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  1. 1-biological-molecules

    1-1-biological-molecules-carbohydrates
    11 主题
  2. 1-2-biological-molecules-lipids
    3 主题
  3. 1-3-biological-molecules-proteins
    5 主题
  4. 1-4-proteins-enzymes
    12 主题
  5. 1-5-nucleic-acids-structure-and-dna-replication
    8 主题
  6. 1-6-atp-water-and-inorganic-ions
    4 主题
  7. 2-cell-structure
    2-1-cell-structure
    7 主题
  8. 2-2-the-microscope-in-cell-studies
    4 主题
  9. 2-3-cell-division-in-eukaryotic-and-prokaryotic-cells
    8 主题
  10. 2-4-cell-membranes-and-transport
    7 主题
  11. 2-5-cell-recognition-and-the-immune-system
    7 主题
  12. 2-6-vaccines-disease-and-monoclonal-antibodies
    6 主题
  13. 3-exchange-and-transport
    3-1-adaptations-for-gas-exchange
    6 主题
  14. 3-2-human-gas-exchange
    10 主题
  15. 3-3-digestion-and-absorption
    5 主题
  16. 3-4-mass-transport-in-animals
    6 主题
  17. 3-5-the-circulatory-system-in-animals
    8 主题
  18. 3-6-mass-transport-in-plants
    6 主题
  19. 4-genetics-variation-and-interdependence
    4-1-dna-genes-and-chromosomes
    7 主题
  20. 4-2-dna-and-protein-synthesis
    6 主题
  21. 4-3-genetic-diversity-mutations-and-meiosis
    7 主题
  22. 4-4-genetic-diversity-and-adaptation
    6 主题
  23. 4-5-species-and-taxonomy
    4 主题
  24. 4-6-biodiversity
    9 主题
  25. 5-energy-transfers-in-and-between-organisms-a-level-only
    5-1-photosynthesis-a-level-only
    5 主题
  26. 5-2-respiration-a-level-only
    7 主题
  27. 5-3-energy-and-ecosystems-a-level-only
    9 主题
  28. 5-4-nutrient-cycles-a-level-only
    4 主题
  29. 6-organisms-respond-to-changes-in-their-environments-a-level-only
    6-1-response-to-stimuli-a-level-only
    12 主题
  30. 6-2-nervous-coordination-a-level-only
    10 主题
  31. 6-3-skeletal-muscles-a-level-only
    6 主题
  32. 6-4-homeostasis-a-level-only
    11 主题
  33. 7-genetics-populations-evolution-and-ecosystems-a-level-only
    7-1-inheritance-a-level-only
    6 主题
  34. 7-2-populations-a-level-only
    3 主题
  35. 7-3-evolution-a-level-only
    5 主题
  36. 7-4-populations-in-ecosystems-a-level-only
    7 主题
  37. 8-the-control-of-gene-expression-a-level-only
    8-1-genetic-mutations-a-level-only
    2 主题
  38. 8-2-regulation-of-gene-expression-a-level-only
    11 主题
  39. 8-3-using-genome-projects-a-level-only
    4 主题
  40. 8-4-gene-technologies-a-level-only
    13 主题
  41. exam-guidance-and-skills
    essay-guidance
    3 主题
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Anaerobic respiration

  • When oxygen is absent, or in short supply, cells may switch to anaerobic respiration; this needs to happen because:

    • there is no final electron acceptor for the electron transport chain

    • the electron transport chain stops

    • oxidative phosphorylation stops producing ATP

  • Examples of anaerobic respiration pathways are:

    • ethanol fermentation, e.g. in yeast cells

    • lactate fermentation, e.g. in mammalian cells

  • Ethanol and lactate fermentation allow cells to produce ATP continuously via glycolysis

    • Without fermentation glycolysis cannot continue indefinitely because cells would run out of NAD

    • Ethanol and lactate fermentation allow NAD to be regenerated so that glycolysis can continue

Ethanol fermentation

  • NAD is regenerated by ethanol fermentation as follows:

    1. pyruvate is decarboxylated to ethanal

      • CO2 is given off as a waste product

    2. ethanal is reduced to ethanol

      • The hydrogen atoms that reduce ethanal come from NADH, so NADH is oxidised to regenerate NAD

      • This involves the enzyme alcohol dehydrogenase

  • The products of ethanol fermentation are:

    • ethanol

    • carbon dioxide

    • NAD

Flowchart of alcoholic fermentation: glucose to pyruvate, pyruvate to ethanal, then ethanol. CO2 is released and NADH is converted back to NAD.
During glycolysis glucose is converted to pyruvate and NAD is reduced (left); NAD can then be oxidised again by the events of ethanol fermentation (right)

Lactate fermentation

  • NAD is regenerated by lactate fermentation as follows:

    • Pyruvate accepts hydrogen from NADH and is reduced to lactate

    • NADH is oxidised to regenerate NAD

  • The products of lactate fermentation are

    • lactate

    • NAD

Diagram of anaerobic respiration shows glucose converting to pyruvate, then lactate. The conversion of pyruvate to lactate involves the oxidation of NADH back to NAD and the donation of hydrogen to pyruvate.
During lactate fermentation pyruvate is reduced to lactate when it accepts hydrogen from NADH

Examiner Tips and Tricks

A common misconception is that the ethanol and lactate fermentation produce ATP; this is not the case. The purpose of fermentation is to regenerate NAD; this allows glycolysis to continue producing ATP.