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AQA Biology Unit 4 Unofficial Mark Scheme

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    • Thread Starter

    1a) What is the term used to describe populations of different species? (1)

    1b) What’s the adaptation of having different niches? (1)
    Less interspecific competition/ competition between species

    1c) Scientist concluded a higher pH results in more beetles. Evaluate. (3)
    Positive correlation. As pH becomes more alkaline (increases) number of beetles increases.
    Correlation does not mean causation.
    Could be due to other factors such as predators.
    Data is widespread and some values overlap. Some points are far away from the line of best fit, so unreliable data.
    Scientists used a representative sample of 30 lakes.
    No data for ph < 4.5 or > 7.5

    1ci) How does presence of fish increase number of beetle species. (1)
    Fish eat predators of beetles. So less beetles eaten by their predators.Beetles feed off fish . So more food.
    More fish present in lake, (naturally) fish die, more dead fish matter in lake available for beetles to feed on.
    More fish species present indicates wider range of aquatic plants for food, which would provide more niches for more beetle species to be present

    2a) What is a phenotype? (2)
    The observable characteristic of an organism resulting from its genotype AND the environment

    2b) Evidence for colour blindness being recessive? (2)
    4 was affected but 2 (wasn’t 1 the mother?) wasn’t - heterozygous carrier mother. 11 was affected but 9 wasn’t - heterozygous carrier mother. OR
    Individuals 9 and 11 produced female child without colour blindness. If recessive, all female offspring would inherit recessive as father was X^bY and an X chromosome is required from both parents to produce offspring of female sex.

    Genotype of individual 8 (1)
    X^B X^b

    Probability of a colour-blind male who cannot roll his tongue? (2)
    Probability of colourblind male is 0.25 - 1 mark for working out 25% probabilityProbability of non-tongue roller is 0.25 0.25 * 0.25 = 0.0625 or 1/16 or 6.25% - second mark

    Hardy-weinberg (2)P = 0.4 ∴ Q = 0.6 ∴ 2pq = 2*0.4*0.6 48%

    3) Why does the presence of microorganisms cause an increase in temperature ?(2)
    when they break down the waste, they respire thus release heat energy

    Why was standard deviation used rather than the range ? (2)
    SD considers all of the data whereas the range doesn’t. Therefore SD is less affected by outliers
    SD shows spread of data around mean, SD less affected by outliers(includes intermediate values.)
    Range excludes intermediate values.
    Shows whether difference in data is significant. Includes all data points so more reliable.
    Range shows the data that was at the furthest deviation either side of the mean. Standard deviation shows the average deviation of data around the mean. Hence, a judgement can be made about the average statistical significance of each sample/measurement made.

    Why rotating the containers? (2)
    Rotating compost so more oxygen so more nitrifying bacteria and less denitrification
    Preventing the temperature from going to high which could denature enzymes in the micro-organisms making them useless.
    Ensures all of the compost is exposed to the bacteria and not settling at the top or bottom (no organic matter is left undigested)
    So bacteria are exposed to new waste because of mixing to ensure uniform decomposition.
    Even distribution of temperature from external environment, so that the conditions inside stayed relatively constant throughout

    4) Where does stage 1 occur? (1)

    Why is stage 2 needed for stage one to continue? (2)
    Regenerate NAD, which is needed to oxidise triose phosphate.Allows glycolysis to continue with little ATPPyruvate was removed by accepting hydrogen from reduced NAD so regenerate NAD to be used in stage 1.

    RQ of aerobic respiration? (1)

    Explain RQ of 1.8? (2)
    More carbon dioxide produced than oxygen take up due to anaerobic respiration. RQ of solely anaerobic respiration is infinity and RQ of solely aerobic respiration is 1, so there must be a mix of both types of respiration. Seeds couldn’t get enough oxygen in to produce sufficient ATP for germination, so needed some anaerobic as well

    Why aerobic produces more ATP? (2)
    More ATP produced in aerobic respiration because in anaerobic only glycolysis occurs as there is no final electron acceptor so link reaction and kreb cycle stop. In aerobic most the ATP produced in oxidative phosphorylation. In aerobic, the full metabolic pathway can be carried out, whereas in anaerobic respiration, only glycolysis occurs.Each glucose molecule = 2 turns of kreb’s cycle.

    5) Where exactly in the cell does the calvin cycle occur. (1)
    Stroma (of the chloroplast)

    Why does gp decrease when co2 is reduced? (1)
    Less CO2 to react with RUBP thus less GP is formed

    2 reasons why stay constant when co2 conc increased? (2)
    Another limiting factor such as temperature or light intensity or ATP/NADPH availability. The amount of NADPH/ATP is unchanged, thus the same amount of RuBP is provided to combine with CO2.No available rubisco to catalyse reaction between co2 and RuBP as all active sites are filled.(The stomata close at high Co2 concentration - maybe volume of co2 absorbed changes )

    H2S in bacteria acts like water in photosynthesis. Explain? (2)
    Photolysis. H2S is split into 2H+ + 2e + S, H+, electrons used for ETC to make ATP, H+ combines with electrons from ETC to make hydrogen that's used to reduce coenzymes. NADP + 2e- + 2H+ = NADPH2

    6) How can you randomly choose rocks to study for algae? (2)
    Use a grid area/ number the rocks. Method of obtaining random numbers table/generator to get coordinates. Number the rocks and use random number generator.

    Why was percentage cover used rather than counting the alga?(1)
    Algae are grown in clumps, and it is hard to differentiate individual alga. Impossible to distinguish between different alga so far more efficient to do % cover.Rocks are different sizes so percentage cover allows direct comparison between different sized rocks.Collect more data quickly + individuals do not need to be counted

    2 Abiotic factors affecting diversity of species of algae?(2)
    Nutrient/ minerals
    Light intensity- species which cannot photosynthesise in lower light intensities will die
    The lower shore is covered by water at high tide so light intensity will decrease at low tide so only algae which can survive in a lower light intensity can live on the lower shore.
    Temperature - temperature affects the activity of enzymes, enzymes are involved in photosynthesis. High temp = enzymes denatured Low temp. = less kinetic energy = less photosynthesis pH
    CO2 conc - different species photosynthesise with different co2 concentrations, species which cannot photosynthesis in lower co2 concentrations will die
    CO2 is required/used in the light independent stage of photosynthesis Salinity - Lower shore has higher salinity than upper shore due to proximity to sea water so only algae which can grow in high salinity can grow on the lower shore.

    Why is there a greater diversity of consumers at site B? (2)
    greater variety of food sources/habitats + greater variety of available niches.
    Greater overall amount of food at B reducing competition - less hostile environment.

    How did the giraffe subspecies develop? (5)
    Giraffes were split into groups, geographically isolated,
    Cannot interbreed/Separation of gene pools.
    Each group may be subject to different abiotic and biotic factors, therefore different selection/environmental pressures.
    Some mutations may make some giraffes more likely to survive than others, for example, skin markings may make provide camouflage.
    These giraffes are more likely to survive so go onto reproduce, and pass on their alleles.
    Allele frequency changes
    Allele frequencies for each group will be different so they are reproductively isolated.

    How can you use mitochondrial DNA to see if species are related ? (2)
    Members of same species have similar alleles (similar alleles maybe instead of DNA?
    Compare base sequence If complementary base pairs form, they are a similar species (hydrogen bonds form between base pairs)
    Dna hybridisation
    Mitochondrial DNA always passed on from the maternal gamete

    Bar Graph showing bacteria populations (3)
    Succession - cocci pioneer species changed environment made it warmer less hostile. Allowed more species to migrate and survive.
    Cocci out competed. Positive rod became dominant specie.
    Species diversity increases
    At the end of succession, the Positive rod was the most dominant with negative rod seeing a decrease in population size after previously being on the increase, indicates the start of climax community. I.e. a single dominant species is established.
    Towards the end, the temperature was similar to the start but the composition of bacteria was completely different, thus some other internal conditions like concentrations etc must have changed, and become more habitable for the other bacteria.

    7) Why were the consumers starved for 5 days? (2)
    Consumers starved may be to make sure they all will have an increased appetite and so will eat the maximum that they can making a reliable mean. This put appetite at a baseline level for each species. Standardise. Ensures the mass is due to the algae and not other food. Prevents any feeding prior to the test having an affect on results.Prevented differing food consumption (1), due to an extraneous variable (1).

    Suggest how the researchers found the mass of algae lost by respiration? (3)
    Measure mass of algae and change in mass on consumer and work out the difference .
    Control experiments
    Biomass Gross production equation (net productivity = gross productivity - respiratory loss)
    Photosynthometer? Sodium hydrogencarbonate to absorb CO2. Work out mass change of sodium hydrogencarbonate (net release of CO2 due to respiration). Mass gain of sodium HCO3 = mass loss from alga

    Using p values write a conclusion.(3)
    probability due to chance... less than 1% and 5% and greater than 5% accordingly etc.. can't remember the names but talked about 2 of the differences in means were significant and 2 weren't.

    8a) How crops use light in LDR. (5)
    Chlorophyll absorbs light, excites electrons /Photoexcitation
    Passed on to ETC and passed down at decreasing energy levels by redox reactions
    Energy from redox reactions used to combine ADP and Pi to make ATP/Photophosphorylation
    Photolysis of water makes protons electrons and oxygen
    Light absorbed by PSI which excites electron to reduce NADP
    Protons and electrons used to reduce NADP to NADPH
    Some electrons from photolysis replace electrons in chlorophyll
    Some O2 produced from photolysis can be used in respiration/diffuses out of leaf as biproduct

    8b) Advantages and disadvantages of chemical pesticides. (5)Advantages:
    • Can be applied to specific area
    • Act quickly
    • Completely eradicate pest
    • Some pesticides are biodegradable
    • Pest may develop resistance
    • Need to be reapplied which is costly
    • Cannot be used in organic farming due to bioaccumulation
    • Can affect the crops make it harmful for consumption
    • May kill other organisms as well other than the pest
    8c) Source of nitrates by bacteria in soil. (5)
    Microorganisms are Saprobiotic bacteria/Saprobionts/Saprophytes
    Secrete enzymes which break down proteins (DNA in specific)/extracellular digestion
    Into more smaller, soluble molecules and convert them into ammonium ions
    Ammonia into nitrites to nitrates (Nitrification)
    By nitrifying bacteria
    By oxidation
    The ploughing of the soil aerates the soil so that the nitrifying bacteria can respire aerobically

    Grade predictions?

    A* - 60, A - 55, B - 50, C - 45

    Massive thanks to everyone who contributed! Apologies for the long post, but it seems people can't be trusted to read a document without editing it.

    Good luck to all for Unit 5.

    2b. Great post! For the pedigree chart one, I'm pretty sure i used parents 10 and 11 and child 12?
    • Thread Starter

    (Original post by themacity)
    2b. Great post! For the pedigree chart one, I'm pretty sure i used parents 10 and 11 and child 12?
    Yep that's fine

    Don't forget bioaccumulation of chemical pesticides!

    (Original post by themacity)
    2b. Great post! For the pedigree chart one, I'm pretty sure i used parents 10 and 11 and child 12?
    I also did it was ok
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