Anaerobic Respiration and Respiratory Substrates

When free oxygen is not present, respiration must be anaerobic respiration.

In the absence of free oxygen, hydrogen cannot be used up by combining it with oxygen to give water, so reduced NAD cannot be recycled to NAD in this way to allow glycolysis to continue. The stages of respiration inside the mitochondrion cannot occur, but two other pathways allow the recycling of reduced NAD formed during glycolysis:

• conversion of pyruvate to ethanol in alcoholic fermentation e.g. by yeast

• conversion of pyruvate to lactate e.g. mammalian muscle.

In alcoholic fermentation, pyruvate is decarboxylated to ethanal. This acceptts hydrogen from reduced NAD and is reduced to ethanol, releasing NAD. 

In mammalian muscles that are deprived of oxygen, pyruvate itself acts as the hydrogen acceptor and is converted to lactate. Again, NAD is released.

• Both reactions 'buy time' by providing hydrogen acceptors so that NAD is released and glycolysis can continue.
• Both pathways are inefficient and wasteful in that the products have chemical bond energy that is untapped. 
• The ethanol or lactate produced is toxic and restricts the use of the…