Question

The Pasteur Effect

In 1861 Louis Pasteur observed that yeast, a facultative anaerobe, could grow in a sugar and protein broth without air, i.e. Oxygen. For every gram of yeast in a culture, 60-80 grams of sugar disappeared from the broth. When the yeasts are cultured with air 4-10 grams of sugar are consumed for every gram of yeast added to the culture. When the yeasts were cultured only on protein broth, only yeast culture with air grew. Below is a table that summarizes the conditions that allowed the yeast to grow.

1 g of yeast	Without oxygen	60-80 g sugar
1 g of yeast	With oxygen	4-10 g of sugar
1 g of yeast	With oxygen	Protein broth, no sugar

  

• How does this data demonstrate yeast are facultative anaerobes?
• Is the ratio of sugar required without oxygen compared to the sugar required with oxygen like the ratio of ATP produced with fermentation and with aerobic respiration?
• How did the yeast without sugar grow?
• Why did the yeast with only protein in the broth require oxygen to grow?
• Explain the Pasteur Effect.

Answer 1

1.
Facultative anaerobes are the organisms which produces ATP when oxygen is present through respiration. In the absence of oxygen they switch it to the fermentation for energy production; however, less energy is produced in fermentation than respiration. That means facultative anaerobes will grow either in the presence or absence of oxygen.
From the given data, it is clear that yeast utilizes sugar irrespective of the presence or absence of oxygen and it is expected that sugar ulization will be less in the absence of oxygen. Therefore, yeast is the facultative anaerobes.

2.

ATP produced in the aerobic respiration is more than the fermentation. In aerobic respiration from one glucose molecules 36 ATP are produced through glycolysis, pyruvate dehydrogenase complex, Kreb cycle and electron transport chain. In fermentation, only two ATP are produced through glycolysis from one glucose molecule.

3.
In the absence of glucose, yeast utilizes energy from the breakdown of fatty acids, proteins and nucleic acid.

4.

Degradation of many amino acids generates intermediates products of Kreb cycles such as oxaloacetate, succinyl COA and alpha ketoglutarate. These intermediate will produce only in the presence of oxygen. No fermentation will be possible wit the intermediates of Kreb cycle. Therefore for protein utilization, yeast requires oxygen to be present for the growth.

5.
Inhibition effect of the presence of the oxygen on the fermentation, it involves sudden change of anaerobic fermentation to aerobic respiration.