22. When no oxygen
is available for aerobic respiration, the electron transport chain begins to
shut down. This occurs because there is no longer any oxygen molecules
to be reduced by the "spent" electrons and hydrogen ions created by
the electron tranport chain. As the electron transport chain shuts down, this
in turn causes the Krebs cycle to shut down. At this point, any pyruvate that
is created has no place to go other than to proceed through the fermentation
steps of anaerobic respiration.
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Slichter