A2 Biology revision aid

1. AQA A2 Biology Unit 4
Populations

2. Specification
3.4.1 The dynamic equilibrium of populations is affected by a number of factors.
Candidates should be able to:
• carry out experimental and investigative activities, including appropriate risk management
• consider ethical issues when carrying out fieldwork, chiefly those relating to the organisms involved and their environment
• analyse and interpret data relating to the distribution of organisms, recognising correlations and causal relationships
• appreciate the tentative nature of conclusions that may be drawn from such data.
• interpret growth curves, survival curves and age-population pyramids
• calculate population growth rates from data on birth rate and death rate.
• relate changes in the size and structure of human populations to different stages in demographic transition.
Populations and
Variation in population
Investigating populations
Human populations
ecosystems
size
A population is all
the organisms of
one species in a
habitat.
Populations of
different species
form a
community.
Within a habitat a
species occupies
a niche governed
by adaptation to
both biotic and
abiotic
conditions.
A critical appreciation of
some of the ways in
which the numbers and
distribution of organisms
may be investigated.
Random sampling with
quadrats and counting
along transects to obtain
quantitative data.
The use of percentage
cover and frequency as
measures of abundance.
The use of mark–
release–recapture for
more mobile species.
Population size
may vary as a
result of
• the effect of
abiotic factors
• interactions
between
organisms:
interspecific and
intraspecific
competition and
predation.
Population size
and structure,
population growth
rate, agepopulation
pyramids, survival
rates and life
expectancy.

3. Definitions
• Abiotic: Ecological factor that makes up part of the non-biological environment
• Biotic: Ecological factor that makes up part of the living environment
• Ecosystem: More or less self contained functional unit in ecology made up of all
interacting biotic and abiotic factors in a specific area
• Population: A group of individuals of the same species that occupy the same
habitat at the same time
• Species: A group of similar organisms that can breed together to produce fertile
offspring
• Community: The organisms of all species that live in the same area
• Habitat: The place where an organism normally lives, which is characterised by
physical conditions and the species of other organisms present
• Niche: All conditions and resources required for an organism to survive,
reproduce and maintain viable population
• Intraspecific: Competition between organisms of the same species
• Interspecific: Competition between organisms of different species
• Predator: An organism which feeds of another organism known as the prey

4. 1.1 Populations and Ecosystems
The environment can include abiotic and biotic factors such as temperature, light
(abiotic), predation and competition (biotic)
The life supporting layer of land, air and water that surrounds the earth is known as
the biosphere
Ecosystems
• The ecosystem is made up of biotic and abiotic
features
• There are two major processes to consider:
• The flow of energy through the system
• The cycling of elements within the system
• There are a number of species in an ecosystem which
make up many groups of individuals that together
make up a population

5. 1.2 Investigating Populations
When studying a habitat the number of individuals in an individual space needs to be counted this is
the abundance
Only small samples are taken due to the time consuming nature
As long as samples are representative the conclusions will be valid
Factors to consider when
using quadrates:
• Size of quadrat: Larger species need larger quadrats, if a species
occurs in series of groups then a large number of small quadrats is
used
• Number of samples: The larger the number the more reliable the
results
• Position of quadrat: Statistically significant results are obtained by
random sampling
Random Sampling
• To avoid bias random sampling is used
• 1. Lay out two long tape measures at right angles along two sides of
the study area
• 2. Obtain series of coordinates by using random numbers taken
from a computer
• 3. Place a quadrat at the intersection of each pair of coordinates
and record the species within it
Systematic sampling along
Transects
• A line transect can be used in which a string/tape is stretched across
the ground and any organism that passes the line is recorded
• A belt transect is a strip marked using a second parallel line, species
within the belt and between the lines are recorded

6. Measuring
Abundance
• Random sampling and counting along transects can obtain measures of
abundance in several ways:
• Frequency: Likelihood of a particular species occurring in a quadrat. It gives quick
idea of species present and general distribution within an area but doesn’t
provide density or distribution of species
• Percentage Cover: Estimate of the area within a quadrat. Useful where species is
particularly abundant, data can be collected rapidly bur it occurs in overlapping
layers
• To obtain reliable results the sample size needs to be large and a mean needs to
be collected
Mark-releaserecapture
• Due to animals being mobile this method is used
• Once an animal is caught it is marked then released, later on more individuals are
captured and the number marked is recorded
• The technique relies on assumptions:
• Proportion of marked to unmarked individuals in the second sample is the same
as the proportion of marked to unmarked individuals in the population as a
whole
• The marked individuals released from the first sample distribute themselves in
sufficient time
• The population has a definite boundary so no immigration or emigration
• There are few deaths and births
• The marking is not toxic or makes the animal more liable to predation
• The mark or label is not lost or rubbed off
Analysing data
• The first stage is to present data in a table or graph so data can be compared and
then calculate standard deviation
• Correlation and causation can be analysed
• Statistical tests can also be used to calculate strength and direction of any
correlation
• Spearman rank, chi squared and standard error can be used

7. Population = Total number of individuals in the first sample X Individuals in the second sample
Number of marked individuals recaptured
Type of Trap
Method of Marking
Pitfall trap: Insects, Small Mammals
Microchip
Electrofishing : Fish
Paint or permanent marker onto animal
Cage set up with food in: Mice, Voles
Photo showing unique markings
Removal method: blocks of part of
environment so can collect fish
Visible implant or tag
Glue Trap: Rodents
Toe Clippings: Vertebrates
Sweepnet moved through plants: Ground
insects
Fur Removal
Wire funnel trap: Insects in tall vegetation

8. 1.3 Variation in Population Size
Population Growth Curves
• 1. Period of slow growth as the initially small number of
individuals reproduce to slowly build up their numbers
• 2. Period of rapid growth where the ever increasing
number of individuals continue to reproduce. The
population size doubles during each interval
• 3. Period when population growth declines until size
becomes stable, the decline can be due to food shortage
or predation
There are 3 phases:
2.
1.
3.

9. Population Size
Population size depends on
limiting factors:
Abiotic Factors
•
•
•
•
•
Mineral ions
Light
Temperature
Oxygen
Food
• Temperature: Optimum temperature needed for best
survival as otherwise enzymes work slower and
metabolic rate is reduced due to denaturation
• Light: Ultimate source of energy, the stronger the light
intensity the faster the rate of photosynthesis causing a
faster growth of plants thus a larger population
• pH: Optimum pH needed otherwise enzymes don’t work
to full potential as denatured
• Water/Humidity: In low water areas only species well
adapted to dry conditions survive. The more humid an
area the slower the rate of transpiration

10. 1.4 Competition
Intraspecific
Competition
Interspecific
Competition
• Between the same species
• Compete for food, water, mates
• Greater the resources availability the larger the population
• Between different species
• For food, light and water
• If two species occupy in the same niche one will normally have a competitive
advantage
• The population of the stronger species will gradually increase while the other
diminishes eventually to be removed: Competitive exclusion principle
• No species can occupy the same niche indefinitely when resources are
limiting

11. Intraspecific Competition
Interspecific Competition

12. 1.5 Predation
The relationship between prey
and predator:
• Predators eat their prey, reducing prey population
• The predators become in greater competition with
each other over the prey left
• Predator population is reduced causing fewer prey to
be eaten
• The prey population increases
• Due to more prey the predator population then
increases
The fluctuations in population
as also due to disease and
climatic factors not just
predation
Population crashes create
selection pressure where
survival of the fittest
occurs, the survivors will
reproduce
• The population then evolves to be better adapted to
harsh conditions

13. 1.6 Human Populations
There has been an explosion in
human population due to:
• The development of agriculture
• The development of manufacturing and trade that created
industrial revolution
War, disease and famine have only been temporary reversals in the upward trend
Factors affecting Growth
• Birth rate
• Death rate
• Immigration: Individuals join a new population
• Emigration: Individuals leave a population
Population Growth = (Births + Immigration) – (Deaths + Emigration)
Percentage Population Growth Rate: Population change during the period
Population at the start of the period X 100

14. Birth Rate Factors
Death Rate Factors
Birth Rate:
number of births per year
Total population in the same year X1000
Death Rate:
number of deaths per year
Total population in the same year X1000
• Economic conditions: Lower capita
income tend to have higher birth rates
• Cultural/Religious: Some religions
oppose birth control
• Social pressure: Large families can
improve social standing
• Birth Control: Contraception isn't always
available
• Political factors: Government influence
births via education and taxation
• Age Profile: The more elderly people the
higher the death rate
• Life expectancy: ECDLs have lower life
expectancies
• Food supply: Need a balanced diet
• Sanitation: Reduces water-borne deaths
e.g. cholera
• Medical care availability
• Natural disasters: Higher death rates near
droughts
• War

15. Population Structure
• Well economically developed countries have a
higher life expectancy, this has led to change in
societies from short life expectancies to long
life expectancies causing demographic
transition
Stable Population
Population pyramids
can be used to plot
populations:
Survival Rates
• Stable Population: Birth and death rate are
balanced so no decrease or increase in
population
• Increasing Population: High birth rate (shows
wider base) and fewer old people (narrow
apex)
• Decreasing Population: Lower birth rate
(narrow base) and low mortality rate so more
elderly people (wider apex)
Increasing Population
• Plotting as survival curve allows life expectancy
to be calculated
Decreasing Population

16. AQA A2 Biology Unit 4
ATP & Photosynthesis

17. Specification
3.4.2 ATP provides the immediate source of energy for biological processes.
3.4.3 In photosynthesis, energy is transferred to ATP in the light-dependent reaction and the ATP is utilised in the light-independent
reaction.
Candidates should be able to explain how growers apply a knowledge of limiting factors in enhancing temperature, carbon dioxide
concentration and light intensity in commercial glasshouses. They should also be able to evaluate such applications using appropriate data.
ATP
The synthesis
of ATP from
ADP and
phosphate
its role as the
immediate
source of
energy for
biological
processes.
Photosynthesis
The lightindependent
and lightdependent
reactions in a
typical C3
plant.
Light-dependent
reaction
• light energy
excites electrons in
chlorophyll
• energy from
these excited
electrons
generates ATP and
reduced NADP
• the production of
ATP involves
electron transfer
associated with the
electron transfer
chain in chloroplast
membranes
• photolysis of
water produces
protons, electrons
and oxygen.
Light-independent
reaction
• carbon dioxide is
accepted by ribulose
bisphosphate (RuBP) to
form two molecules of
glycerate 3-phosphate
(GP)
• ATP and reduced
NADP are required for
the reduction of GP to
triose phosphate
• RuBP is regenerated in
the Calvin cycle
• Triose phosphate is
converted to useful
organic substances.
Limiting factors
The principle of
limiting factors
as applied to the
effects of
temperature, ca
rbon dioxide
concentration
and light
intensity on the
rate of
photosynthesis.

18. Definitions
•
•
•
•
•
•
•
•
•
•
ATP: An activated nucleotide found in all living cells that acts as an energy carrier
Endothermic: Reaction that requires energy
Activation Energy: Minimum energy required to bring about a chemical reaction
Hydrolysis: The breaking of large molecules to small using water
Condensation: Chemical process where two molecules form a larger molecule
Electron carrier molecules: 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
NADP: Molecule that carries electrons produced in the light-dependent reaction
Stomata: Pores surrounded by guard cells that allow gas diffusion
Stroma: Matrix of chloroplast where the light-independent reaction occurs

19. 2.1 Energy and ATP
All living organisms require energy to stay alive
Plants use solar energy from the sun to combine water and CO₂ in photosynthesis
The organic molecules formed are broken down by plants and animals into ATP
Energy
Why Energy is Needed
• Energy can take many forms such as light and kinetic
• Energy cannot be created or destroyed
• It is measured in joules(J)
• It can change from one form to another
• Metabolism: All reactions in the body require energy
• Movement: For in and out of the body itself
• Active Transport: Ions and molecules need to be transported
against the concentration gradient across plasma membranes
• Repair and Division
• Production of Substances: Such as enzymes and hormones
• Maintenance of body temperature: Mammals and birds are
endothermic and need energy to replace that lost to the
environment

20. Energy and Metabolism
• 1. Light energy from the sun is converted by plants into chemical energy during photosynthesis
• 2. The chemical energy from photosynthesis, organic molecules, is converted into ATP during
respiration
• 3. ATP Is used by cells
Storing ATP
• ATP has 3 phosphate groups which have unstable bonds thus a low activation energy so are easily
broken
• Energy is released when the bonds break
• The reaction uses water to is a hydrolysis reaction
• ATP + H₂O → ADP + Pi (inorganic phosphate) + E (Energy)
Synthesis of ATP
• Adding an inorganic phosphate to ADP can form ATP again in a condensation reaction. It occurs in 3
ways:
• Photophosphorylation: In chlorophyll containing plant cells during photosynthesis
• Oxidative Phosphorylation: In the mitochondria of plants and animal cells during electron transport
• Substrate-level Phosphorylation: Plant and animal cells when phosphate groups are transferred
from donor molecules to ADP making ATP
• In the first two ATP is synthesised using energy released during electron transfer along an electron
carrier chain

21. Roles of ATP
ATP is the immediate energy source as its energy store is not long lasting due to the instability of the
phosphate bonds
Cells don’t store large quantities of ATP however ATP is rapidly re formed from ADP and inorganic
phosphate easily making it go further
ATP is better than glucose as it releases smaller more manageable quantities of energy
The hydrolysis of ATP to ADP is a single reaction releasing energy immediately whereas the process
for glucose is much longer
ATP cannot be stored so is continuously made in the mitochondria, cells such as muscle fibres
contain large mitochondria due to the required energy
ATP as a source of energy
• Metabolic Processes: Forming polysaccharides from
monosaccharides, Polypeptides from amino acids and DNA/RNA
from nucleotides
• Movement: Provides energy for muscle contraction allowing the
muscle filaments to slide over each other
• Active Transport: ATP provides energy to change the shape of
carrier proteins in plasma membranes allowing molecules to
move against the concentration gradient
• Secretion: Forms lysosomes needed for secretion of cell products
• Activation of molecules: When a phosphate molecule is
transferred from ATP to another it makes it more reactive
lowering activation energy. This allows enzyme catalysed
reactions to occur more readily

22. Absorption of Light Energy
• Light energy is captured and
is transferred to chlorophyll a
molecules.
• Electrons in the outer shell of
the chlorophyll a molecule
are excited.
• The electrons are passed
through a series of carrier
molecules and are used to
power,
– Photolysis
– Reduction of NADP
– Photophosphorylation

23. 3.1 Photosynthesis
Leaf Structure
• Large surface area
• Leaves minimise overlapping
• Thin so short diffusion path
• Transparent cuticle and epidermis let light through to
photosynthetic mesophyll cells
• Long narrow mesophyll cells packed with chloroplast
• Large number of stomata able to open and close in light intensities
• Air spaces allow diffusion of CO₂ and O₂
• Xylem brings water and phloem carries away sugars
Photosynthesis Outline
• 6CO₂ + 6H₂O → C6H12O + 6O2
• 1. Capturing light energy by chloroplast pigments e.g. chlorophyll
• Light-dependent Reaction: Light converted into chemical energy,
an electron flow is created and causes water to split (photolysis)
into protons, electrons and oxygen. Products are reduced NADP,
ATP 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.
They contain chlorophyll and also attach to each other via intergranal lamellae
• Stroma is a fluid filled matrix where the light independent reaction
occurs

24. ADP + P
e-
e-
PHOTOPHOSPHORYLATION
e-
e-
ATP
e-
PHOTOLYSIS
light
2H2O
Chlorophyll
a
4H+ + e - + O2
THE LIGHT-DEPENDENT REACTION
4NADP  4NADPH

25. 3.2 The Light-Dependent Reaction
When a substance loses electrons it is oxidised
When a substance gains electrons it is reduced
The Making of ATP
• When chlorophyll absorbs light energy it causes a pair of electrons in it to become
a higher energy level, they are in an excited state
• The electrons then leave the chlorophyll and are taken up by an electron carrier
• The chlorophyll has become oxidised and the electron carrier reduced
• Via a series of oxidation-reduction reactions the electrons pass along the electron
carriers as a transfer chain is formed in the membranes of the thylakoids
• Each new carrier has a slightly lower energy level than the one before causing the
electrons to lose energy
• This energy is used to combine an inorganic phosphate molecule with ADP forming
ATP
Photolysis of Water
• Due to the lose of electrons the chlorophyll they must be replaced
• The electrons are replaced via water molecules being split using light energy
• 2H₂O → 4H+ + 4e- +O₂
• The hydrogen ions are taken up by NADP causing it to be reduced
• The reduced NADP and electrons from the chlorophyll enter the light independent
reaction
• Reduced NADP gives the plant a source of chemical energy
• The oxygen by-product from the photolysis of water is diffused or used in
respiration

26. Photolysis
Water molecules are
split using energy from
excited electrons in
chlorophyll a molecules.
2H2O  4H+ + 4e- + O2
Oxygen is released into
the atmosphere.
Hydrogen ions and
electrons are now
available to be used to
produce a reducing
agent.

27. Site of Light-Dependent Reaction
Origin: Thylakoids of Chloroplast
Adaptations of Chloroplast:
• Thylakoid membranes provide a large surface area for
chlorophyll attachment, electron carriers and enzymes
• Network of proteins in the grana hold the chlorophyll in a
precise manner for maximum absorption of light
• Granal membranes have enzymes for ATP production
• Contain both DNA and ribosomes so there is easy
manufacture of proteins needed

28. Reduction of NADP
 Electrons and Hydrogen ions
produced during photolysis
are used to reduce NADP to
Reduced NADP (NADPH).
 Excited electrons and
hydrogen ions are transferred
to NADP.
NADP + H + + e-  NADPH
 NADPH can donate electrons
and hydrogen ions to carbon
dioxide and so is a reducing
agent.

29. 3.3 The Light-Independent Reaction
It takes place in the stroma of the chloroplasts
It doesn’t require light to occur
ATP and reduced NADP are used to reduce carbon dioxide
The Stages
• Carbon dioxide from the atmosphere diffuses into the leaf
through stomata and dissolves in water around the walls of the
mesophyll cells. It then diffuses through the plasma
membrane, cytoplasm and chloroplast membranes to the
stroma
• In the stroma, CO₂ combines with ribulose bisphosphate(RuBP)
using an enzyme
• Glycerate 3-phosphate (GP) is formed, 2 molecules per one
combination
• ATP and reduced NADP activate the GP into triose
phosphate(TP)
• The NADP is reformed and goes back to the light dependent
reaction to be reduced again by accepting more hydrogen
• Some TP is converted into useful organic substances such as
glucose
• Most TP is used to regenerate RuBP using ATP from the light
dependent reaction

30. Photophosphorylation
• Energy from the
excited electrons is
used to make ATP.
• A phosphate group is
added to ADP.
ADP + P --energy from excited electrons  ATP

31. Products of the Light Dependent Stage
• Photolysis
– H+ ions
– Electrons
– Oxygen
Used to produce NADPH
• Photophosphorylation
– ATP

32. The Calvin Cycle

33. Site of the Light-Independent Reaction
The chloroplast is adapted for this reaction as:
• The fluid of the stroma contains enzymes needed
• The fluid surrounds the grana so the products can diffuse
quickly to it
• It contains both DNA and ribosomes so can easily
manufacture proteins needed

34. 3.4 Factors affecting Photosynthesis
Limiting Factors
Light Intensity
Carbon Dioxide
Temperature
• A limiting factor restricts the rate at which a process can occur
• It is the slowest reaction that determines the overall rate of
photosynthesis
• ‘At any given moment, the rate of a physiological process is limited by
the factor that is at its least favourable value’
• When light is the limiting factor photosynthesis is directly
proportional to light intensity
• As light intensity increases the volume of oxygen produced and
carbon dioxide absorbed will increase till it balances the oxygen
absorbed and carbon dioxide produced
• This point is the compensation point due to no net exchange of
gases into or out of the plant
• When increasing light intensity has no effect on rate of
photosynthesis there is another limiting factor
• 0.1% CO₂ will give the optimum concentration for photosynthesis to
occur
• CO₂ concentration affects enzyme activity especially the enzyme that
catalyses the combination of ribulose bisphosphate and carbon
dioxide in the light independent reaction
• From 0 to 25°C the rate of photosynthesis doubles for each 10°C
• 25°C is the optimum temperature and after this the rate of
photosynthesis declines due to enzymes becoming denatured
• Photosynthesis isn't purely photochemical as if it was it wouldn’t be
affected by temperature

35. The Light-Independent Reaction
Does not require light energy.
However, requires the products produced in the lightdependent reaction, therefore photosynthesis cannot
occur without light energy.
Takes place in the stroma.
Enzyme controlled, therefore it is affected by
temperature.
Uses energy from ATP, and the electrons and
hydrogen ions from NADPH to reduce CO2 to
glucose.

36. Fixing Carbon Dioxide
• Ribulose Biphosphate
(RuBP), a 5 carbon
molecule, combines with
carbon dioxide via the
enzyme RuBISCO.
• This forms 2 molecules of
glycerate-3-phosphate
(GP), a 3 carbon organic acid.

37. Reducing glycerate-3-phosphate (GP)
 NADPH and ATP from the lightdependent reaction are required
for this stage.
 NADPH transfers electrons and
hydrogen ions to GP to form 2
molecules of Triose phosphate.
 The energy for this is provided
by the ATP.
 The NADPH has now been
oxidised back to NADP and can
be reused in the light-dependent
reaction.
 The ATP has lost energy and so
returns to ADP + P which can
also be reused in the lightindependent stage.

38. Producing Glucose and Regenerating RuBP
Producing Glucose
Regenerating RuBP
• For every 6 CO2 molecules
entering the cycle, 12
Triose phosphates will be
produced.
• 2 of these molecules will
be converted into glucose.
• Of the 12 Triose phosphates
that are produced, 10 will be
used to regenerate RuBP.

39. Law of Limiting Factors: “ The overall rate of the process will be limited by
the factor which is at the least favourable value”
FACTORS AFFECTING THE RATE OF
PHOTOSYNTHESIS

40. Light Intensity
 At low light intensities, the rate of
photosynthesis is directly proportional to
the light intensity.
 Because as more light becomes
available, more chlorophyll molecules can
absorb light so more electrons are excited
leading to photolysis and
photophosphorylation.
 More ATP and NADPH are produced so
the light-independent reactions can occur
at a higher rate so more product is
produced.
 Eventually a maximum rate is reached
and so increasing light intensity has no
effect so the graph levels off.
 This can be because all available
chlorophyll molecules are absorbing light.
Or some other factor is now the limiting
factor.

41. Temperature
 When light is not a limiting factor (i.e.
high light intensities), increasing the
temperature increases the rate of
photosynthesis.
 Above the optimum temperature, any
further increase causes the rate to
decrease rapidly.
 Because the Calvin Cycle is enzyme
controlled, when the temperature
increases both enzymes and substrates
gain kinetic energy, so more collisions
occur, so more enzyme substrate
complexes form, so more product
forms.
 When the temperature exceeds the
optimum, the enzymes will denature
and the specific shape of the active site
will change and no longer be
complementary to the substrate so
fewer enzyme-substrate complexes can
form.

42. Carbon Dioxide Concentration
 At low CO2 levels an increase in
concentration causes a directly
proportional increase in the rate
of photosynthesis.
 A maximum rate is eventually
reached and further increase has
no effect and so the graph levels
off.
 This is because atmospheric CO2
levels are lower than the
optimum value so when
concentration is increased more
CO2 is absorbed so more product
is made.
 Eventually, there is no more
RuBP available to absorb
anymore CO2 so there is no
further effect.

43. IMPLICATIONS FOR COMMERCIAL GLASSHOUSE
MANAGEMENT

44. What does glasshouse cultivation allow?
• Better yields can be achieved because
conditions for photosynthesis can be kept at
an optimum.
• Crops can be grown out of season all year
providing a better economic return.
• Crops can be grown in regions where they
might not grow well naturally.

45. Factors to be Considered
 For maximum yields to be achieved, limiting factors must be kept at an optimum
because the faster the plant photosynthesises the more carbohydrates it produces
which means the maximum yield will be achieved in the shortest time.
 Carbon dioxide levels
 High levels of CO 2 are the optimum however if the levels are too high over a long period of time then
the stomata will close resulting in a drop in the rate of photosynthesis. A compromise level must
therefore be used.
 Temperature
 An optimum temperature should be used to ensure that the plants photosynthesise rapidly without
any damage to cells.
 Water
 Need to be well watered to ensure the stomata remain open to absorb CO 2. However the soil must
not become waterlogged as it will reduce the uptake of mineral by active transport. The plants must
not become to wet either as this will promote fungal disease to spread.
 Light
 Artificial lighting is used when natural light intensity falls. Specific wavelengths are chosen so they are
absorbed by the plants (i.e. red and blue).
 Minerals
 Soil must be supplemented with essential minerals. Potassium is particularly important in stomatal
mechanisms and so must be kept at an optimum.

46. AQA A2 Biology Unit 4
Respiration

47. Specification
3.4.4 In respiration, glycolysis takes place in the cytoplasm and the remaining steps in the
mitochondria. ATP synthesis is associated with the electron transfer chain in the
membranes of mitochondria.
Aerobic Respiration
glycolysis takes place in the
cytoplasm and involves the
oxidation of glucose to
pyruvate with a net gain of
ATP and reduced NAD
pyruvate combines with
coenzyme A in the link
reaction to produce
acetylcoenzyme A
in a series of oxidationreduction reactions the
Krebs cycle generates
reduced coenzymes and
ATP by substrate-level
phosphorylation, and
carbon dioxide is lost
Aerobic Respiration Conc
acetylcoenzyme A is
effectively a two carbon
molecule that combines
with a four carbon
molecule to produce a six
carbon molecule which
enters the Krebs cycle
synthesis of ATP by
oxidative phosphorylation
is associated with the
transfer of electrons
down the electron
transport chain and
passage of protons across
mitochondrial
membranes.
Anaerobic respiration
Glycolysis followed by
the production of
ethanol or lactate
and the regeneration
of NAD in anaerobic
respiration.

48. Definitions
•
•
•
•
•
•
•
•
•
•
Hydrolysis: Breaking down of large molecules into smaller ones by the addition of water
Activation Energy: Energy required to bring about a chemical reaction
Oxidation: Lose of Electrons
Glycolysis: First part of cellular respiration in which glucose is broken down anaerobically in
the cytoplasm to 2 molecules of pyruvate
Substrate-Level Phosphorylation: The formation of ATP by the direct transfer of a phosphate
group from a reactive intermediate to ADP
Aerobic: Connected with the presence of oxygen, aerobic respiration requires oxygen to
release energy from glucose and other foods
Adenosine Triphosphate: An activated nucleotide found in all living cells that acts as an
energy carrier.
Redox: Reaction in which oxidation and reduction take place
Krebs Cycle: Series of aerobic biochemical reactions in the matrix of the mitochondria of
most eukaryotic cells by which energy is obtained through the oxidation of acetylcoenzyme A
produced in the breakdown of glucose
NAD: (Nicotinamide adenine dinucleotide phosphate) Molecule that carries electrons and
hydrogen ions during aerobic respiration

49. 4.1 Respiration Overview
Glucose cannot be used directly by cells as an energy source so they use ATP
There are two different forms of
respiration:
• Aerobic Respiration: requires oxygen and produces carbon
dioxide, water and much ATP
• Anaerobic Respiration ((fermentation): Takes place in the
absence of oxygen and produces lactate (inn animals) or
ethanol and carbon dioxide in plants, very little ATP is produced
Aerobic Respiration steps:
• Glycolysis: Splitting of the 6 carbon glucose molecule into 2 3carbon pyruvate molecules
• Link Reaction: Conversion of the 3-carbon pyruvate into
carbon dioxide and a 2-carbon molecule called
acetylcoenzyme A
• Krebs Cycle: Introduction of acetylcoenzyme A into a cycle of
oxidation-reduction reactions that yield some ATP and a large
number of electrons
• Electron Transport Chain: Use of the electrons produced in the
Krebs Cycle to synthesis ATP with water produced as a byproduct

50. Glycolysis
Glycolysis is the initial stage in both aerobic and anaerobic respiration
Occurs in the cytoplasm of all living cells
A hexose sugar is split into 2 molecules of 3-carbon pyruvate
It has four stages:
Energy Yield:
• Activation of Glucose by phosphorylation: Glucose is made more
reactive by adding 2 phosphate molecules, these come from the
hydrolysis of 2 ATP molecules to ADP. This provides energy to
activate the glucose as the activation energy has been lowered
• Splitting of the Phosphorylated glucose: Each glucose molecule is
split into 2 3-carbon molecules known as triose phosphate
• Oxidation of Triose Phosphate: Hydrogen is removed from each
triose phosphate molecule and transferred to a hydrogen-carrier
molecule (NAD) to form reduced NAD
• Production of ATP: Enzyme controlled reactions convert each
triose phosphate into another 3-carbon molecule called pyruvate,
2 molecules of ATP are regenerated from ADP
• 2 molecules of ATP
• 2 molecules of reduced NAD
• 2 Molecules pyruvate
As glycolysis occurs in the cytoplasm of cells it doesn’t require an organelle or membrane for it to occur
It doesn’t require oxygen and without oxygen pyruvate is converted to lactate or ethanol by anaerobic respiration

51. Glycolysis
 Glycolysis takes place in the cytoplasm
of the cell.
 Glucose is first phosphorylated by 2
phosphate groups from 2 molecules of
ATP to produce 2 molecules of
glyceraldehyde-3-phosphate (GALP).
 GALP is then oxidised and
dephosphorylated into pyruvate.
 In this process, the phosphate groups are
transferred to ADP producing 2 molecules
of ATP. A hydrogen is transferred to a
molecule of NAD producing NADH.
 The net yield of glycolysis per glucose is
 2ATP
 2NADH
 2 pyruvate
 The pyruvate produced then diffuses
into the mitochondria.
1 x Glucose
2ATP
2ADP + 2P
2 x Glyceraldehyde
3-phosphate
2NAD
4ADP + 4P
2NADH
4ATP
2 x Pyruvate

52. 4.2 The Link Reaction
For pyruvate molecules to enter the Krebs cycle they need to by oxidised
Occurs in the mitochondria
Pyruvate produced in the cytoplasm is actively transported into the matrix of mitochondria
Pyruvate undergoes a series of
reactions:
• Hydrogen is removed from the pyruvate, the hydrogen is
accepted by NAD to form reduced NAD
• 2-carbon molecule, acetyl group, is formed and then combines
with coenzyme A ((CoA) to produce acetylcoenzyme A
• A carbon dioxide molecule is formed from each pyruvate
Pyruvate + NAD + CoA → acetyl CoA + reduced NAD + CO₂

53. The Link Reaction
 Takes place in the matrix.
 Pyruvate undergoes oxidative
decarboxylation.
 Oxidation
2 x Pyruvate
NAD
 Electrons and hydrogen from the pyruvate
are transferred to NAD producing NADH.
 Decarboxylation
 Carbon dioxide is removed which converts
the pyruvate into acetate.
 The acetate then combines with
CoenzymeA to produce Acetyl
CoenzymeA.
 Since 2 molecules of pyruvate were
produced in glycolysis, the net yield of
the link reaction per glucose is
 2 Acetyl CoenzymeA
 2NADH
 2 Carbon dioxide
NADH
Carbon dioxide
CoenzymeA
Acetyl
CoenzymeA

54. The Krebs Cycle
Series of oxidation-reduction reactions in the matrix of mitochondria
• 2-carbon acetylcoenzyme A from the link reaction combines with 4-carbon
molecule to produce a 6-carbon molecule
• The 6-carbon molecule loses carbon dioxide and hydrogen to give a 4-carbon
Process:
molecule and a single ATP molecule produced as a result of substrate-level
phosphorylation
• The 4-carbon molecule can now be combined with another acetylcoenzyme A to
repeat the cycle
• Reduced coenzymes e.g. NAD/FAD have the potential to produce ATP molecules
Products:
• 1 molecule of ATP
• 3 molecules of carbon dioxide
Due to 2 pyruvate molecules being produced from each original glucose the yield of a single glucose molecule is double
the quantities above
Coenzymes: some enzymes
require to function
Significance of the Krebs
Cycle
• Major role in photosynthesis and respiration as carry hydrogen atoms from one
molecule to another e.g.
• NAD, important throughout respiration
• FAD, important in the Krebs cycle
• NADP, important in photosynthesis
• NAD is the most important carrier, it works with dehydrogenase enzymes that
catalyse the removal of hydrogen ions from substrates and transfers them to other
molecules such as hydrogen carriers involved in oxidative phosphorylation
• It breaks down macromolecules into smaller ones e.g. pyruvate into carbon dioxide
• It produces hydrogen atoms carried by NAD to the electron transport chain for
oxidative phosphorylation . This leads to the production of ATP for metabolic
energy in the cell
• It regenerates 4-carbon molecule that combines with acetylcoenzyme A
• It is a source of intermediate compounds used by cells to manufacture substances
such as fatty acids and chlorophyll

55. The Krebs Cycle
•
•
•
•
•
Takes place in the matrix.
Closed cycle of enzyme controlled reactions.
Provides a continuous supply of reduced electron
carriers for the electron transport chain.
AcetylCoA combines with a 4-C compound to produce
citric acid, a 6-C compound.
The citric acid then undergoes
–
–
•
•
A decarboxylation reaction which removes carbon dioxide.
A series of oxidation reactions which remove hydrogen ions
and electrons. H + ions and e – are picked up by NAD and
FAD and they become NADH and FADH.
At the end of the cycle the 4-C compound is recycled so
the cycle can continue.
Since each glucose molecule produced 2 molecules of
pyruvate and so 2 molecules of AcetylCoA, the yield per
glucose for the Krebs cycle is
–
–
–
–
NADH
4 carbon dioxide
2FADH
6NADH
2ATP
ADP
NADH
ATP
FADH
NADH

56. 4.3 Electron Transport Chain
Occurs in the mitochondria
Enzymes are attached to the cristae that are involved in the electron transport chain
Synthesis of ATP
Importance of Oxygen
• Hydrogen atoms produced in glycolysis and the Krebs cycle combine with
coenzyme NAD and FAD that are attached to the cristae
• Reduced NAD and FAD donate the electrons of the hydrogen atom to the first
molecule in the electron transport chain
• This releases the protons from the hydrogen atoms and these are actively
transported across the inner mitochondrial membrane
• The electrons pass along the chain via oxidation-reduction reactions in which they
lose energy that combines ADP and an inorganic phosphate to make ATYP,
remaining energy is given off as heat
• The protons accumulate in the space between the mitochondrial membranes
before they diffuse back into the matrix through special protein channels
• At the end electrons combine with the protons and oxygen to form water
• It is the final acceptor of hydrogen atoms
• Without it the hydrogen ions and electrons would ‘back up’ along the chain and
respiration would cease
• Cyanide is a non-competitive inhibitor of the final enzyme in the electron transport
chain
• It catalyses the addition of the hydrogen ions and electrons to oxygen to form water
• Its inhibition causes hydrogen ions and electrons to accumulate on the carriers
stopping cellular respiration

57. Electron Transport Chain
 Found on the cristae of the mitochondria which
provide a large surface area for this to take place.
 The electron carriers are arranged in descending
energy levels.
 When electrons pass through the carriers, the
energy released is used to move hydrogen ions
from the matrix into the intermembrane space.
 This creates a large concentration gradient of H+
ions and so they diffuse back into the
mitochondrial membrane by diffusion via ATP
synthase.
 As the H+ ions diffuse through the enzyme, they
attach P groups to ADP to produce ATP.
 At the end of the chain, the electrons are picked
up by the terminal electron acceptor, which is
oxygen, to produce water.
- + 2H + + ½O  H O
2e
2
2
 This process is called oxidative phosphorylation.

58. 4.4 Anaerobic Respiration
Without oxygen the Krebs Cycle and the Electron Transport Chain cannot occur
Only glycolysis is a source of ATP, for it to continue its products of pyruvate and hydrogen must be constantly
removed
The hydrogen must be released from the reduced NAD in order to regenerate NAD as if it wasn’t converted no NAD
could take up the newly produced hydrogen from glycolysis and it would stop
The replenishment of NAD is achieved by pyruvate accepting hydrogen from the reduced NAD

59. Production of Ethanol
• Bacteria, fungi and plants can produce ethanol
• The pyruvate molecule formed in glycolysis loses a molecule of carbon
dioxide and accepts hydrogen from reduced NAD to produce ethanol
• Yeast is grown in anaerobic conditions to produce ethanol for brewing
• Pyruvate + reduced NAD → Ethanol + Carbon dioxide + NAD
Production of Lactate
• Anaerobic respiration I animals leads to lactate production in order to
overcome temporary shortage of oxygen
• Lactate production occurs most commonly in muscles as a result of
strenuous exercise as there is not enough oxygen being supplied causing
an oxygen debt
• Reduced NAD must be removed in order for energy to be released
• This is achieved as each pyruvate molecule produced takes up 2 hydrogen
atoms from the reduced NAD produced in glycolysis to form lactate
• Lactate needs to be oxidised back to pyruvate
• It can either further to release energy or converted into glycogen
• Lactate build up can cause cramp and muscle fatigue. Muscles do have a
certain tolerance however it has to be removed by the blood and taken to
the liver to be converted to glycogen
• Pyruvate + reduced NAD → lactate + NAD
Energy Yields
• In anaerobic respiration, pyruvate is converted to either ethanol or lactate.
• Therefore in anaerobic respiration neither the Krebs cycle nor the electron
transport chain can take place
• The only ATP that can be produced anaerobic respiration is formed by
glycolysis which is a very small amount compared to aerobic respiration

60. 1 x Glucose
2NAD
2ADP + 2P
2NADH
2ATP
2 x Pyruvate
ANAEROBIC RESPIRATION
2NADH
2NAD
2 x Lactic Acid

61. Anaerobic Respiration
When oxygen isn’t available, the electron transport
chain cannot operate so the initial supply of NAD run
out.
To regenerate this, pyruvate produced during glycolysis
must be reduced.
Pyruvate is converted into lactic acid in animal cells.
Pyruvate + NADH  Lactic Acid
The net yield from anaerobic respiration is simply the
2ATP produced in glycolysis and is therefore much less
energy efficient.
In some plants and microbes, pyruvate is converted
into ethanol.
Pyruvate + NADH  Ethanol + Carbon Dioxide + NAD

62. AQA A2 Biology Unit 4
Nutrient Cycles

63. Specification
3.4.6 Chemical elements are recycled in ecosystems. Microorganisms play a key role in
recycling these elements.
Candidates should be able to analyse, interpret and evaluate data relating to evidence of
global warming and its effects on the yield of crop plants, the life-cycles and numbers of insect
pests, the distribution and numbers of wild animals and plants.
Candidates should be able to analyse, interpret and evaluate data relating to eutrophication.
Nutrient cycles
The role of
microorganisms in the
carbon and nitrogen
cycles in sufficient detail
to illustrate the
processes of saprobiotic
nutrition,
ammonification,
nitrification, nitrogen
fixation and
denitrification.
Carbon
The importance of
respiration,
photosynthesis and
human activity in giving
rise to short-term
fluctuation and longterm change in global
carbon dioxide
concentration.
The roles of carbon
dioxide and methane in
enhancing the
greenhouse effect and
bringing about global
warming.
Nitrogen
The environmental issues
arising from the use of
fertilisers.
Leaching and
eutrophication.

64. Definitions
•
•
•
•
•
•
•
•
•
•
•
•
Active Transport: Movement of a substance across a membrane from a region of low
concentration to high concentration using ATP
Aerobic: In the presence of oxygen
Anaerobic: Without oxygen
Biomass: Total mass of living material in a specific area at a given time, usually
measured as dry mass since water value is variable
Consumers: Organism that obtains energy by eating another
Decomposer: An organism, e.g. fungus that breaks down organic material.
Ecosystem: Unit in ecology made up of all interacting biotic and abiotic factors in a
specific area
Greenhouse Gas: Such as methane and carbon dioxide, they cause heat to be trapped
in the atmosphere raising the Earth’s temperature
Niches: All conditions and resources required for an organism to survive, reproduce
and maintain population
Oxidation: Chemical reaction causing the loss of electrons
Producers: Organism that synthesises organic molecules from simple inorganic ones
Saprobiotic Microorganisms (Saprophyte): Organism that obtains food from dead or
decaying remains of other organisms

65. Basic Nutrients Cycle
• The flow of nutrients such as carbon and nitrogen is cyclic
When both the
producer and
consumer die
saprobiotic
microorganisms
break down the
molecules releasing
the nutrients
Nutrients taken up
by producers
(plants) as simple
inorganic
molecules
The nutrients are
then passed along
a food chain
Producers
incorporates the
nutrient into
complex organic
molecules
The producer is
eaten and
nutrients pass to
the consumers

66. 6.1 The Carbon Cycle
The main source of carbon for terrestrial organisms is
carbon dioxide in the atmosphere
Photosynthetic organisms remove it from the air to form
macromolecules e.g. carbohydrates, fats and proteins
Respiration returns carbon dioxide back to the air
The concentration of CO₂ is higher at night than day due to
no photosynthesis occurring while respiration still occurs

67. The Increase in Carbon Dioxide
Main Reasons due to human
activities:
• Combustion of Fossil Fuels: Coal, oil and peat
releases CO₂ previously trapped
• Deforestation: Removes photosynthesising biomass
so less CO₂ is removed
CO₂ is a greenhouse gas and contributes to global warming
The ocean is a CO₂ sink so keeps it constant
When organisms die Saprophytes break them down
into small soluble molecules using enzymes
• The decomposers then
absorb the molecules via
diffusion
The carbon is then released as CO₂ during respiration of the decomposer
When decay is prevented fossils are formed

68. The Carbon Cycle Diagram

69. 6.2 The Greenhouse Effect and Global Warming
The Greenhouse
Effect
• Natural process that occurs all the time
• Due to solar radiation from the sun reaching the
earth
• Greenhouse gases trap the heat in the Earth’s
atmosphere causing it to heat up
Greenhouse
Gases
• The major greenhouse gas is CO₂ which is
increasing due to human activities
• Methane is also produced when microorganisms
break down organic molecules , it occurs in two
situations:
• Decomposers break down dead remains of
organisms
• Microorganisms in intestines of primary
consumers e.g. cattle digest food
Global Warming
• Due to the layer of greenhouse gases building up it
traps the heat from the sun causing the Earth to
heat up

71. Consequences of Global Warming
Changes in temperature and precipitation, the timing of seasons and frequency of
extreme events e.g. storms
Climate change will effect niches available due to organisms being adapted to
particular niches
Animals could migrate to new areas causing competition and loss of native species
Melting ice gap could cause extinction of wild plants and animals e.g. polar bears and
sea levels will rise
Low land would be flooded and sea water would extend further up rivers making
cultivation difficult
Droughts could occur due to higher temperatures meaning xerophytes could only
survive
Greater rainfall would occur in some areas
Insect lifecycles will be altered and due to them carrying human and crop pathogens
tropical diseases could spread toward poles
Benefit could be more rainfall filling reservoirs, higher temperatures causing higher
rate of photosynthesis so more productivity and a larger harvest

72. 6.3 The Nitrogen Cycle
All living organisms require a source of nitrogen to form nucleic acids and
proteins
Plants take most of their nitrogen up via nitrate ions (NO₃-) from the soil
The ions are absorbed by active transport from the root hairs
Animals obtain their nitrogen compounds by eating the plants
Nitrate ions are soluble
When plants and animals die decomposition occurs and the nitrates are
restored to the soil

74. The Stages of the Nitrogen Cycle
Ammonification
• The production of ammonia from organic ammonium compounds e.g. urea, proteins and nucleic acids
• Saprobiotic microorganisms e.g. fungi feed on these materials releasing ammonia which forms ammonium ions in
the soil
• This is where nitrogen returns to non living components of the ecosystem
Nitrification
• Ammonium ions to nitrate ions is an oxidation reaction so releases energy
• It is carried out by nitrifying bacteria in two stages:
• 1. Oxidation of ammonium ions to nitrite ions (NO₂¯)
• 2. Oxidation of nitrite ions to nitrate ions (NO₃¯)
• Nitrifying bacteria require oxygen to carry out the conversions so the soil needs to have air spaces
• Farmers keep soil light and aerated by ploughing and having good drainage
Nitrogen Fixation
• Nitrogen gas is converted to nitrogen containing compounds
• 1. Free living nitrogen fixing bacteria reduce gaseous nitrogen to ammonia to manufacture amino acids, when the
bacteria dies and decay they release the nitrogen compounds
• 2. Mutualistic nitrogen fixing bacteria live in nodules on roots of plants and they obtain carbohydrates from the
plant while in return the plant acquires amino acids from the bacteria
Denitrification
• When soil is waterlogged there is a shortage of oxygen and the type of microorganism present changes
• Fewer aerobic nitrifying and nitrogen fixing bacteria are found meaning more anaerobic denitrifying bacteria are
present
• This bacteria converts soil nitrates into gaseous nitrogen reducing availability of nitrogen containing compounds for
plants
• To prevent the build up of denitrifying bacteria the soil has to be well aerated

75. 6.4 Use of Natural and Artificial Fertilisers
Intensive food production makes large demands on the soil
Due to this the minerals are removed from the soil, in agriculture the remains
of the consumer are rarely returned to the same area so mineral ions fall
The mineral ions need to
be replenished so
fertilisers are added
• Natural (organic): consist of dead/decaying
remains as well as animal waste
• Artificial (inorganic): Mined from rocks and
deposits then converted into different forms
to give the appropriate balance of minerals ,
compounds contain nitrogen, phosphorus
and potassium

76. Fertilisers Increasing Productivity
Plants require minerals for growth, nitrogen is needed for proteins and
DNA
With nitrogen plants grow taller and have a greater leaf area
This increases the rate of photosynthesis and improves crop productivity

77. 6.5 Environmental Consequences of using
Nitrogen Fertilisers
Effects of Nitrogen Fertilisers
• Nitrogen is essential for proteins and growth and causes the
increase in leaf area
• This increases the rate of photosynthesis and improves crop
productivity
The nitrogen containing fertilisers have bad effects to:
• Reduced species diversity as nitrogen rich soils favour growth
of grasses so they out compete other species that then die
• Leaching leads to pollution of watercourses
• Eutrophication caused by leaching of fertiliser into
watercourses

78. Leaching and Eutrophication
Leaching
• The process by which nutrients are removed from the soil
• Rain water will dissolve soluble nutrients e.g. nitrates and carry them into the soil
beyond plant roots
• The leached nitrates reach the watercourses e.g. rivers that drain into freshwater
lakes
• They can harm drinking water, prevent efficient oxygen transport in babies and
cause stomach cancer
• The leached nitrates are harmful to environment as they cause eutrophication
Eutrophication
• The process by which nutrients build up in bodies of water
• Most rivers contain low nitrate levels so it is a limiting factor for plant/algae
growth
• Nitrate concentration increases due to leaching so the plants grow exponentially
• Algae grow at the surface so the upper layers of water become densely
populated with algae, ‘algae bloom’
• The layer absorbs light and prevents it from reaching the lower depths
• Light becomes the limiting factor for growth so plants at deeper depths die
• The lack off dead plants and algae is no longer limiting for the growth of
saprobiotic algae so they grow exponentially
• Saprobiotic bacteria require oxygen for respiration creating a demand for oxygen
• The concentration of oxygen in the water is reduced and nitrates are reduced
from decaying organisms
• Oxygen then becomes the limiting factor for aerobic organisms e.g. fish so they
die
• Without aerobic organisms there is less competition for anaerobic organisms
• These organisms further decompose dead material realising more nitrates and
toxic waste like hydrogen sulphide
• Animal slurry, human sewage, ploughing and artificial fertilisers cause leaching

79. AQA A2 Biology Unit 4
Ecological Succession

80. Specification
3.4.7 Ecosystems are dynamic systems usually moving from colonisation to climax communities in the
process of succession.
Succession
Succession from pioneer
species to climax
community.
At each stage in
succession, certain species
may be recognised which
change the environment so
that it becomes more
suitable for other species.
The changes in the abiotic
environment result in a less
hostile environment and
changing diversity.
Conservation of habitats
frequently involves
management of succession.
Candidates should
be able to
• use their knowledge
and understanding to
present scientific
arguments and ideas
relating to the
conservation of species
and habitats
• evaluate evidence and
data concerning issues
relating to the
conservation of species
and habitats and
consider conflicting
evidence
• explain how
conservation relies on
science to inform
decision-making.

81. Definitions
•
Ecosystem: More or less a self-contained functional unit in ecology made up of all
the biotic and abiotic factors of a specific area
•
Abiotic: An ecological factor that makes up part of the non-biological environment
of an organism e.g. temperature
•
Biotic: An ecological factor that makes up the living environment e.g. food
•
Communities: The organisms of all species that live in the same area
•
Deciduous: Plants that shed their leaves in one season
•
Habitats: The place where an organism lives, characterised by physical conditions
and the species of other organisms present
•
Climax Community: The organisms that make up the final stage of ecological
succession
•
Biodiversity: The range and variety of living organisms within a particular area
•
Biomass: The total mass of a living material in a specific area at a given time,
usually measured as dry mass as amount of water is variable
•
Conservation: The management of the Earth’s natural resources

82. 7.1 Succession
Ecosystems constantly change, this is known as succession
The first stage of succession is the colonisation of an inhospitable environment by the pioneer species
Pioneer Species
Adaptations:
Succession Stages
Succession Features:
• Product vast quantities of wind dispersed seeds/spores
• Rapid germination of seeds on arrival
• Ability to photosynthesise
• Ability to fix nitrogen from the atmosphere
• Tolerant of extreme conditions
• Over these stages the environment becomes more hospitable and new species begin
to grow which can outcompete other species
• The pioneer species e.g. lichen, grow in inhospitable environments and as time
progresses the lichen die and decompose producing nutrients to support the
community
• Lichens have changed the abiotic environment by creating soil and nutrients
• Mosses are the next followed by ferns causing an increase in organic matter, due to
the dying plants the soil becomes thicker
• The hospitability is increased till the ultimate community is formed (climax
community)
• Due to the variation of plants a variation of animals is also increased
• Non-living environment becomes less hostile due to nutrients being increased
• Greater number and variety of habitats
• Increased biodiversity
• More complex food webs
• Increased biomass

83. Climax Communities
• Have a stable equilibrium with the climate
• Abiotic factors determine the dominate species in the community
Secondary Succession
• If land has been cleared for agriculture or a forest fire the process of succession
still occurs
• It is a faster process as spores and seeds remain alive in the soil and there is an
influx of animals and plants via migration
• There is no need for a pioneer species

84. 7.2 Conservation of Habitats
Conservation involves active intervention from humans to maintain biodiversity
Reasons for conservation:
• Ethical: Species should be allowed to coexist with humans
• Economic: Living organisms have a large pool of genes that
could be valuable. Long term productivity is greater if
ecosystems are maintained
• Cultural and Aesthetic: Habitats and their organisms enrich our
lives
Managing Succession
• Many of the organisms present in the series of succession are
no longer present at the climax community
• Due to being outcompeted or their habitat being no more the
species often migrate
• In order to combat this problem succession is stopped e.g. the
land my be burnt or grazed on by sheep stopping tree saplings
from growing
• If the factor preventing further succession is removed then the
climax community will grow in secondary succession