Methodology: What does it say?

Dr. Margulis was doing reserarch on the origin of eukaryotic cells. She looked at all the data about prokaryotes, eukaryotes, and organelles. She proposed that the similarities between prokaryotes and organelles, together with their appearance in the fossil record, could best be explained by "endo-symbiosis".

[Endo = "within"] [Endocytosis = (cyto = cell) a process of 'cell eating' - cells are engulfed, but then usually digested as food....] [Endosymbiosis = cells are engulfed, but not digested...cells live together is a mutually benefitting relationship, or symbiosis]

Her hypothesis originally proposed that:

• mitochondria are the result of endocytosis of aerobic bacteria

• chloroplasts are the result of endocytosis of photosynthetic bacteria

• in both cases by large anaerobic bacteria who would not otherwise be able to exist in an aerobic environment.

• this arrangement became a mutually beneficial relationship for both cells (symbiotic).

Margulis' original hypothesis proposed that aerobic bacteria (that require oxygen) were ingested by anaerobic bacteria (poisoned by oxygen), and may each have had a survival advantage as long as they continued their partnership.

Therefore, Endosymbiotic Theory proposed that mitochondria developed from proteobacteria, or Rickettsia, (respiring Bacteria) and that chloroplasts were originally from cyanobacteria. Anaerobic bacteria engulfed the aerobic bacteria, which was not completely digested. They formed a symbiotic relationship, and both cells benefited from each other mutually. The anaerobic bacteria procured food for the aerobic bacteria and provided it with a safe shelter. The engulfed bacteria was able to provide a new aerobic method for converting oxygen that was toxic to the anaerobic bacteria into ATP. Aerobic bacteria eventually became mitochondria, which is supported by the fact that mitochrondria have their own cellular mechanisms and DNA. In addition to these cells, the heterotrophic host cell also engulfed photosynthetic cyanobacteria, which were able to use photosynthesis to convert sunlight into ATP. This additional trait was beneficial for the newly formed eukaryotic cell. This symbiont cell became the chloroplast.

Over numerous years, mitochondria and chloroplast became gradually more specialized to such an extent that they now cannot survive outside of the cell. These cells had a greater advantage over other cells, and, through natural selection, became more prevalent than other types of cells. Eukaryotes with only mitochondria became animal and fungi cells, whereas those with both mitochondria and chloroplasts became plants cells.

Each would have performed mutually benefiting functions from their symbiotic relationship. The aerobic bacteria would have handled the toxic oxygen for the anaerobic bacteria, and the anaerobic bacteria would ingested food and protected the aerobic "symbiote".

The result = a cell with a double-membrane bound organelle. The inner lipid bilayer would have been the bacterial cell's plasma membrane, and the ouler lipid bilayer came from the cell that engulfed it.