What is the Difference Between Primary and Secondary Endosymbiosis

The main difference between primary and secondary endosymbiosis is that primary endosymbiosis is the engulfing and absorbing a prokaryotic cell by a eukaryotic cell, whereas secondary endosymbiosis is the engulfing and absorbing of a eukaryotic cell by another eukaryotic cell that has already undergone primary endosymbiosis. Furthermore, primary endosymbiosis is thought to have occurred first, before secondary endosymbiosis. Moreover, in primary endosymbiosis, either cell does not depend on each other while in secondary endosymbiosis, the engulfed cell depends on the larger cell.  

In brief, primary and secondary endosymbiosis are two evolutionary theories thought to occur sequentially. They describe the emergence of certain organelles, including mitochondria and plastids, such as chloroplasts in eukaryotic cells.  

Key Areas Covered 

1. What is Primary Endosymbiosis
     – Definition, Characteristics, Significance
2. What is Secondary Endosymbiosis
     – Definition, Characteristics, Significance
3. What are the Similarities Between Primary and Secondary Endosymbiosis
     – Outline of Common Features
4. What is the Difference Between Primary and Secondary Endosymbiosis
     – Comparison of Key Differences

Key Terms 

Chloroplasts, Double Membrane Structure, Eukaryotic Cells, Mitochondria, Primary Endosymbiosis, Secondary Endosymbiosis

What is Primary Endosymbiosis 

Primary endosymbiosis is the process in which a eukaryotic cell engulfs another living prokaryote. After that, the eukaryotic cell may use that prokaryotic cell to its advantage. Therefore, when it engulfs a photosynthetic cyanobacterium, the eukaryotic cell or the larger organism uses the products of the cyanobacteria and becomes an autotrophic organism. Meanwhile, the cyanobacterium becomes a plastid inside the eukaryotic cell, further evolving into the chloroplast. Thus, the main feature of primary endosymbiosis is the coexistence of the two cells. That means; although the larger cell dies, the smaller or the prokaryotic cell may not die and may leave the remains of the old cell to survive. Although the smaller cell dies, the larger organism can stay alive. 

Figure 1: The Model of Amitochondriate Eukaryote Engulfing an Aerobe and a Cyanobacterium

Along with the secondary endosymbiosis, primary endosymbiosis is responsible for the creation and evolution of organelles in eukaryotic cells such as mitochondria and chloroplast-like plastids. Further, the resultant organelles also share genetic information with the nucleus of the eukaryotic cell. However, primary endosymbiosis is thought to occur only a few times in evolutionary history.

What is Secondary Endosymbiosis 

Secondary endosymbiosis is the process of the engulfment of a eukaryote cell by another eukaryote cell. Most importantly, the smaller cell or the cell that is subjected to engulfment here has already undergone primary endosymbiosis. Unlike, primary endosymbiosis, the secondary endosymbiosis occurred more frequently in evolutionary history. It was responsible for generating a greater genetic diversity found on the earth. Furthermore, the main characteristic feature of secondary endosymbiosis is the dependence of the smaller cell on the larger cell. Therefore, the smaller cell cannot leave the larger cell, or it cannot return into its original state. 

Figure 2: Primary and Secondary Endosymbiosis of Chloroplast

After the initial process of endosymbiosis, which is the primary endosymbiosis, the original hybrid undergoes reproduction. Then, one of its new offspring is engulfed by a second eukaryotic cell. Here, the second eukaryotic cell is non-photosynthetic and secondary endosymbiosis allows it to become photosynthetic. Similarly, primary endosymbiosis of an aerobic bacteria by an anaerobic eukaryotic cell, which is then followed by the secondary endosymbiosis results in the formation of an aerobic eukaryotic cell. Further, the evolution of the aerobic bacteria inside the eukaryotic cell forms the mitochondrium.

Figure 3: Chloroplast-Cyanobacterium Comparison

The organelles formed at the end of the secondary endosymbiosis can have three membranes. Basically, the first membrane of the organelles belongs to its ancestor. While the second membrane is formed from the first eukaryotic cell, the third membrane is formed from the second eukaryotic cell.  

Similarities Between Primary and Secondary Endosymbiosis  

• Primary and secondary endosymbiosis are two evolutionary theories which describe the emergence of certain organelles, including mitochondria and plastids such as chloroplasts in eukaryotic cells.  
• Moreover,  they describe the emergence of the double-membrane structure of eukaryotic organelles.  
• The plastid formed from the smaller cell always shares genetic information with the nucleus of the large cell. 

Difference Between Primary and Secondary Endosymbiosis 

Definition 

Primary endosymbiosis refers to the process in which a eukaryotic cell engulfs another living prokaryote. Meanwhile, secondary endosymbiosis refers to the process in which a eukaryotic cell engulfs another eukaryotic cell that has already undergone primary endosymbiosis.  

Sequence 

Moreover, primary endosymbiosis is thought to have occurred first while secondary endosymbiosis occurred later. 

Dependence of the Engulfed Cell 

In primary endosymbiosis, either cell does not depend on each other. But, in secondary endosymbiosis, the smaller cell depends on the larger cell.  

Membrane Structures

Primary endosymbiosis results in the formation of the first membrane, which belongs to the smaller cell. Meanwhile, secondary endosymbiosis results in the formation of three plasma membranes; the second membrane belongs to the first eukaryotic cell and the third membrane belongs to the second eukaryote. 

Conclusion 

Primary endosymbiosis is the engulfment of a prokaryotic cell by a eukaryotic cell to get the use of functions of the prokaryotic cell. Generally, it occurred less frequently in evolutionary history. However, it formed a plastid of the smaller cell surrounded by the first membrane. Further, a second eukaryotic cell engulfs the offspring of this eukaryotic cell, forming the second membrane and the third membrane. This occurred more frequently in the evolutionary history and allowed the second eukaryotic cell to gain the function of the prokaryotic cell. Therefore, the main difference between primary and secondary endosymbiosis is their process. 

References:

1. “Primary versus Secondary Endosymbiosis.” The Endosymbiotic Hypothesis, 2 Dec. 2010, Available Here.
2. “Primary vs. Secondary Endosymbiosis.” The Endosymbiotic Hypothesis, 3 Dec. 2012, Available Here.
3. “Plastid Evolution.” Wikipedia, Wikimedia Foundation, 30 Sept. 2019, Available Here.

Image Courtesy:

1. “Serial endosymbiosis” By Kelvinsong – Own work (CC BY-SA 3.0) via Commons Wikimedia   
2. “Chloroplast endosymbiosis” By Kelvinsong – Own work (CC BY-SA 3.0) via Commons Wikimedia  
3. “Chloroplast-cyanobacterium comparison” By Kelvinsong – Own work (CC BY-SA 3.0) via Commons Wikimedia