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Objectives
Explain how fermentation in muscle cells is different from cellular
respiration.
Give examples of products that depend on fermentation in
microorganisms.
Key Terms
fermentation
anaerobic
When you walk down the street, your lungs supply your cells with oxygen
at a rate that keeps pace with ATP demand. But what happens when you
sprint to catch a bus? Your leg muscles are forced to work without
enough oxygen because you are spending ATP more quickly than your
lungs and bloodstream can deliver oxygen to your muscles for cellular
respiration. Fortunately, some of your cells can produce ATP and
continue working for short periods without oxygen.
Fermentation in Human Muscle Cells
If you exercise for a certain amount of time, your muscles must
regenerate ATP. Normally, the cells can produce ATP through cellular
respiration. But when you sprint, your lungs and bloodstream can't supply
oxygen fast enough to meet your muscles' need for ATP. In such
situations, your muscle cells use another process, called fermentation,
that makes ATP without using oxygen. Cellular respiration still continues,
but it is not the main source of ATP while fermentation is occurring.
Fermentation makes ATP entirely from glycolysis, the same process that
is the first stage of cellular respiration. Note in Figure 7-21 that glycolysis
does not use oxygen.
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Wednesday, October 19, 2011 9:19:40 AM CT
Figure 7-21
When little oxygen is available in muscle cells, fermentation allows
glycolysis to continue.
As you read in Concept 7.5, glycolysis directly produces a net of two
molecules of ATP from each molecule of glucose it consumes. Remember
that glycolysis produces 4 ATP but that 2 ATP molecules are required to
power this stage, yielding a net of 2 ATP. This may not seem very
efficient compared to the 38 molecules of ATP generated during all of
cellular respiration. However, by burning enough glucose, fermentation
can regenerate enough ATP molecules for short bursts of activity such as
a sprint to catch the bus.
Fermentation in muscle cells produces a waste product called lactic acid.
The temporary buildup of lactic acid in muscle cells contributes to the
fatigue you feel during and after a long run or a set of push-ups. Your
body consumes oxygen as it converts the lactic acid back to pyruvic acid.
You restore your oxygen supply by breathing heavily for several minutes
after you stop exercising.
Fermentation in Microorganisms
Like your muscle cells, yeast (a microscopic fungus) is capable of both
cellular respiration and fermentation. When yeast cells are kept in an
anaerobic environment—an environment without oxygen—they are
forced to ferment sugar and other foods. In contrast to fermentation in
your muscle cells, fermentation in yeast produces alcohol, instead of
lactic acid, as a waste product (Figure 7-23). This fermentation reaction,
called alcoholic fermentation, also releases carbon dioxide. For thousands
of years, humans have put yeast to work producing alcoholic beverages
such as beer and wine. The carbon dioxide is what makes champagne and
beer bubbly. In another example of "taming" microbes, the carbon
dioxide bubbles from baker's yeast make bread rise.
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Wednesday, October 19, 2011 9:19:40 AM CT
Figure 7-23
Fermentation in yeast produces ethyl alcohol. The carbon dioxide that is
released during fermentation creates bubbles and pockets that make bread
rise. The alcohol evaporates during baking.
There are also fungi and bacteria that produce lactic acid during
fermentation, just as your muscle cells do. Humans use these microbes to
transform milk into cheese and yogurt. The sharpness or sour flavor of
yogurt and some cheeses is mainly due to lactic acid. Similar kinds of
microbial fermentation turn soybeans into soy sauce and cabbage into
sauerkraut.
Yeast cells and muscle cells are versatile in their ability to harvest energy
by either respiration or fermentation. In contrast, some bacteria found in
still ponds or deep in the soil are actually poisoned if they come into
contact with oxygen. These bacteria generate all of their ATP by
fermentation. If you had to do that—though you don't and you can't—you
would have to consume almost 20 times more food than normal. Oxygen
enables you to get the most energy from your food. In the next chapter,
you'll learn about the original source of this energy—photosynthesis.
Concept Check 7.6
1. How is fermentation different from cellular respiration?
2. Describe one example of how fermentation in microorganisms
produces human foods.
3. What is the waste product of fermentation in your muscle cells?
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Wednesday, October 19, 2011 9:19:40 AM CT