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FERMENTATION
Respiration without Air
Where does fermentation fit into cellular respiration?
When glucose enters the cell, it undergoes glycolysis – always. In the space of ten
reactions, a single glucose molecule is transformed into two molecules of pyruvate, with
a net yield of two ATP. In its most basic form, glucose can be described thus:
After a glucose molecule has become pyruvate, a cell can proceed with any of three
choices: aerobic respiration, lactic acid fermentation, or alcohol fermentation (illustrated
below, respectively).
There are three key differences between the processes of cellular respiration and those
of fermentation.
1.
No oxygen is required for fermentation.
2.
Fermentation involves no Krebs cycle or electron transport chain.
3.
The reactions of fermentation occur completely in the cytosol.
1. No oxygen is required for fermentation.
Aerobic respiration is typically the first path pyruvate will take because it is far more
efficient in terms of energy capture than the other methods (capturing (40% of the
original energy in glucose versus the 2% that fermentation captures).
However, aerobic respiration requires oxygen to move forward, as its name suggests.
When no air is present or readily available, a cell must still create ATP so it can perform
other functions of life. This is where fermentation comes in to play.
Glycolysis is the only reaction in aerobic respiration that requires no oxygen to move
forward; it requires only a constant source of NAD+ to oxidize the glucose. This
oxidation indirectly allows the creation of two ATP that occurs during glycolysis. In an
anaerobic situation, the two ATP from glycolysis are the only energy molecules a cell can
produce. Because of this, a cell wants to push as many glucose molecules through
glycolysis as possible.
This poses a problem: there needs to be a way for NAD+ to lose its electrons and
become available to oxidize glucose again, but the usual place for electron dropoff is
being blocked by lack of oxygen.
2. Fermentation involves no Krebs cycle or electron transport chain.
Key to the function of glycolysis is
NAD+, an electron transport usually used
to take electrons to the Krebs cycle. Two
NAD+ arrive at the glucose molecule and
oxidize it (removing four electrons and four
hydrogen atoms), becoming two molecules
of NADH. This is a key step in the
breakdown of glucose. Usually, NAD+ can
be reused: after dropping its electrons into
the Krebs cycle, NAD+ returns to
glycolysis to oxidize a new glucose
molecule.
Right: NAD+ usually returns to
glycolysis after dropping its electrons and
protons into the ETC.
When there is no oxygen, however, the Krebs cycle is not moving forward. The NAD+
still needs to be regenerated if the cell is to create energy. The cell has two methods of
relieving the NAD+ of its hydrogen atoms and electrons. These are the two pathways of
fermentation: alcohol fermentation and lactic acid fermentation.
In this first method, one carbon dioxide molecule is
removed from each pyruvate molecule at the end of
glycolysis.
The resulting molecules, acetaldehyde, are reduced
by NADH, accepting its hydrogens and its electrons.
The NAD+ is now available to return to glycolysis.
Below: Alcohol fermentation in simplified steps: a. Pyruvate loses a molecule of CO2 to
the cytosol; b. Pyruvate is now acetaldehyde; c. NADH oxidizes Acetaldehyde;
d. Acetaldehyde becomes ethanol.
The removal of carbon dioxide, and the oxidation of acetaldehyde, yields no extra ATP,
but the NAD+ is now available to return to glycolysis.
The second method is simpler. In lactic acid
fermentation, pyruvate is directly given the electrons
from NADH. It rearranges to create lactate, the ionized
form of lactic acid.
Below: Lactic acid fermentation in simplified steps: a. Pyruvate is oxidized by NADH;
b. Pyruvate becomes lactic acid.
The oxidation of pyruvate yields no extra ATP, but NAD+ is now available to return to
glycolysis.
3. The reactions of fermentation occur completely in the cytosol.
Because of its increased efficiency, aerobic respiration is generally the preferred path
for cells to take when they need to produce energy. However, in environments where
oxygen is scarce, and sugar is plentiful, many organisms thrive on fermentation. Yeast is
an example of such an organism: yeast live in environments devoid of oxygen but rich in
sugars that are available to go through glycolysis.
This use of fermentation, along with the fact that it occurs outside of any organelles,
has led scientists to believe that fermentation evolved as an energy creation agent long
before oxygen was present in the atmosphere.
The only form of fermentation that eukaryotic cells are capable of performing is lactic
acid fermentation. Our cells are not able to produce ethanol. A common example of lactic
acid fermentation use in eukaryotic cells is the soreness we feel after engaging after
prolonged exercise to which we are not accustomed.