Biology Unit 4 Revision - AQA

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Chapter 1.1 - Populations and Ecosystems

Key Definitions:

  • Ecosystem - An area which is made up of all the interacting Biotic and Abiotic factors, within each area there is a number of species, each species has a groupd of individuals which make up the population.
  • Populations - A groud of interbreeding organisms of one species in a habitat. Populations of different species iform a community.
  • Community- All of the populations of various organisms, living and interacting in a particular place at the same time.
  • Habitat - Where a community of Organisms live, each habitat has smaller units called Microhabitats.
  • Ecological Niche - How an organisms fits into an environment, and refers to where an organism lives and what it does there. It includes Abiotic and Biotic factors.
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Chapter 1.2 - Investigating Populations.

Quadrats -

Split the area into grids and use a random number generator to generate a series of co-ordinates. Once the co-ordinates are obtained, place the quadrat at the intersect of the co-ordinates. Count the abundance of species in the Quadrat.

Factors to consider when using Quadrats are:

  • The size of Quadrat to use, depending on the species that is being investigated.
  • The number of Quadrates to use, depending on the area of investigation.
  • The position of each quadrat within the study area, use random sampling.

Types of Quadrats;

  • Frame Quadrat
  • Point Quadrat 
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Chapter 1.2 - Investigating Populations.

Mark-release-recapture techniques

- Catch a known number individuals in a specific area and mark a few of them. Release the first sample and wait a while for the individuals to be integrated back into the population. Catch another known number of individuals, known as the second sample, and record the number of marked individuals recaptured.

Population size= (total no. in 1st sample x total no. in 2nd) / no. of marked recaptured

The technique relies on a number of assumptions:

  • Ratio of unmarked to unmakred is the same in both samples.
  • The marked individuals distribute themselves evenly amongst the population.
  • No immigration or emigration.
  • There are few, deaths and births.
  • Marking is not toxic. marked is not liable to predation.
  • The mark is not rubbed off during the investigation.
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Chapter 1.2 Investigating Populations.


  • If it is necessary to remove the individuals from their habitat, the numbers should be kept to a minimum.
  • Any organisms removed from the habitat must be returned as soon as possible, to the same place where they were removed. Even if they are dead.
  • A sufficient period of time should pass before a site is used again for future investigations.
  • Disturbance and damage of the habitats should be avoided.


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Chapter 1.3 Variation in Population Size.

Abiotic Factors:

  • Temperature: If the temperature if too low the enzymes are too slow. If the temperature is too high, the enzymes become denatured. The further away the temperature is from the optimum, the more energy is spent on maintaining the temperature as opposed to growth and reproduction.
  • Light: Increased light intensity means an increased rate of photosynthesis, which then leads on to increased growth and so, and overall increased reproduction. More plants, mean that there is more food for other organisms to consume.
  • PH: This affects the action of enzymes. Each Enzyme has an optimum PH. A change in the PH could mean a change in the tertiary structure of an enzyme, and so, making the active site uncomplimentary to a specific substrate, and making it non-functional.
  • Water and Humidity: Where water is scarce, populations are small and consist of only species who are well adapted to living in dry conditions. Humidity affects the transpiration rates of plants.
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Chapter 1.4 Competition

Intraspecific Competition:

  • Occurs when individuals of the SAME species compete with one another for resources such as: Food, Water and Breeding sites.
  • It is the availability of such resources that determines the size of the population. Thus, with a greater availability of the resources, the larger the population of the species.

Interspecific Competition:

  • Occurs when individuals of DIFFERENT species compete for resources such as Food, Water and Breeding Sites.
  • Where populations of 2 species occupy the same Niche, then one species will usually have a competitive advantage over the other.
  • This type of competition will usually lead to the complete removal of one species from a Niche.  - This is known as the Competitive exclusion principle.
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Chapter 1.5 - Predation


  • Occurs when one organism is consumed by another.

The relationship between Predator and Prey:

  • Predators eat their prey, thereby reducing the population of the prey.
  • With fewer prey available, the predators are in greater competition with each other for the prey that are left.
  • The predator population is reduced as some individuals are unable to obtain enough prey for their survival.
  • With fewer predators left, fewer prey are eaten.
  • The Prey populations increases.
  • With more prey now available as food, the predator population in turn increases again.

Example of a predator and prey - The Canadian Lynx and the Snowshoe Hare.

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Chapter 1.6 - Human Populations.

Factors affecting population size:

  • Birth rate
  • Death rate
  • Immigration
  • Emmigration

Factors affecting Birth Rates:

  • Economic Conditions - Low GDP = (generally) high birth rate
  • Culture and Religion - Some religions encourage large families (catholicism)
  • Social Pressures- Large family = Social standing.
  • Birth Control- the extent at which contraception is available and used.
  • Political factors-  Government policies on education and taxation for large families. (china)
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Chapter 1.6 - Human Populations.

Factors affecting Death rates:

  • Age profile - High population of elderly = high death rate.
  • Life expectancy at birth - LEDC's = low life expectancy.
  • Food Supply - Balanced diets improve reduces death rate.
  • Safe water supply- Reduced diseases (cholera) = reduced death rate.
  • Medical care- Access to healthcare reduces death rate.
  • Natural disasters - higher risk of disaster = higher death rate.
  • War- Immediate drop in population.

2 Major recent events that have stimulation population growth:

  • The development of agriculture
  • The development of Manufacturing that was stemmed from the industrial revolution.
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Chapter 2.1 - Energy and ATP

Why do organisms need energy?

  • Metabolism
  • Movement
  • Active Transport
  • Maintainence, repair and division of new cells.
  • Production of substances/
  • Maintainence of Body temperature.

Energy and Metabolism

  • Light energy from the sun is converted by plants into chemical energy (photosynthesis).
  • Chemical energy is then converted into ATP during respiration in all cells.
  • ATP is used by cells to perform useful work.
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Chapter 2.1 - Energy and ATP

Synthesis of ATP

  • Conversion of ADP + Pi to ATP = Condensation reaction
  • Conversion of ATP to ADP + Pi = Hydrolysis reaction.
  • Occurs in 3 ways : Photophosphorylation, Oxidative Phosphorylation, Substrate-level, Phosphorylation.

Roles of ATP:

  • Immediate source of energy
  • Cells do not store large quantities of ATP and so can be readily used.
  • Is better than Glucose, as it does not have to undergo a series of reactions to release energy.
  • Energy released is in small, manageable quantities.
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Chapter 3.1 - Photosynthesis

Stucture of the Leaf:

  • Large surface area
  • Arrangement of leaves - minimises overlapping.
  • Thin - Short diffusion pathway.
  • Transparent Cuticle.
  • Packed with Choloplasts.
  • Many Stomata - gas exchange, and respond to the light intensity.
  • Many air spaces, allows diffusion to take place.
  • Network of Xylem and Phloem.

Stages of photosynthesis:

  • Capturing light energy in the Cholorplasts.
  • Light Dependent reaction.
  • Light independent reaction.
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Chapter 3.1 - Photosynthesis

Structure and Role of the Cholorplasts.


  • Disc- like structures called Thylakoids.
  • Where the light-dependent reaction takes place.
  • Some have inter-granal lamellae.


  • Fluid Filled Matrix
  • Where the light- independent stage takes place.
  • also has a number of Starch Grains.
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Chapter 3.2 The Light-dependent reacion.

Photolysis -  The loss of electrons when a photon of light strikes a Chlorophyll molecule leaves it short of electrons. These electrons are replaced, when a water molecule is split using light energy.

Stages of the Light-Dependent Reaction (summarised):

  • Photon of Light Strikes the Chlorphyll.
  • Electrons are excited, and passed onto Photosystem II
  • Photolysis occurs to replace the lost electrons.
  • Electrons are oxidised and are passed onto Electron carries.
  • Plastoquinone.
  • B6-F Complex.
  • ATP Synthase - Synthesises ATP
  • Photosystem I
  • NADP Reductase.
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Chapter 3.3 - The Light-Independent Reaction

The site of the light-independent reaction

  • It does not require light directly.
  • Takes place in the Stroma of the Cholorplasts.
  • This stage is often referred to as the 'Calvin Cycle'.
  • Choloroplasts are adapted.
  • Fluid of the stroma has enzymes needed to carry out the reaction.
  • Stroma surrounds the Grana and so the products in the Grana can easily diffuse into the Stroma.
  • Contains DNA and Ribosomes.
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Chapter 3.3 - The Light-Independent Reaction.

Stages of the Calvin Cycle:

  • CO2 from the atmosphere diffuses into the leaf through the Stomata, diffuses through the cells of the leaf until it reaches the Stroma of the Cholorplast.
  • CO2 in the Stroma combines with the 5C compound using an enzyme = RuBP
  • The Combination of RuBP + CO2 produces 2 Molecules of the 3 Carbone Glycerate Triphosphate (GP).
  • ATP and reduced NADP from the light dependent reaction are used to reduce the activated GP to Triose Phosphate.
  • The NADP is reformed and goes back to the light-dependent reaction to be reduced again by acception more hydrorgen ions.
  • Some triose phosphate molecules are converted ot useful organic substances such as Glucose.
  • Most Triose Phosphate molecules are used to regenerate RuBP using ATP.
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Chapter 3.4 - Factors Affecting Photosynthesis.

Limiting Factors include:

  • Light Intensity.
  • CO2 and O2
  • Water
  • Temperature
  • PH
  • Nutrient Availability.

The law of Limiting Factors:

''At any given moment, the rate of Physiological process is limited by the factor that is at its least favourable value''.

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Chapter 3.4 - Factors Affecting Photosynthesis

Effect of Light Intensity on the rate of Photosynthesis

  • When light is the limiting factor, the rate of Photosynthesis is directly proportional to light intensity.
  • As light intensity increases, the volume of O2 + CO2 produced will increase to a certain point.
  • A point will be reached at which light intensity will have no impact on Photosynthesis.
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Chapter 3.4 - Factors Affecting Photosynthesis

Effect of CO2 on the rate of Photosynthesis:

  • Often a factor that limits the rate of Photosynthesis under normal conditions.
  • CO2 Concentration affects enzyme activity. 
  • The enzyme that catlyses the combination of Ribulose Biphosphate (RuBP) with CO2 in the light independent reaction. 
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Chapter 3.4 - Factors Affecting Photosynthesis

Effect of Temperature on the rate of Photosynthesis: 

  • Provided there are no other factors limiting the rate of Photosynthesis.
  • Rate of Photosynthesis is increased in direct proportion to the rise in Temperature.
  • The rate of photosynthesis is approximately doubled for each 10 degrees rise in temperature.
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Chapter 4.1 - Respiration

Stages of Aerobic Respiration: 

  • Glycolysis
  • Link Reaction 
  • Krebs Cycle
  • Electron Transport Chain

Stages of Glycolysis: 

  • Activation of Glucose by Phosphorylation - uses 2 ATP molecules. 
  • Splitting of the Phosphorylated Glucose into Triose Phosphate (3 Carbon Molecule).
  • Oxidation of Triose Phosphate.
  • Production of ATP.

The Link Reaction:

  • Pyruvate is oxidised by removing H+ ions. The H+ is accepted by NAD to form NADH and later for ATP. 
  • Product is Acetyle Coenzyme A.
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Chapter 4.2 - Link Reaction and Krebs Cycle

The Krebs Cycle: 

  • Substrate level phosphorylation. 
  • For each molecule of Pyruvate the link reaction and the Krebs cycle occurs twice and produces: 
  • Reduced Coenzymes - NADH and FADH
  • 4 ATP Molecules
  • 5 Molecules of CO2


  • Molecules that some molecules need in order to function, such as NAD, FAD and NADP. 

The significance of the Krebs Cycle: 

  • It breaks down molecules into smaller ones.
  • Regenerates the 4 carbon molecule that combines with Acetyle CoA.
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Chapter 4.3 Electron Transport Chain

  • H+ Ions combine/attaches to the Cristae of the Mitochonidria. 
  • NADH and FADH donate electrons to first molecule in ETC. 
  • Releases protons from H+ atoms and are actively transported across the inner mitochondrial membrane. 
  • Electrons pass along a chain of ETC molecules through oxidation-reduction reactions.
  • Electrons lose energy along the ETC.
  • Some of the energy lost is used to make ATP. 
  • The Protons accumulate in between the Membranes of the Mitochondria before they can diffuse back into the Mitochondrial matrix through protein channels.
  • Electrons combine with these protons and O2 to form water, 
  • O2 is the final electron acceptor. 
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Chapter 4.4 - Anaerobic Respiration

Production of Ethanol (plants). 

  • Pyruvate + NADH = Ethanol + CO2 + NAD

Production of Lactate (animals). 

  • Pyruvate + NAD = Lactate + NAD

Energy Yield - Anaerobic and Aerobic 

  • Substrate level phosphorylation - Glycolysis, Link Reaction and Krebs cycle.
  • Oxidative Phosphorylation - ETC - Produces ATP using H+ atoms from Glycolysis + Krebs Cycle that are carried with NAD and FAD.
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Chapter 5.1 - Food Chains and Food Webs


  • Photosynthetic organisms that manufacture organic substances. 


  • Obtain their energy by feeding on other organisms. Primary consumers. 


  • Death = releases valuable nutrients = detrivores. 

Food Chains: 

  • Food Chains describes a feeding relationship which the producers are eaten by Primary Consumers. 
  • Primary Consumers are then eaten by Secondary. 
  • Each stage in this chain is referred to as a Trophic level. 
  • Arrows reflect the direction of the energy flow. 
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Chapter 5.2 - Energy Transfer between Trophic Leve

Energy Losses in Food Chains

  • Most of the suns energy is not converted to organic matter by photosynthesis.
  • 90% of the sun's energy is reflected back into space by clouds and dust. 
  • Not all wavelengths of light can be absorbed.
  • Light may not fall on a chlorophyll molecule. 
  • Low CO2 levels.

Low % of energy transferred at each stage: 

  • Some of the organisms is not eaten. 
  • Some parts of the plant/animal are eaten but cannot be digested/ lost in faeces.
  • Lost as excretion. 
  • Lost through heat in respiration, 

Inefficiency of Energy transfer explains why: 

  • Most chains have only 4/5 trophic levels - total mass is less at higher trophic levels.
  • Total amount of energy is reduced at higher trophic levels.
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Chapter 5.3 - Ecological Pyramids

Pyramids of Numbers

  • Numbers of organisms at lower trophic levels are higher than numbers at higher levels. 

Pyramids of Biomass: 

  • The total mass of the plants/animals in a particular place. 
  • Measured in grams per cubic metre (gm-3). 

Pyramids of Energy: 

  • Measure energy stored in organisms. 
  • Results are much more reliable than those for Biomass. 
  • Measured in KJM-2year-1 
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Chapter 5.4 - Agricultural Ecosystems

What is productivity? 

  • The rate at which something is produced. 
  • The rate at which plants assimilate this chemical energy = gross productivity. 
  • Remainder energy= net productivity. 
  • Net Productivity= gross productivity- respiratory losses. 

Natural Ecosystems: 

  • Solar energy only - no additional energy input.
  • Lower productivity 
  • more species diversity
  • more genetic diversity
  • nutrients are recylced naturally within the ecosystem with little addition from outside. 
  • populations are controlled by natural means.
  • natural climax community.
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Chapter 5.4 - Agricultural Ecosystems

Agricultural Ecosystems

  • Solar Energy pulls energy from food and fossil fuels
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Chapter 5.5 - Chemical and Biological Control of P

What is a pest?

  • An Organism that competes with humans for food or space, or it could be a danger to health.
  • It is just an organism growing or living where we do not want it.

Pesticides should:

  • Be Specific to a plant/animal
  • Be Biodegradeable
  • Be Cost-effective
  • Not Accumulate in a specific area.
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Chapter 5.5 - Chemical and Biology control of Pest

Biological Control

  • They do not act as quickly, so there is some interval of time between introducing the contol organism and a significant reduction in the pest problem.
  • A control organism may itself become a pest.
  • It is very specific.
  • Once introduced the control organism reproduces.
  • Pests do not become resistant.

Chemical Control

  • Always have some effect on non-target species.
  • Must be reapplied at intervals.
  • Expensive.
  • Pests= genetic resistance= new pesticides.
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Chapter 5.5 - Chemical and Biology control of Pest

Integrated pest-control systems

  • Choosing animal of plant varieties that suit the local area.
  • Managing the environment to provide suitable habitats.
  • Regularly monitoring the crops for signs of pests = early actions are taken.
  • Removing pests mechanically.
  • Use Biological agents if necessary.
  • Using Pesticides as a last resort.

Controlling pests/productivity.

  • Pests reduce productivity and compete for water, mineral ions, CO2, space and light.
  • Pests may become the limiting factor in photosynthesis.
  • Pest control = limit the effect of pests on productivity.
  • Balance the cost of pest control.
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Chapter 5.6 - Intensive rearing of Domestic Livest

Increases Energy Conversion:

  • Movement is restricted = less muscle contraction = less energy is used.
  • Environment can be kept warm = reduces heat loss.
  • Feeding can be controlled so that animals recieve optimum amount = no wastage.
  • Predators are excluded so that there is no loss to other organisms in the food web.

Features of Intensive Rearing:

  • Efficient energy conversion.
  • Low cost.
  • Quality of food.
  • Use of space, drugs, fossil fuels = pollution (from fossil fuels.
  • Safety
  • Diseases reduced
  • Reduced genetic diversity
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Chapter 6.1 - The Carbon Cycle.

The Sequence of all nutrient Cycles:

  • Nutrient taken up by producers.
  • Producer incorporates this into a complex molecule.
  • Producer is eaten by the consumer, meaning the nutrient is passed on.
  • Nutrient passes along the food chain when animals are eaten by consumers.
  • Death = saprobiotic microorganims = release the nutrient.

Exam tips for chapter 6:

  • All organisms die and so decomposers feed on all trophic levels.
  • Global warming changes the niches = affects communities.
  • Availability of nitrates is the factor that limits plant growth. Plants are primary producers, nitrate availability impacts the whole ecosystem.
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Chapter 6.2 - The Greenhouse effect & Global Warmi

Greenhouse Gases:

  • CO2 = Most important, it remains in the atmosphere for much longer than other gases.
  • 50 - 70% of global warming is due to CO2.
  • Mainly a result of human activities.


  • Natural greenhouse gas.
  • Produced when micoorganisms break down the organic molecule which organisms are made.
  • When decomposers break down the dead remains of organisms.
  • When microorganisms in the intestines, primarily of primary consumers such as cattle digest the food that has been eat = release of Methane.
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Chapter 6.2 - The Greenhouse effect & Global Warmi

Consequences of Global Warming:

  • Melting of Ice caps = extinction of some wild plants and animals.
  • A rise in sea level due to thermal expansion = flood low lying land.
  • Extension of salt water = difficult for crop cultivation.
  • Higher temperatures + less rainfall = more drought resistant species. Only Xerophytes will be able to survive.
  • Greater rainfall and intense stroms would occur in some areas due to the disturbance of weather patterns.
  • Life cycles and populations of insect pests would alter as they adapt to the changed conditions.
  • Tropical diseases could spread towards the poles, such as Malaria.
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Chapter 6.3 - The Nitrogen Cycle.

The Nitrogen Cycle:

  •  78% of the atmosphere is N2.
  • Plants absorb N2 ions by active transport in the root hairs.
  • Nitrate ions are very soluble.

Main stages of the Nitrogen Cycle:

  • Nitrogen Fixation
  • Ammonification
  • Nitrification
  • Denitrification.
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Chapter 6.3 - The Nitrogen Cycle.


  • Production of Ammonia from organic compounds.
  • These compounds include: Urea, Proteins, Nucleic Acids and Vitamins.
  • Saprobiotic microorganisms mainly fungi and bacteria feed on these materials = releasing ammonia which then forms ammonium ions.


  • Oxidation reactions and so, releases energy.
  • Carried out by free living soil microorganisms.
  • 1 - Oxidation of Ammonium ions to NITRITE ions (NO2-)
  • 2 - Oxidation of Nitrite ions to NITRATE ions (NO3-).
  • Important to keep soil light and well aerated to prevent Denitrification from occuring.
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Chapter 6.3 - The Nitrogen Cycle.

Nitrogen Fixation:

  • Nitrogen gas is converted into nitrogen compounds.
  • Free living NF bacteria - reduce gaseous N2 into NH4.
  • Mutualistic NF bacteria live in root nodules of plants. They obtain carbohydrates from the plant, aquires amino acids from the bacteria.


  • Soils become water logged and short of O2.
  • Less Aerobic NF bacteria are found.
  • Increase in anaerobic denitrifying bacteria.
  • Conver nitrates into gaseous nitrogen.
  • Vital to prevent the build up of denitrifying bacteria through aeration.
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Chapter 6.4 - Use of Natural and Artificial Fertil

The need for Fertilisers and how they increase productivity.

  • Intensive food production = mineral ions are continually removed from the soil by crops.
  • These minerals are not replaced to the same area of land.
  • It is important to replace the nutrients - may be the main limiting factor to plant growth.
  • Productivity will be reduced. To offset this, fertlisiers will be needed to be added to the soil.
  • Nitrogen is an essential component for proteins and DNA which is needed for Plant Growth.
  • Nitrates that are readily available = Stimulate plant growth/ Photosynthesis.

Types of Fertilisers:

  • Natural/ Organic = Manure, Urea, Bone Meal.
  • Inorganic = Ammonium Nitrate , Ammonium Sulphate, NPK Fertilisers.
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Chapter 6.5 - Environemental Consequences of N2 Fe

Effects of Nitrogen Fertilisers:

  • Reduces Species diversity - Nitrogen Soils favour grasses, nettles and other rapidly growing species.
  • Leads to leaching.
  • Also leads to Eutrophication - caused by leaching of fertilisers into water courses.


  • Nutrients are removed from the soil.
  • Rain water will dissolve any soluble nutrients.
  • The leached nitrates find their way into the water courses.
  • Harmful to humans.
  • Can cause Eutrophication.
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Chapter 6.5 - Environemental Consequences of N2 Fe

Stages of Eutrophication:

  • Most lakes = very little nitrate = limiting factor for algae.
  • N2 increases from leaching = algae grows.
  • Algal Bloom.
  • Algae absorbs light and prevents it from penetrating lower depths.
  • Light = limiting factor = plants at lower depths die.
  • Saprobiotic algae grow exponentially.
  • Increased demand for O2.
  • The concentration of O2 in the H2O is reduced and nitrates are released from the decaying organisms.
  • O2 = limiting factor for the population of aerobic organisms such as fish.
  • Less competition for the aerobic organisms who's population decreases.
  • Anaerobic organsims further compose dead material and toxic waste.
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Chapter 7.1 - Succession

Definition of Succession:

  • This is the changes overtime, in the species that occupy a particular area.

Features of  Pioneer Species:

  • Vast quanitities of wind dispersed seeds.
  • Rapid Germination of seeds on arrival.
  • Ability to photosynthesise.
  • Ability to fix Nitrogen from the atmosphere.
  • Tolerance to extreme conditions.
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Chapter 7.1 - Succession.

Features of Climax Communities:

  • The organisms that make up the final stage of ecological succession.
  • Within the climax community there is normally a dominant plant species and a dominant animal species.

Features that emerge:

  • Abiotic environement becomes less hostile.
  • A greater number and variety of habitats.
  • Increased biodiversity.
  • More complex food webs.
  • Increased biomass.
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Chapter 7.2 - Conservation of Habitats.

What is Conservation?

  • Management of the Earth's natural resources in such a way that maximum use of them can be made in the future.

What are the main reasons for Conservation?

  • Ethical - Other species have occupied the Earth for longer, and so, we must respect all living things.
  • Economic - Massive Gene pool - with the capacity to make millions of substances.
  • Cultural and Aesthetic - Organisms enrich our lives, variety adds interest to everyday life.

Conserving habitats by managing succession:

  • If the factor that is preventing further succession is removed, then the ecosystem develops naturally into its climax community.
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Chapter 7.2 - Conservation of Habitats.

Ways to Conserve Species and Habitats:

  • Plants can be conserved using seedbanks - stores lots of seeds from different plant species.
  • Fish Species can be conserved using Fishing Quotats - these are limits to the amount of certain fish species that fisherman are allowed to catch.
  • Animals can be conserved by using Capitive Breeding Programmes - these involve breeding animals in controlled environment in order to increase the population of endangered species.
  • Any organism can be conserved by relocation - This is moving a population of species to a new location because they are directly under threat.
  • Habitats can be conserved by using Protected areas - This includes areas such as national parks and nature reserves. It restricts urban development, industrial development and farming.
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Chapter 8.1 - Inheritance and Selection.


  • This is the genetic make-up of an individual.
  • It describes all the alleles that an organism contains.
  • Sets the limits which the characteristics of an individual may vary.
  • Any change to the Genotype as a result of a change to the DNA is called a Mutation.


  • Observable Characteristics of an organism.
  • Result of the Interaction between the expression of the Genotype and the environment.
  • The environment can alter an organism's appearance.
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Chapter 8.1 - Inheritance and Selection.


  • Section of DNA, sequence of nucleotide bases that usually determine a single characteristic of an organism.
  • Position of a Gene on a chromosome is known as the 'Locus'.
  • A Gene exists in 2 forms = Alleles.


  • 1 of the different forms of Gene.
  • Only 1 Allele can occur at the locus of any one chromosome.
  • Chromosomes occur in pairs - Homologoud pairs.

Homozygous Dominant =   R R        Heterozygous Dominant R r

Homozygous Recessive = r r

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Chapter 8.1 - Inheritance and Selection.


  • Where 2 alleles contribute to the Phenotype. In which case the phenotype may be a blend of the 2 alleles.

Key Terms in Genetics:

  • Genotype - Genetic composition of an Organism.
  • Mutation - Due to a change in DNA. May be inherited by future generations.
  • Genes - Portion of DNA.
  • Alleles - Usually 2 for each Gene.
  • Co-Dominance - if both alleles contribute to the Phenotype.
  • Multiple Alleles  - If more than 2 alleles for each Gene.
  • Dominant Allele - Expresses itself even in the presence of a recessive allele.
  • Recessive Allele - Expresses itself only when there is another identical allele.
  • Phenotype - Actual appearance of an organism.
  • Modification - Any change to the Phenotype - not inherited.
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Chapter 8.2 - Monohybrid Inheritance.

Tips for Representing Genetic Crosses:

  • Chose a single letter to represent each characteristic.
  • Choose the first letter of one of the contrasting features.
  • Choose the letter in which the higher and lower cases differ.
  • Let the higher case letter represent the dominant allele.
  • Represent the parents with the appropriate pairs of letters and label them 'parents'.
  • State the gametes produced by each parent - circle them to show meiosis has occured.
  • Use a Punnett square to show the results of random crossing over of Gametes.
  • State the phenotypes of each different phenotype and indicate the numbers of each type.
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ahmed ameen

AMAZING! really helpful I couldn't make better note in a million years!


Thank you! x

Lacey Read

This is literally amazing! Thank you!!


Lacey Read

This is literally amazing! Thank you!!



really good notes (: but i think you mean climate patterns and not sleeping patterns in 6.2 :p


Thank you so much ! These are so concise and to the point!


Thankyou! :) 

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