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Cell Theory

Cell Theory tells us that all Cells come from pre-existing Cells, but where did the first Cell come from?

Wow, what a question!  It leads us to the Origins of Life, which is huge question for Scientists (eg Biologists and Astrophysicists) and Theologians, alike.  Here, I write about Endosymbiosis, a theory for the development of Eukaryotic Cells.

First, what is a Eukaryotic Cell?  Eukaryote refers to “true nucleus”, and refers to Cells that have membrane bound organelles, ie sub-cellular structures that are enclosed in membrane (a phospholipid bilayer), and which perform specific and different cellular metabolic functions.  This is in comparison to a Prokaryotic Cell, which is a Cell that does not have membrane bound organelles, ie the cytoplasm inside the Cell is not divided into specific and individual sub-cellular structures.  Bacteria, eg Escherichia coli (E. coli), are Prokaryotic Cells, and Humans are Eukaryotes, that is, our Cells are Eukaryotic in nature.

Endosymbiosis refers to one Cell living inside another Cell (endo-) in a mutually beneficial arrangement (symbiosis).  It is postulated that an ancient Cell type, called Archaebacteria, engulfed (swallowed; “ate”) a Prokaryote via phagocytosis, and the Prokaryote escaped being killed (eaten; digested), and instead resided in the larger, Archebacteria.  The Prokaryotic endosymbiont provided benefit to the Host Cell by, for example, providing improved energy production, or better reactive compound detoxification; and received from the Host Cell safety and access to food.  Over evolutionary time the two Cells became dependent on each other, and worked together as one.

It is proposed that Mitochondria and Chloroplasts, two types of organelles within Eukaryotic Cells, are the evolutionary end-product of Endosymbiosis.  Mitochondria are organelles that assist Eukaryotic Cells with Aerobic Respiration, which is producing cellular energy in the presence of Oxygen (O2).  And, Chloroplasts are organelles that carry out Photosynthesis in Plants, which is capturing light energy, from the Sun, and converting it to (bio-)chemical energy.  Why do Scientists think these organelles are the end-product of Endosymbiosis?  There is some evidence to support this theory:

  1. These organelles are surrounded by a double membrane.
  2. These organelles contain their own genetic material, which is a singular, circular chromosome.
  3. These organelles reproduce via binary fission; and control their own division (replication).

The characteristics (b) & (c) reflect Prokaryotes, and (a) reflects the process of phagocytosis.

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