Alcoholic Fermentation

Glycolysis

Pyruvic acid

NAD+

CO2 C

2-carbon compound figure 7-6

Some cells engage in alcoholic fermentation, converting pyruvic acid into ethyl alcohol. Again, NADH is oxidized to NAD+.

Alcoholic Fermentation

Some plant cells and unicellular organisms, such as yeast, use a process called alcoholic fermentation to convert pyruvic acid into ethyl alcohol. After glycolysis, this pathway requires two steps, which are shown in Figure 7-6. In the first step, a CO2 molecule is removed from pyruvic acid, leaving a two-carbon compound. In the second step, two hydrogen atoms are added to the two-carbon compound to form ethyl alcohol. As in lactic acid fermentation, these hydrogen atoms come from NADH and H+, regenerating NAD+ for use in glycolysis.

Alcoholic fermentation by yeast cells such as those in Figure 7-7 is the basis of the wine and beer industry. Yeasts are a type of fungi. These microorganisms cannot produce their own food. But supplied with food sources that contain sugar (such as fruits and grains), yeast cells will perform the reactions of fermentation, releasing ethyl alcohol and carbon dioxide in the process. The ethyl alcohol is the 'alcohol' in alcoholic beverages. To make table wines, the CO2 that is generated in the first step of fermentation is allowed to escape. To make sparkling wines, such as champagne, CO2 is retained within the mixture, "carbonating" the beverage.

Bread making also depends on alcoholic fermentation performed by yeast cells. In this case, the CO2 that is produced by fermentation makes the bread rise by forming bubbles inside the dough, and the ethyl alcohol evaporates during baking.

figure 7-7

The yeast Saccharomyces cerevisiae is used in alcohol production and bread making.

figure 7-7

The yeast Saccharomyces cerevisiae is used in alcohol production and bread making.

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