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 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
    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


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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


  • 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


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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