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In 1861, Louis Pasteur observed that when yeasts grow in a sugar and protein medium, completely free of air, they ferment vigorously; and for every gram of yeast that forms, 60 to 80 grams of sugar disappeared. If the experiment is carried out in the presence of air, for one gram of yeast that forms, only 4 to 10 grams of sugar are removed. The yeasts again ferment if transferred to a sugar-containing medium absent air.
When the experiment is repeated with a protein medium, the yeasts grow only in the presence of oxygen. Pasteur concluded that the yeasts can take oxygen from air, and in the absence of air, the yeasts take oxygen from sugar.
Pasteur applied quantitative methods to his studies of fermentation and was the first to report on organisms that could live and reproduce in the absence of oxygen.
His conclusions were, however, incorrect. These different behaviors of yeasts are known today as the Pasteur effect.
a. Explain the two yeast behaviors based on modern concepts of microbial metabolism.
b. What was incorrect about Pasteur's conclusion?
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a. Based on modern concepts of microbial metabolism, the two yeast behaviors can be explained as follows:
1. Fermentation in the absence of air: In the presence of oxygen, yeasts can metabolize glucose through aerobic respiration, where glucose is broken down into carbon dioxide and water, releasing energy. However, in the absence of oxygen, yeasts switch to anaerobic respiration or fermentation. During fermentation, yeasts metabolize glucose to produce energy in the absence of oxygen. This metabolic pathway does not involve the complete break down of glucose and does not require oxygen. This is why yeasts can still ferment vigorously even when grown in a sugar medium without air.
2. Inhibition of fermentation in the presence of air: When yeasts are exposed to air or oxygen, aerobic respiration becomes possible. In this case, yeasts preferentially switch from fermentation to aerobic respiration. This is because aerobic respiration is a more efficient process, yielding more energy per unit of glucose compared to fermentation. Therefore, when given the option to respire aerobically, yeasts consume less sugar during fermentation since they can produce more energy through aerobic respiration.
b. What was incorrect about Pasteur's conclusion is his explanation of how yeasts obtain oxygen. Pasteur proposed that yeasts take oxygen from the air, but this was found to be incorrect. In reality, yeasts do not directly extract oxygen from air. Instead, yeasts can obtain oxygen through the breakdown of glucose. Even in the absence of air, yeasts can still obtain the necessary oxygen by breaking down sugar molecules.
a. The two yeast behaviors can be explained based on modern concepts of microbial metabolism as follows:
1. Fermentation in the absence of air: Yeasts have the ability to carry out fermentation, which is a metabolic process that occurs in the absence of oxygen. During fermentation, yeasts convert sugars into alcohol and carbon dioxide, releasing energy in the process. This allows yeasts to grow and reproduce even in the absence of oxygen. In the given experiment, when yeasts were grown in a sugar and protein medium without air, they underwent vigorous fermentation. This means that they were able to derive energy and sustenance solely from the sugar present in the medium, converting it into alcohol and CO2.
2. Oxidative metabolism in the presence of air: Yeasts also have the capability to carry out oxidative metabolism in the presence of oxygen. This process, also known as aerobic respiration, is more efficient in terms of energy production. In oxidative metabolism, yeasts fully oxidize the sugar molecules, breaking them down into carbon dioxide and water, and releasing a larger amount of energy compared to fermentation. When the experiment was conducted in the presence of air, the yeasts utilized this oxygen to carry out oxidative metabolism, resulting in the conversion of less sugar into alcohol and CO2.
b. What was incorrect about Pasteur's conclusion: Pasteur concluded that yeasts could obtain oxygen from sugar in the absence of air. This conclusion was inaccurate based on our modern understanding of microbial metabolism. Yeasts cannot directly extract oxygen from sugar molecules. Instead, they carry out fermentation in the absence of oxygen as a means to generate energy when oxidative metabolism is not possible. In reality, yeasts do not need oxygen to obtain energy from sugar; they can derive energy through fermentation even in the absence of air. Pasteur's conclusion that yeasts could extract oxygen from sugar was a misunderstanding of the metabolic processes involved.