Endosymbiotic Theory | Ask A Biologist
A symbiont is an organism living in a relationship with another species in which descendents of symbiotic relationships which began at least a billion years ago. The definition of symbiosis is controversial among scientists. The endosymbiotic theory describes how a large host cell and ingested bacteria dependent on one another for survival, resulting in a permanent relationship. The Endosymbiotic Theory first postulated by Lynn Margulis in the biology can now offer a plausible explanation for the evolution of eukaryotes not digested cells live together is a mutually benefitting relationship.
When one organism actually lives inside the other it's called endosymbiosis. The endosymbiotic theory describes how a large host cell and ingested bacteria could easily become dependent on one another for survival, resulting in a permanent relationship. Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell. It's Just a Theory In everyday speech, people use the word theory to mean an opinion or speculation not necessarily based on facts.
But in the field of science, a theory is a well established explanation based on extensive experimentation and observation. Scientific theories are developed and verified by the scientific community and are generally accepted as fact.
And both organelles use their DNA to produce many proteins and enzymes required for their function. A double membrane surrounds both mitochondria and chloroplasts, further evidence that each was ingested by a primitive host. The two organelles also reproduce like bacteria, replicating their own DNA and directing their own division.
It is passed down directly from mother to child, and it accumulates changes much more slowly than other types of DNA. Because of its unique characteristics, mtDNA has provided important clues about evolutionary history.Evolution: It's a Thing - Crash Course Biology #20
We think we know part of the answer. Eukaryotic cells may have evolved when multiple cells joined together into one.
They began to live in what we call symbiotic relationships. The theory that explains how this could have happened is called endosymbiotic theory. An endosymbiont is one organism that lives inside of another one. All eukaryotic cells, like your own, are creatures that are made up of the parts of other creatures.
Mitochondria, the important energy generators of our cells, evolved from free-living cells. They were prokaryotes that ended up inside of other cells host cells. They may have joined the other cell by being eaten a process called phagocytosisor perhaps they were parasites of that host cell.
The Evolution of the Cell
Rather than being digested by or killing the host cell, the inner cell survived and together they thrived. The host cell provides a comfortable, safe place to live and the organelle pays rent by making energy that the host cell can use. This happened a long time ago, and over time the organelle and the host cell have evolved together.
Now one could not exist without the other. Today they function as a single organism, but we can still find evidence of the free-living past of the organelles if we look closely. What Evidence Supports Endosymbiotic Theory? As early asbotanist Andreas Schimper was looking at the plastid organelles of plant cells using a microscope.
He watched the plastids divide and noticed something odd. The process looked very similar to the way some free-living bacteria divided.
During the s and 60s, scientists found that both mitochondria and plastids inside plant cells had their own DNA.
It was different from the rest of the plant cell DNA. When scientists looked closer at the genes in the mitochondrial and plastid DNA, they found that the genes were more like those from prokaryotes.
This tells us that organelles are more closely related to prokaryotes. The green chloroplasts in this cell are now a critical part of plant cells, but they evolved from an entirely different organism than the plant cell. The chloroplast is thought to have evolved from a cyanobacterial cell that managed to survive the cell's defenses. The idea that the eukaryotic cell is a group of microorganism was first suggested in the s by the American biologist Ivan Wallin.
The endosymbiont theory of mitochondria and chloroplasts was proposed by Lynn Margulis of the University of Massachusetts Amherst.
BBC Bitesize - Higher Biology - Symbiosis - Revision 1
InMargulis published Symbiosis in Cell Evolution in which she proposed that the eukaryotic cells originated as communities of interacting entities that joined together in a specific order. The procaryote elements could have enterered a host cell, perhaps as a indigested prey or as a parasite.
Over time, the elements and the host could have developped a mutually beneficial interactionlater evolving in an obligatory symbiosis. Margulis has also proposed that eukaryotic flagella and cilia may have arisen from endosymbiotic spirochetesbut these organelles do not contain DNA and do not show any ultrastructural similarities to any prokaryotes, and as a result this idea does not have wide support.
Margulis claims that symbiosic relationships are a major driving force behind evolution. According to Margulis and Sagan"Life did not take over the globe by combat, but by networking" i. She considers Darwin notion of evolution driven by competition to be incomplete. Evidence for the theory Evidence that mitochondria and chloroplasts arose via an ancient endosymbiosis of a bacteria is as follows: