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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.


<p><strong>SF Fig. 2.4.</strong> 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.</p><br />

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.

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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.