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The energy-releasing process in which the substrate is oxidized without an external electron acceptor is called:
A
aerobic respiration
B
glycolysis
C
fermentation
D
photorespiration
Detailed Solution
In aerobic respiration the electrons removed from the substrate are finally passed to oxygen, which is an external electron acceptor.
Fermentation takes place in the absence of oxygen, and no external electron acceptor is used.
The $NADH + H^+$ produced in glycolysis is reoxidised to $NAD^+$ by passing its electrons to an organic molecule formed from the substrate itself: pyruvic acid (giving lactic acid) or acetaldehyde (giving ethanol and $CO_2$).
So in fermentation the substrate is only partially oxidised, and an internal organic compound acts as the electron acceptor. Only a small amount of energy (a net gain of 2 ATP per glucose) is released.
Glycolysis is only the first step, common to both aerobic respiration and fermentation, and by itself does not include the fate of the reduced coenzymes.
Photorespiration is not an energy-releasing process; it consumes energy.
So the process is fermentation.
Fermentation takes place in the absence of oxygen, and no external electron acceptor is used.
The $NADH + H^+$ produced in glycolysis is reoxidised to $NAD^+$ by passing its electrons to an organic molecule formed from the substrate itself: pyruvic acid (giving lactic acid) or acetaldehyde (giving ethanol and $CO_2$).
So in fermentation the substrate is only partially oxidised, and an internal organic compound acts as the electron acceptor. Only a small amount of energy (a net gain of 2 ATP per glucose) is released.
Glycolysis is only the first step, common to both aerobic respiration and fermentation, and by itself does not include the fate of the reduced coenzymes.
Photorespiration is not an energy-releasing process; it consumes energy.
So the process is fermentation.