Why does yeast breath anaerobically

Experiments on alcoholic fermentation

Pasteur effect ... a nice experiment on fermentation, breathing and brewing beer :-)

The Pasteur effect represents an exciting experimental application of your knowledge from breathing and fermentation!

Here is a picture of an experiment with yeast cells. The values ​​in the table were taken from Schlegel -> Textbook for Microbiology from Thieme Verlag in a modified form (see literature; 1992 edition).

Energy generation: Many roads lead to ATP

Energy can be obtained through fermentation, of course also through the processes of the respiratory chain. There are organisms that can do both. The baker's / brewer's yeast (Saccharomyces cerevisiae) can this.

Material and question

The yeast ferments intensively under anaerobic conditions, but hardly grows. With aeration, fermentation declines in favor of breathing. There are yeasts in which fermentation can be almost completely suppressed by vigorous aeration (Pasteur effect).

a.) What does “facultative anaerobic” stand for?

b.) Compare the results when the yeasts are aerated or kept without oxygen. Why do the yeast cells need much less glucose in the first case? Compare the amount of energy produced in cases 1 and 2 and discuss the use of the broken down glucose.

c.) 2,4-Dinitrophenol (2,4-DNP) is a decoupler of the respiratory chain. What happens when using decouplers? How does the yeast react to the decoupler, how would we humans react to it?




aerobic + 2,4-DNP

Glucose consumption




Degradation by












Glucose breakdown








Numbers in relative units

Proposed solution

a.) “Facultative anaerobic” describes the ability to carry out both breathing and fermentation. We humans have a “fermentation path” from pyruvate to lactate, but unfortunately we cannot use it to react to a lack of oxygen in the long term.

b.) Respiration and fermentation differ in the presence / absence of oxygen. But that's not all: During fermentation (whether for ethanol like yeast or like humans in their skeletal muscles to lactate), 2 moles of ATP (from glycolysis) are produced by breaking down 1 mole of glucose. During the oxidative breakdown of glucose via glycolysis, the citric acid cycle and the respiratory chain, 38 moles of ATP are produced per mole of glucose. This is a clear difference and the main reason why less glucose is broken down overall. 1/9 of the amount of glucose is enough to generate the same amount of energy through breathing that could result from fermentation.

If you look at the table shown above, the values ​​correspond to this calculation quite well. The energy gained is used in the body to build biomass (this corresponds to assimilation). In this way, the same biomass can be built up in the presence of oxygen, although approx. 3 times less glucose is used.

c.) DNP prevents the build-up of the proton gradient across the mitochondrial membrane during the respiratory chain. This reduces the possibility of gaining ATP. Heat is released as the high-energy bonds "fizzle out".

DNP was used as a slimming aid in the last century (as it was in the 1920s), but it caused body temperature to rise slowly.

The yeast catches the "loss" of ATP through increased fermentation; we could hardly do that ... it would be practical, however.