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Evolution and the brain

Help Endosymbiotic Theory Origine et évolution des mitochondries Formation de le cellule Eucaryote par symbioses successives
Evolution’s Double Take Protistes
Lynn Margulis

Mitochondria are responsible for cellular respiration, the process that enabled eukaryotic cells to break their dependency on the older, relatively inefficient, anaerobic process of glycolysis.

The fermentation of alcohol is an example of an anaerobic process involving glycolysis. In fermentation, in the absence of oxygen, yeast converts the initial substrate, glucose, into ethyl alcohol. The alcohol thus serves as an internal acceptor for the electrons. Fermentation is still used for energy production by certain micro-organisms. It yields a net energy balance of two molecules of adenosine triphosphate (ATP).

In the cellular respiration that takes place in a mitochondrion, the oxygen molecule is used as the final electron acceptor. This molecule offers the advantage of allowing the glucose molecule to be fully oxidized and completely converted into water and carbon dioxide. The net energy balance is 38 ATP molecules: 19 times more than in glycolysis!

The mitochondrion's matrix contains the enzymes needed to break down the pyruvic acid (obtained from glucose and fatty acids), while its cristae (folds in its inner membrane) contain the electron transporters where the ATP is synthesized.

Link : Compartimentage et isolement Link : La respiration cellulaire Link : La respiration cellulaire
Link : Les mitochondries Link : La glycolyse anaérobie

Eukaryotic cells, with their many intracellular organelles, were long regarded as descendants of prokaryotes that grew more complex as the result of genetic mutations. But in the 1960s, biologist Lynn Margulis proposed an alternative explanation that initially got a cold reception from the scientific community. Her endosymbiotic theory, presented more formally in a book published in 1981, proposes that eukaryotic cells as we know them today are the result of a series of symbiotic associations with various prokaryotes.

According to this theory, not only mitochondria, but also chloroplasts and possibly other organelles, such as lysosomes and flagella, were originally prokaryotes that took refuge in larger, anaerobic cells that offered them a rich supply of nutrients. In exchange, these anaerobic host cells benefitted from the prokaryotes’aerobic or photosynthetic capabilities. Over time, the aerobic prokaryotes became mitochondria, while the photosynthetic prokaryotes became the chloroplasts now found in plant cells. This symbiosis between aerobic prokaryotes and the anaerobic ancestors of eukaryotic cells would have given both of them advantages for surviving in an environment where oxygen levels had recently increase substantially.

Margulis's theory is now supported by most biologists, and much evidence has been found in its favour. For instance, phylogenetic analyses have clearly demonstrated that plastids and mitochondria derive from bacterial lines related to modern-day cyanobacteria and proteobacteria, respectively.

The double membrane found in mitochondria and chloroplasts appears to be a relic of the absorption of the prokaryotic bacteria by the eukaryotic host cells. The inner membrane, which now contains numerous folds, apparently came from the bacterial membrane, while the outer membrane came from the host cell itself.

Just like prokaryotes, mitochondria and chloroplasts have their own DNA that is not trapped inside a nucleus. However, the proteins encoded by this DNA do not account for all of the mitochondrial proteins. The prokaryotes are believed to have relinquished certain genes to the nuclei of their host cells, a process known as endosymbiotic gene transfer. For this reason, mitochondria and chloroplasts now depend on their hosts to synthesize most of their components.

Source: Saunders College Publishing

Thus eukaryotic cells are a sort of genetic hybrid, assembled from parts of various organisms. This endosymbiosis, this founding partnership, is certainly one of the most important events in the history of evolution.

But this endosymbiosis also has some negative implications for the host cell. While the mitochondria provide this cell with highly efficient energy production, in the same process they also produce waste materials, the infamous free radicals, which are highly toxic to such cells and are considered one of the main causes of ageing.

All of the mitochondria in the cells of every human being come from the ovule of that person’s mother, who received them from her mother, who received them from her mother, and so on. On the basis of the rate of mutation of mitochondrial DNA, we can estimate the number of years that separate human beings from a common ancestor. Using this method, the search for the “"mitochondrial Eve" scientists have calculated that the species Homo sapiens, which includes all modern humans, first appeared about 200 000 years ago. However, this estimate is still the subject of debate.

Also, it is the people of Africa who show the greatest diversity in their mitochondrial DNA, which supports the hypothesis that human beings originated in Africa. This hypothesis is very well supported by the fossil record.

Link : Scandale chez les pharaons Link : Evolutionary Origin of Mitochondria Link : Mitochondrial DNA (mtDNA) Link : ADN mitochondrial Link : Mitochondrial DNA Clarifies Human Evolution Link : L'arbre phylogénique (VI) Link : LA PHYLOGENIE MOLECULAIRE History Module: Hominization, or The History of the Human Lineage Link : Origines et position de l'homme dans l'évolution : la connexion chromosomique
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