By Dulara Janani Kuruppu
Until the 1970s, prokaryotes were viewed as the predecessors of the more complex eukaryotes. However, the prevalent hypothesis that eukaryotic cells evolved gradually from simpler prokaryotic cells through natural selection was challenged when American researcher Lynn Margulis posited the endosymbiotic theory, introducing a drastically different means by which eukaryotic cells would have evolved.
By definition, prokaryotic cells are those that do not contain membrane-bound organelles such as mitochondria or chloroplasts. Prokaryotic cells are generally much smaller than eukaryotic cells, and a defining difference between the two types of cell is that prokaryotic cells do not have a nucleus, whereas eukaryotic cells do. Instead, prokaryotic cells store their genetic material in a circular loop of DNA (deoxyribonucleic acid) called a nucleoid.
Most respiring cells require mitochondria, which are popularly referred to as the ‘powerhouses’ of a cell. This is due to its pivotal function in carrying out the Krebs cycle to generate ATP (adenosine triphosphate), a form of chemical energy which cells use to execute cellular processes. Chloroplasts are organelles which contain the green pigment chlorophyll, which is typically associated with photosynthesizing plants and algae. It follows that chloroplasts absorb light and enable photosynthesis to occur.
The Endosymbiotic Theory
According to Margulis, early eukaryotic cells were anaerobic and did not rely on oxygen to produce ATP and release energy. In hypothesizing the evolutionary origins of mitochondria, she proposed that such an anaerobic eukaryotic cell engulfed an aerobic prokaryotic cell that was thought to be a heterotypic proteobacterium.
By chance, this prokaryotic cell is not ingested by the larger cell that engulfed it, but the two parties develop a relationship that is advantageous for each other. Oxygen produced by the prokaryote enables the eukaryotic cell to use the energy to grow and carry out cellular processes more efficiently; the prokaryote is given protection and access to further nutrients. Over time, the prokaryote evolves into what is now recognized as a mitochondrion. As mitochondria and eukaryotic cells become increasingly codependent, mitochondria become a permanent fixation in eukaryotic cells.
Similarly, in the evolutionary pathway of chloroplasts, a prokaryotic cyanobacterium is ingested by an ancestral eukaryotic cell. By absorbing light energy and carrying out photosynthesis, the prokaryote releases energy which can be used by the eukaryotic host cell. This establishes a symbiotic relationship between the host cell and the prokaryote, which now resides within the eukaryotic cell. When the cell divides by mitosis, the daughter cells produced will contain the photosynthesizing organelle. Over time, chloroplasts became widespread in plants and algae, which photosynthesize to sustain life.
Thus, the endosymbiotic theory describes the mutually beneficial relationship that arises between two biological entities, namely an eukaryotic host cell and a prokaryotic cell, when the prokaryotic cell exists within the larger host cell.
Evidence for the Endosymbiotic Theory
One of the most striking pieces of evidence backing up the endosymbiotic theory is that mitochondria and chloroplasts contain DNA that is independent of the genetic material in the nucleus of the host cell. For Margulis, discovering that the genomes of these organelles did not match that of the nuclear DNA was one of the first indicators that the historic hypothesis of cell evolution needed refinement. Further, it was found that mitochondria and chloroplasts reproduce independently of their host cell, unlike most other organelles.
Moreover, both mitochondria and chloroplasts share several structural similarities, some of which are remnants of their common evolutionary origins. Mitochondria and chloroplasts both have double membranes. Scientists believe that the inner membrane belonged to the prokaryotic cell that was engulfed, and that the outer membrane was provided by the eukaryotic host cell. Both organelles also have folded inner membranes such as cristae in mitochondria, which provide large surface areas that enable enzyme-catalyzed reactions to occur more efficiently. Furthermore, both mitochondria and chloroplasts contain smaller 70S ribosomes, which are commonly associated with prokaryotic organisms, as opposed to the larger 80S ribosomes that are present in eukaryotes.
Molecular phylogeny, which involves analyzing sequences in biological molecules, suggests that proteins, RNA and DNA in mitochondria and chloroplasts are distinct from those present in their host cells. DNA sequencing has revealed an ancestral lineage between mitochondrial DNA and certain bacterial species, enabling us to establish the distinct evolutionary pathways of eukaryotic host cells and their endosymbionts.
When Margulis first postulated the endosymbiotic theory, her findings were disregarded by traditional evolutionary biologists who followed the principles of natural selection laid down by Charles Darwin. However, in the decades to follow, it has become increasingly clear that the endosymbiotic theory is more successful in delineating the evolutionary origins of mitochondria and chloroplasts.
References
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