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Emily Young

Higher IB Biology

Summarise the origin of cells with reference to the endosymbiotic theory and Pasteur’s
experiments
Since the 1880’s cell theory has stated that cells can only be formed by the division of pre-existing
cells, this occurs by either meiosis or mitosis and there is a lot of evidence to support this
...

The suggestion that all living things could be traced back generation by generation to one cell seems
mind-blowing, also that the DNA in the first cell was able to replicate and the cell able to divide
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We trace our own life back to our parents and this goes back until our
origins, we have all evolved from the first original cells on earth and the continuity of life remains
...
In 2010 biologists were able to
recreate a slightly modified DNA base sequence for a bacterium, however this had to then be
transferred to a living cell
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Before biology in the natural world was at all understood, spontaneous generation was a theory of
Greek botanists and philosophers
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Later, in the 16th century it was also believed that animals such
as mice and eels being formed from water, air and decaying matter
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Francesco Redi and Lazzaro
Spallanzani were two such biologists
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Despite these experiments some
scientists still believed that life could form with only air, it was Louis Pasteur’s experiments that put
an end to this
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He created a broth from sugar and yeast, he then placed the broth in sealed flasks
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Then he used a pad of cotton to collect
bacteria and fungi spores from the air, when he added this cotton pad to the flasks microorganisms
grew on the broth surface
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He set
up several swan necked flasks with a mixture of boiled and unboiled broths within them
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What he saw was that in the unboiled
flasks microorganisms rapidly grew on the surface of the broth, yet in flasks with bent swan necks
there was no growth
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When Pasteur snapped the necks off of the flasks microorganisms did start to grow, this shows that
the swan necks prevented microorganisms in the air from reaching the broth and therefore before
nothing grew in the broth
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As a result of this experiment by
Pasteur and other evidence put forward it became universally accepted that cells were only formed

Emily Young

Higher IB Biology

from pre-existing cells for three main reasons: the cell was a highly complex structure, cell numbers
do not increase without cell division, and viruses (which are not living) can only reproduce inside
living cells
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However, as we previously acknowledged, the cell is a complex structure and it is
difficult to imagine how this could have arisen through evolution
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Firstly by creating a mixture of methane, hydrogen and
ammonia we can create an approximation of the original atmosphere of the earth, by charging these
with electricity (a simulation of lightning) it is possible that carbon and amino acids necessary for life
were formed
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Then follows
the assembly of polymers, it is suggested that a mixture of inorganic chemicals combine with the
energy from cracks in the earth’s surface in deep sea vents allowed the accumulation of carbon
compounds into polymers
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Lastly, the theory for developing the ability to replicate and inherit
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However, what has
been suggested is that RNA used to contain the genetic material in the same way DNA now does, yet
RNA is able to self-replicate using itself as a catalyst
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Another development that we have theorised over is the origin of eukaryotic cells, cells with a
nucleus, this we have explained through endosymbiosis
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Mitochondria were small free-living prokaryotes with the ability to
aerobically respire, whereas large prokaryotes could only respire anaerobically
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So the mitochondria provided energy from aerobic respiration,
whilst the large prokaryote supplied the mitochondria with food
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It is also thought that a prokaryote that evolved to be able to
photosynthesis was taken in by a larger prokaryote and today these have developed into
chloroplasts in plant cells
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Through the
experiments of the likes of Pasteur the theory of spontaneous generation was falsified and thus cell
theory came to be
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We have also tracked the development of prokaryotes to
eukaryotes through the introduction of small prokaryotes to create an endosymbiotic relationship
which benefits both organs and has allowed for the evolution of the more complex structure of
specialised cells that we have today

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