The theory of endosymbiosis

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The theory of endosymbiosis

(Endo Within, Symbiosis Living together! )

I am going to analyze the theory of endosymbiosis over the next few pages.  I am going to give

arguments for and against so I can make a reliable conclusion.  First of all I am going to give

some information about the theory of endosymbiosis.  According to the theory of

endosymbiosis, billions of years ago mitochondria and chloroplasts were free-living bacteria

(prokaryotes) which somehow became part of an early cell.  The primitive Earth did not have

oxygen in its atmosphere so these early cells must have been able to survive with out oxygen.  

However, oxygen gradually oxygen built up in the atmosphere and it is possible that some

bacteria evolved which were able to use oxygen for respiration (aerobic bacteria).  According

to the theory, an aerobic bacterium became engulfed by an anarobic amoeba-like bacterium,

and the amoeba-like bacterium navigated through the newly oxygen rich waters in search of

food.  In support of this theory of endosymbiosis, scientists have shown that oxygen began to

accumulate between the first fossil records of prokaryotes and the later fossil records of

eukaryotes.

Anaerobic amoeba-like         Anerobic bacterium                      Aerobic bacterium

bacterium.                           engulfs aerobic bacterium.          becomes symbiotic inside

                                                                                            the anarobic bacterium.

When the original anerobic bacterium reproduced, so did the aerobic bacterium inside it, so all

offspring were an association of the two bacteria.  Over time, the two bacteria developed

mutually beneficial existence and eventually both partners lost their ability to function without

each other.  A relationship like this, in which both partners benefit, is called symbiosis.  

According to the endosymbiotic theory, this is how mitochondria evolved.  It is thought that

chloroplasts evolved in a similar way, When a primitive cell engulfed a photosynthetic bacterium.

We can never be sure about how life began on Earth but many scientists belive that the first cells

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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 ...

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