Biology (Holt)

Alcoholic Fermentation
In other organisms, the three-carbon pyruvate is broken down to
ethanol (ethyl alcohol), a two-carbon compound, through alcoholic fer-
mentation. Carbon dioxide is released during the process. As shown in
Figure 15, alcoholic fermentation is a two-step process. First, pyruvate
is converted to a two-carbon compound, releasing carbon dioxide.
Second, electrons are transferred from a molecule of NADH to the two-
carbon compound, producing ethanol. As in lactic acid fermentation,
NADis recycled, and glycolysis can continue to produce ATP.
Alcoholic fermentation by yeast, a fungus, has been used in the
preparation of many foods and beverages. Wine and beer contain
ethanol made during alcoholic fermentation by yeast. Carbon
dioxide released by the yeast causes the rising of bread dough and
the carbonation of some alcoholic beverages, such as beer. Ethanol
is actually toxic to yeast. At a concentration of about 12 percent
ethanol kills yeast. Thus, naturally fermented wine contains about
12 percent ethanol.

SECTION 3 Cellular Respiration 109

http://www.scilinks.org

Topic:Fermentation

Keyword:HX4080

Muscle Fatigue and Endurance Training

nyone who runs or exercises

for a long period of time

soon learns about muscle fatigue.

As you continue vigorous exer-

cise, the muscles you are using

become fatigued—that is, tired

and less able to generate force.

The reasons for muscle fatigue are

not fully understood, but in most

cases the fatigue increases when

the production of lactic acid by

the exercising muscle increases.

Anaerobic Threshold

Why does an exercising muscle

produce lactic acid? A resting

muscle obtains most of its energy

from aerobic respiration. A con-

tinuously exercising muscle, how-

ever, soon depletes its available

oxygen. At this point, called the

anaerobic threshold, the exercis-

ing muscle begins to obtain the

ATP needed anaerobically. In the

absence of oxygen, glycolysis

extracts the required ATP from

glycogen in the muscle. Glycogen

is a storable form of glucose

that acts as an energy reserve.

Glycolysis converts the muscle

glycogen to pyruvate, which is

then fermented to lactic acid.

The ability to perform con-

tinuous exercise is limited by

the body’s stored glycogen. So,

physical endurance can increase

if glycogen stored in muscles is

spared during exercise. Trained

athletes such as cyclist Lance

Armstrong, shown at right, get a

relatively large portion of their

energy from aerobic respiration.

Thus, their muscle glycogen

reserve is depleted more slowly

than that in untrained individuals.

In fact, the greater the level of

physical training, the higher the

proportion of energy the body

derives from aerobic respiration.

Athletic Endurance

Endurance-trained athletes gen-

erally have more muscle mass

than untrained people. But it is

endurance-trained athletes’ high

aerobic capacity—rather than

their greater muscle mass—that

allows these athletes to exercise

more before lactic acid produc-

tion and glycogen depletion

cause muscle fatigue.

http://www.scilinks.org

Topic:Anaerobic Threshold

Keyword:HX4192

A

Lance Armstrong