,
proteins, nucleic acids and vitamins
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Nitrification 
Some bacteria obtain energy from chemical reactions
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• Nitrifying bacteria need oxygen – need soil with air spaces
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• Two stages 
1
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2
...


Nitrogen Cycle
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Nitrogen fixation 
Nitrogen gas is converted into nitrogen-containing compounds
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Carried out by microorganisms 
Free-living nitrogen-fixing bacteria – reduce gaseous nitrogen to ammonia – used
to manufacture amino acids – nitrogen-rich compounds released in decay
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Denitrification 
Different bacteria present in water logged soils
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Converts soil nitrates into gaseous nitrogen
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Soil must be kept well aerated
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• In agricultural systems, plants are harvested and removed
...

• Natural (organic) fertilisers  consists of dead and decaying remains of
plants and animals as well as animal wastes
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• Combination of natural and artificial gives the greatest long-term increase in
productivity
...

• Increases photosynthesis rate – improves crop productivity
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• Leaching
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• Leaching 
• Rain water dissolves nitrates – carries them deep into the soil
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• It can prevent efficient oxygen transport in babies
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Consequences of Nitrogen Fertilisers
• Eutrophication 
• Nitrate is a limiting factor for plant and algal growth
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• Algal blooms form
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• Light becomes the limiting factor for growth of plants – they then die
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• Saprobiotic bacteria require high demand of oxygen
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• Oxygen is the limiting factor for anaerobic organisms e
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Fish
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Chapter Seven

Definitions
• Climax community: the organisms that make up the final stage of ecological
succession
...

• Biomass: the total mass of a living material in a specific area at a given time
...


Succession
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Ecosystems constantly change  succession
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Pioneer species adaptions 
Product vast quantities of wind dispersed seeds/spores
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Ability to photosynthesise
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Tolerant of extreme conditions
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Greater number and variety of habitats
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More complex food webs
...


Succession
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Succession stages 
Over these stages, the environment becomes more hospitable and new species
begin to grow which can outcompete other species
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The pioneer species changes the abiotic environment by creating soil and nutrients
...

The hospitality is increased until the ultimate community is formed – a climax
community
...

Secondary succession 
If land has been cleared for agriculture or a forest fire, the process of succession still
occurs
...

There is no need for a pioneer species
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Reasons for conservation 
Ethical – species should co-exist with humans
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Cultural and aesthetic – habitats and their organisms enrich our lives
...

Due to being outcompeted or their habitat being no more, the species often
migrate
...

If the factor preventing further succession is removed, then the climax community
will grow in secondary succession
...

• Genotype sets the limits within which characteristics may vary
...

• Mutation  any change to the genotype as a result of a change to the DNA
...

• Gene  a section of DNA, a sequence of nucleotide bases, that usually
determines a single characteristic of an organism – codes for particular
polypeptides
...

• Allele  one of the different forms of a gene
...


Studying Inheritance
• In sexually reproducing organisms, chromosomes occur in pairs –
homologous chromosomes
...

• Homozygous  if both alleles are the same
...

• Dominant  the allele of the heterozygote that expresses itself
...


• Homozygous dominant  two dominant alleles
...

• Co-dominant  two different alleles that both contribute to the phenotype
– blend of both features or both features are represented
...


• Pure breeding  homozygous for a particular gene
...

• Law of segregation 
• In diploid organisms, characteristics are determined by alleles that occur in
pairs
...


Sex Inheritance & Linkage
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Sex is determined by chromosomes, not genes
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Males have one x and one y-chromosome
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For most the length of the x-chromosome, there is no equivalent homologous
portion on the y-chromosome – this chromosome is smaller
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Haemophilia 
Carried on the x-chromosome
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Almost entirely confined to males
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Females are carriers if they’re heterozygous for it – passed on through mothers
...


Co-Dominance & Multiple Alleles
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Co-dominance  both alleles are equally dominant
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Multiple alleles  there are more than two alleles for a gene – only two can be
present in the loci of the gene
...

Allele IB codes for antigen B
...

Blood Group

•

Possible genotypes 

Genotype

A

IAIA or IAIO

B

IBIB or IBIO

AB

IA IB

O

IOIO

Allelic Frequencies & Population
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Gene pool  all of the alleles of all of the genes of all the individuals in a
population
...


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Hardy-Weinberg Principle 
The principle predicts that the proportion of dominant and recessive alleles of any
gene in a population remains the same from one generation to the next,
provided 
No mutations arise
...

No selection
...

Mating within the population is random
...

Let allele a frequency = Q
...


• P2 + 2PQ + Q2 = 1
...


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Differences between reproductive success affects allele frequency 
All organisms produce more offspring than can be supported
...

Hence, intraspecific competition arises
...

These are more likely to obtain available resources, survive and reproduce
...

New individuals inherit “advantageous” alleles
...


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“Advantageous” alleles depend upon environmental conditions
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Stabilising selection  favours average individuals and preserves characteristics
...


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If two populations become separated, the flow of alleles between them may cease
...

In time, the gene pools may become so different that, even if reunited, they would
be incapable of successfully breeding with each other
...

Climate changes over centuries lead to drier conditions which reduce the forest area
and separate it into two regions many kilometres apart
...

In A, phenotypes are selected to survive colder and wetter conditions
...

Type and frequency of alleles in gene pools of each groups of species X become
increasingly different
...


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