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Endosymbiosis and the evolution of eukaryotes

In order to understand eukaryotic organisms fully, it is necessary to understand that all extant eukaryotes are descendants of a chimeric organism that was a composite of a host cell and the cell(s) of an alpha-proteobacterium that “took up residence” inside it. This major theme in the origin of eukaryotes is known as endosymbiosis    , one cell engulfing another such that the engulfed cell survives and both cells benefit. Over many generations, a symbiotic relationship can result in two organisms that depend on each other so completely that neither could survive on its own. Endosymbiotic events likely contributed to the origin of the last common ancestor of today’s eukaryotes and to later diversification in certain lineages of eukaryotes ( [link] ). Before explaining this further, it is necessary to consider metabolism in prokaryotes.

Prokaryotic metabolism

Many important metabolic processes arose in prokaryotes, and some of these, such as nitrogen fixation, are never found in eukaryotes. The process of aerobic respiration is found in all major lineages of eukaryotes, and it is localized in the mitochondria. Aerobic respiration is also found in many lineages of prokaryotes, but it is not present in all of them, and many forms of evidence suggest that such anaerobic prokaryotes never carried out aerobic respiration nor did their ancestors.

While today’s atmosphere is about one-fifth molecular oxygen (O 2 ), geological evidence shows that it originally lacked O 2 . Without oxygen, aerobic respiration would not be expected, and living things would have relied on fermentation instead. At some point before, about 3.5 billion years ago, some prokaryotes began using energy from sunlight to power anabolic processes that reduce carbon dioxide to form organic compounds. That is, they evolved the ability to photosynthesize. Hydrogen, derived from various sources, was captured using light-powered reactions to reduce fixed carbon dioxide in the Calvin cycle. The group of Gram-negative bacteria that gave rise to cyanobacteria used water as the hydrogen source and released O 2 as a waste product.

Eventually, the amount of photosynthetic oxygen built up in some environments to levels that posed a risk to living organisms, since it can damage many organic compounds. Various metabolic processes evolved that protected organisms from oxygen, one of which, aerobic respiration, also generated high levels of ATP. It became widely present among prokaryotes, including in a group we now call alpha-proteobacteria. Organisms that did not acquire aerobic respiration had to remain in oxygen-free environments. Originally, oxygen-rich environments were likely localized around places where cyanobacteria were active, but by about 2 billion years ago, geological evidence shows that oxygen was building up to higher concentrations in the atmosphere. Oxygen levels similar to today’s levels only arose within the last 700 million years.

Recall that the first fossils that we believe to be eukaryotes date to about 2 billion years old, so they appeared as oxygen levels were increasing. Also, recall that all extant eukaryotes descended from an ancestor with mitochondria. These organelles were first observed by light microscopists in the late 1800s, where they appeared to be somewhat worm-shaped structures that seemed to be moving around in the cell. Some early observers suggested that they might be bacteria living inside host cells, but these hypotheses remained unknown or rejected in most scientific communities.

Questions & Answers

A closed circulatory system is a closed-loop system, in which blood is not free in a cavity. Blood is separate from the bodily interstitial fluid and contained within blood vessels. In this type of system, blood circulates unidirectionally from the heart around the systemic circulatory route, and th
Peter Reply
what is meiosis
Ram Reply
meiosis is a reduction Division in which four hapliod off springs are produced
Nandanee
what is electrochemical gradient?
Joneth Reply
what is mitosis?
Aliness Reply
(cytology) the division of cell nucleus in which the genome is copied and separated into two identical halves.it is normally follow by cell division
Adekunle
it Is the division of the mother cell into two daughter cells,genetically identical to each other and to their parent cell
Joneth
it is a study of living organisms
Prudence Reply
What are other types of cell
ATAMA Reply
plant and animal cell
Jessy
prokaryotic and eukaryotic cell
Joneth
meaning inside and outside cells
Joneth
external feature of earthworm
Ajas Reply
what is the reason behind that before F2 generation is no segregation?
asmamaw Reply
what is mitosis and meiosis
Rabiu Reply
hypothesis theory law
Tamba Reply
what is hypothesis theory law
Tamba
Briefly describe the process of mitosis and meiosis.
Lilian Reply
what is chromosome
Deborah Reply
Chromosome is the thread-like structure containing DNA and found in the nucleus of a cell. DNA contains gene which has genetic information.
Lilian
if a cell is killed by strain why then is it use
uchenna Reply
what is an atom
Finda Reply
an atom is the must smallest element in the world.
Young
please help me friends. I wont a good example of lab report based on carbohydrate,lipid,reducing and non reducing sugar.
Ng
that can't be split by any chemical means
uchenna
No an atom is the smallest particles of an element which can take part in a chemical reaction
Solayemi
An atom is the smallest indivisible particle of an element which can take part in a chemical reaction.
Samuel
sure atoms can't be splited
Solayemi
atom can't be split
john
what is molecule
john
Molecules consists of one or more tiny particles called atom
Rofiah
atoms joined together by covalent bonds
Bad
an atom is a particle that cannot be divided further by a chemical reaction
Bad
how do earthworms excrete
Ndlovu
what is genotype
Precious Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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