Fermentation
Fermentation is linked with glycolysis, so it is important to review some key
pieces of glycolysis. A net total of only two ATP molecules per molecule of
glucose are produced in glycolysis, and no more is produced during fermentation.
Out of many possible types of fermentation processes, two of the most common
types are lactic acid fermentation and ethanol (alcohol) fermentation. The various
fermentation processes are all anaerobic — none of them use oxygen.
Notice that these values are per pyruvic acid, so since there are two
molecules of pyruvic acid formed from each glucose, these numbers would be
doubled per glucose. Notice that no more ATP is formed, so the various
fermentation processes are only able to harvest 2 ATPs’-worth of energy from a
glucose molecule (as compared with 30-ish ATPs’-worth harvested via cellular
respiration, as described below) through the coupling of glycolysis and
fermentation.
Lactic acid fermentation is done by some fungi, some bacteria like
the Lactobacillus acidophilus in yogurt, and at certain times by our muscles.
Normally our muscles do cellular respiration like the rest of our bodies, using
O2 supplied via our lungs and blood. However, under greater exertion when the
oxygen supplied by the lungs and blood system can’t get there fast enough to keep
up with the muscles’ needs, our muscles can switch over and do lactic acid
fermentation. In the process of lactic acid fermentation, the 3-carbon pyruvic acid
molecules are turned into lactic acid and NADH is oxydized into NAD+. It is the
presence of lactic acid in yogurt that gives it its sour taste, lactic acid produced by
the bacteria and/or fungi in many cheeses that gives those cheeses their
characteristic flavors, and it is the presence of lactic acid in our muscles “the
morning after” that makes them so sore. Once our muscles form lactic acid, they
can’t do anything else with it, so until it is gradually washed away by the blood
stream and carried to the liver (which is able to deal with it), our over-exerted
muscles feel stiff and achy even if they haven’t been physically injured.
Note that, as mentioned, it is the hemoglobin in our red blood cells (RBCs)
that carries oxygen from the lungs to all the cells in the body so they can do
cellular respiration. However, if someone is anemic (technically, there are several
different kinds of anemia), that typically means that person doesn’t have enough
RBCs/hemoglobin to carry enough oxygen to the cells so they can perform as much
cellular respiration as is needed to harvest all the energy they require. In that case,
the muscles will more frequently rely on lactic acid fermentation to obtain at least a
little bit of energy to keep moving. Thus an anemic person may be more likely to
have stiff, achy muscles. This can be a concern for people with cancer if either
their cancer and/or their chemotherapy has supressed production of RBCs. There
are also some rare genetic disorders in which a person’s body lacks one of the
enzymes needed to do cellular respiration, and those people’s muscles will also
depend on lactic acid fermentation to obtain energy.
A related bit of information: while most textbooks focus on the role of
cellular respiration in our muscles’ ability to contract and their ability to use lactic
acid fermentation when they’re short on oxygen, little mention is made of the role
of cellular respiration in our other cells’ ability to “do their jobs” properly. One
example of another very important use for cellular respiration is that our nervous
system’s ability to function properly depends on cellular respiration. All of the
nerve cells (neurons) in our brain and the rest of our nervous system depend on
cellular respiration in their mitochondria to provide them with the energy they
need to think and to process other incoming and outgoing sensory messages
(rembember the Sodium-Potassium Pump to re-set the neuron?). One extremely
important difference between nerve cells and muscle cells is that nerve
cells cannot switch to lactic acid fermentation if oxygen is low, but rather, our
nervous system is minute-to-minute, second-to-second, totally dependent on the
oxygen delivered by the blood. That means that if someone is anemic, therefore
lacking the hemoglobin needed to transport oxygen, therefore lacking proper
oxygen levels in his/her brain, his/her brain function will be impaired. That person
will think more slowly and perhaps be unable to follow conversation occuring at a
normal speed. Also, his/her reaction time will be slowed, possibly to the point
where it might be dangerous to drive. That person may seem to do everything in
“slow motion,” and take a long time between each step in a task to “think about it”
and process information before (s)he is able to go on to the next step: for example,
opening the refrigerator door, then just standing there, “staring at it” for a couple
minutes before being able to remove the desired item(s). (Also note that while
muscles can also store some of the sugar they need for fuel, the brain cannot store
that, either, and so is also totally dependent on the sugar delivered by the blood.
Thus, someone who is both anemic and hypoglycemic could, potentially, have
significant problems with brain function.)
Alcohol fermentation is done by yeast and some kinds of bacteria. Again,
pyruvic acid is used, but, this time, the products of this process are ethanol and
carbon dioxide (CO2). Additionally, NADH is oxydized to NAD+. Humans have
long taken advantage of this process in making bread, beer, and wine. In bread
making, it is the CO2 which forms and is trapped between the gluten (a long
protein in wheat, rye, and barley) molecules that causes the bread to rise, and the
ethanol (often abbreviated as EtOH) evaporating that gives bread its smell while
baking.
On a somewhat-related note, more and more people, these days, believe
that they are gluten-intolerant, and thus, cannot eat wheat, barley, or rye. Since it is
the gluten in “regular” bread that holds in the CO2, allowing bread to rise,
manufacturers of gluten-free bread have experimented with using other, perhaps
questionable health-wise, additives to trap the CO2. Also, unfortunately, those
manufactured breads often also have relatively large amounts of simple starches
and sugars added, making them unsuitable for people who have hypoglycemia or
diabetes.
(reference: Carter, J, 1996. Retrieved from: http://biology.clc.uc.edu/courses/bio104/cellresp.htm. Edited by Flanigan, P.)
Questions
(write out and answer each question in complete sentences in your notebook)
1. What does lactic acid fermentation generate?
2. What does ethanol fermentation generate?
3. Both types of fermentation start with the same molecule and the same
electron/H carrier. Identify what the molecule is and what the electron/H
carrier is.
4. What energy process, utulized by almost all organisms on Earth, creates the
“molecule” from question #3?
5. Neither lactic acid fermentation nor ethanol fermentation, in and of
themselves, actually makes ATP. However, when coupled with another
process, fermentation and the other process work harmoniously to make
ATP. Identify this other process.
6. Fermentation is an anaerobic process that, eventually leads to the
continuous production of ATP. Explain why the fact that fermentation is
anaerobic is crucial. (Hint: think of the role of oxygen in cellular
respiration).
7. Explain all of the reasons why there are air pockets in bread.
8. Explain all of the reasons why your muscles are sore after going for a long
run.
Please do NOT steal this packet. If you do not
finish, it is posted on our class web site so you
can finish this after class on your own time.