Primary endosymbiosis is the process which involves the engulfment of a prokaryote by another living cell. The engulfed organism may be used as an advantage, supplying the larger cell with its products. For example, if a eukaryotic cell engulfs a photosynthetic alga cell, the eukaryotic cell can then use the products of the alga and become an autotrophic organism.
Either cell can be affected if one of the cells dies. If the larger cell dies the engulfed cell may leave the remains of the larger cell in order to survive. If the originally engulfed cell dies instead, the larger organism will continue to live.
Following primary endosymbiosis, the larger cell may be engulfed itself. This is the process of secondary endosymbiosis.
Scientists believe that endosymbiosis has lead to the creation and evolution of both cholroplasts and mitochondria. This theory is called the Endosymbiotic theory.
1. Primary versus Secondary Endosymbiosis. Word Press Blog 2010. <http://endosymbiotichypothesis.wordpress.com/primary-versus-secondary-endosymbiosis/>