ATP & Respiration

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Structure Of ATP

  • Consists of:
    -Organic base,
    -5C-Sugar,
    -Ribose,  
    -3 Phosphate groups. 
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How ATP Stores Energy

  • Bonds between the phosphate groups are unstable and easily broken, releasing energy.
  • This is catalysed by the enzyme ATPase.
  • ATP + H20 < - > ADP + Pi + Energy
  • Addition of Pi to ADP is called phosphorylation; ATP transfers energy.
  • There are three forms of phosphorylation:
    -Oxidative phosphorylation: which occurs on the membranes of mitochondria in aerobic respiration.
    -Photophosphorylation: which occurs on the membranes of chloroplasts during photosynthesis.
    -Substrate-level phosphorylation: which occurs when phosphate groups are transferred from donor molecules to ADP to make ATP.
  • The first two release ATP by the transfer of electrons along a chain. 
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Importance And Role Of ATP

  • Instead of an uncontrolled release of energy like in glucose - which would destroy cells - ATP allows for small amounts of energy in steps when required.
  • ATP can be hydrolysed for when it is needed; ATP -> ADP + Pi + 30kJmol-1
  • Further advantages of ATP include:
    -The hydrolysis is one reaction that releases immediate energy.
    -Only one enzyme is required.
    -A common source of energy throughout the cell in many chemical reactions, it is a universal molecule. 

  • ATP provides energy for:
    -Metabollic processes: to build large molecules from  smaller ones; ie, DNA.
    -Active transport: to change shape of carrier proteins to allow ions/molecules to move through.
    -Movement: for muscle contraction.
    -Nerve transmission: Na/K pumps actively transport across.
    -Synthesis of materials within cells.
    -Secretion: the packaging of transport of products into vesicles in cells. 
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Respiration

  • Respiration can occur either aerobically or anaerobically.
  • Aerobic occurs as such:
    -Glycolysis
    -The link reaction
    -Krebs cycle
    -Electron transport chain
     
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Glycolysis

  • Initial stage in both aerobic and anaerobic respiration.
  • Happens as follows:
    -Glucose is activated by addition of 2 ATP's, lowering activation energy.
    -Glucose is converted to GC hexose phosphate, which splits into 2 triose phosphates.
    -Hydrogen is removed from each of the two TP molecules, and forms reduced NAD.
    -The TP is thus converted into pyruvate, these steps generate 4 ATP molecules by substrate-level phosphorylation - but 2 are used, producing a net of 2 ATP.
    -2 molecules of red. NAD are also formed.
  • Energy still remains in pyruvate that can be released in Krebs Cycle. 
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Link Reaction

  • Links glycolysis to Krebs cycle:
    -Pyruvate diffuses from CYTOPLASM to MITOCHONDRIAL MATRIX.
    -Pyruvate is then decarboxylated.
    -It is also dehydrogenated, where the H is accepted by NAD.
    -The remaining molecule then combines with coenzyme A to form acetyl coenzyme A.
  • Pyruvate + NAD + CoA -> acetyl CoA + red. NAD + CO2 
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Krebs Cycle

  • The function of Krebs cycle is to liberate energy from the C bonds to provide ATP/NADH.
  • This occurs by:
    -Acetyl CoA enters Krebs and combines with a 4C acid, to form a 6C compound.
    -The 6C undergoes reactions during which CO2 and H are removed, after which the remaining 4C compound is regerated to reform the original 4C acid.
    -Two steps include decarboxylation, and four involve dehydrogenation.
  • So for each molecule of glucose:
    -2 ATP are formed by substrate-level phosphorylation.
    -6 NADH and 2 FADH2.
    -2 molecules of CO2.
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Krebs Cycle Diagram

(http://images.tutorvista.com/content/respiration/citric-acid-cycle.jpeg)

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Electron Transport Chain

  • A series of pumps and carriers, releasing energy as ATP in the INNER MEMBRANES.
  • CO2 is waste, but H are carried by coenzymes NAD/FAD and synthesise 3 or 2 ATP accordingly; for each red. NAD 3 ATP are formed, each red. FAD there are 2 formed.
  • This is decribed by the chemiosmotic theory:
    -NADH donates the e- from the H to the electron carriers, and the H+ remains in solution.
    -The e- provide energy to pump H+ into mitondrial inter-membrane space.
    -The e- move along providing energy for each pump in turn.
    -Eventually a concentration gradient is set up as the membrane is impermeable to H+.
    -The channel where H+ can move through is associated with ATP synthetase.
    -When H+ diffuse back into the matrix, kinetic energy causes the generation of ATP.
    -At the end of the chain the e- and H+ combine with O2 to form H20. 
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Anaerobic Respiration

  • If there is no O2 available then only gycolysis can take place.
  • For glycolysis to occur pyruvate and H must be constantly removed and NAD regenerated.
  • So pyruvate accepts the H from the NAD.
  • This all occurs in two ways:
    -The first is in animals and occurs mainly in muscle tissues, during exercise O2 cannot be supplied enough and so pyruvate accepts H and converts into LACTATE.
    -The second occurs in micro-organisms such as yeast; the pyruvate is first decarboxylated to produce ethanal which is then converted to ethanol with the addition of H from NAD. 
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ATP Yields

  • Glycolysis:
    -2 formed by substrate-level phosphorylation.
    -Also 2 NAD are formed, producing 6 ATP in the electron transport chain.
    TOTAL = 8 ATP
  • Link reaction:
    -2 NAD are formed.
    TOTAL = 6 ATP
  • Krebs Cycle:
    -2 formed by substrate-level phosphorylation.
    -6 NAD are formed, producing 18 ATP.
    -2 FAD are formed, producing 4 ATP.
    TOTAL = 24 ATP

    GRAND TOTAL = 38 ATP  

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Alternative Respiratory Substrates

  • Lipids: 
    -Fat provides an energy store and is used for when carbs are low.
    -It's split into glycerol and fatty acids.
    -The first is phosphorlyated with ATP, and dehydrogenated with NAD; and converted into TP.
    -The second ones are formed into 2C fragments which enter Krebs as acetyl CoA.
  • Proteins:
    -This can be used under extreme circumstances, when tissue protein is mobilised.
    -It is hydrolysed into amino acids and deanimated in the liver, where the amino group forms urea and the residue forms acetyl CoA or pyruvate. 
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