A Comparison of Mitosis and Meiosis
efficient breakdown with less ATP produced. We will now
look at these two anaerobic processes.
Fermentation
Fermentation is the process by which yeast breaks down
glucose anaerobically (in the absence of oxygen). The final
products of fermentation are carbon dioxide gas (CO 2 ),
ethyl alcohol (CH 3 CH 2 OH), and ATP. In yeast cells,
glucose breaks down, as in glycolysis, to produce two
molecules of pyruvic acid, a net gain of two ATP and two
NADH 2. However, because oxygen is not used, the pyru-
vic acid molecules do not proceed to the citric acid cycle.
Instead a yeast enzyme called a decarboxylase breaks down
the pyruvic acid to CO 2 and a C 2 compound,
acetaldehyde- (ass-et-AL-deh-hyde) (CH 3 CHO). It is the
CO 2 gas that causes bread to rise and is the reason we add
yeast to our flour (glucose), water, and eggs (which makes
dough) when we bake bread. Because this pro-cess occurs
without oxygen, the NADH 2 does not give its electrons to
oxygen through the electron transport system as it does in
aerobic respiration. Instead the NADH 2 donates its two
hydrogen atoms to the acetalde-hyde through the action of
another yeast enzyme called an alcoholic dehydrogenase.
This reaction regenerates the NAD and forms the final
product ethyl alcohol. This product is what is produced in
the beer, wine, and liquor industries to convert the sugars in
grapes and the sugars in grains to alcohol.
In conclusion, the fermentation process produces only
two ATP per glucose molecule. Obviously, this -energy-
capturing mechanism is much less efficient than aerobic
respiration.
Anaerobic Production of ATP by Muscles
The second situation that can occur in anaerobic res-
piration is the breakdown of glucose in human muscle cells
when not enough oxygen becomes available due to muscle
fatigue such as when an athlete sprints. Again, this process
starts with glycolysis. However, the pyruvic acid formed
undergoes a different fate. Again glycolysis yields two
pyruvic acid molecules, a net gain of two ATP molecules,
and two NADH 2 per glucose molecule. As it was in
fermentation, the two NADH 2 cannot donate their electrons
to oxygen. Instead the NADH 2 donates them to pyruvic
acid to form lactic (LAK-tik) acid. It is the ac-cumulation
of lactic acid that causes the momentary fatigue- in muscles
that are overexercised. When muscles are overworked, the
muscle cells need to produce extra energy in the form of
ATP. Aerobic respiration produces
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much of this energy. However, if the muscle is worked
more rapidly than oxygen (O 2 ) can be supplied to it from
the bloodstream, the muscle cells will begin to produce the
ATP anaerobically and lactic acid accumulates. Once
oxygen gets to the muscle, the fatigue diminishes as -lactic
acid is broken down.
When we overexercise and our muscles get sore and
we experience muscle fatigue, we notice that our heart-beat
and breathing rates are accelerated. We sit down, breathe
faster (to get more O 2 into our bodies), and the fatigue
slowly diminishes. When O 2 again becomes avail-able, the
lactic acid is converted back to pyruvic acid and aerobic
respiration proceeds as normal. We note that anaerobic
formation of ATP by muscles is much less -efficient than
aerobic respiration. Only two molecules of ATP are
produced per glucose molecule.Production of ATP from General
Food Compounds
Obviously, we do not only eat glucose. So where do the
other food compounds in our diet fit into the res-piration
cycle to produce ATP? If we think of the steps in
biochemical- respiration as parts of a very efficient cellular-
furnace where fuel (food) is converted to another- form of
chemical energy, ATP, then we can grasp a better
understanding of how other food molecules are burned to
produce ATP (Figure 4 - 4).
Glucose is a simple carbohydrate. Other carbohy-
drates such as starch (plant carbohydrate) and glycogen
(animal starch) as well as other types of sugars such as
monosaccharides and disaccharides fit into the cellular
furnace at the level where glucose enters the glycolytic
sequence. If after digestion the food molecules are not
needed immediately, they can be stored in the body (in
food vacuoles or the liver, or converted to fat cells) until
needed later to produce more ATP.
Digestion decomposes fat into fatty acids and glycerol.
They, too, will enter the cellular furnace at a stage related
to their chemical structure. Glycerol, a C 3 molecule, is
similar to PGA and will enter at the PGA stage of
glycolysis. Fatty acids enter the Krebs citric acid -cycle.
Proteins are broken down by digestion into amino acids-.
Again, they will enter the cellular furnace at a level -related
to their chemical structure. Alanine, a C 3 amino acid, and
lactic acid enter at the pyruvic acid stage. -Glutamic acid, a
C 5 amino acid, is similar to alpha--ketogluteric acid.
Aspartic acid, a C 4 amino acid, -resembles oxaloacetic
acid. These amino acids enter into the citric acid cycle at
different stages. So when you put