Innovations in Biotechnology and Medical Sciences

Endosymbiotic Theory and Symbiogenesis


From UPSC perspective, the following things are important :

Prelims level: Endosymbiotic Theory, Nitrogen Cycle

Mains level: NA

Why in the news?

  • The endosymbiotic theory suggests that tiny structures in cells called mitochondria and chloroplasts were once independent bacteria.
  • A biologist named Lynn Margulis came up with the idea that cells merged with bacteria, challenging the usual belief that evolution happens mainly through genetic changes.

 What is the Endosymbiotic Theory?

  • The endosymbiotic theory proposes that organelles like mitochondria and chloroplasts were once free-living bacteria engulfed by recipient cells.
  • American biologist Lynn Margulis introduced Symbiogenesis, challenging the Neo-Darwinist consensus on genetic mutations driving evolution.
  • Margulis’s Struggle:
  1. Margulis’s manuscript on symbiogenesis faced rejection by academic journals before finally being published in The Journal of Theoretical Biology in 1967.
  2. It took years for mitochondria and chloroplasts to be acknowledged as former free-living bacteria turned endosymbionts.

Recent Discoveries and Endosymbiotic Theory

  • Recent papers published in Science and Cell have reignited interest in the endosymbiotic theory.
  • The focus is on nitrogen fixation, crucial for proteins and DNA in living organisms.
  • Despite abundant atmospheric nitrogen, plants lack the means to utilize it efficiently.
  • Legumes host nitrogen-fixing bacteria in root nodules, aiding in ammonia production for plant use.

Evolution of Nitroplast

  • Cyanobacterium UCYN-A was found in marine algae, establishing a symbiotic relationship.
  • Nitroplast, a new organelle, co-evolved with its host cell, satisfying criteria for organelle classification.
  • Nitroplast integrates into host cell function and architecture, imports host cell proteins, synchronizes growth, and is inherited during cell division.
  • Nearly half of nitroplast proteins are derived from the host cell.

Nitrogen Cycle:

The nitrogen cycle is a crucial biogeochemical cycle that describes the transformation and circulation of nitrogen in various forms within ecosystems. It involves several key steps:

  1. Nitrogen Fixation: This is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3) or nitrate (NO3-) ions, which are forms of nitrogen that plants can utilize. Nitrogen fixation can occur through biological, atmospheric, and industrial processes. In biological nitrogen fixation, certain bacteria like Rhizobium spp. in the root nodules of leguminous plants or cyanobacteria convert N2 into ammonia.
  2. Ammonification: Ammonification is the process by which organic nitrogen compounds from dead organisms, feces, and other waste materials are converted into ammonia by decomposer organisms like bacteria and fungi during the decay process.
  3. Nitrification: Nitrification is the biological oxidation of ammonia (NH3) or ammonium (NH4+) to nitrite (NO2-) and then to nitrate (NO3-) by nitrifying bacteria such as Nitrosomonas and Nitrobacter. This process occurs in aerobic conditions and plays a crucial role in making nitrogen available to plants.
  4. Assimilation: Plants and other organisms take up ammonia, nitrate, or organic nitrogen compounds from the soil to use them in synthesizing proteins, nucleic acids, and other vital nitrogen-containing compounds.
  5. Denitrification: Denitrification is the process by which nitrate (NO3-) is converted back into nitrogen gas (N2) or nitrous oxide (N2O) by denitrifying bacteria under anaerobic conditions. This step completes the cycle by returning nitrogen to the atmosphere, closing the loop.
  6. Ammonium Ionization: In soils and aquatic environments, ammonium ions (NH4+) may undergo ionization, converting to ammonia gas (NH3), which can volatilize and return to the atmosphere.

Significance of the Nitroplasts

  • Agriculture: Nitroplasts offer potential solutions for reducing the harmful effects of industrial ammonia production.
  • Biotechnology: Biotechnological applications may include engineering host cells and nitroplasts for efficient nitrogen fixation in plants.


[2021] In case of which one of the following biogeochemical cycles, the weathering of rocks is the main source of release of nutrient to enter the cycle?​

(a) Carbon cycle

(b) Nitrogen cycle

(c) Phosphorus cycle

(d) Sulphur cycle

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