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Types of Cellular Processes
Aerobic vs. Anaerobic
Processes
by Heather Scoville
All
living things need a continuous supply of energy to keep their cells
functioning normally and to stay healthy.
Some
organisms, called autotrophs, can produce their own energy using sunlight or
other energy sources through processes such as photosynthesis.
Others,
like humans, need to eat food in order to produce energy.
However,
that is not the type of energy cells use to function. Instead, they use a
molecule called adenosine triphosphate (ATP) to keep themselves going.
The
cells, therefore, must have a way to take the chemical energy stored in food
and transform it into the ATP they need to function.
The
process cells undergo to make this change is called cellular respiration.
Two
Types of Cellular Processes
Cellular
respiration can be aerobic (meaning "with oxygen") or anaerobic
("without oxygen").
Which
route the cells take to create the ATP depends solely on whether or not there
is enough oxygen present to undergo aerobic respiration.
If
there is not enough oxygen present for aerobic respiration, then some organisms
will resort to using anaerobic respiration or other anaerobic processes such
as fermentation.
Aerobic
Respiration
In
order to maximize the amount of ATP made in the process of cellular
respiration, oxygen must be present.
As
eukaryotic species evolved over time, they became more complex with more organs
and body parts.
It
became necessary for cells to be able to create as much ATP as possible to keep
these new adaptations running properly.
Early
Earth's atmosphere had very little oxygen. It wasn't until after autotrophs
became abundant and released large amounts of oxygen as a byproduct
of photosynthesis that aerobic respiration could evolve.
The
oxygen allowed each cell to produce many times more ATP than their ancient
ancestors that relied on anaerobic respiration. This process happens in the
cell organelle called the mitochondria.
Anaerobic
Processes
More
primitive are the processes that many organisms undergo when not enough
oxygen is present. The most commonly known anaerobic processes are known as
fermentation.
Most
anaerobic processes start out the same way as aerobic respiration, but
they stop partway through the pathway because the oxygen is not available
for it to finish the aerobic respiration process, or they join with another
molecule that is not oxygen as the final electron acceptor.
Fermentation
makes many fewer ATP and also releases byproducts of either lactic acid or
alcohol, in most cases. Anaerobic processes can happen in the mitochondria or
in the cytoplasm of the cell.
Lactic
acid fermentation is the type of anaerobic process humans undergo if there is a
shortage of oxygen.
For
example, long distance runners experience a buildup of lactic acid in their
muscles because they are not taking in enough oxygen to keep up with the demand
of energy needed for the exercise.
The
lactic acid can even cause cramping and soreness in the muscles as time goes
on.
Alcoholic
fermentation does not happen in humans. Yeast is a good example of an organism
that undergoes alcoholic fermentation.
The
same process that goes on in the mitochondria during lactic acid fermentation
also happens in alcoholic fermentation.
The
only difference is that the byproduct of alcoholic fermentation is ethyl
alcohol.
Alcoholic
fermentation is important for the beer industry. Beer makers add yeast which
will undergo alcoholic fermentation to add alcohol to the brew.
Wine
fermentation is also similar and provides the alcohol for the wine.
Which
is Better?
Aerobic
respiration is much more efficient at making ATP than anaerobic processes like
fermentation.
Without
oxygen, the Krebs Cycle and the Electron Transport Chain in
cellular respiration get backed up and will not work any longer.
This
forces the cell to undergo the much less efficient fermentation.
While
aerobic respiration can produce up to 36 ATP, the different types of
fermentation can only have a net gain of 2 ATP.
Evolution
and Respiration
It
is thought that the most ancient type of respiration is anaerobic.
Since
there was little to no oxygen present when the first eukaryotic
cells evolved through endosymbiosis, they could only undergo
anaerobic respiration or something similar to fermentation.
This
was not a problem, however, since those first cells were unicellular. Producing
only 2 ATP at a time was enough to keep the single cell running.
As
multicellular eukaryotic organisms began to appear on Earth, the larger and
more complex organisms needed to produce more energy.
Through natural
selection, organisms with more mitochondria that could undergo aerobic
respiration survived and reproduced, passing on these favorable adaptations to
their offspring.
The
more ancient versions could no longer keep up with the demand for ATP in the
more complex organism and went extinct.
Heather Scoville
Introduction
Five years working in the medical research field
High school science teacher since 2005
College Board certified to teach Advanced Placement
biology
Curriculum writer for online high school science
courses
Experience
Heather Scoville is a former writer for ThoughtCo where
she wrote about evolution and other science-related topics for more than five
years. Her writing experience also includes the creation of science curriculum
for use in online high schools. These courses include Advanced Placement
Chemistry and Biology and Physics. Her work aligns with the Next Generation
Science Standards (NGSS)
In 2005, Heather began her teaching career. She taught
science to students in grades 7th to 12th, in both private and public school
settings in Iowa. Heather teaches high school general biology, chemistry,
physics, physical science, anatomy, physiology, and earth science. Before
starting her teaching career, Heather worked in research laboratories studying
ion channels and stem cell research in breast and brain cancer.
In 2009, Heather started an Advanced Placement
biology program in her high school and became College Board certified to teach
the course in the same year.
Education
Heather Scoville obtained her Master Arts (M.A.) in
Technological Teaching and Learning from Ashford University. She also holds a
Bachelor Arts (B.A.) in Biochemistry and Molecular Biology from Cornell
University.
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