5. Yeast is used as a leavening agent in making bread. Explain, in biochemical t

Question

5. Yeast is used as a leavening agent in making bread. Explain, in biochemical terms, why the bread dough rises when placed in a warm place. 6. Yeast cells grown on nonfermentable substrates which are then abruptly switched to glucose exhibit substrate-induced inactivation of several enzymes. Which enzymes would glucose cause to be inactivated, and why? 7. During anaerobic fermentation, the majority of the available glucose is oxidized via the glycolytic pathway and the rest enters the pentose phosphate pathway to generate NADPH and ribose. This occurs during aerobic respiration as well, except that the percentage of glucose entering the pentose phosphate pathway is much grunter in aerobic respiration than during anaerobic fermentation. Explain why.

Explanation / Answer

5. Yeast cells mixed with dough start to feed on sugars, such as sucrose, glucose, fructose, or maltose. When placed in a warm place, yeast cells start utilizing sugars, they produce carbon dioxide and alcohol during fermentation. The carbon dioxide released in this process gets trapped in the elastic dough, causing it to rise.

7. During anaerobic fermentation, either ethanol or lactic acid is formed. In the formation of ethanol, pyruvate is converted to acetaldehyde and carbon dioxide, by the enzyme pyruvate decarboxylase. Then alcohol dehydrogenase converts acetaldehyde to ethanol.

Lactic acid fermentation is another form of anaerobic fermentation. In this, cells convert pyruvate into L-Lactate using the enzyme lactate dehydrogenase. As the process of anaerobic fermentation is used to restore NAD+ to allow cell to continue generation ATP through glycolysis, glucose entering the pentose phosphate pathway is much lesser compared to that of aerobic respiration.

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