In his studies of glucose metabolism by yeast, Louis Pasteurdiscovered that the

ID: 4255 • Letter: I

Question

In his studies of glucose metabolism by yeast, Louis Pasteurdiscovered that the
sudden addition of oxygen (O2) to a previously anaerobic suspensionof yeast in
grape juice resulted in a dramatic decrease in the rate of glucoseconsumption. This
“Pasteur effect” can be counteracted by the addition of2,4-dinitrophenol (DNP), an
uncoupler of the mitochondrial electron transport chain. (a) Why doyeast cells
consume less glucose in the presence of oxygen? Can you estimatehow much less
glucose they use in the presence of oxygen? (b) Why does DNPcounteract or
prevent the Pasteur effect?

Explanation / Answer

[A] Yeast cells have 2 ways of producing energy one is anaerobic and the other is aerobic In the anaerobic type, there is no oxygen/less oxygen,pyruvate from glucose is converted to ethanol and CO2. This gives only 2 ATP. But in the aerobic type, the concentration of glucose is more.So the pyruvate gets converted to acetyl coA which goes into theKrebs cycle. After entering the Krebs cycle and the ETC, the totalenergy obtained per glucose molecule is 36 ATP. So under aerobic conditions, the yeast will think of producingmore energy rather than through the process of fermentation whichgives only 2 ATP. So under aerobic conditions, the ATP production increases to36ATP. This ATP acts as an allosteric inhibitor of the glycolyticpathway. This reduces the rate of glucose breakdown. Therefore yeast cells consume less glucose in the presence ofoxygen. Aerobically one glucose produces 36 ATP. anaerobically one glucose produces 2 ATP. So to get the 36 ATP 18 molecules of glucose are broken downanaerobically Therefore they use 18 times more glucose in the absenceof oygen[fermentation] The amount of less glucose consumed in the presence of oxygenwould be 18 times less than under anaerobic conditions [B] DNP acts as an uncoupler of the ETC. It acts as an agent that can transport protons acrossbiological membranes. This transport is usually done by electroncarriers in the ETC with the generation of ATP. But in the presence of DNP, no ATP is produced. The energy gained is lost as heat. So when the yeast does not get energy, it breaks down moreglucose and this results in the spurt of glucose breakdown.

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