Figure 6.22 Fermentation Pathways Use Pyruvate or a Derivative As a Terminal Electron Acceptor (a) In lactic acid fermentation, pyruvate serves directly as a terminal electron acceptor, producing lactate. (b) In ethanol fermentation, pyruvate is first converted to acetaldehyde, which then serves as the terminal electron acceptor, producing ethanol.

used as a marker to aid in identification. Some end products are commercially valuable. In fact, much of chapter 32 is devoted to the fermentations used to produce certain beverages and food products. Note that organic acids produced during fermentation are traditionally referred to by the name of their undissociated form. Important end products of fermentation pathways include (figure 6.23):

■ Lactic acid. Lactic acid (the ionized form is lactate) is produced when pyruvate itself serves as the terminal electron acceptor. The growth of a group of Grampositive organisms called the lactic acid bacteria is encouraged to produce fermented food products. Their end products are instrumental in creating the flavor and texture of cheese, yogurt, pickles, cured sausages, and other foods. On the other hand, lactic acid causes tooth decay and spoilage of some foods. Some animal cells use this fermentation pathway on a temporary basis when molecular oxygen is in short supply; the accumulation of

Butyric acid Butyl alcohol Acetone Isopropyl alcohol CO2 H2

Ethyl alcohol CO2



Acetic acid Lactic acid Succinic acid Ethyl alcohol CO2 H2



Propionic acid Acetic acid CO2

Streptococcus Lactobacillus

Formic acid Ethyl alcohol Lactic acid 2,3-Butanediol

Lactic acid cO2

Figure 6.23 End Products of Fermentation Pathways Because a given type of organism uses a characteristic fermentation pathway, the end products can be used as an identifying marker. Some end products are commercially valuable.

lactic acid in muscle tissue causes the pain and fatigue sometimes associated with strenuous exercise. ■ lactic acid bacteria, p. 275 ■ cheese, yogurt and other fermented milk products, p. 804

■ pickled vegetables, p. 806 ■ fermented meat products, p. 806

■ Ethanol. Ethanol is produced in a pathway that first removes CO2 from pyruvate, producing acetaldehyde, which then serves as the terminal electron acceptor. The end products of these sequential reactions are ethanol and CO2, which are used to make wine, beer, spirits, and bread (see figures 32.4, 32.5, and 32.6). Members of Saccharomyces (yeast) and Zymomonas (bacteria) use this pathway. ■ wine, p. 807 ■ beer, p. 808 ■ distilled spirits, p. 809

■ Butyric acid. Butyric acid (the ionized form is butyrate) and a variety of other end products are produced in a complex multistep pathway used by species of Clostridium, an obligate anaerobe. Under certain conditions, some species use a variation of this pathway to produce the organic solvents butanol and acetone.

■ Propionic acid. Propionic acid (the ionized form is propionate) is generated in a multistep pathway that first removes CO2 from pyruvate, generating a compound that then serves as a terminal electron acceptor. After NADH reduces this, it is further modified to form propionate. Members of the genus Propionibacterium use this pathway; their growth is encouraged in the production of Swiss cheese. The CO2 they form makes the holes, and propionic acid gives the cheese its characteristic flavor. ■ cheese, p. 805

■ 2, 3-Butanediol. This is produced in a multistep pathway that uses two molecules of pyruvate to generate two molecules of CO2 and acetoin. The latter then serves as the terminal electron acceptor. The primary


E. coli

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