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AQA A2 Biology Unit 4
Transcript of AQA A2 Biology Unit 4
Topics: Photosynthesis, Respiration, Nutrient Cycles and Energy and EcoSystems
Stage 1: Glycolysis
Population = group of organisms in a habitat that produce fertile offspring when they breathe.
How the leaf is adapted:
Large surface area to collect sunlight
Thin for short diffusion pathway
Upper mesophyll packed with chlorophyll
Stomata that open and close according to light intensity
Loads of stomata
Transparent Cuticle to let light in
Light Dependent Reaction
A photon of light enters the leaf and enters the stroma of a cell, colliding with a chlorophyll molecule in carrier protein Photosystem 2, this excites electrons into a higher energy level, a high energy electron is then lost and travels to the electron transport chain.
Electron Transport Chain
The electron then travels from Photosystem 2, to Cytochrome, where the electron is further excited and more protons are pumped into the stroma, then to Photosystem 1, to Ferredoxin. This releases energy for ATP to be made from ADP and Pi.
When a photon of light collides in photosysytem 2. it creates enough energy to split 2 water molecules
Water molecule splitting releases protons (H+) into the stroma
The proton pumping and the release of H+ from the photolysis of water cause a proton build up in the stroma, causing a proton gradient. They then fall through the enzyme ATP Synthase to the outside stroma, this makes enough energy for more ADP to be bonded to Pi.
The electrons are then taken on by an H+ to make a Hydrogen atom, which is then taken on by NADP to make NADPH, which is then used in the next step in photosynthesis.
The Calvin Cycle
Glucose and other organic substances
Reduced by NADPH
4 Carbon Molecule with 2 Phosphates
Makes unstable 6 Carbon intermediate
3-Carbon with one phosphate
3 Carbon with 3 phosphates
Fructose 1-6 bisphosphate
Use 1 ATP
Use 1 ATP
2 ADP -> ATP
2 ADP -> ATP
NADPH -> NADP
NADPH -> NADP
The Link Reaction
Pyruvate is reduced by removing a hydrogen, the molecule then becomes unstable and forms a 2-Carbon molecule called Acetyl, this combines with Co-enzyme, to make Acetyl Co Enzyme A
Acetyl Co Enzyme A
Ecosystem = An interaction of biotic and abiotic factors within a specific area. The flow of energy and cycling of elements are the two processes in an eco system
Community = all the different living populations interacting in a specific area
Habitat = A place where a community of organisms live, it can contain many microhabitats, where specific populations live, which can often have their own microclimate i.e. damp and cold underground
Elogical Niche = Where a particular organism/population lives, what it eats, and what it does.
Random sampling requires the use of a quadrat, there are 3 main things to consider when using a quadrat:
Size of the Quadrat to use- This is relative to the size of the thing you're measuring, you don't want to use a huge quadrat to measure how many daisies there are, by the same token, you dont want to use a tiny quadrat to measure the amount of wild garlic (?!) there is in an area.
The Number of Quadrats you use- more quadrats = more reliable results, but it is also time consuming, so you need to do as many quadrats in the time allotted.
The Position of the Quadrat- it must be completely unbiased to acheive a valid result, so random sampling is used to achieve that
The first step in random sampling is to create a transect along two sides of the area you want to study.
The second step is to use a random number calculator to generate random coordinates
The final step is to merely place the quadrat on the co ordinates and record the number of specied in there
Systematic Sampling Along Transects
Two lines (both about a metre in width) are used to make a 'belt', anything that enters the belt is counted
Random sampling and measuring along a transect calculate abundance, which can be interpreted in several ways, the two main ones are:
Frequency- Likelihood of a member of a species will appear in a quadrat.
Percentage Cover-The area within a quadrat covered by the species, rather than a specific number of organisms
Mark- Release- Recapture Techniques
The other two methods work with plants but not really animals.
To measure animal population size, take a specific area, capture a set number of animals, tag them with an individual tag, then come back at a later date, do the same thing, then come back again, capture more of the same species and count how many recaptured were tagged. The population size is then estimated with this equation:
Estimated Population Size
Total number of individuals captured in first sample x Total number of individuals caught in second sample
Number of individuals recaptured that were marked
Don't take them away if you don't have to.
Return them to where you found them if you have to move them
Don't come back for a bit
Don't damage the place you measured
Tagging shouldn't damage animal
domesticated animals and plants
produce food for mankind
Net Productivity = Gross Productivity- Respiritory losses in
Is not as quick.
Can itself become a pest
Are more environmentally friendly.
Must be have right animas for environment.
Have habitats nearby for natural predators of pest
Regular crop monitoring.
Mechanically move pests
Pesticides last resort.
Animals in enclosed environment
Efficient energy conv, Use of space, safety, drugs, disease, welfare, pollution, reduced diversity, fossil fuels
Can be a long term solution
Must be continuously re applied to soil
Can get pests that become resistant to this
Will be effective as long as the pests remain
Agent can be come pest if it multiplies too much
This describes the colonisation of baron land, to become a climax community, with a wide variety of plant and animal life
Years prior to ice retreating
Stage lasts around 20 years
Stage lasts around 50 years
Stage lasts around 50 years
The land consists of bare rock, conditions are very hostile and there is little to none species diversity
Stage lasts around 30 years. PRIMARY COLONISERS SETTLE IN
Soon after the ice has retreated simple photosynthetic organisms such as bacteria lichen appear, these fix nitrogen, and provide a tough stable ground, and when they die become a nutrient rich humus, which allows for mosses to form.
Levels of nitrogen rise slowly from 0 to around 50g/m-2
Species diversity increases sharply from 0 to around 15 different species
Biomass levels rise almost unnoticably
Secondary colonies form
The ground is a mat of herbaceous material whose roots give support to the unstable soil beneath that is made from degraded rocks.
Drayas is also nitrogen fixing so when it dies it adds more nutrients to the soil.
Nitrogen concentration in the soil steadily climbs at roughly the same rate as in the pioneer stage, so there is 100g/m-2 by the end of the stage, however, it doubles in a much shorter space of time as there are more dead organisms contributing to it.
Number of species rises very gently, around 20 different species by the end.
Above ground biomass also increases slightly, around 50 tonnes by the end.
Tertiary and scrubland species appear.
Alder is a type of shrub with nitrogen fixing nodules at it's roots. When it's leaves drop off, they decompose into the soil and leave nutrients.
Nitrogen soil concentration in soil increases dramatically, at the end there is around 200g/m-2
Number of species remains pretty constant, rising by 3-4
Biomass increases dramatically however, to around 200 tonnes per hectacre, as more shrubland takes over
Evergreen spruce begins to appear, this is taller and has a bigger leafspan than shrubbery, so starts to outcompete the Alder for things like space, sunlight, water and soil nutrients, so starts to displace it.
Nitrogen levels in the soil reach a peak at about 120 years post glacier retreat as a large quantity of shrubs die, then the spruce begins taking nitrogen up from the soil and not replacing it soon after, so nitrogen levels decrease
Species diversity of plants drops dramatically as the spruce outcompetes everyone, then it becomes constant.
Above ground biomass increases dramatically, but reaches a stopping point, and biomass remains constant after 125 years.
Spruce has now completely displaced the alder.
Plant species diversity remains constant
Nitrogen levels continue to steadily decrease
Above ground Biomass remains constant
Spruce are the Climax Community
ATP is how the body stores energy, when ATP molecules are split into ADP+Pi, energy is released
In a living organism, energy is used for:
Cell maintenance, repair and division
Maintenance of body temp
Energy put in
Nitrogen in atmosphere
Ammonium containing molecules in producers
Ammonium containing molecules in consumers
Ammonium containing molecules in Decomposers
Feeding and Digestion
The production of ammonia from ammonia containing components, such as urea. Saprobiotic Microorganisms feed on the materials and release ammonia
Nitrifying bacteria in the soil oxidise the Ammonia to NO
They then oxidise that to make NO
Where nitrogen is converted from nitrogen gas to nitrogen containing compounds by either:
Mutualistic Bacteria- These are attached to nodules on roots on plants, they give amino acids to the plant and get carbs from it
Free Living Bacteria- Living in the soil and use the nitrogen to make amino acids.
The Carbon Containing Molecule Cycle
CO in atmosphere
Higher Level Consumers
Detritavores and Decomposers
Factors Affecting Growth and Size of Human Population
I'm a saprobiotic organism!
Where individuals from outside join a population
Where individuals leave a population
Percentage Population Growth Rate
(births + immigration) - (deaths + emigration)
Population change over the period
Population at the start of the period
Factor Affecting Birth Rate
I'm a nitrifying bacteria!
Countries that have lower a GNP often also have higher birth rates
Cultural and Religious Backgrounds
Some religions dictate the size of the family a person may have, and some religions are also opposed to birth control
Social Pressures and Conditions
In some countries large families improve social standing.
Governments i.e. China can dictate the amount of children you have by adding taxes for extra children or manipulating the education system.
The extent to which contraception and abortion are used heavily dictates the amount of births.
Factors Affecting Death Rate
Lower life expectancy means more frequent deaths. Those in MEDC's have longer life expectancies.
Access to good medical care dramatically increases life expectancy and the general age profile of a country. Countries without access to medical help have much lower life expectancies and age profiles.
Death in war causes an immediate rise in death rate and overall a lower population later as there are fewer fertile adults.
Larger proportion of old people, higher death rate of country.
Access To Safe Drinking Water
Unsanitary drinking water carries parasites, and diseases such as cholera and dysentery, which dramatically increase death rate left untreated.
Natural Disasters have the potential to kill a huge amount of people at any one time, and so can sifnificantly increase death rate.
Less food means a lower death rate as a result of malnutrition. A more balanced diet increases life expectancy
Use population pyramids to analyse whether a population is growing, shrinking or stable
Typically growing if an LEDC
Typically shrinking if a Stage 5 MEDC
Survival Rates and Life Expectancy
Survival curves are plotted on a graph and often plot % of population alive against age. Can often be used to find the populations after an event such as a natural disaster.
Average life expectancy is the point at which 50% of the population are still alive
Types of Competition
Individuals of different species compete.
Compete for things essential to life, such as breeding rights, living space food and water
The more of the
for, the more of that species there is.
Here two species can occupy the same niche, in this instance, the species that has the most advantageous qualities will out. Say 2 birds eat the same nut, and live in the same tree, the one with the better shaped beak for stabbing will win, and that species will grow, while the other will decline. This is the competitive exlcusion principle
Individuals of same species compete.
Some members of the species can grow to have more advantageous qualities than another, and so take nutrients, and the weaker form dies out, so there are fewer, better adapted organisms
Big tree will outcompete little trees, little trees will die, big trees left over
Predation occurs when one animal is consumed by another.
Data about predation is often obtained in a lab, however, lab settings do not always represent the wild, and where prey is completely vulnerable in a lab, it would not be in the wild, so data is not always reliable.
Effect of Predator- Prey Relationship on Population Size
Predator eat prey, lowers prey population
With less prey, the predator population reduces as a result of starvation
With less predator influence, the prey population grows unrestricted.
More plentiful prey so population of predator increase, as more survive to breed.
Variation in Population Size
Biotic Factors are living factors that dictate the size of a population, such as competition or predation
Abiotic Factors are non-living parts of an ecosystem that dictate how well organisms can function.
Conservation Of Habitats
Conservation is the management of natural resources so that they are available in the future. Allows for the maintenance of habitats and therefore biodiversity.
Main Reasons for Conservation
Cultural and Aesthetic
Organisms contain the ability to make a vast amount of substances in their genes, these abilities may become of use in the future, and so to maintain productivity ecosystems should be maintained
Allows respect for living beings that have been on the planted longer than we have.
Enrichment of human life, inspire people, nice to look at essentially
Conservation via managing succession
Habitats and therefore species can be conserved by preventing succession, by preventing a more superior plant from outcompeting the smaller plants, the smaller plants and organisms that live in them can continue to thrive.
Energy and Ecosystems
Food Chains and Food Webs
There are different types of organisms in a food chain:
Producers: Organisms that get their energy from photosynthesis, like a tree.
Organisms that get their energy from other organisms
Eat the plants
Eat the primary
Top of chain and eat secondary
Organisms that break down the former two to release the nutrients.
Food chains often cross over each other as animals don't tend to rely on one food source, so food webs are formed.
Each level in a food chain is called a trophic level
Energy Transfer Between Trophic Levels
Decomposers and Detritavores
Reasons All Energy Is Not Transferred
Only 1%-3% of sunlight is absorbed by plants because
It is reflected by clouds or is absorbed by the atmosphere.
Isn't the correct wavelength to be absorbed,
Light doesn't fall on every cholorophyll molecule
Make a mnemonic!
Only 50% of absorbed sunlight is converted into organic materials.
Even though all of the organism is consumed it's not digestible so energy is lost in faeces.
Some organism not eaten
A LOT of energy is lost in respiration of warm blooded animals, maintaining body heat, and this is constantly lost.
Equations You Need:
Net Production = Gross Production- Respiratory Losses
Energy Transfer =
energy available after transfer
energy available before transfer
Percentage efficiency for an energy transfer.
Pyramids of Number
Measure raw numbers
Show quantitative data about animals at each trophic level
Data complicated to get: In pyramid of biomass, the animal must be killed and dried, in pyramid or energy, complex maths
Pyramid of Biomass
Pyramid of Energy
Compares the raw numbers of each organism.
Not very reliable because:
Hard to represent actual figures
No account is taken of the actual size of the organism, so one oak tree is the same size as one insect,
Measure the mass of each organism in a food chain and represent that on a pyramid.
However because of varying levels of water must be used,
the animals must be dead and dried to measure their dry
mass, this often means a small sample is used to represent
a very large population,
and it may not be representative.
More accurate than pyramid of biomass because animals of the same dry mass can have different energy values
Very time consuming as data is normally collected over a year
kJ m year
Only organisms there at the particular time are measured, so no seasonal differences observed
Measure data in specific area
The Greenhouse Effect
The earth is kept warm because a small percentage of the sun's heat is trapped near the earth's surface by clouds and greenhouse gases
However, there is an increasing amount of greenhouse gas in the atmosphere, especially CO2, which is causing more heat to be trapped near the earth surface
Consequences are rising sea level, flooded low-lying areas, areas turned
to desert, crops failing, greater rainfall and storms in some areas, and different ecosystem structure as animals able to withstand environment, and disease spread.
Need nitrogen fertilisers because crops are grown on same patch of land, they take up nitrogen and then it is not returned.
Nitrogen, phosphorous and potassium always present
Dead and decaying remains of plants and animals, and waste products like manure and bone meal
Made of minerals drilled from rocks and converted into different forms and blended with other things, can be specifically altered to specially suit any crop to make it grow at
Combination of both allows for plants to grow at maximum efficiency
Effects of Nitrogen Fertilisers
Lower species Diversity:
Excess nitrogen promotes the growth of faster growing plants, which out-compete other plants and as a consequence the die out
Where rain causes all the nutrients in the soil to be pushed deeper into the soil, where it may find it's way into water courses or fresh water lakes, which is harmful to humans.
Where nutrients build up in freshwater lakes and rivers
1. Nitrates allow for an increase of algal growth on water surface where it was a limiting factor before.
2. Algae on surface stops light penetrating bottom so the plants on the bottom.
3. Saprobiotic plants thrive as a result.
4. Oxygen conc reduced as more nitrogen released so fish die
5. Anaerobic organisms thrive with lack of competition, and make water putrid by release hydrogen sulphide.
Inheritance and Selection
The observable characteristics of an organism
Genetic make up of an organism
Made up of a series of nucleotide bases that code for a specific characteristic
A variation of a gene, It codes for the same functional thing, just a slight variation. Found on the same loci of a chromosome
get hydrogens from Citric acid COOH group, causing the unstable COO group to leave as CO2
Lost during the cycle
Co-Dominance & Multiple Alleles
Where two homozygous organisms (one dominant, one recessive) are crossed, they both have an allele for a phenotype, i.e. size. The dominant for tall would be TT, the recessive for small would be tt. Both organisms are crossed, and a genetic cross is drawn
No matter the cross of alleles from both parents, a dominant allele will always be inherited, and so all of the offspring will be tall
X chromosomes are big and strong, Y chromosomes are small, and where an X chromosome has a defective recessive allele, there is more often than not, no homologous loci on the Y chromosome to house a dominant chromosome, so it is expressed.
You can tell a genetic disease is ex-linked if only one gender get it.
A genetic disease is recessive if you can have unaffected female carriers.
Normal Blood Clotting:- B
Only boy will have haemophilia, as a girl will always ave the dominant allele to cause it to not be expressed. 25% chance offspring will have haemophilia.
: where two alleles that code for different phenotypes are equally dominant
Where more than one allele that codes for a particular phenotype reside at the same loci on the chromosome
Co Dominant Flower Colour
Multiple Allele Blood Type
Red C C
White C C
Calculated with the Hardy Weinberg Principle
The principle is valid as long as there
are no mutations
is no immigration or emigration, so no flow of alleles
is no selection
p + 2pq + q = 1.0
p + q = 1
The values given are always decimals, so the square roots are larger numbers, so p + q = 1.0, use 1-number give to find number you're looking for, then at both numbers together, and multiply it by two to find the frequency of heterozygous alleles
Allelic frequency varies because organisms with the more favourable alleles survive while those with the less favourable alleles die.
Directional selection occurs when there is a shift in the environmental conditions. One group of animals will posses an allele that makes them more suited to the new conditions than the other organisms of their species, they survive and breed, while the others die and do not pass on their alleles. Over generations, the mean frequency of that allele increase, and the phenotype changes accordingly.
Those with the favourable phenotype, which causes their phenotype to be around the mean appearance for survival, are continually selected and their allele passed on.
Evolution of new species from an existing species
All species breed with each other, and they all have populations, if one population becomes segregated, over time, their gene pools differ as mutations occur and allelic frequencies change. Once reunited, the two species will not be able to breed to make fertile offspring.