cells were tiny prokaryotes, and for
more than two billion years they were
the only life forms on earth.
These cells were anaerobic, which means
that oxygen was poisonous to them. These
cells were well adapted to living with
the early atmosphere, which was probably
a mixture of nitrogen, carbon dioxide,
and water vapor. Because they
lived in the water, they were sheltered
from the ultraviolet radiation that came
from the sun.
Some of these prokaryotes began to
capture the sun's energy using
photosynthesis. The waste products
of photosynthesis are oxygen and
water. The photosynthesizing
prokaryotes released oxygen, which
dissolved in the water. There was
iron dissolved in the water,as well, and for
millions of years the iron combined with
the oxygen and so did not escape into
the atmosphere. Geologists still
see evidence of this in the banded
iron formations, which are very
old rocks with alternating pale
and rust-colored stripes.
The blue-green cyanobacteria were
especially notable for their oxygen
production. They still thrive,
virtually unaltered, and release an
important part of the oxygen produced on
Important Environmental Change
After two billion years of prokaryote
life, the oxygen concentration in the
atmosphere reached about 2% of its
present concentration in our atmosphere
today. (Our present oxygen
concentration is about 21%, so this
works out to 0.42% in the ancient
atmosphere. We wouldn't even
notice that 0.42%: we would just die in about
5 minutes!) This wasn't very much
oxygen, but its appearance coincides
with the emergence of the eukaryotes.
This was a gigantic step forward in the
development of life on earth. Not
all eukaryotes use oxygen, but being
able to use oxygen increased the
diversity possible for eukaryotes.
Forms Responds to Change
Environmental change creates new
opportunities and presents new problems.
For the prokaryotes there was a problem:
oxygen was poisonous to many of them.
However, cell chemistry using oxygen can
be more efficient and productive.
This represented an opportunity that
some of the
eukaryotes grasped, although they had
problems of their own to solve.
Eukaryotic cells are microscopic in
size, but they have about ten times the
diameter of prokaryotes.
Back on the circle
math page we could see that when you
want to know the surface of the sphere,
you square the radius. This means
that the eukaryote cell has about
(10*10) 100 times the surface area of a
prokaryote. That sounds pretty
good! But wait a minute. To
get the volume of a sphere, you cube the
radius. That would mean that the
eukaryote had (10*10*10) a thousand
times the volume but only a hundred
times the surface area of a
prokaryote. Getting oxygen and
chemicals to all the parts inside the
cell became a very challenging
Eukaryotes are much
more complex inside than
prokaryotes. They contain intricately
folded membranes that
separate the contents of the cell and
direct the flow of materials within
it. The DNA, the instructions to
the cell of how to reproduce and
maintain itself, changed from a single
slender loop into pairs of chromosomes
that were kept together in the nucleus
of the cell. More DNA was needed
because the larger, more complex cell
needed more instructions. However,
"Today's solution becomes
tomorrow's problem" (Viau's
Law). The eukaryotes needed to
develop a way to divide up the
chromosomes when the cell divided.
This led to a complex, beautiful,
dance-like process that duplicated and
divided the chromosomes, providing each
cell with a full compliment of DNA .
This process is called mitosis.
Eukaryotes also took some prokayrotes
into their own cells. These prokaryotes
live in a symbiotic relationship
with the host cell. The organisms
making up the cell work together and
help each other. After so many
millions of years of living together,
the pieces of the cell could not survive
alone now. The prokaryotes have
become endosymbionts, life forms
living within a host organism.
They still have their own DNA, and
divide by binary fission just as they
did when they were wild and free.
Now, the interior of the
eukaryotic cell is a very comfortable
environment for them. The
eukaryotic cell protects them from
contact with oxygen, which is poisonous
to prokaryotes, and provides a stable
environment with water and the necessary
chemicals for their life processes and
reproduction. These organelles perform
tasks that help their host cell
Here is a diagram of a eukaryote cell
compared with a prokaryote cell. I
have not put in all the parts, just a
few parts to show you some of the
progress that was made when the
eukaryotes formed. Scientists have
studied the chemical activity that goes
on inside the cell; the chemistry is
interesting but complex. Let's
comparison between eukaryotes and
Header by Viau
in a lily
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