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    Alright, um this is just another one of my revision threads which I am making in advance for tomorrow morning. :woo:

    I really don't want to revise for this exam bc I hate it but tbh I don't actually know what I hate bc I don't know what it consists of since I barely went to class which means I don't have many class notes to work from.. and it feels disastrous but the exam is in a week's time so I'm hoping the textbooks will save me + I'll manage to cover everything way before then.

    Ahem. I can do this. :'3
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    (Original post by Anon_98)
    Alright, um this is just another one of my revision threads which I am making in advance for tomorrow morning. :woo:

    I really don't want to revise for this exam bc I hate it but tbh I don't actually know what I hate bc I don't know what it consists of since I barely went to class which means I don't have many class notes to work from.. and it feels disastrous but the exam is in a week's time so I'm hoping the textbooks will save me + I'll manage to cover everything way before then.

    Ahem. I can do this. :'3
    :rave: Good luck!
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    Wait what do you mean? AQA biology unit 4 is next week thursday, I thought
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    (Original post by SeanFM)
    :rave: Good luck!
    :cube:
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    Populations + ecosystems - Topic 1.

    Population- All the organisms of one species found in a particular habitat at a particular time.

    Community - All the populations found in a particular habitat at a particular time.

    Ecosystem - A self-contained unit consisting of a community and it's non-living environment. E.g pond/rainforest.

    Habitat - The place in an ecosystem where a community of organisms is found.

    Niche - A description of how the species functions in that habitat.


    A description of a niche takes into account:

    - Abiotic conditions: Non living aspects of the environment like the temperature a species can tolerate.

    - Biotic conditions: Biological aspects such as the availability of a suitable food source or the presence of competitors.
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    Investigating populations - Topic 1.

    The number of any one species in an ecosystem can be estimated using:

    - Quadrats. (For relatively stationery organisms)

    - Mark-release-recapture techniques (for mobile species)

    Okay so there are three sampling techniques we need to know. - Random quadrats, along a transect and mark-release recapture. I'm going to be going through all three methods in steps. First we'll start with the quadrats which I'll do in a separate post.
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    Using random quadrats to estimate population size

    You would use this method if you suspected there was no environmental change in the area. E.g. field.

    1) Divide the area into a grid and give the grid coordinates.

    2) Use the random number generator on a calculator to produce a pair of coordinates, for example A5, B6.

    3) Repeat this procedure until you have identified a sufficient number of samples to make your data reliable.

    4) Place a quadrat frame with it's top left hand corner on the intersection of one pair of coordinates.

    5) Estimate the abundance of the different organisms in each quadrat using one of the following methods:

    - Count the number of organisms in each quadrat.

    - Calculate the percentage frequency of occurrence. (This is done by recording the presence or absence in each quadrat and converting the number of occurrences into a percentage. E.g. A species that occurs in 8 out of 10 quadrats has a frequency of 80%)

    - Calculate the percentage cover. (This is done by either making an estimate of the percentage of each quadrat covered by the species or by using a quadrat that is subdivided into smaller squares. You multiply how many squares the organism is present in by the percentage of what each square represents of the total area. E.g If an organism covers 7 squares and each square represents 4% supposing the quadrat is made up of 25 squares then the percentage cover is 4% x 7 = 28% )


    6) Find the mean abundance per quadrat.

    7) Knowing the area of the quadrat, convert the mean abundance per quadrat to mean abundance per standard area, for example per m2
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    Okay, now we'll do transects.

    Using quadrats along a belt transect to estimate population size.

    You would use this method if you are investigating an area which is not uniform. E.g. rocky shore. The reason being is if a random sample were to be taken, then a sample area may be missed. However, we can overcome this by taking samples systematically along a transect.

    1) Lay a tape measure across the sample area. (E.g. From point A to point B)

    2) At regular intervals, lay five quadrats to one side of the tape.

    3) Estimate the abundance of different organisms at each sampling point along the transect using one of the methods described above.
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    Finally, the last technique.

    Using mark-release-recapture techniques for mobile species.

    This method is suitable for animals that are v mobile.

    1) Collect a sample of the animals from the area and count them.(N1)

    2) Put a small mark in an unobtrusive place on each animal.

    3) Release the marked animals and allow time for them to disperse among the population.

    4) Collect a second sample and count both the total size of the sample (N2)and the number that are marked (n)

    5)Estimate the population size using the formula:


    \displaystyle \dfrac{N1 *  N2 }{n}



    ~

    For example:

    If you caught and released 50 woodlice + later caught 40 of which 10 were marked then you would estimate the population as:

    \displaystyle \dfrac{50 * 40}{10} = 200


    When using this technique we assume that, during the time the marked individuals disperse among the population:

    - There is no migration.

    - There are no births or deaths.

    - There is random mixing of the marked + unmarked individuals.

    - Marking does not affect the behaviour.
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    Variation in population size - Topic 1.

    Alright so, population size is dependent on the availibility of resources. IF resources are unlimited then a population is capable of growing exponentially. However, resources are usually not unlimited and both abiotic + biotic factors restricts population growth.


    Abiotic factors. (Non living components of an eco-system.)

    - Carbon dioxide concentration in the atmosphere -- Affects populations of photosynthesisers.

    - Mineral ion availability in the soil -- affects populations of photosynthesisers.

    - Light intensity-- Affects populations of photosynthesisers + some animal populations.

    - Water availibility-- Affects all populations.

    - Temperature -- Affects all populations.



    Biotic factors. (Biological components of an eco-system)

    - Disease-causing organisms -- slow population growth.

    - Intraspecific competition -- competition between members of the same species slows population growth.

    - Interspecific competition -- competition between members of different species usually results in one of the species out-competing the other or the two species coexisting but with smaller numbers of both.

    - Predation-- the presence of predators ( or of herbivores in the case of plants) reduces the size of the prey (or plant) population.


    ~

    Predators + their prey.

    - An increase in the population of the prey means more food for the predators.

    - With more food, the predator population increases.

    - More predators kill more prey, so the prey population decreases.

    - There is now less food for the predators, so the predator population decreases.

    - The reduced number of predators kills fewer prey so the prey population increases.
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    Human populations - Topic 1.

    Okay so, any change in the size of a human population is a result of the difference between numbers born and numbers dying and there are three possibilities to this that you need to know:

    - Births > Deaths; The population increases.

    - Births < Deaths; The population decreases.

    - Births = Deaths; The population remains the same size.

    A change in population size is usually expressed as a population growth rate or rate of natural increase.

    This is determined from:

    Birth rate (Which is the number of births per 10 000 people)
    Death rate (Which is the number of deaths per 10 000 people)

    Growth rate( rate of natural increase) = Birth rate - Death rate.


    ~

    For example, if the birth rate is 14 per 10 000 + the death rate is 8 per 10 000 then the population growth rate is:

    14 - 8 = 6 per 10 000 or +0.06%

    I.E. The population is increasing.

    ~

    The growth in human population has mainly resulted from reduction in death rates due to:

    - Increased quality + quantity of food available.

    - Improved sanitation.

    - Improved medical care.
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    ATP (Adenosine triphosphate)- Topic 2.

    A molecule of ATP contains:

    - One molecule of adenine. (a nitrogenous base found in DNA/RNA)

    - One molecules of ribose.(pentose found in RNA)

    - And finally, 3 phosphate groups.

    ~



    ATP ----------------- ATPase----------------> ADP + Pi


    ADP and Pi can be joined again to make ATP, which requires an input of energy and this energy required to synthesise ATP from ADP and Pi can come from:

    - Cellular respiration.
    - The transduction of light energy in photosynthesis.


    ~


    ATP is the molecules that releases energy to drive biological processes. It is an ideal molecule for this function bc:

    - Energy is released from the molecule quickly, in a single-step hydrolysis reaction.

    - Energy is released in small amounts that are matched closely to the amounts used in coupled cellular reactions.

    - The molecule is moved around easily within the cell, but cannot leave the cell.
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    Photosynthesis - Topic 3.

    Photosynthesis is a process with two main stages:the light dependent reaction + the light independent reaction.



    - In the light dependent reaction, light energy is absorbed by pigments in the chloroplasts + is used to synthesise ATP and reduced NADP.


    - In the light independent reaction, the ATP and reduced NADP from the light-dependent reaction are used to drive reactions that result in a synthesis of glucose.

    ~


    NADP can act as an electron carrier. So when it accepts electrons, we say it's been reduced.


    In the light independent reaction, ATP and reduced NADP are used in the following ways:

    - ATP supplies energy to drive endergonic reactions + provides phosphate groups to phosphorylate other compounds.

    - Reduced NADP gives up it's electrons which are used to reduce glycerate-3-phosphate.
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    Chloroplasts are adapted for their function in the following ways:

    Okay, so the light dependent reaction takes place in the membrane of the thylakoids.

    - The thylakoids are stacked in grana which maximises the ability of the pigments to absorb light energy.
    - Chlorophyll are organised into photosystems on the thylakoid membranes.


    The light independent reaction takes place in the liquid stroma. Many chemical reactions take place most efficiently in a fluid medium.
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    The light dependent reaction - Topic 3.

    The light dependent reaction takes place in the grana. There are two different types of photosystems in the membranes of the grana that absorb different wavelengths of light. - Photosystem I and Photosystem II.


    ~

    - Light energy is absorbed by the pigments in photosystem II and passed to the chlorophyll.


    - The absorbed energy raises the energy levels of electrons in the chlorophyll molecule so that a pair of electrons leaves the chlorophyll molecule .


    - These electrons are then taken up by the primary electron acceptor and passed along an electron transfer chain. As they pass along the chain, energy is released and used to generate ATP from ADP and Pi.

    - The electrons are transferred to the chlorophyll in photosystem I.


    - When further light energy is absorbed by photosystem I, two more electrons are excited + escape from this chlorophyll molecule.


    - The electrons from photosystem I are taken up by a different primary electron acceptor + passed along a second electron transfer chain.


    - At the end of this second electron transfer chain, the electrons react with hydrogen ions and NADP to form NADPH.(reduced NADP)


    -The ATP and NADPH leave the granum + enter the stroma. (which is where the LIR takes place)
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    Photolysis

    Okay so, photolysis is basically the splitting of water by light and what happens is that water molecules are split into hydrogen ions, electrons + oxygen atoms.

    ~

    - The hydrogen ions are used to reduce NADP, as describe in the post above.

    - The electrons are taken up by the chlorophyll, replace those it lost in photosystem II.

    - And the oxygen atoms combine to form molecular oxygen which leaves the chloroplast. (Oxygen is basically a waste product)


    The equation for it is as follows:

    2H2O ---------------------------------------------> 4H+ + 4e- + O2
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    The light independent reaction - Topic 3.

    Thelight independent reaction takes place in the stroma.


    ~


    - Carbon dioxide enters the stroma and reacts with ribulose bisphosphate ( RuBP), which has 5 carbon atoms.


    - This reaction produces 2 molecules of glycerate-3-phosphate (GP), which have 3 carbon atoms.


    - ATP and NADPH from the light dependent reaction are used to reduce GP to triose phosphate (TP), which also has 3 C atoms.

    - Some of the TP is used to synthesise hexoses + ,from them, starch + other organic compounds like cellulose, amino acids + lipids)


    - The rest of the TP is used to synthesise more RuBP to keep the Calvin cycle going.


    - ADP, Pi and NADP leave the stroma + enter the granum.
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    Limiting factors - Topic 3.

    Alright so there are 3 limiting factors that have an influence on the rate of photosynthesis that we need to know about.

    Light intensity, Carbon dioxide + Temperature.

    ~

    Light intensity - Light energy drives the light dependent reaction therefore increasing the intensity of light, increases the rate of this reaction.

    Carbon dioxide. - Carbon dioxide reacts with RuBP in the light independent reaction therefore increasing the concentration of carbon dioxide, increases the rate of this reaction.

    Temperature. - Many of the reactions of photosynthesis are controlled by enzymes therefore increasing the temperature to the optimum temp increases the rate of photosynthesis. However, above this temp, enzymes will start to denature + the process will quickly slow down.
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    Respiration - Topic 4.

    Respiration releases energy from organic molecules + this energy is used to synthesise ATP.

    So there are two kinds of respiration:


    Aerobic respiration. (Which takes place in the presence of oxygen)

    Anaerobic respiration. (Which takes place in the absence of oxygen)

    ~

    Some of the ATP in aerobic respiration + all of the ATP in anaerobic respiration is produced by substrate level phosphorylation.
    (In this process, a compound with a phosphate group attached, transfers its phosphate group to ADP, producing a molecule of ATP)


    Most of the ATP produced in aerobic respiration is by oxidative phosphorylation.
    (This is associated with the transfer of electrons down an electron transfer chain.)
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    Aerobic respiration- Topic 4.

    Alright so, there are four stages in the aerobic respiration of glucose + they are:

    Glycolysis, The link reaction, Krebs Cycle + Electron transport and Chemiosmosis.

    ~


    Glycolysis.

    Glycolysis takes place in the cytoplasm. It is the first stage of aerobic respiration and involves the oxidation of one molecule of glucose to two molecules of pyruvate.

    Glucose can't enter a mitochondrion, but pyruvate can.

    ~

    During glycolysis:

    - Two molecules of ATP are used to energise one glucose molecule ( 6C) + kick start the process.

    - Using phosphate from the two hydrolysed ATP molecules, the glucose molecule is converted into two 3-carbon molecules called triose phosphate.

    - The two molecules of triose phosphate are oxidised by NAD to form two molecules of pyruvate and reduced NAD

    - During this oxidation of two molecules of triose phosphate to two molecules of pyruvate, sufficient energy is released to convert four molecules of ADP into four molecules of ATP.



    Overall, the products of glycolysis:

    - 4 x ATP (Net profit of 2ATP)

    - 2 x Pyruvate.

    - 2 x Reduced NAD.
 
 
 
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