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Photosynthesis and aerobic respiration are opposite sides of a very
important cellular dance. Photosynthesis is the process by which plants use the
pigment chlorophyll to absorb energy from the sun and convert it into glucose.
Cellular respiration is the process animals and plants use to break down sugars in to
usable forms of energy (eHow, 2011).
Plants have a specialized organelle known as the chloroplast, and this is
where the reactions of photosynthesis take place. Chloroplasts take up the solar
energy of the sun and convert it to useable energy that takes the form of glucose.
Chlorophyll is a pigment found in these unique organelles in plants that absorbs
mostly red and blue light and is found in the chloroplast membranes.
As mentioned above, cellular respiration is another crucial process that is
basically the opposite of the system described above. Respiration uses oxygen
obtained from the air we breathe to break down the glucose molecules described
above into more readily useable energy forms that the cells of plants and animals
utilize to drive necessary functions. For humans, this process of aerobic respiration
takes place in the mitochondria, our own specialized organelle. Fortunately, each of
our cells has thousands of these powerhouses, and we use these to change the sugar
glucose into adenosine triphosphate (ATP). The process of respiration produces
carbon dioxide, water, and ATP (energy) as left over products.
Interestingly, many cells can still perform respiration even when there is no
oxygen available. Although this may seem impossible, it is actually quite vital for our
continued existence as a species. In addition, other organisms use this adapted
process as well. This amazing transformation is known as fermentation and takes
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place even in the complete absence of oxygen. Furthermore, the processes of aerobic
respiration (in the presence of oxygen) and fermentation work very differently;
nevertheless, they both produce nearly the same result. In both cases, the ideal end
product is the release of energy.
Anaerobic respiration, on the other hand, is unable to take the sugar
molecules to the mitochondria, and therefore yields energy through the cytoplasm
in the chemical process known as glycolysis. Glycolysis is the first step of the
respiration cycle in both aerobic and anaerobic respiration. During glycolysis,
glucose molecules are broken down into a substance called pyruvate. In an aerobic
process, the pyruvate is then broken down more and ultimately goes through many
more chemical reactions to generate a large amount of energy. If oxygen is not
available, the pyruvate goes through the process of fermentation, which generates a
small amount of energy along with alcohol or lactic acid (Silberstein, 2011). Even
though organisms make much more energy from the reactions processed in oxygen,
many higher species have developed a way to live for a short period of time even
when oxygen is low. Many plants are even more adept at this process. In fact, this is
where some of the more popular alcoholic drinks come from!
The fermentation of glucose, chemical formula C6H12O6., is summarized in the
chemical reaction listed below:
C6H12O6 → 2 C2H5OH + 2 CO2.
In summary, one glucose molecule is converted into two ethanol molecules and two
carbon dioxide molecules: C2H5OH is the chemical formula for ethanol. However,
before fermentation can even take place, one glucose molecule first needs to be
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broken down into two molecules of pyruvate (Purves, 2004). This process also
generates two molecules of ATP as an immediate energy yield and two molecules of
the intermediary molecule, NADH. That process is listed here:
C6H12O6 + 2 ADP + 2 Pi + 2 NAD+ → 2 CH3COCOO− + 2 ATP + 2 NADH + 2 H2O + 2H+
(Anestis 2006).
Enzymes are fantastic methods for decreasing the activation energy needed
for a chemical reaction to take place. In other words, these proteins allow the
reaction to move forward at a faster speed. One of the greatest features of these
proteins is that they are not altered or used up during the catalyzed reaction. To
explain in a more clear fashion, the enzyme works upon the interaction between the
beginning substance, or substrate, and the eventual product. The enzyme for the
reaction works in a very specific way for a particular reaction. Of course, there are
ways to alter this interaction, including molecules that interfere with the reaction,
known as inhibitors. In contrast, some molecules actually enhance the activity of
enzymes, and logically, are known as activators. Each of these reactions has three
main parts. The first is the enzyme and its substrate. This leads to the enzymesubstrate complex, and finally, the enzyme and ultimate product are produced.
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References
Anestis, M. (2006) AP Biology. 2nd Edition. McGraw-Hill Professional. p. 61
eHow (2011) How Are Photosynthesis & Aerobic Respiration Linked?
http://www.ehow.com/facts_5900543_photosynthesis-aerobic-respirationlinked_.html Last accessed July 28, 2011.
Purves, W. (2004) Life, the science of biology. 7th Edition. Macmillan
Publishers. 2004. pp. 139–140.
Silberstein, S. (2011) How Is Fermentation Different From Cellular
Respiration? http://www.ehow.com/about_6472230_fermentation-differentcellular-respiration_.html Last accessed July 28, 2011.