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Weird Science: Serial Endosymbiosis

The prokaryotic cells that live inside eukaryotic cells are called endosymbionts. Endosymbiosis is a term used to describe two organisms living together with one inside the other. The word endosymbiont comes from two Greek root words: endo, meaning within, and symbios meaning, living together. Symbios itself comes from syn, meaning with, and bios, meaning life.


SF Fig. 2.4. The process of serial endosymbiosis is explained in the diagram above. Mitochondria and chloroplasts were likely created when a eukaryotic cell engulfed smaller, prokaryotic cell, which then became an organelle.

Image courtesy of Kelvinsong, Wikimedia Commons

Scientists speculate that chloroplasts and algae arose through a series of endosymbiotic events. The first endosymbiotic event occurred when a eukaryotic cell engulfed a prokaryote (SF Fig. 2.4 Step 3). This process, known as primary endosymbiosis, created the mitochondrion.


Chloroplasts likely evolved when a eukaryotic cell containing mitochondria engulfed a photosynthetic cyanobacteria cell (SF Fig. 2.4 Part 5). This is also called primary endosymbiosis. Chloroplasts that evolved from primary endosymbiosis have two sets of cell membranes surrounding them: one from the host cell and one from the endosymbiont. The chloroplasts from green and red algae are derived from primary endosymbiosis.


Secondary endosymbiosis occurs when a eukaryotic cell engulfs a cell that has already undergone primary endosymbiosis. They have more than two sets of membranes surrounding the chloroplasts. The chloroplasts of brown algae are derived from a secondary endosymbiotic event.

Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.