AQA Unit 4 A2 Biology

1. Biology
Unit 4
AQA

2. Ecology Definitions
Habitat – The place where an organism lives
Population – A group of organisms belonging to the same species
Community – All the populations of different organisms living and interacting
in the same space at the same time
Ecosystem – A community of living organisms and the abiotic factors which
affect them
Abiotic – The physical and chemical features of the environment
Biotic – The biological features of the environment (living)
Niche – A species role within it’s habitat
Adaptation – A feature that members of a species have to increase their
chance of survival

3. Investigating
Populations
Quadrats:
- Set out 2 tape measure at right angles, forming the axes for the
chosen area
- Generate 2 random numbers (using calculator) to use as co-
ordinates
- Place quadrat where co-ords meet
- Find mean number of species per quadrat
- Multiply by size of area being sampled
Transects:
- It’s a line through an area to be studied to identify changes through
an area
- Line Transects – a tape measure is placed along the transect and
the species that touch the tape measure are recorded
- Belt Transects – quadrats are placed next to each other along the
transect to work out species frequency &
percentage cover along a transect

4. Measuring Abundance
Quadrats:
- Have a known dimension
- Used to:
- Estimate population density
- Estimate % cover of an organism
- Estimate the frequency of an organism
Factors:
- Size of quadrat – More small quadrats = more representative results
- Number of quadrats – more quadrats = more reliable results
- Position of quadrat – must be placed randomly to avoid bias
At least 20 samples taken. Eventually a sample size is big enough that the
number of species doesn’t increase much more the sample is said to be
representative.

5. Sampling Strategies
Random Sampling:
• When area is uniformed
• Use random co-ordinates
Systematic:
• When there’s an environmental gradient
• Use a transect
Stratified:
• Compare two areas
• Select two areas & sample within them

6. Mark-Release
Recapture
A known number of animals are caught and marked. They’re then released back.
Later another sample are caught and the number of marked individuals is recorded
Assumptions:
- No reproduction
- No migration
- Enough time for both marked & unmarked animals to mix
- Marking doesn’t affect behaviour

7. Variation in Population Size
Abiotic Factors:
- Affected by factors such as temperature, light, space, water etc…
- When conditions are ideal an organism will thrive and vice versa
Biotic Factors:
- Interspecific Competition:
- Competition between different species
- Intraspecific Competition:
- Competition between the same species
- Predation – Predator & Prey populations are linked
- Prey increases, more food, so predator increases.
- Predator eats prey, prey decreases as they’re eaten
- Predator decreases due to lack of food
- Predator peaks after prey

8. Human Populations
Population Growth = (BR + Immigration) – (DR + Emigration)
% Population Growth Rate = x 100
Demographic Transition Model:
- Shows the change in BR, DR & population size over along period of time
Population Change
Population Start

9. Survival Curves
Show the percentage of all individuals that were born in a population that are still
alive at a given age.
Life Expectancy – is the age someone is expected to live to
- it’s the age at which 50% of the population are still alive
e.g. the life expectancy of this example is
81 as that is the age when 50% of the
population are still alive

10. Age-Sex Population
Pyramids
West Africa:
- High BR
- Short Life Expectancy
- High DR
- Developing Country
West Europe:
- Lower BR
- Long Life Expectancy
- Lower DR
- Developed Country

11. Ecosystem Definitions
Producer – They’re photosynthetic organisms that manufacture organic
substances using light energy, water and CO2
Consumer – They’re organisms that obtain their energy by feeding on other
organisms
Decomposers – When consumers & producers die, the energy can be used
by organisms that break down the complex materials into single
components again
Food Chains – Describes a feeding relationship in which the producer are
eaten by the primary consumers. They’re then eaten by secondary
consumer
Trophic Level – The level between each stage in the food chain
Food Web – More than one food chain linked together
Grass  Sheep  Human
(Producer) (1° Consumer) (2° Consumer)
Trophic Level

12. Energy Transfer Between
Trophic Levels
Little solar energy converted to chemical energy in PS:
- Some is reflected due to wrong wavelength/frequency/colour
- Doesn’t hit chlorophyll molecule
- Lost as heat during evaporation
Energy is lost along a food chain:
- Not all the organism is eaten
- Not all organism digested – lost in faeces
- Urine
- Heat in respiration
- Movement
- Birds & Mammals – energy used to maintain a constant body temperature
(homeostasis)
Not enough energy to support further trophic levels, so rarely more than 4
trophic levels present in a food chain

13. Gross Primary Productivity (GPP) – Amount of light energy that plants
convert to chemical energy
Net Primary Productivity (NPP) – Total amount of energy stored in a plant
that is available to the next trophic level
NPP = GPP - Respiration
Measured in kJ m-2 Year -1
Energy Energy after Transfer
Transfer (%) Energy before Transfer
Net Primary Productivity
= 100X

14. Ecological Pyramids
Pyramids of Numbers:
Pyramids of Biomass:
• Fresh Biomass less accurate as different amounts of H2O present in organism
• Dry Biomass better but organism has to be killed
Fox
Rabbit
Grass
Ladybird
Aphid
Oak Tree Oak Tree
Caterpillar
Parasite
Caterpillar
Large Fish
Small Fish
Zooplankton
Phytoplankton
This happens as zoo plankton is rapidly reproducing
Pyramids of Energy:
• Always pyramid shaped
• Amount of energy stored in organisms at each trophic level

15. Agricultural Ecosystems
Natural Ecosystem:
• Haven’t been changed by human activities
Agricultural Ecosystem:
• Have changed by controlling abiotic & biotic conditions to make it more
favourable for crops or livestock
Natural Ecosystem
• Solar Energy only
• Lower Productivity
• More Species & Genetic Diversity
• Nutrients Recycled
• Populations controlled naturally
• Natural Climax Community
Agricultural Ecosystem
• Solar Energy + Food
• Higher Productivity
• Less Species & Genetic Diversity
• Nutrients Supplemented by Fertilisers
• Population Controlled by Pesticides Too
• Prevented From Reaching Natural Climax
Intensive Rearing Of Livestock:
• Increases efficiency of energy conversion – movement restricted & warmth provided to
reduce respiration – more energy available for growth
• Increase energy input – more energy available for growth (optimum amount/type of food
provided)
• Animal produces more growth in a shorter period of time earning more money

16. Biological & Chemical Controls
Herbicide:
- Chemicals that kill plants
Fungicide:
- Chemicals that kill fungi
Insecticide:
- Chemicals that kill insects
Pesticide:
- Chemicals that kill pests
Features of effective chemical pesticide:
• Specific – Only toxic to specific organism not to humans
• Biodegrade – Breakdown into harmless compounds in
soil and chemically stable
• Cost Effective – Only useful until pest becomes resistant
or needs reapplying
• Bioaccumulation – No build up of chemicals in the crop
Biological Control
Chemical Control
Control using organisms that are the pest’s parasite or predator
Disadvantages:
Not as quick
Doesn’t completely remove pest
Control organism may become pest
Biological Control Chemical Control
Specific Always Effect Non-Target Species
Control Organism
Reproduces
Chemicals Must Be Reapplied
Pests Don’t Become Pests Develop Resistant
Integrated Pest-Control Systems:
- Combines Bio & Chem

17. Carbon Cycle
In Carbohydrates,
Lipids, Proteins &
Nucleic Acids
Stages:
• Photosynthesis:
• CO2 from atmosphere & dissolved in oceans is absorbed by producers
• This produces carbon-containing compounds e.g. sugar
• Feeding:
• Carbon-containing compounds pass along food chain
• Death:
• Dead remains digested by saprobiotic microorganisms
• Decay:
• Saprobiotic organisms secrete enzymes that breakdown large carbon-containing
compounds into smaller ones
• When the microorganisms respire they release CO2
• Respiration:
• All organism respire and release CO2
• Fossilisation:
• When organisms don’t decay fully due to conditions in the soil
• Fossil fuels form (Oil, Coal, Gas)
• Combustion:
• Fossil fuels are burnt & CO2 is released into the atmosphere

18. Carbon Cycle
Carbon
Containing
Compounds
In Producers
Carbon
Containing
Compounds
In Primary
Consumers
Carbon
Containing
Compounds
In Secondary
Consumers
Feeding Feeding
Carbon
Containing
Compounds In
Dead Remains
Death
Saprobiotic
Microorganism
s
Decay
Death Death
Fossil Fuels
CO2 In
Atmosphere &
Dissolved In
Oceans Combustion Fossilisation
Respiration
Photosynthesis
Respiration

19. [CO2]
Short-Term Fluctuations:
• Day:
• Plants respire & photosynthesise
• Usually photosynthesis > respiration
• More CO2 taken up
• [CO2] falls
• Night:
• Plants respire
• More CO2 released
• [CO2] rises
• Winter:
• Cold temp, short days, leaf loss means photosynthesis rate decreases
• Less CO2 taken up
• [CO2] rises
• Summer:
• Warm temp, long days means photosynthesis rate increases
• More CO2 taken up
• [CO2] falls

20. [CO2]
Long-Term Fluctuations:
• Burning Fossil Fuels:
• Releases CO2 into atmosphere from locked up sources
• [CO2] rises
• Deforestation:
• Trees remove carbon from atmosphere (Carbon Sinks)
• Less trees – less PS – less CO2 taken up
• Forests cleared by burning – releasing CO2
• Forests cleared by chopping down – leaving the stumps –
stumps are decomposed releasing CO2 by respiration

21. Global Warming
Greenhouse Effect:
Trapping of the sun’s warmth, increasing the global temperature
Main Greenhouse Gases:
CO2 Burning FF’s, respiration…..
CH4 Livestock, rice farming, landfill…..
CFC’s Aerosols, refrigerants…..
H2O
Consequences:
- Rising Sea Levels
- Extreme Weather
- More Disease
- Distribution of Species

22. Nitrogen Cycle
In DNA, RNA, Proteins
Nitrogen Fixation:
• Lightening
• N2(g)  NO3
-
• Free-Living Nitrogen Fixing Bacteria (azotobacter)
• N2(g)  NH4
+
• Symbiotic Nitrogen Fixing Bacteria (rhizobium)
• Found in root nodules of legumes (peas, clover…)
• N2(g)  NH4
+
Nitrification:
• Carried out aerobically by nitrifying bacteria
• NH4
+  NO2
- (nitrosomonas)
• NO2
2-  NO3
- (nitrobacter)
Ammonification:
• Dead remains/waste  NH3
• Saprobiotic Microorganisms breakdown the nitrogen containing compounds
Denitrification:
• NO3
-  NO2(g) Anaerobically (waterlogged soil)
• Denitrifying bacteria

23. Nitrogen Cycle
N2(g) in
Atmospher
e
Symbiotic N2
fixing bacteria
in roots
Free-living N2
fixing bacteria
in soil
NH4
+
NO2
-
NO3
-
N2 in dead
remains,
waste
N2 in
Producer
s
N2 in Primary
Consumers
N2 in Secondary
Consumers
Lightening
Nitrogen Fixation
Nitrification
Nitrification
Denitrification
Absorption
DeathDeath Death
Ammonification

24. Nitrogen Fixation in Roots
N2(g)  NH4
+  NO3
-
Nitrogen to Ammonium ions requires nitrogenase (anaerobic)
This requires ATP & rNAD
Sugar used by bacteria to produce ATP & rNAD
Plants supply bacteria with sugar
Symbiotic relationship
Plant then gets NO3
- from soil
Catalyst

25. Deforestation & Nitrogen Cycle
Trees cut down & burnt:
• Ash provides temporary high of N2
• This is rapidly leached from the soil
Trees are removed:
• More rain hits soil
• More leaching
• Soil is waterlogged allowing denitrifying bacteria to work
• No decomposition so no N2 returned to soil
• [NO2] falls

26. Fertilisers
Chemicals that provide crops with minerals for growth
More N2 provided  More growth  more PS  more productivity
Natural Fertilisers:
• Organic
• Dead and decaying remains of plants & animals
• e.g. manure
Artificial Fertilisers:
• Inorganic (e.g. NH4NO3)
• Mined from rocks
• Converted into different forms to provide correct mineral balance for
particular crop
• More easily leached from soil
Artificial fertiliser use can lead to eutrophication & bioaccumulation
Build up of
chemical in
body

27. Eutrophication
• In most water sources nitrate levels are low so it limits algal/plant growth
• Added nitrates mean the growth of plants is not limited
• Algal blooms appear on water’s surface
• This ‘bloom’ absorbs light and prevents it reaching the bottom
• Light becomes a limiting factor, so plants at a lower depth die
• Lots of dead organisms, saprobiotic organisms grow exponentially
• Saprobiotic organisms require O2
• [O2] reduced as more [NO3
-] is released from decaying organisms
• O2 is now a limiting factor so aerobic organisms die (e.g. fish)
• Less aerobic organisms, so less competition for anaerobic organisms, so there
population rises exponentially
• Anaerobic organisms further decay dead remains releasing toxic waste, making the
water putrid

28. Leaching
• Rain dissolves soluble nutrients (e.g. nitrates)
• They’re carried deep into the soil
• Dissolved nutrients may find there way into water sources
• May cause harm if the water is consumed
• Leached nitrates can cause eutrophication
Reduces Species Diversity:
• N2 rich soils favour growth of grasses etc…
• These species out compete other species
• Other species die out
• Reducing the species diversity

29. Succession
Succession – describes the change of ecosystems over time
Seres – various stages of succession
Pioneer Species – 1st species to bring about colonisation in an area
Climax Community – ultimate stable community that is in equilibrium with the
abiotic conditions
Primary Succession – when succession starts from bare, previously uncolonised
ground or water with no life
Secondary Succession – when succession starts again in an area where
communities have been destroyed – it occurs more quickly
Lithosere/Halosere/Hydrosere – succession from rocks/seawater/freshwater
Plagioclimax – when succession is stopped

30. Succession
- 1. Originally hostile – only pioneer species can live here
- 2. Pioneer species die – increases organic content of soil e.g. mineral, humus
- 3. Gradually environment changes & becomes less hostile
- 4. More nutrients present
- 5. Other species now grow, with more habitats present
- 6. Climax Community eventually reached (usually oak trees)

31. Conservation
Conservation – protection & management of species & habitats
Methods:
- Seed Banks
- Fishing Quotas
- Captive Breeding Programmes
- Relocation
- Protecting Areas/National Parks
Conservation Moorland:
- Sheep graze & controlled burning prevents climax community from
being reached
- Preventing loss of habitat for many species

32. Production of ATP
• ATP- Adenine TriPhosphate
• Made from ADP + Pi
• Energy stored in the phosphate bond
• ATPase catalyses the breakdown of ATP into ADP + Pi
• ATP synthase catalyses the production of ATP
• The ADP + Pi is recycled and the process starts again
Properties:
• Small compound – easily transported around the
cell
• Easily broken down (Hydrolysed)
• Cell has instant energy supply

33. Photosynthesis
Inner &
Outer
membrane
Granum
Contains
Chlorophyll
Thylakoid Stroma Starch Grain
Loop of DNA
2 Photo Systems capture light in a chloroplast PSI (best at 700nm) & PSII (best at
680nm)
Lamellae (Membrane
joining Thylakoids)
6CO2 + 6H2O + Energy = C6H12O6 + 6O2
Number of
Chloroplasts
Substomatal
Cavity
Lower Epidermis
Spongy Mesophyll
Airy Cells,
lots of
space
Palisade Layer
Upper Epidermis
Waxy Cuticle
Absorption Spectrum
Plants absorb red & blue
wavelengths only
reflecting green. It’s why
they’re green

34. LDS (Non-Cyclic Photophosphorylation)
Photolysis Of Water:
2H2O = 4H+ + 4e- + O2
Requires a photon to split water
Occurs in the Thylakoids of
chloroplasts
Thylakoids adapted for their function:
• Large SA, large area for attachment of chlorophyll, electron carriers and enzymes
• Proteins in grana hold chlorophyll to allow max light intake
• Granal membranes contain enzymes that help make ATP
• Chloroplast contain DNA & Ribosomes to manufacture proteins for LDS quickly
Electron CarrierElectron Acceptor

35. Cyclic Photophosphorylation
Happens when lack of NADP
No light wasted
Only uses Photo System 1
Only ATP produced

36. LIS (Calvin Cycle)
In Stroma
RuBp – Ribulose
Bisphosphate
TP – Triose Phosphate
(GALP)
GP – Glycerate 3-Phosphate
RUBISCO – Enzyme used in
CO2 Fixation
ATP and rNADP from LDS
6 Cycles = 1 Glucose
Molecule

37. Respiration
C6H12O6 + 6O2 = 6CO2 + 6H2O + Energy
1. Glycolysis:
• Makes Pyruvate from Glucose
• In cytoplasm
• Anaerobic Process
• Net Yield of 2ATP
Dehydrogenation – Removal of H2
- Using dehydrogenase enzyme
Substrate Level Phosphorylation
- ADP + Pi  ATP

38. 2. Link Reaction:
• Pyruvate oxidised by removing H
• Acetyl CoEnzyme A produced
• Per Pyruvate a CO2 molecule produced
Pyruvate + NAD + CoA = Acetyl CoA + rNAD + CO2
Decarboxylation – Removal of CO2
- Using Decarboxylase enzyme

39. 3. Krebs Cycle:
• Acetyl CoA + oxaloacetate (4C) = Citrate
• Citrate converted to 5C compound ( 2H+ & CO2 removed)
• 5C to 4C Produces:
• 2 x rNAD
• ATP
• rFAD
• CO2
NAD – Nicotinamide Adenine Dinucleotide
FAD – Flavine Adenine Dinucleotide
In Mitochondrion

40. Electron Transfer Chain
When rFAD & rNAD are oxidised they release 2H & 2e-
Electrons used in transfer chain
Energy/ATP produced in ETC is used to power chemiosmosis
Hydrogen used in chemiosmosis
Oxygen is the last electron acceptor.
O2 + 2e- + 2H  H2O

41. Chemiosmosis
In Photosynthesis & Respiration Energy (ATP) from ETC used to power Chemiosmosis
Active Transport
If ATP synthase
not present energy
lost in the form of
Heat instead of
forming ATP
Electro – Chemical
Gradient

42. Respiration

43. Anaerobic Respiration
Instead of pyruvate being converted into Acetyl CoA it’s
converted into ethanol (in plants and yeast) and lactic acid (in
animals and some bacteria)

44. Genetics
Allele: Variant of a gene
Phenotype: Characteristics of an organism, often visible, resulting from both it’s
genotype and the effects of the environment
Genotype: Genetic composition of an organism
Poly-genetic Inheritance: More than one gene to produce a characteristic
Gene: Section of DNA that codes for a protein
Gamete: Reproductive cell that fuses with another gamete during fertilisation
Trait: A characteristic
Multiple Allele: Many forms of a gene (e.g. Blood group)
Heterozygous: One Dominant & One Recessive e.g. Rr
Homozygous: Both recessive or dominant e.g. RR or rr
Codominant: Both dominant or recessive contribute to final trait

45. Monohybrid Genetics
Red x White
RR rr
OR OR Or Or
R R
r Rr Rr
r Rr Rr
All Rr
Red x Red
Rr Rr
OR OROr Or
R r
R RR Rr
r Rr rr
1RR : 2Rr : 1rr

46. Sex Linked Diseases
Carrier Girl x Well Boy
XHXh XHY
XH Y
XH XHXH XHY
Xh XHXh XhY
1 Carrier Girl XHXh
1 Well Girl XHXh
1 Well Boy XHY
1 Ill Boy XhY
Male = XY
Female = XX

47. Dihybrid Crosses
Yellow Smooth Green Wrinkled
YYSS yyss
x
All YySs
Yellow Smooth Yellow Smooth
YySs YySs
x
9YYSS : 3YySS : 3YYSs : 1yyss
F1 Generation
F2 Generation

48. Hardy-Weinberg Equation
Assumptions:
No mutations
No migration/Population Isolated
Large populations
Random Mating
No natural selection
P2 + 2PQ + Q2 = 1
P + Q = 1
1RR : 2Rr : 1rr
e.g. 1 in 10,000 Albino (Recessive) Population = 10,000
Q2 = 1/10,000
Q = 0.01
P = 1 – Q
P = 0.99
(2 x P x Q) x 10,000 = 198 Heterozygotes
P2 x 10,000 = 9801 Homozygous Dominants

49. Speciation
• Development of a new species
• Population separated – e.g. geographical
isolation
• Abiotic conditions in each location is
different, so there are different selection
pressures
• In colder climates polar bears with longer fur
are more likely to survive, reproduce & pass
allele to offspring
• Eventually the polar bears become so
different that a new species develops – so
they’re no longer able to breed with the other
group
Polar Bears
Variation in Fur Length
Stabilising
Directional