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ESSAY SAMPLE ON "A LOOK AT CHANGES IN THE ATMOSPHERE, EUKAROTES AND MULTICELLULARITY" |
About 2.5 billion years ago, oxygen began slowly to accumulate in the atmosphere, as a
result of the photosynthetic activity of the cyanobacteria. Those prokaryotes that were
able to use oxygen in ATP production gained a strong advantage, and so they began to
prosper and increase. Some of these cells may have evolved into modern forms of aerobic
bacteria. Other cells may have become symbionts with larger cells and evolved into
mitochondria. As the amount of oxygen and other atmospheric gasses increased, they
started blocking out deadly u.v. rays from the sun. The sun's rays made life outside of
water nearly impossible. These changes made life on land possible and evolution occurred
as prokaryotes gave rise to land living eukaryotes.
The microfossil record indicates that the first eukaryotes evolved at least 1.5
billion years ago. Eukaryotes are distinguished from prokaryotes by their larger size,
the separation of nucleus from cytoplasm by a nuclear envelope, the association of DNA
with histone proteins and its organization into a number off distinct chromosomes, and
complex organelles, among which are chloroplasts and mitochondria. Scientists believe
that eukaryotic organisms such as the protists evolved from the prokaryotes. There are
two main theories which describe how this transition may have occurred. The first is the
endosymbiotic theory, or enosymbiosis, and the other is the autogenous theory, or
autogenisis. These two theories are not mutually exclusive, meaning one or the other
could account for different parts of eukaryotic cells. The endosymbiotic theory states
that the formation of eukaryotic cells were symbiotic associations of prokaryotic cells
living inside larger prokaryotes. The endosymbiotic hypothesis accou
nts for the presence in eukaryotic cells of complex organelles not found in the far
simpler prokaryotes. Many modern organisms contain intracellular symbiotic bacteria,
cyanobacteria, or photosynthetic protists, indicating that such associations are not
difficult to establish and maintain. Endosymbiosis is said to be responsible for the
presence of chloroplasts and mitochondria in eukaryotes. Autogenisis, the alternative to
the endosymbiotic theory is specialization of internal membranes derived originally from
the plasma membrane of a prokaryote. Autogenisis could be responsible for structures
like the nuclear membrane and endoplasmic reticulum in eukaryotes.
There are two scenarios for which multicellularity may have occurred. The first is
unicellular organisms came together to form a colonial organism, then some tissue
developed specialized functions and the cells became differentiated, forming a
multicellular organism. The other scenario starts with a coencytic organism forming
cellulorization with individual cells developing membranes, then tissues became more
specialized forming a multicellular organism. There are some advantages of
multicellularity such as having specialization of cells which creates a division of
labor, leading to greater efficiency. Another advantage is multicellular organisms have
a larger size which provides protection from predators. Fungi are large and have a large
surface area to volume ratio, allowing them to absorb nutrients more efficiently.
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