Explain Evolution of Plastids? Biology notes for Class 11
Evolution of Plastids : Plastids are thought to have originated from endosymbiotic cyanobacteria (are bacteria that obtain their energy through photosynthesis, and they are the only photosynthetic prokaryotes able to produce oxygen). As a by-product of photosynthesis, producing and releasing oxygen, cyanobacteria are thought to have converted the early oxygen-poor, reducing atmosphere into an oxidizing atmosphere, causing the Great Oxygenation Event and the “rusting of the Earth”,which dramatically changed the composition of the Earth’s life forms and led to the near-extinction of anaerobic organisms. The most common examples of obligate endosymbioses are chloroplasts and mitochondria.
This endosymbiosis of plastids developed around 1.5 billion years ago and enabled eukaryotes to carry out oxygenic photosynthesis. Three evolutionary lineages have since emerged in which the plastids are named differently:
- Chloroplasts in green algae and plants,
- Rhodoplasts in red algae and
- Muroplasts in the glaucophytes (also known as glaucocystophytes or glaucocystids, are a small group of rare unicellular freshwater microscopic algae)
The plastids differ both in their pigmentation and in their ultrastructure. For example, chloroplasts have lost all phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae, and glaucophytes, but instead contain stroma and grana thylakoids, structures found only in plants and closely related green algae. The glauco cystophycean plastid—in contrast to chloroplasts and rhodoplasts—is still surrounded by the remains of the cyanobacterial cell wall. All these are primary plastids that are surrounded by two membranes.
Complex plastids start by secondary endosymbiosis (where a eukaryotic organism engulfs another eukaryotic organism that contains a primary plastid resulting in its endosymbiotic fixation) when a eukaryote engulfs a red or green alga and retains the algal plastid, which is typically surrounded by more than two membranes. In some cases, these plastids may be reduced in their metabolic and/or photosynthetic capacity.
Algae with complex plastids derived by secondary endosymbiosis of a red alga include the
- Heterokonts: The heterokonts (stramenopiles) are a major line of eukaryotes currently containing more than 25,000 known species. Most are algae, ranging from the giant multicellular kelp to the unicellular diatoms, which are a primary component of plankton.
- Haptophytes: Haptophytes are a phylum of algae, also called the Prymnesiophyta. All or most of them are single-celled photosynthetic phytoplankton.
- Cryptomonads: The cryptomonads are a group of algae, most of which have plastids. They are common in freshwater, in marine and brackish habitats.
- Dinoflagellates (= rhodoplasts): The dinoflagellates, are a large group of flagellate eukaryotes. Most are marine plankton, but they are also common in freshwater habitats.
The symbionts that endosymbioses a green alga include the:
- Euglenids: Euglenids are marine and freshwater green or colorless flagellate organisms.
- Chlorarachniophytes(= chloroplasts): The chlorarachniophytes are a group of single-celled phototrophic, mixotrophiceukaryotes (mixotrophs, include a combination of phagotrophy and phototrophy) in marine environments. They are most common in tropical and temperate seas.
The Apicomplexa, a phylum of obligate parasitic protozoa which includes the causative agents of toxoplasmosis (Toxoplasma gondii), malaria (Plasmodium spp.), and many other human or animal diseases also harbor a complex plastid (although this organelle has been lost in some apicomplexans, such as Cryptosporidium parvum, which causes cryptosporidiosis). The ‘apicoplast’ is an essential organelle, but no longer capable of photosynthesis, and a promising target for antiparasitic drug development.
Some sea slugs and dinoflagellates, in particular of the genus Elysia, take up algae as food and keep the plastid of the digested alga to profit from the photosynthesis; meanwhile, the plastids are also digested. This process is known as kleptoplasty, (Greek – kleptes means thief).
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Tag: Evolution of Plastids, Plastids, Complex plastids, Biology notes for Class 11