Plant Physiology - Lecture Notes - O. M. Tarnopilska 2019

2. Structural Components of the Plant Cell
2.2 Symbiotic Theory of the Origin of Mitochondria and Chloroplasts

While studying the ontogeny of Organelles—specifically Cell/35.html">Mitochondria and METABOLISM/14.html">Chloroplasts—in 1912, the Russian botanist B. M. Kozo-Polyansky suggested that The Cell is a symbiotrophic system in which several organisms live in Symbiosis. According to the symbiogenesis hypothesis, mitochondria and chloroplasts were once independent organisms. This hypothesis has many proponents, such as L. Margulis, A. A. Takhtajan, D. Kirk, and M. Rutten. Both mitochondria and chloroplasts exhibit a certain degree of autonomy and are capable of self-duplication. Both organelles contain RNA and specific DNA, are capable of Protein Synthesis, and possess structurally similar Ribosomes and Phospholipids. Prokaryotes lack mitochondria, whereas eukaryotes display A wide variety of forms and sizes of these organelles.

The symbiotic theory is based on the premise that tricarboxylic acids remain as "waste products" during The Glycolytic Pathway and could have served as a food source for certain aerobic microorganisms. It is possible that such microorganisms, having penetrated a Introduction/4.html">Prokaryotic Cell, entered into a symbiosis with it and evolved to function as mitochondria. This assumption is further supported by the similarities between mitochondria and microorganisms (circular DNA, small ribosomes, and sensitivity to bacterial Antibiotics). In primitive eukaryotes, Mitochondrial Genomes are longer than in higher ones. According to the symbiogenetic origin hypothesis of mitochondria, the transition to aerobic Respiration resulted from the invasion of a primary aerobic prokaryotic form into a free-living, autotrophic, yet anaerobic Organism. According to this theory, chloroplasts arose through the symbiosis of an autotrophic microorganism capable of transforming solar energy with a heterotrophic host cell. Chloroplasts contain only a few dozen identical DNA molecules, which is why their autonomy is limited in duration.



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