Question

Describe/explain the endosymbiont theory for the evolution of mitochondria & plastids in Eukaryotic cells .

-Who was involved (Who engulfed who??)

-What organelles were formed and how

-What were the advantages

-What are the five (5) pieces of evidence that support this theory

Answer 1

The endosymbiotic theory, that holds that eukaryotic mitochondria and plastids arose from the engulfment and integration of a microorganism by another cell, has long been a matter of polemical (controversial) discussion, however growing proof over time has led to the substantiation and universal acceptance of the speculation.

1). Who was involved (Who engulfed who??)

In 1905, Konstantin Mereschkowsky, a distinguished Russian scientist, printed his piece of work in “The nature and origins of chromatophores within the plant kingdom” wherever he projected that the photosynthetic organelles of plants (plastids, chromatophores) originate from Single cellular alga that lives in a symbiotic relationship with their host cells.

Mereschkowsky was a number one lichenologist and his theory was galvanized by his work on lichens wherever he had shown that lichens are organisms composed of fungi and alga Mereschkowsky primarily based his theory on the symbiogenesis of the plastids of green plant cells (chloroplasts) on the work of the German scientist Andreas Schimper (1856-1901) who had antecedently discovered that pigment bodies in plant cells don't develop de novo, however, reproduce by dividing and are distributed to the offspring cells once the plant cell divides. In 1967, Lynn Margulis was distinguishing that eukaryotes organelles evolved through the symbiosis of individual single-celled prokaryotes.

2). What organelles were formed and how

The primitive eukaryotic cell was also eventually able to eat prokaryotes, a marked improvement to absorbing small molecules from its environment. The process of endosymbiosis commenced when the eukaryote engulphed but did not digest an autotrophic bacterium. Evidence suggests this engulfed bacterium was an alphaproteobacterium, an autotroph that uses photosynthesis to acquire energy. The eukaryote then began a mutually beneficial (symbiotic) relationship with it whereby the eukaryote provided protection and nutrients to the prokaryote, and in return, the prokaryotic endosymbiont provided additional energy to its eukaryotic host through its respiratory cellular machinery. The relationship became permanent over time completing primary endosymbiosis as the endosymbiont lost some genes it used for independent life and transferred others to the eukaryote's nucleus. The symbiont thus became dependent on the host cell for organic molecules and inorganic compounds. The genes of the respiratory machinery became a mitochondrion. Endosymbiotic theory hypothesizes the origin of chloroplasts similarly, where chloroplasts a eukaryote with mitochondria engulfs a photosynthetic cyanobacterium in a symbiotic relationship ending in the chloroplast organelle.

3). What were the advantages

The phenomenon of endosymbiosis, or one organism living within another, has deeply impacted the evolution of life and continues to shape the ecology of countless species. Mitochondria and chloroplasts are relicts of ancient bacterial endosymbionts that ultimately extended the range of acceptable habitats for life by allowing hosts to thrive in the presence of oxygen and to convert light into energy.

4). What are the five pieces of evidence that support this theory

1. Mitochondria have very similar characteristics to purple-aerobic bacteria. They both use oxygen in the production of ATP, and they both do this by using the Kreb’s Cycle and oxidative phosphorylation. Similarly, chloroplasts are very similar to photosynthetic bacteria in that they both have similar chlorophyll that harnesses light energy that is converted into chemical energy. Although there are many similarities between mitochondria and purple aerobic bacteria and chloroplasts and photosynthetic bacteria, they appear to be slight and explainable by subsequent evolution.
2. Mitochondria and chloroplasts are similar in size to bacteria, 1 to 10 microns.
3. Mitochondria and chloroplasts DNA, RNA, ribosomes, chlorophyll (for chloroplasts), and protein synthesis are similar to that for bacteria. This provided the first substantive evidence for the endosymbiotic hypothesis. It was also determined that mitochondria and chloroplasts divide independently of the cell they live in. Mitochondria having their own DNA and dividing independently of the cell is what ultimately results in only mitochondrial DNA being inherited by one’s mother since only an egg cell has DNA while a sperm cell does not.
4. Both mitochondria and chloroplasts have double phospholipid bilayers. This appears to have arisen by mitochondria and chloroplasts entering eukaryotic cells via endocytosis. Both purple, aerobic bacteria (similar to mitochondria) and photosynthetic bacteria (similar to chloroplasts) only have one phospholipid bilayer, but when they enter another cell via endocytosis, they are bound by a vesicle which forms the second layer of their double phospholipid bilayer.