Biology (Holt)

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Glucose

In lactic acid fermentation, pyruvate is converted to lactate. In alcoholic fermentation, pyruvate is broken down

to ethanol, releasing carbon dioxide, CO 2.

Lactic acid fermentation Alcoholic fermentation

Glycolysis Pyruvate Glycolysis

NAD+ NADH + H+ NAD+ NADH + H+

Glucose Pyruvate

CO 2

Lactate Ethanol compound2-carbon

Fermentation in the Absence

of Oxygen

What happens when there is not enough oxygen for aerobic respi-

ration to occur? The electron transport chain does not function

because oxygen is not available to serve as the final electron accep-

tor. Electrons are not transferred from NADH, and NADtherefore

cannot be recycled. When oxygen is not present, NADis recycled in

another way. Under anaerobic conditions, electrons carried by

NADH are transferred to pyruvate produced during glycolysis. This

process recycles NADneeded to continue making ATP through gly-

colysis. The recycling of NADusing an organic hydrogen acceptor

is called .Prokaryotes carry out more than a dozen

kinds of fermentation, all using some form of organic hydrogen

acceptor to recycle NAD. Two important types of fermentation are

lactic acid fermentation and alcoholic fermentation. Lactic acid fer-

mentation by some prokaryotes and fungi is used in the production

of foods such as yogurt and some cheeses, as shown in Figure 14.

Lactic Acid Fermentation

In some organisms, a three-carbon pyruvate is converted to a three-

carbon lactate through lactic acid fermentation, as shown in

Figure 15. Lactate is the ion of an organic acid called lactic acid. For

example, during vigorous exercise pyruvate in muscles is converted

to lactate when muscle cells must operate without enough oxygen.

Fermentation enables glycolysis to continue producing ATP in mus-

cles as long as the glucose supply lasts. Blood removes excess lactate

from muscles. Lactate can build up in muscle cells if it is not

removed quickly enough, sometimes causing muscle soreness.

fermentation

108 CHAPTER 5Photosynthesis and Cellular Respiration

Figure 14 Fermentation.

In cheese making, fungi or

prokaryotes added to milk

carry out lactic acid fermenta-

tion on some of the sugar in

the milk.

When oxygen is not present, cells recycle NAD+ through fermentation.

Figure 15 Two types of fermentation