Primary Endosymbiosis is the first step in the process that eventually led to the formation of mitochondria and chloroplasts as we know them today. Primary Endosymbiosis first occurred when a large anaerobic cell engulfed a smaller aerobic bacteria. This aerobic bacteria was able to use the growing amount of oxygen in the atmosphere to sustain itself and produce energy. The larger anaerobe is then able to utilize the organic products that the aerobic bacteria produced using this oxygen.
Perhaps the most important difference between primary and secondary Endosymbiosis is that in the case of primary, the engulfed endosymbiont remains relatively autonomous. If the host cell dies the aerobic bacteria can exit the cell and continue to live on its own and vis versa; the host cell can continue to function if the engulfed bacteria dies. Primary Endosymbiosis is also believed to have only occurred a relatively small number of times over the course of the Earth’s lifetime, but these few times were enough to jump start the rise of eukaryotic cells.
Secondary Endosymbiosis occurs when the host cell in primary Endosymbiosis is itself engulfed by another cell. This process is illustrated in the diagram above. A green algae, which descended from the product of primary Endosymbiosis, is engulfed by a larger heterotrophic cell. The green algae then becomes a red algae inside the host cell by losing the nucleus and mitochondria that had been present before the algae engaged in primary Endosymbiosis. The result is a double membrane bound organelle containing all the structures necessary for photosynthesis. Over time these red algae evolved to become the plastids known as chloroplasts.
These two separate phases of Endosymbiosis, both primary and secondary, explain why chloroplasts and mitochondria have two phospholipid bi-layers. The inner layer is the membrane originally containing the photosynthetic bacteria that was engulfed in primary Endosymbiosis, and the second layer is the membrane of the original host cell that was in turn engulfed by the final heterotroph. Secondary Endosymbiosis has occurred a great number of times and this has led to the rise of a great diversity of eukaryotic species. Another important difference between secondary and primary is that in secondary Endosymbiosis the new plastid and its host cell become entirely dependent on each other.
An example of this can be seen in the work of scientists Okamoto and Inouye in 2005. They observed a heterotrophic predator called Hatena that engulfs green algae. After the green algae is engulfed the Hatena loses its vital feeding apparatus and the green algae lose parts of its cytoskeleton and other important structures that are crucial to the integrity of an autonomous cell. Once the Hatena becomes a host cell is ceases predation and begins using its locomotion to move towards light in order to provide energy for the algae living inside. In this scenario neither the Hatena nor the green algae can return to autonomous living. The Hatena has lost its ability to hunt like a heterotroph and the green algae has lost its cytoskeleton and other structures. This is a perfect example of secondary Endosymbiosis because it happens relatively frequently and the two cells become truly dependent on their symbiotic relationship.