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4.5 Anaerobic respiration in mammals and yeast
Oxidative phosphorylation, the `end' process of the electron transport chain, uses oxygen as its final electron acceptor.
This means that when oxygen is not present, the electron transport chain stops, and Krebs cycle (and the link reaction)
must also stop too. This leaves only the anaerobic process of glycolysis to produce ATP.
The electron transport chain reoxidises any reduced NAD that have gained hydrogen during glycolysis, the link reaction
and Krebs cycle, so without this process, how does the NAD which is reduced under anaerobic glycolysis get reoxidised? It
has to be reoxidised to give the organism any chance of surviving, otherwise they will run out of NAD to use for glycolysis.
Animals will use lactate fermentation and fungi such as yeast will use alcoholic fermentation.
Neither method produces any ATP, but they do reoxidise the reduced NAD, which allows it to be used once more for
glycolysis, which in itself has a net yield of 2 ATP (although this is not a great deal of ATP).
Anaerobic respiration is just a modified version of
glycolysis. Lactate fermentation occurs in mammalian glucose
muscle tissue, during times of vigorous activity when
ATP demand is high (for muscle contraction), and
there is an oxygen deficit. ADP + Pi
This pathway begins with glycolysis as
normal, and the pyruvate molecules
end up being the hydrogen acceptors
to reoxidise the reduced NAD, 2ADP + 2Pi 2ADP + 2Pi
forming lactate which is carried away
NAD 2ATP NAD 2ATP
from the muscle tissue by the blood,
to the liver. When oxygen becomes NADH2 NADH2
later available again, the lactate may pyruvate pyruvate
be converted back to pyruvate (so CH3COCOOH CH 3COCOOH
that it can be aerobically respired and
go through Krebs cycle). However, if lactate NADH2 lactate NADH2
not, a build up of lactate can result in dehydrogenase dehydrogenase
a decrease of the pH of muscles, so
enzyme activity is reduced (it is not CH3CHOHCOOH
the lactate build up directly which
causes muscular fatigue, it is this pH reduction). The enzyme which catalyses the oxidation of NADH2 and the reduction of
pyruvate to lactate is called lactate dehydrogenase.
Under anaerobic conditions in yeast cells, each pyruvate molecule is decarboxylated (loses one carbon dioxide molecule),
becoming ethanal. This reaction is catalysed by the enzyme pyruvate decarboxylase. The ethanol will begin to accept the
hydrogen atoms from reduced NAD, reoxidising the NAD, but reducing the ethanal to ethanol, a reaction catalysed by the
enzyme ethanol dehydrogenase. This reoxidised NAD can therefore be used once more to produce ATP in glycolysis.
CO2 NADH2 NAD
pyruvate ethanal ethanol
CH3COCOOH pyruvate CH3CHO ethanol CH3CH2OH
Although yeast is able to survive in anaerobic conditions (without oxygen), it dies if the ethanol concentration builds up to
around 15% as ethanol is a poisonous substance.