arose spontaneously about 3.5 billion years ago. Conditions were a lot different and the
atmosphere contained hydrogen, methane, ammonia and water with very little oxygen. This is
just right for forming the chemicals we find in living organisms. In the 1950s, Miller and Urey
passed electrical sparks through a mixture of these gases. The sparks stimulated lightning.
Several days later they detected organic compounds.
A key point to the theory is that some aerobic bacteria resisted digestion. They actually thrived
in the protected, nutrient-rich environment. In time, they were releasing growth, increased
activity, and the assembly of hard body parts and other new structures. The guests were no
longer duplicating metabolical functions that the hosts performed for them. The anaerobic and
aerobic cells were incapable of in dependant life. The guests had become mitochondria,
supreme suppliers of ATP.
Prokaryots
These cells most likely had no organelles but because there was no oxygen the almost certainly
respired anaerobically. This process produces carbon dioxide and releases it into the
atmosphere and this levels of carbon dioxide slowly rose over the next 500 million years. This
made the levels of infra red and ultra violet radiation fall and the atmosphere cooled. This made
the rate of organic compounds fall so if it weren't for eukaryotes who can produce there own
organic compounds then the prokaryotes would most likely have died out. The eukaryotes
used light energy to convert carbon dioxide into carbohydrates.About 2.5 million years ago, as
oxygen levels increased, some prokaryotes evolved the ability to use oxygen to generate ATP
using a mechanism that is very similar to that used by mitochondria today. So, by this time there
were three types of prokaryote
* Those that could photosynthesis;
* Those that could respire aerobically;
* Those that could respire anaerobically.
Eukaryotes
The first eukaryote cells appears approximately 1.5 million years ago. The endosymbiotic
theory. The aerobic bacteria would ingest organic material and aerobic bacteria would oxidize
this to provide ATP for both organisms. The embedded aerobic bacteria also formed symbiotic
consortia with cyanobacteria. These cells were ancestors of eukaryotic plant cells.
At first this theory was ridiculed, but there was an important test. If chloroplasts and
mitochondria were prokaryotic symbionts, they would have their own DNA (more explanation
later). This turned out to be the case. The division of mitochondria and chloroplasts is not
under control of the nucleus of the cell, but is controlled by DNA in the organelles.
Arguments for the theory of endosymbiosis.
In 1960 a scientist called Lynn Margullis came up with a theory of endosybiosis. She said that if
the theory of endosymbiosis was true the mitochondria and chloroplasts would have there own
DNA because they come from different places and is unable to be produced by the other part
of the symbiotic cell. In the 1980s it was found out that these organelles do indeed have their
own DNA. Also like bacteria they have there own singular circular piece of DNA without any
proteins attached. A main argument for the theory of endosymbiosis is that we know
mitochondria and chloroplasts can only be formed from pre-existing mitochondria and
chloroplasts. They cannot be formed in a cell which lacks these organelles because nuclear
genes code for only some of the proteins of which they are made. Both mitochondria and
chloroplasts have their own protein-synthesizing machinery, which resembles that of
prokaryotes, but these are smaller than (70s) type found in bacteria, not the larger kind found in
eukaryotic cells. Strong evidence favors Margulis's theory. There are plenty of examples of
nature continuing to tinker with endosymbionts, the mitochondria are like bacteria in size and
structure. The inner mitochondrial membrane is like a bacterial plasma membrane. Each
mitochondrion replicates it's own DNA and divides independently of the host cell's division. A
few genetic code words in it's DNA and mRNA have unique meanings. They are translated
into few proteins required for specialized mitochondrial tasks. Thus, compared to the genetic
code of cells, the "mitochondrial code" have few distinct differences. Also consider
chloroplasts, which may be the stripped down descendants of oxygen-evolving, photosynthetic
bacteria. Perhaps predatory aerobic bacteria engulfed such photosynthetic cells, which escaped
digestion, absorbed nutrients from their host's cytoplasm, and continuing to function. By proving
their respiring host with oxygen, their endosymbiotic existence would have been favored. In
metabolism and overall nucleic acid sequence, chloroplasts resemble some eubacteria. Their
DNA is self replicating, and they divide independently of the cell's division. Chloroplasts vary in
shape and their array of light-absorbing pigments, just as various photosynthetic eubacteria
do.They may have originated a number of times, in a number of different lineages.
One early and important discovery in support of the SET occurred in the laboratory of Kwang
W. Jeon, a biologist at the University of Tennessee. Jeon witnessed the establishment of an
amoeba-bacteria symbiosis in which new bacterial symbionts became integrated in the host
amoeba (Jeon 1991). In 1966, when the bacteria first infected the amoebas, they were lethal to
their hosts. However, as time progressed, some of the infected amoebas survived and became
dependent on their newly acquired endosymbionts within a few years. Jeon was able to prove
this dependency by performing nuclei transplants between infected amoebas and amoebas
lacking the bacteria. If left alone, the hybrid amoebas died in a matter of days. Yet if he re
infected these hybrids with the once-lethal bacteria, the amoebas recovered and once again
began to grow (Margulis and Sagan 1987). This discovery served to demonstrate that
endosymbiosis could provide a major mechanism for cellular evolution and explain the
introduction of new species (Jeon 1991). Recent research by Martin and Müller (1998) into
the origin of the mitochondrion has led to a new theory of endosymbiosis called the "hydrogen
hypothesis." In the current picture of the origin of the eukaryotic cell, the mitochondrion was a
"lucky accident" (Vogel 1998). The ancestral host cell simply engulfed the mitochondrion
ancestor, did not fully ingest it, and an even more successful cell resulted. According to the
hydrogen hypothesis, however, the first eukaryotic cell did not form simply by accident. Instead,
it was the result of a purposeful union between an archaebacterial host cell, a methanogen that
consumed hydrogen and carbon dioxide to produce methane, and a future mitochondrion
symbiont that made hydrogen and carbon dioxide as waste products of anaerobic metabolism.
Thus, although the symbiont was probably capable of aerobic respiration, the symbiosis began
as a result of the products of anaerobic metabolism. The host’s dependence upon hydrogen
produced by the symbiont is identified as the selective principle that consolidated the common
ancestor of eukaryotic cells (Martin and Müller 1998).
Arguments against the theory of endosymbiosis.
Of course there will be alternative hypothesizes because we can't ever really know if this theory
is the because we weren't there at the time the symbiotic arrangement occured. There are other
theories to explain the evolution of eukaryotic cells. One argument is that mitochondria and
chloroplasts can produce only a few proteins. Most of the enzymes found in the organelles are
coded for by the nuclear DNA. Supporters of the endosymbiotic theory suggest that some
organelle DNA "jumped" to the nuclear DNA during evolution. This is why neither chloroplasts
or mitochondria are able to live independently of the eukaryotic cell. Some people belive that
mitochondria and chloroplasts developed by some kind of "pinching off" of the cell membrane.
Others belive that some early bacteria developed a "casing" around them selves, which is what
we now call a eukaryotic cell.
Conclusion
I conclude that the theory of endosybiosis has enough evidence to show that it is true. Although
we can't be 100 percent sure that this is the was it occured. I think that Lynne Margullis' theory
is pretty conclusive as the cell cant reproduce a different set of DNA also the mitochondria and
chloroplasts cannot be reproduced unless its by pre existing mitochondria and chloroplasts.
This shows that the cell cannot produce these organelles making it pretty safe to assume that
these two cells are symbiotic. So the strength of this evidence leads me to belive that the theory
is true but I'm not going to say that is for sure the case as there is no solid evidence that proves
this to be the case.
Bibliography
http//www.geocities.com/jjmohn/endosymbiosis.htm
Eukaryotic and prokaryotic cells sheet.
Biology The unity and diversity of life. Starr and Taggart, 1998
By Michael Finnigan.