AQA A2 Biology Unit 4

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Populations & Ecosystems

Ecosystem- A self-contained system in ecology made up of both biotic (living) and abiotic (non-living) factors.

Population- The number of individuals of a species in a given area at a given time.

Community- All of the populations of different species in a given area at a given time.

Habitat- The place where a community lives.

Niche- The role a species plays in the upkeep of an ecosystem. 

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Investigation Populations

Quadrats, three factors to consider- size of quadrat, No of sample quadrats to use in study, Position of quadrats.

Random Sampling- e.g. draw up a grid of the area and generate coordinates randomly to sample quadrats.

Sampling along transects- tape measure places in a line and quadrats placed evenly along the transect.

Measuring Abundance- after obtaining results using quadrats or along transects, find either a frequency (likelihood of a species being found in a quadrat) or a percentage cover (estimate the area a species covers in a quadrat and then find an average over all of the quadrats).

Mark-Release-Recapture- Estimated          individuals in 1st capture X individuals in 2nd capture                                      Population Size                        Marked individuals recaptured

      (must make sure the mark left on individuals does not disadvantage them in their ecosystem)

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Variation in Population size / competition

Population Growth Curves- a natural population has three phases:                                                         1) a period of slow growth as the initially small number of individuals reproduce slowly.                 2) a period of rapid growth where the ever increasing individuals continue to reproduce.                 3) a period when the population growth declines until the size remains more or less                          stable. decline may be due to food supply or increased predation.

Population Size- A population may grow or decrease due to limiting factors, these can be either                            biotic (predation) or abiotic (temperature, light,  pH, water, humidity) factors.


Intraspecific competition- Individuals of the same species competing with each other for resources, this could mean food, water or mates.

Interspecific- Individuals of different species competing for resources. Where 2 species occupy the same niche one will normally have an advantage over the other.

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Predation- when one organism in consumed by another. In a natural ecosystem the area in which the predator and prey species live is so great that the prey species rarely becomes extinct.

Effect of predator-prey relationships on population size- Population of prey decreases due to predation, Fewer prey available therefore predator population decreases due to greater competition, prey population increases due to reduced number of predators, predator population increases due to increase in prey population.

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Human Populations

Human Population Growth Rate- originally human growth has been kept in check by disease and famine however due to developments in technology and agriculture human populations have exploded. 

Factors affecting human populations- (Births+immigration)-(deaths+emigration)= Growth                                                     Birth rate:           No births per year                  X1000                                                                                                                                Total population of same year                                                                                                                    (Affected by Economic conditions, religious backgrounds, society,politics and birth control.)                                            Death rate:          No Deaths per year               X1000                                                                                                                                 Total population in same year                                                                                                               (Affected by life expectancy, food supply, sanitation, medicine, war and natural disasters)                                                 emigration and immigration.

Population Structure-  stable populations - where birth and death rates are in balance
                                     increasing population - birth rate high giving a wider base at the bottom of the pyramid and very                            narrow at the top. countries with less economically developed 
                                     decreasing populaiton - where there is a low birth rate (narrow base) and lower mortality rate                                 leading to more elderly people (wider apex). usually cxountries which are ecomically developed.


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Energy- the ability to do work

Why do organisms need energy?- metabolism, movement, active transport,maintenance, repair and division of cells, production of substances, maintanance of body temperature.

Flow of energy through living systems- Light fro sun converted into chemical energy, chemical energy in the form of organic molecules are converted into ATP during respiration, ATP used by cells in important reactions.

How ATP stores energy-              ATP + water <--> ADP + Phosphate ion + energy                   The forward reactions = hydrolysis reaction and the backward reaction = condensation reaction.

ATP Synthesis-  can happen in 3 ways: 1) phosphorylation - takes place in chlorophyll.                                         2) oxidative phosphorylation - occurs in mitrocondria in electron transport.                                   3) substrate level phosphorylation -  e.g: formation of pyruvate.       

ATP efficiency- small amounts of energy released                                                                                              one step reaction so very fast release of energy

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Overview of Photosynthesis

Overall Equation- 6CO2 + 6H2O  -->  C6H12O6 + CO2                                                               Structure of leaf- large surface area- collects as much sunlight as possible.                                               thin -  most light is absorbed in first few mm & diffusion pathway kept short.                            transparent cuticle and epidermis - lets light through to photosynthetic cells.                            long, narrow, upper epidermis cells packed with chloroplasts to collect sunlight.                      numerous stomata for gaseous exchange.                                                        Structure of chloroplasts- grana - stacks of up to 100 disc like structures                                                                    called thylakoids where the light dependent stage of                                                              photosynthesis take place, within these are chlorophyll.                                                          stroma - a fluid filled matrix where the light independent stage                                                takes place. within in it are other structures such as starch 

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Light-Dependent Reaction

 ATP Production- Chlorophyll molecule absorbs light and electrons excite, electrons taken up by electron carrier and pass down transport chain(in thylakoid membrane), energy released used to produce ATP.                                                                                                        

Photolysis- water molocule split into oxygen, protons and electrons. Electrons produced replace ones exited from chlorophyll molecule. The oxygen given off as waste product. Chlorophyll molecules go through another excitation, losing electrons, electrons leave chlorophyll molecule and join with electron acceptor (NADP) and are used along with the protons from photolysis reaction to form NADPH.    

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Light-independent Reactions

Calvin Cycle- CO2 joined with RuBP to produce 2xGP, 2xGP is reduced with ATP & NADPH from the light dependant reaction, to form 2xTP. 2xTP converted into amino acids, lipids, starch or glucose by plant. unused TP  converted into RuP. RuP then made back into RuBP by reacting it with ATP. (RuP + ATP ---> RuBP + ADP)

                      (                    Site of light-independant reaction- takes place in stroma of the chloroplasts:                          stroma fluid ( which contains all of the enzymes needed)surrounds the grana and so the            products of the light-dependant reaction can easily diffuse into the stroma. The                        fluid contains DNA and ribosomes so it can quickly and easily manufacture some of the            proteins needed for the light-independent reactions.

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Photosynthesis summary / Limiting Factors

                                              (                                    ----------------------------------------------------------------------------------------------------------------------------------  Limiting Factors: Light- when limiting factor, rate of photosynthesis is directly                                                             proportional to light intensity. as light intensity increases the net gas                                       exchange will decrease as O2 produced and CO2 taken in are equal.                             CO2effects enzyme activity, in particular increases the enzyme that                                              catalyse the joining of RuBP & CO2 in light-independant reactions.                   Temperature- photosynthesis increases as temperature increases until optimum                                         temperature, then enzymes start to denature & rate of photosynthesis                                     decreases. 

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Respiration: Glycolysis

1) Glucose + 2ATP --> Phosphorylated Glucose + 2 Phosphate ions

2) Phosphorylated Gucose --> 2 TP

3) 2TP + 2NAD --> 2 Oxidised TP + 2 Reduced NAD

4) 2 Oxidised TP + 4 ADP --> 2 Pyruvate + 4 ATP

Energy Yield- 

2 molecules of ATP (4 molecules of ATP are produced but 2 were used up in the phosphorylation of glucose.

two molecules of NAD 
two molecules of pryruvate

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Respiration: Link Reaction & Krebs Cycle

Link Reaction:                                                                                                                     1) pyruvate actively transported into matrix from cytoplasm.                                                     2) pyruvate oxidised by removing a hydrogen and CO2 released to form acetyl, hydrogen then       accepted by NAD to form NADH.                                                                                   3) acetyl then joined with CoA to form acetylcoenzyme A

Krebs Cycle:                                                                                                         1) AcetylCo A joined with a 4C molecule to form a 6C molecule.                                             2) 6C molecule loses 2CO2 & H2 to form 4C molecule, 1 ATP molecule, NADH & FADH.         3) 4C molecule can then join with acetylCo A to restart the cycle.                       (

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Respiration: Electron Transport Chain

The Electron Transport Chain is found in the cristae of mitochondria. This is where most of the ATP is produced in respiration.

 1) H2 join with NAD and FAD                                                                                                 2) FADH & NADH donate electrons of H2 atoms to first carrier protein in chain.                          3) The H+ ions are then actively transported across the inner mitochondrial membrane.              4) electrons pass along carrier proteins gradually losing energy, this energy is then used to              combine ADP and a phosphate to form ATP.                                                                      5) At the end of the chain the H+ ions and the electron combine with O2 to form water. (this              means oxygen is the final acceptor in the chain)

Importance of oxygen being the final acceptor- without it the H+ and electrons would 'back                                                                                 up' along the chain and respiration would                                                                                   come to a halt. 

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Anaerobic Respiration

This is when respiration happens in the absence of oxygen, also anaerobic reactions happen in some bacteria and fungi and in some plant cells (e.g. root cells in water logged conditions).

End of Glycolysis- pyruvate + NADH --> ethanol + CO2 + NAD

In animals the reaction is slightly different:

                  Pyruvate + NADH --> Lactic Acid + NAD

The lactate is oxidised into glycogen when oxygen is next available. (this is what causes cramp in muscles)

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Food Webs/energy transfer between trophic levels/

Food Web- systems of interlocking and interdependent food chains.                                          ------------------------------------------------------------------------------------------------------------------------------------------Energy loss: From Producers- light reflected, not a wavelengths absorbed, limiting factors.                             From Consumers- don't consume all of prey, bodily waste, heat loss.

Net Production = Gross Production - Respiratory Losses

Efficiency of energy transfer -       energy available after the transfer           X100                                                                           energy available before the transfer

Percentage efficiency-          % of energy available after transfer            X100                                                                       % of energy available before transfer                                                 ---------------------------------------------------------------------------------------------------------------------------------     Pyramid of Numbers- measured in numbers, no acount taken for size and some data can be                    to large to represent.                                                                                    Pyramid of Biomass- Biomass is the total mass of the plants or animals in a particular                           place, measured in grams per square meter, seasonal differences are not apparent  Pyramid of Energy- Most accurate,difficult to collect data,results are more reliable than                           pyramids of biomass.

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Agricultural Ecosystems / Fertilisers

   Agricultural ecosystem- made up of domesticated animals used to produce food for                                                    humans, used to minimise energy loss so humans get as much                                              energy as possible.                                                           Productivity- Rate at which something is produced                                                                                    net productivity = gross productivity - respiratory losses                                         (affected by effeciency of photosynthesising crop and area of ground it covers) 

Comparison of natural and agricultural ecosystems-                                                                         Natural                                                                       Agricultural            solar energy the only input, lower productivity,    solar energy + fossil fuels, higher productivity, more species diversity, more genetic diversity,     less species diversity, less genetic diversity, nurients naturally recycled, population                 artificial fertalisers used, populaion controlled controlled by natural means, natural climax          by natural and artificial (pesticides)means,   community.                                                        preveted from reaching natural climax.         ---------------------------------------------------------------------------------------------------------------------------------------Fertilisers are chemicals that provide crops with minerals for growth e.g. nitrates, they replace minerals lost through plant uptake so more energy from ecosystem can be used to grow, increasing the efficiency of conversion.                                                                           1. Natural fertilsers are organic matter- including mnure and sweage sludge               2. Artificial are inorganic- contain pure chemicals e.g. ammonium nitrate

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Chemical & Biological control of pests

Pesticide- poisonous chemical that kills pests                                                                                         should be specific to pest, biodegrade, cost-affective, not accumulate in soil.

Biological control- controlling pests by the introduction og a predator species.                                                         very specific, control organism reproduced itself, no resistance. 

Agricultural Pests:Integrated pest control-choosing crops/animals that are resistance to pests,provide habitats for natural predators, look for signs of pests, only use pesticides if necessary. 

How pests affect productivity- Reduce productivity, Compete for resourceslacking                                       resources may become limiting factor for, photosynthesis or  growth,                                       damaging to crops,disease causing to animalsconsuming crops, competition                         to  humansmonocultures enable  rapid spread of pests or fungi.

Effect of intensive rearing on energy conversion: movement restricted (less energy lost in                        muscle contraction) warm environment (no heat loss to surroundings), controlled                      feeding (optimum diet, minimum waste)Predators excluded (no loss to other                          organisms in food web)selective breeding (animals that are most efficient at                          converting food to body mass are bred with eachother to encourage this trait). 

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Carbon Cycle

- CO2 is absorbed by plants when they carry out photosynthesis- it becomes organic                      compounds in plant tissues.                                                                                     - Carbon is passed on to primary consumers when they eat plants. It's passed on through the          food chain.                                                                                                                   - Carbon compounds in dead organisms digested by microorganisms called decomposers e.g.        bacteria and fungi. Feeding on dead organic matter is called saprobiontic nutrition.     - Carbon is returned to the air (and water) as all living organisms (including decomposers) carry       out respiration, which produces CO2.                                                                               - If dead organic matter in places where there aren't any decomposers e.g. deep                            oceans,carbon compounds are turned into fossil fuels.                                                         - The carbon in fossil fuels is released when they're burnt- called combustion.                               

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Nitrogen Cycle

Ammonification- Nitrogen compounds found in dead organisms and bodily waste are broken down by saprobiotic microorgamisms. NH4+ --> NH3

Nitrogen Fixation- Nitrogen is converted into NH3. This can happen when lightning passes through the atmosphere, by bacteria living in soil, or bacteria living in root nodules of plants.

Nitrification- This is the conversion of ammonium ions to nitrate ions by nitrifying bacteria, to be used by the plant. Firstly nitrfying bacteria oxidise ammonium ions to nitrite ions (N02-) and Secondly other nitrifying bacteria oxidise nitrite ions to nitrate ions (N03-).

Denitrification- Nitrates in soil converted to N2 by denitrifying bacteria, they use nitrates in soil to carry out respiration and produce N2. Happens under anaerobic conditions.                                                                (

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Leaching and Eutrophication

Leaching- Rain water dissolves and washes nutrients through of the soil so they are so deep,                        they cant be reached by plant roots. From here they can be washed into ground water                  and eventually pollute bodies of water like streams and fresh water lakes.

Eutrophication-1. Nitrates leached from fertilised fields stimulate growth of algae in ponds etc.                             2. Large amounts of algae block light from reaching plants below                                                 3. Plants die as they are unable to photosynthesise                                                                   4. Bacteria feed on the dead plant matter                                                                                   5. Increased numbers of bacteria reduce the oxygen concentration in water by                                 carrying out aerobic respiration                                                                                             6. Fish etc. die as there isn't enough dissolved oxygen

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Succession- the name given to the process of a ecosystem as it changes through out time.       1) Where there is bare land a pioneer species will colonise the area e.g. lichens, this could be         by the lichens dying and when they decompose release nutrients into the soil.                   2) The pioneer species will then change the area so it becomes habitable for other species.       3) This will carry on until a climax community has been reached.                                          During succession you will see: A greater number and variety of habitats, Increases                                                                biodiversity, More complex food web, Increased biomass              (                                                                         Conservation of Habitats- conservation is the management of the earths natural resources                                              so that maximum use of them can be made in the future.                                                        Done for ethical, economic, cultural and aesthetic purposes.


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Gene- length of DNA which codes for a poly peptide, a gene may also code for a characteristic. 

Allele- one from of a gene, of which their may be many.

Homologous Chromosomes- pair of chromosomes that have the same gene loci and so                                                          determine the same features.

Sex Linkage- A Gene which is carried on the X or Y chromosomes, however due to the X                                   chromosome being longer many genes are only seen on the X and there is no                             equivalent gene on the Y.  

Haemophilia- The blood clots slowly. Although some females have haemophilia it mostly occurs                       in males as it is a recessive allele on a sex-linked gene.

Pedegree chart- A way to trace sex-linked genes across generations.

Co-dominance- where 2 alleles share dominance over a characteristic.theresult could be that                                both alleles are expressed or a mix of them is expressed.

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Population Genetics / Selection

Hardy-Weinberg Principle: P + 2PQ + Q = 1    and  P + Q = 1                                                                                         Provided that: no mutation, isolated population, no selection, large                                               population, random mating.                                                              -------------------------------------------------------------------------------------------------------------------------------------------Directional Selection - favours individuals that vary one side of the mean

Stabalising Selection-  favors individuals about the mean


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Occurs when populations of the same species are seperated, and due to different environmental conditions, over time become different species altogether.

speciation:  The eveolution of two or more species from an existing species.                      1) Due to climate change, a population living in an area is seperated to become two               separate populations.                                                                                                 2) The climates are different and so favour different phenotypes that are survival                    advantages in these conditions.                                                                                     3) Over time, the type and frequency of alleles of the gene pool in each population is so        different that they become different species.

The two populations could be isolated by either sympatric (breeding seasons) or allopatric means (geographical).

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this is amazing! thank you so much!

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