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Some scientists
have said that the prokaryote, a simple unicellular is the ancestor
of all life forms. The prokaryote is an organism that lacks a
nucleus and other features found in the more complex cells of all
other organisms called eukaryotes.
It is very
difficult to imagine the size of a prokaryotic cell. The size of the
cells is relatively small, each ranging from 0.0001 millimeter to
0.003 millimeter in diameter. Like all organisms, prokaryotes too
are complex molecules built from simple molecules.
Prokaryotes too,
again like other organisms, require carbon and energy to create the
molecules of life: carbohydrates, proteins, lipids and nucleic
acids. They obtain these from a variety of sources. Some types of
prokaryotes use carbon dioxide as their source for carbon and they
derive their energy from different sources. This type of prokaryotes
is called autotrophs. Among these are photoautotrophs, which obtain
energy from light (and hence the name), while chemoautotrophs draw
energy from inorganic compounds such as hydrogen sulphide, ammonia
and iron.
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Almost all types
of prokaryotic cells are surrounded by a protective cell wall. The
cell walls of archaebacteria and bacteria (An American
microbiologist, Carl Woese, proposed in the year 1990 that bacteria
may be divided into two groups, namely, archaebacteria or the
archaea and bacteria, based on their structural and physiological
differences. In some classification systems, the archeae are
considered prokaryotes.) contain forms of peptidoglycan, a
protein-sugar molecule not present in the cell walls of fungi,
plants, and certain other eukaryotes. The archaebacteria cell wall
has a more diverse chemical composition than the cell wall of
bacteria.
It is generally
considered that prokaryotes are the ancestors of all life forms.
Although scientists debate the events of early evolution, evidence
suggests that the archaebacteria, the first cells on earth, evolved
at least 3.5 billion years ago, about a billion years after the
earth was formed, possibly in waters with very high temperatures.
Researchers have
put forth that the environment of the archaebacteria lacked free
oxygen, which did not accumulate in the atmosphere or water for
another billion and a half years. Prior to and during the time
archaebacteria evolved, frequent volcanic eruptions poured mixtures
of hot gasses into the air, which eventually dissolved in the
boiling seas, constantly changing their chemical composition. As a
result, natural selection favored the evolution of diverse
metabolic pathways in the archaebacteria. The composition of water
and the atmosphere was further altered by the biochemical activity
of the archaebacteria, paving the way for evolution of bacteria.
Among the early
bacteria are the cyanobacteria or the blue green algae. Fossils of
the algae, found in ancient rock forms called stromatolites,
indicate that the cyanobacteria evolved from 2.5 to 3.4 billion
years ago.
Scientists,
through a widely held theory known as endosymbiosis, have proposed
that simple eukaryotes evolved from prokaryotes that engulfed other
prokaryotes. They have said that the engulfed prokaryotes, which
remained active in the cells, underwent changes over time and became
the energy-producing organelles of protozoa, from which animals
evolved.
The endosymbiosis
theory further holds that when photosynthetic bacteria were engulfed
by other prokaryotes, the bacteria continued to photosynthesize
within the cells that had engulfed them. The engulfed photosynthetic
bacteria evolved into the chloroplasts of photosynthetic protists,
the ancestors of plants. The bacteria-like DNA and ribosomes found
in mitochondria, the energy producing organelles of protozoa, and
chloroplasts provide evidence for this theory, they have said.
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