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Biochemistry of Respiration

  • Oxidative breakdown of organic molecules to store energy as ATP
  • Animals and plants respire; FAD and NAD are coenzymes

Aerobic respiration

  • C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
  • Complete oxidation of an organic substrate to CO2 and H2O using free O2
  • Production of CO2, NADH + H+ and FADH + H+, 38ATP

1) Glycolysis → cytoplasm

  • Glucose enters cell by facilitated diffusion
  • ATP activates glucose to produce 2 unstable compounds
  • Substrate-level phosphorylation produces 4ATP
  • Net yield of 2ATP and 2reducedNAD per glucose molecule

2) Link reaction → matrix of mitochondria

  • Pyruvate enters matrix of mitochondrion for further reaction
  • Net yield of 2reducedNADH per glucose

 3) Krebs cycle → matrix of mitochondria

  • Citrate is gradually broken down to re-form oxaloacetate
  • Substrate-level phosphorylation forms 2ATP
  • Removal of hydrogen from respiratory substrate
  • Net yield of 2ATP, 2reducedFADH, 6reducedNADH per glucose

4) Electron Transport Chain ETC → inner membrane/cristae of mitochondria

  • Reduced coenzymes arrive at ETC
  • Split into coenzyme + 2H+ + 2e- by hydrogen carriers
  • 2e- are transferred to electron carriers (cytochrome)
  • Pass down ETC by redox reaction and release energy as they go
  • Energy produces ATP by oxidative phosphorylation
  • Final electron acceptor 1/2O2 is reduced by 2H+ and 2e- to produce H2O
  • Net yield of 34ATP (30NADH, 4FADH) per glucose
  • //Cytochromes are iron-containing proteins → cytochrome a3 also contains copper and is irreversibly damaged by cyanide

IMG 5-14-8

Anaerobic respiration (fermentation)

  • Substrate-level phosphorylation: 2ADP + 2Pi → 2ATP directly by enzymes in glycolysis
  • No O2 to accept electrons from NADH + H+ → no Krebs cycle or ETC
  • NADH + H+ reduces (gives off H+ ions to) pyruvate to produce
    • Lactate C3 in animal cells → can be re-oxidised
    • Ethanol C2 in yeast cells → irreversible, CO2(g) lost
  • Regenerates NAD
  • NAD can be re-used to oxidise more RS/allows glycolysis to continue
  • Can still form ATP/release energy when O2 is in short supply

Role of ATP

  • Adenosine (ribose + adenine) triphosphate (3 phosphate groups)
  • Produced by adding Pi to ADP → phosphorylation
  • Breaks down to ADP (adenosine diphosphate) and Pi (inorganic phosphate ion) by hydrolysis
  • ATP is useful as an immediate energy source/carrier because
    • Energy release only involves a single reaction
    • Energy released in small quantities
    • Easily moved around inside cells, but cannot pass through cell membranes
  • Light-dependent reaction cannot be the only source of ATP
    • "Photosynthesis cannot produce ATP in the dark
    • Need more ATP than can be produced in photosynthesis
    • Cannot be produced in plant cells lacking chlorophyll
    • ATP cannot be transported"1
  • Central molecule in metabolism (ATP hydrolysis)
    • Muscle contraction → changes of position of myosin head relative to actin
    • Protein synthesis → ATP "loads" amino acids onto tRNA
    • Active transport → driven by phosphorylation of membrane-bound proteins
    • Calvin Cycle → cyclic reduction of CO2 to TP
    • Nitrogen fixation → involves ATP-driven reduction of molecular nitrogen
  • ATP in liver is used for active transport / phagocytosis / synthesise of glucose, protein, DNA, RNA, lipid, cholesterol / urea in glycolysis / bile production / cell division

Brown fat

  • White fat insulates the body and reduces heat loss
  • Brown fat cells in mitochondrial membrane produce heat
  • Mitochondria in other tissue / chemiosmosis
    • H+ ions pass back from space between two mitochondrial membranes into matrix
    • Through pores which are associated with the enzyme ATP synthetase
    • Energy from the ETC will be used to produce ATP
  • Mitochondria in brown fat
    • H+ ions flow back through channels not associated with ATP synthetase
    • Energy produces heat instead of ATP
    • Found in chest, larger arteries for heat distribution round the body or in hibernating mammals