FERMENTATION AND ANAEROBIC RESPIRATION


ATP that is yielded from aerobic respiration depends on the electronegative oxygen to pull the electrons down the transport chain - without the oxygen phosphorylation ceases
  • there are, however, two mechanisms that can be used in certain cells to oxidize organic fuel and generate ATP withoutoxygen
    • determining between these two processes is based on the presense of the electron transport chain
    • with the electron transport chain - anaerobic respiration occurs
    • without the transport chain - fermentation occurs
  • anaerobic respiration takes place in certain prokaryotic organisms that live in an environment without oxygen
    • these organisms have an electron transport chain but do not use oxygen as the electron acceptor at the end of the chain
    • other less electronegative substances are used as final electron accpetors

FERMENTATION

  • fermentation harvests chemical energy without both oxygen and the electron transport chain
    • glycolysis generates 2 ATP whether oxygen is present or not
    • fermentation is an EXPANSION of glycolysis that allows continuous generation of ATP using substrate-level phosphorylation
    • there must be a sufficient supply of NAD+ to accept electrons during the oxidation step of glycolysis
    • the anaerobic alternative to the electron transport chain is to transfer electrons from NADH to pyruvate which is the end product of glycolysis
  • fermentation consists of glycolysis plus reactions that regenerate NAD+
    • the NAD+ is regenerated by transferring electrons from NADH to pyruvate
    • the NAD+ can then be reused to oxidize sugar through glycolysis
  • there are TWO TYPES OF FERMENTATION
    1. alcohol fermentation
      • pyruvate is converted to ethanol in two steps
      • the first step releases carbon dioxide from the pyruvate which is converted to acetaldehyde
      • the second step reduces acetaldehyde to ethanol through NADH
      • this regenerates the supply of NAD+ needed to continue glycolysis
      • many bacteria use alcohol fermentation under anaerobic conditions
    2. lactic acid fermentation
      • pyruvate is reduced directly by NADH to form lactate as an end product
      • there is no release of carbon dioxide
      • lactic acid fermentation is used by certain fungi and bacteria in the dairy industry as well as human muscle cells


THE TWO TYPES OF FERMENTATION

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  • obligate anaerobes - organisms that carry out ONLY fermentation or anaerobic respiration and can not survive in the presence of oxygen
  • facultative anaerobes - organisms that makes ATP by aerobic respiration if oxygen is present but that can switch to anaerobic respiration or fermentation if there is no oxygen

GLYCOLYSIS AND THE CITRIC ACID CyCLE CONNECT TO OTHER METABOLIC PATHWAYS


  • free glucose molecules are not common in the diets of humans and other animals - we obtain our calories in the forms of fats, proteins, sucrose and starch
    • all of these organic molecules can be used as fuel for cellular respiration to make ATP
  • proteins can be used for fuel but they must be digested into amino acids first
  • catabolism can harvest energy stored in fats from either food or storage cells in the body
    • fats are digested into fatty acids and glycerol
    • most of the energy form the fat is stored in the fatty acids
    • the metabolic sequence called beta oxidation breaks the fatty acids down to two-carbon fragments which enter the citric acid cycle as acetyl CoA
  • cells need substance as well as energy
    • food must also provide the carbon skeletons that cells requireto make their own molecules
    • the bod needs some molecules that are not present in most foods
    • for these molecules, compounds are formed as intermediates of glycolysis and the citric acid cycle can be diverted to anabolic pathways to sythesize these molecules
    • glycoysis and the citric acid cycle function as metabolic interchanges that enable cells to convert some types of molecules to others as they are needed


CATABOLISM OF VARIOUS MOLECULES FROM FOOD

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  • the principles of supply and demand apply to metabolic pathways
    • a cell will not waste energy making more than it needs of a substance
    • most common mechanism for this control is feedback inhibition - the end product inhibits the enzyme that catalyzes an early stage of the pathway
    • the cell also controls its catabolism
      • if the cell is working hard and ATP concentration drops then respiration speeds up
      • if there is an excess of ATP respiration slows down
    • control is based on regulating the activity of enzymes at strategic points
    • one important regulator is phosphofructokinase
      • this enzyme catalyzes step 3 of glycolysis
      • it is inhibited by ATP and stimulated by AMP which is derived from ADP
        • as ATP accumulates glycolysis slows down as the enzyme is inhibited
        • the enzyme becomes active again as cellular work converts ATP to ADP faster than ATP is regenerated
      • it is also sensitive to citrate which is the end product of the citric acid cycle
        • as citrate accumulates, glycolysis slows down and the supply to the citric acid cycle decreases
        • as citrate is consummed, glycolysis accelerates and meets the demand


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