Anaerobic respiration – Ethanol and Lactate pathways

Anaerobic respiration is a type of respiration
that does not use oxygen. It is used when there
is not enough oxygen for aerobic respiration.
In the absence of free oxygen:
 Oxidative phosphorylation cannot take
place, as there is nothing to accept the electrons
and protons at the end of the electron transport
 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 mitochondrion quickly runs out of NAD
or FAD that can accept hydrogens from the
Krebs cycle reactions. The Krebs cycle and the
link reaction therefore come to a halt.
 Glycolysis, however, can still continue, so long as the pyruvate produced
at the end of it can be removed and the reduced NAD can be converted back to
Two other pathways allow the recycling of reduced NAD formed during

The Cori cycle serves two purposes:
• it ‘rescues’ lactate and prevents the wasteful loss of some of its chemical bond
• it prevents a potentially disastrous fall in plasma pH.
The lactate that is produced in muscles diffuses into the blood and is carried in
solution in the blood plasma to the liver. Here, liver cells convert it back to
pyruvate. This requires oxygen, so extra oxygen is required after exercise has
finished. The extra oxygen is known as the oxygen debt.

Later, when the exercise has finished and oxygen is available again, some of the
pyruvate in the liver cells is oxidised through the link reaction, the Krebs cycle
and the electron transport chain. Some of the pyruvate is reconverted to glucose in the liver cells. The glucose may be released into the blood or converted to
glycogen and stored.

• Both reactions ‘buy time’ by providing hydrogen acceptors so that NAD is
released and glycolyis can continue.
• Both pathways are inefficient and wasteful in that the products (ethanol or
lactate) have chemical bond energy that is untapped.
• The ethanol or lactate produced is toxic and restricts the use of the pathways.
• While the lactate pathway is reversible (by the Cori cycle) in the mammalian
liver, the ethanol pathway is irreversible.
• There is a net gain of only two ATP molecules per glucose molecule (from
glycolysis) during anaerobic respiration.







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