The Endosymbiotic Theory

                                              The theory of Endosymbiosis explains the origin of
                                      chloroplasts and mitochondria and their double membranes.

      This concept postulates that chloroplasts and mitochondria
             are the result of years of evolution initiated by the
               endocytosis of bacteria and blue-green algae.
      According to this theory, blue green algae and bacteria
          were not digested; they became symbiotic instead.  

             Endocytosis is when a substance gains entry
        into a cell without passing through its cell membrane.
                  A cell's plasma membrane invaginates
               and fuses to lock foreign material inside.
                An intracellular vesicle is formed as a result.

                           The ribosomes of the inner structure (in chloroplasts and  mitochondria)
                                                     resemble prokaryotic ribosomes.

                     There are two               One cell "engulfs"          A double membrane                     
                     prokaryotic cells             the other cell               can be found inside

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