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    The link reaction.

    The link reaction takes place in the matrix of the mitochondria. It is the second stage of aerobic respiration. Pyruvate formed in glycolysis enters the matrix of a mitochondrion where it combines with co-enzyme A to form acetylcoenzymeA. It involves no further gain of ATP but does produce reduced NAD. Oxygen is also required ( this is an oxidation reaction) and carbon dioxide is made.

    2 x Pyruvate + 2 x CoenzymeA --------2 x NAD turns into 2 x Reduced NAD----------> 2 x AcetylcoenzymeA + 2CO2


    Overall, the products of the link reaction:

    - 2 x AcetylcoA

    - 2 x CO2

    - 2 x Reduced NAD.
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    Krebs Cycle.

    The Krebs cycle also takes place in the matrix of the mitochondria. It is the third stage of aerobic respiration. AcetylcoenzymeA reacts with a 4-carbon compound (oxaloacetate) to form a 6-carbon compound (citrate). In a series of reactions, the 6-carbon compound is converted to a 5-carbon compound then back to the 4-carbon compound (oxaloacetate).

    So, it's a cyclical series of enzyme catalysed reactions involving organic compounds which are continuously broken down + regenerated. This also forms some ATP (via substrate level phosphorylation), more reduced NAD and CO2 as well as some reduced FAD.


    Overall, the products of the Krebs cycle (one turn):

    - 3 x reduced NAD

    - 1 x reduced FAD

    - 2 x CO2

    - 1 x ATP.


    The cycle turns twice for every glucose molecule, therefore the products are double that of above per glucose.
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    Electron transport and Chemiosmosis.

    The electron transport chain occurs on the inner membrane of the cristae of the mitochondrion. It is the fourth stage of aerobic respiration. The most important products of the Krebs cycle are reduced NAD + FAD. They are passed to the electron transfer chain where the chemical potential energy they contain is used to produce ATP by oxidative phosphorylation.

    It involves a series of electron carrier proteins arranged down an energy gradient. Hydrogen ions + electrons are removed from the reduced co-enzymes.(FAD + NAD) + eventually the hydrogen ions, electrons all combine with O2 at the end of the chain to produce H2O. (This process releases lots of energy to synthesise lots of ATP via oxidative phosphorylation)

    The equation is as follows:


    1/2O2 + 2H+ + 2e- -----------------------> H2O

    ~

    Cyanide inhibits the final step in the electron transfer chain as an enzyme is involved. This enzyme is called cytochrome oxidase + it prevents the re-oxidation of reduced NAD ------> NAD and reduced FAD -----------> FAD.


    The chemiosmotic synthesis of ATP.

    As the electrons from a reduced NAD molecule pass along the transport chain, proton pumps move three hydrogen ions into the intermembrane space.

    The continual pumping of H+into the intermembrane space creates a diffusion gradient between the intermembrane space and the matrix.

    This gradient results in hydrogen ions diffusing through ATP synthase molecules.

    When a hydrogen ion moves through ATP synthase, it releases sufficient energy to enable the synthesis of one molecule of ATP.

    Electrons from a molecule of reduced FAD only move two hydrogen ions through the proton pumps + so only two molecules of ATP are synthesized rather than three.


    The formation of ATP by chemiosmosis depends upon the electron transfer chain creating the proton gradient. Without oxygen to accept the electrons at the end of this chain, the electron transfer + proton pumping would cease. ( And bc it depends upon oxygen, remember that this method of producing ATP is called oxidative phosphorylation)
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    Anaerobic respiration- Topic 4.

    In the absence of oxygen, the electron transfer chain cannot take place bc oxygen is not present to act as the terminal electron acceptor.As a result:

    - There is a build up of reduced NAD + FAD bc they cannot be oxidised.

    - The link reaction + Krebs cycle cannot take place.

    However, Glycolysis can still occur.


    So, in aerobic respiration:

    - Glucose is converted to pyruvate by glycolysis.

    - The net yield of ATP is two molecules per molecule of glucose.

    - ATP is only generated by substrate level phosphorylation during glycolysis.

    - The reduced NAD generated in glycolysis cannot pass down the electron transfer chain bc there is no oxygen to acts as the terminal electron acceptor.

    - Reduced NAD is used to reduce pyruvate to either ethanol or lactate. This makes NAD available again + allows the glycolysis reactions to continue.
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    Energy Transfer- Topic 5.

    Energy cannot be created or destroyed, it can only be transferred + in the transfer converted from one form to another. This is known as the conservation of energy.

    The amount of usable energy in an ecosystem decreases as energy is transferred through it.

    - Energy enters ecosystems as light + is transduced to chemical energy in the biological molecules within producers as a result of photosynthesis.

    - Some of the chemical energy in the biological molecules of producers is released in respiration and is used to drive other processes in the producer and is then lost as heat.

    - Some of the chemical energy in the biological molecules of producers is transferred to herbivorous animals when they feed.

    - The same is true when carnivorous animals eat other animals + when decomposers break down dead remains.

    - Animals release the chemical energy from their food + use it to drive other processes, losing some as heat.


    (The transfer of energy through ecosystems can be represented in food chains/webs, pyramids of numbers/biomass + energy flow diagrams)

    ~

    The biological molecules an organism takes in are used in one of two main ways:

    - They are respired to produce ATP which is used to drive processes such as muscle contraction + active transport; some of the energy is lost as heat.

    - They are assimilated into the structure of the organism; the energy in the molecules remains within the organism.
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    During feeding, energy is transferred from one trophic level to another. This is represented in a food chain, for example:

    Grass (producer) -----------------> Gazelle (Primary consumer) -----------------------> Cheetah. (Secondary consumer)

    The number of trophic levels in an ecosystem does not often exceed five, this is bc:

    - Only around 5% of the light shining on plants is used in photosynthesis.

    - Only around 10% of the energy in trophic levels is passed to the next level.

    - There is too little energy in the fifth trophic level to support another trophic level.
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    So, when a primary consumer eats a producer, not all of the materials in the producer end up as a consumer. There are losses bc:

    - Some parts of the producer are not eaten.

    - Some parts are not digested + therefore are not absorbed. (lost in faeces)

    - Metabolism of some of the absorbed substances leads to the formation of excretory products which are released into the environment.

    - Many of the absorbed substances are respired, releasing energy: The carbon dioxide formed at the same time is lost to the environment.

    This means that only a small fraction of the substances in the producer becomes incorporated into new cells in a primary consumer.

    ~
    The loss in biomass at each level means that a smaller biomass is available for growth at successive levels which is usually associated with smaller numbers. The exceptions to this occur when:

    - Very large producers are eaten by smaller consumers.

    - An organism is parasitised.
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    Okay so, food chains can be represented as any of the following ecological pyramids:


    - A pyramid of numbers represents the total number of the organisms at each trophic level in a food chain, at a given moment, irrespective of biomass. (size)

    - A pyramid of biomass represents the total biomass of the organisms at each trophic level in a food chain, at a given moment, irrespective of numbers.

    - And finally, A pyramid of energy represents the total amount of energy present in each trophic level of a food chain, irrespective of numbers + biomass, in a given period of time.


    Note: Pyramids of biomass and energy ALWAYS have a true pyramid shape, whereas the pyramids of numbers DO NOT.
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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
    If you need help, you can PM me

    I used to hate this unit so much, but now I find it easier than expected (except for data questions). Here are some common questions and model answers you should remember.
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    Energy and food production - Topic 5.

    Productivity is the energy entering a trophic level that remains as energy in biomass:

    - In a given period of time (often a year)

    - For a given area of the ecosystem (often a square meter)

    The units can either be in kg or kJ.


    Primary productivity -The productivity of producers (plants)

    Gross primary productivity (GP) - All the biomass produced by the plants per m2 per year.

    Net primary productivity (NP) - The biomass that is left per m2 per year, after losses in respiration (R) are taken into account. This is the biomass of the plants that will pass either to the primary consumers or to the decomposers following death of the plants.

    We can express this relationship in the formula below:

    NP = GP - R.


    Secondary productivity -The productivity of animals.The biomass remaining in the animals needs to take into account:

    - The amount of energy ingested in food.

    - The amount of energy lost in respiration.

    - The amount of energy lost in urine.

    - The amount of energy lost in faeces.
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    (Original post by kkboyk)
    If you need help, you can PM me

    I used to hate this unit so much, but now I find it easier than expected (except for data questions). Here are some common questions and model answers you should remember.
    Thanks ever so much! Yeah, I'm hoping that once I've completed this thread then it will feel slightly easier. I'll definitely use those key questions again, possibly after I've finished writing up my notes bc they were v helpful last time. :cube:
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    Alright so, farmers aim to get as much biomass production for as little input as possible. To do this, farmers can adopt any of the following practices:

    Monoculture

    This means growing a single type of crop over a wide area. This practice:

    - Allows easier control of pests. - Number of different pests is often limited by the crop + so specific pesticides can be used.

    - Reduces plant competition for nutrients, space + solar radiation. - Only the crop plant is taking nutrients from the soil + absorbing light energy.

    - Maximises profit by growing crops with a high gross margin.


    ~

    The use of fertilisers.

    Okay so there are two types; inorganic + organic.

    Organic fertilisers are materials produced directly from animals, plants + other living organisms + must be decomposed to release mineral ions. There are therefore slow-release fertilisers. They include farmyard manure, seaweed, dried blood, sewage sludge + poultry manure.

    ON THE OTHER HAND.

    Inorganic fertilisers do not need to be broken down bc they are already in the form of soluble mineral ions. They are, therefore quick release fertilisers. A consequence of overusing them, however, is that it can lead to eutrophication bc the high solubility means ions are leached easily into waterways.
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    The use of pesticides.

    Pests reduce the productivity of crops. Insect pests can reduce the yield of the crop in a number of ways:

    - They can feed on the leaves; this reduces the leaf area + therefore the capacity of the plants for photosynthesis.

    - They can feed on the roots, lowering the uptake of mineral ions essential for growth.

    - They can feed on sap from the phloem + so disrupt the transfer of sugars manufactured in photosynthesis to other organs.

    - They can spread organisms that cause disease.



    Pesticide - A chemical that helps to control a pest population.

    Pesticides can be classified according to the type of organism they control, for example:

    - Insecticides. (kill insects)

    - Herbicides (kill plants - weed killers)

    - Fungicides (kill fungi)

    - Molluscicides (kill slugs + snails)



    Some pesticides have effects on organisms in the environment other than the pests they are used to control. They might:

    - Kill useful insects as well as the targeted harmful insects.

    - Persist in the environment for many years before they are finally broken down; they may be taken up by crop plants + so enter humans through the food chain.

    - Accumulate along food chains. (bioaccumulation)
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    Biological control.

    This involves introducing a natural parasite or predator of the pest into the area. The aim is to reduce the pest population to a level that does not cause major damage.

    The following are examples of biological control methods:

    - Introduce a predator. -- e.g. ladybirds have been used to control aphid populations in orange groves.

    - Introduce a herbivore. -- e.g. a moth native to South America was introduced to Australia to control the 'prickly pear' cactus.

    - Introduce a parasite. -- e.g. larvae of the wasp are introduced into greenhouses to control whiteflies on tomato crops.

    - Introduce sterile males. -- this reduces the number of successful matings + so reduces the pest numbers.

    - Use pheromones -- these animal sex hormones are used to attract the males or females, which are then destroyed -- e.g. male-attracting pheromones are used to control the damson-hop aphid, reducing damage to plum crops.



    Biological control has several advantages over the use of pesticides:

    - Pests do not usually develop resistance to a predator or parasite.

    - Biological control agents are usually more specific than pesticides; for example, a carefully chosen predator will target only the pest, whereas a pesticide might target all the animals of a particular group (an insecticide might kill many kinds of insect)

    - Once a natural predator or parasite has been introduced, no further introductions are necessary, whereas pesticides must be applied regularly.



    Biological control is not always appropriate since:

    - The proposed control agent might affect populations other than the pest.

    - The control agent might not be able to reproduce in the new environment.

    - Reducing the numbers of one specific pest might allow another pest to fill the original pest's ecological niche.

    - It is not an appropriate method for controlling pests of stored grain. The grain would become contaminated with the dead bodies of both pest + control agent.
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    Integrated control systems.

    Usually neither chemical control nor biological control alone is really effective enough in controlling pests. Biological control is often enhanced when low levels of pesticides are used at the same time. This is a simple example of an integrated control system.

    In integrated crop management, most or all of the following would be considered as methods of maximising productivity:

    - Selecting crops that are adapted to the type of soil + the climate in the area.

    - Selecting crops that have some resistance to known pests in the area.

    - Choosing appropriate methods of pest control.

    - Rotating crops grown in a particular field so that the same pests do not build up in the soil + the same ions are not continually removed by the crop.

    - Using fertilisers (organic, inorganic or a combination) that are appropriate to the conditions, to replace the mineral ions removed by cropping.

    - Appropriate treatment + storage of the final crop (to minimise damage by pests)

    - Irrigation of the soil (where necessary)


    By using a combination of these techniques, a farmer can reduce damage by pests + ensure that crops have a continuous supply of mineral ions.
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    Intensive rearing of livestock.

    The main principles of intensive farming practices include:

    - Feeding a precisely controlled diet that ensures less of the food is lost in faeces.

    - Restricting the movement of the animals so that less energy is lost this way + more energy is used in growth.

    - Keeping the animals in a warm environment so that less energy is lost as heat to the environment + more is used in growth.

    - Using hormone injections or supplements to increase the rate of growth.

    - Using antibiotic injections to increase the rate of growth.
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    Nitrogen cycle - Topic 6.

    Alright so there are basically 4 stages to the nitrogen cycle: Nitrogen fixation, Ammonification, Nitrification and finally, denitrification.


    Nitrogen fixation
    Atmospheric oxygen is reduced to ammonia by a bacteria called Rhizobium.


    Ammonification
    Decomposers convert nitrogen compounds in dead animals + plants into ammonium compounds.


    Nitrification.
    Ammonium is converted first to nitrites (NO2-) then nitrates (NO3-) by nitrifying bacteria.


    Denitrification.
    Nitrates in the soil are converted into nitrogen gas by denitrifying bacteria. This occurs anaerobically.


    Okay, so overall:

    Organic compounds are broken down via aerobic sapribiotic bacteria and fungi to ammonium ions via ammonification. Ammonium ions are converted to nitrite ions then nitrate ions via aerobic nitrifying bacteria in the process nitrification. The nitrate ions are taken up by the roots and used to synthesise plant proteins.
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    Eutrophication.

    This is the result of leaching of both nitrates (NO3-) and phosphates (PO42-) from artificial fertilisers + phosphates from chemical detergents. (Basically can happen if organic fertilisers are overused)


    1)Leaching of NO3- and PO42- ions from farmland/gardens via heavy rainfall.


    2) Surface run off ( "stream" of running water containing the dissolved ions) and drainage into rivers, lakes, streams or ponds.


    3) Excessive growth of photosynthetic algae on the surface of the water. ("Algal bloom")


    4) This reduces light intensity for submerged aquatic plants and therefore they stop photosynthesising and eventually die.


    5) Increased numbers of sapribiotic bacteria and fungi in the decomposition of dead organisms using up dissolved oxygen via aerobic respiration.


    6) O2 levels continue to fall as algae die through lack of nutrients + other aquatic organisms.(e.g. invertebrates)


    7) Eventually fish also die + the only remaining organisms after time are anaerobic bacteria.
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    Carbon cycle - Topic 6.

    The main processes involved in cycling the element carbon through ecosystems are:


    1) Photosynthesis - fixes carbon atoms from carbon dioxide into organic compounds such as glucose.

    2) Feeding + assimilation - feeding passes carbon atoms in organic molecules to the next trophic level in the food chain where they are assimilated into the body of that organism.

    3) Respiration. - Releases carbon dioxide from organic compounds.

    4) Fossilisation - sometimes dead material does not decay fully due to the conditions in the soil, and fossil fuels (e.g.coal, oil etc) are formed.

    5) Combustion. - Fossil fuels are burned, releasing carbon dioxide into the atmosphere.
 
 
 
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