The endosymbiotic origin of chloroplast and plastid

Anec  > Biology > Evolution

The origin of chloroplasts or plastid is an important biological question, and the scientific community has proposed several hypotheses to explain their origin. Among them, the most widely accepted hypothesis is the endosymbiotic theory.

By comparing the DNA of chloroplasts and mitochondria, we have learned that they originated from similar independent events. Chloroplasts appeared later than mitochondria. They were not engulfed by prokaryotes but rather by eukaryotes that had already acquired mitochondria. Around 1.5 billion years ago, a certain eukaryote engulfed a blue green algae(cyanobacterium), but did not completely digest it. The blue green algae utilized the carbon dioxide produced by the host and sunlight for photosynthesis. The host could not only engulf organic compound from the outside, but also use the ATP and organic compound produced by algae photosynthesis. They formed a perfect endosymbiosis and quickly gained an advantage in competition with other organisms. Eventually, the blue green algae transferred most of its DNA to the host. The lipids on the outer side of the cell wall became the chloroplast outer membrane, and the peptidoglycan on the inner side of cell wall disappeared. The cell membrane became the chloroplast inner membrane. Thylakoids transformed into flattened disks and filled the chloroplasts. Blue green algae became specialized organelle for producing nutrients. Over time, the endosymbiotic organisms evolved into red algae and green algae. Green algae are the ancestors of all terrestrial plants.

The chloroplasts of higher plants are surrounded by two membranes and considered to be produced by primary endosymbiosis. However, scientists have discovered chloroplasts in some protists have four membranes recently. It is believed that chloroplasts with four membranes did not directly evolve from blue green algae, but were formed when eukaryotes engulfed algae containing chloroplasts. The engulfed algae were gradually digested to leave behind the cell membrane and chloroplasts which the four-membrane chloroplasts came from. This is also known as the secondary endosymbiosis.

In addition to the endosymbiotic theory, there is the non-endosymbiotic hypothesis. This hypothesis suggests that chloroplasts did not originate from endocytosis events, but rather developed and evolved from folded cell membrane that enclosed region of photosynthesis. However, it cannot explain why mitochondria, chloroplasts and bacteria share so many similarities in DNA structure and protein synthesis. It is also challenging to explain the sources of DNA and ribosomes for mitochondria and chloroplasts. Therefore, the endosymbiotic theory remains the most widely accepted explanation for the origin of chloroplasts.

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