Martin taggart

Researchers propose that certain organelles, such as mitochondria and chloroplasts, originated from free-living prokaryotic cells that were engulfed by a larger host cell. This process, known as endosymbiosis, is depicted in Figure 1, where a larger cell engulfs a smaller cell in three stages. The claim is supported by several lines of evidence that highlight structural, functional, and genetic similarities between these organelles and free-living prokaryotes.

1. Double Membranes
Mitochondria and chloroplasts have double membranes, consistent with the engulfment process. The inner membrane resembles the plasma membrane of prokaryotes, while the outer membrane represents the engulfing vesicle of the host cell.

2. Presence of Circular DNA
Both mitochondria and chloroplasts contain their own circular DNA, similar to that of prokaryotes. This DNA is independent of the host cell’s nucleus and encodes some of the proteins required for the organelle’s function.

3. Ribosomes Similar to Prokaryotes
The ribosomes found within mitochondria and chloroplasts closely resemble those of prokaryotic cells in size and structure, supporting their prokaryotic origin.

4. Independent Division
Mitochondria and chloroplasts divide by binary fission, a process characteristic of prokaryotic cells, further indicating their ancestral link to free-living prokaryotes.

5. Genetic and Functional Evidence
Genomic analyses show that mitochondrial and chloroplast genes are closely related to those of certain bacteria. For example, mitochondrial genes are similar to those of alpha-proteobacteria, while chloroplast genes are related to cyanobacteria.

6. Biochemical Similarities
These organelles possess unique biochemical pathways and enzymes similar to those found in prokaryotes. For example, chloroplasts conduct photosynthesis using mechanisms nearly identical to those of cyanobacteria.

Conclusion
The evidence supporting the endosymbiotic theory aligns with the researchers’ claim that certain organelles originated from free-living prokaryotic cells engulfed by a larger cell. This remarkable evolutionary event not only explains the origin of mitochondria and chloroplasts but also highlights the intricate relationships between organisms that shape life on Earth.