<< Chapter < Page Chapter >> Page >

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

what is an organ
rab Reply
a part of a organism
Devinayasha
A group of cell makes organ
Hamza
Organ is part of the body.
MR
a group of tissues that perform a specific function
Divya
What is pseudopodia
Mmesoma Reply
a temporary protrusion of the surface of an ameboid cell for movement and feeding.
Black
This help the animals to move from one place to another
Francess
what is the live ?
AZHKIR Reply
what
Hamdi
awkward🤒
bix
What is biology?
khan Reply
what is alliminatary canal
Shaaibu
i don't know but i wana to laern please tell me
Hamdi
there is something called googling as far as i know
bix
biology_it's science that study of living things
Black
alimentary canal_the whole passage along which food passes through the body from mouth to anus. It includes the esophagus, stomach, and intestines.
Black
is a branch of science whith deals the study of living things
AZHKIR
what element in colors purple
Mikaela Reply
what are the function of sympathetic nervous system and parasympathetic nervous system
Ubon Reply
what is preganglionic
Ubon
don't know
Sweety
what is reproduction
Aben
reproducing specifically : the process by which plants and animals give rise to offspring and which fundamentally consists of the segregation of a portion of the parental body by a sexual or an asexual process and its subsequent growth and differentiation into a new individual.
Black
is the process where by organism produce their new organism of the same species from those who a ready in existence.
Prudent
what is unicellular cell
Abigail
unicellular orgnism* meaning consisting of a single cell
Black
How does reproduction take place in human being
Aben
Is a single celled organism
Brian
What is the Antibiotic
Tamara Reply
antibiotics is any medication that stop the growth of bacteria
onuoha
what is biomolecules
Lawerence Reply
This is a group of molecules produced by a living organism
Odion
thank alot,I had a hard time getting the answer
Lawerence
wat is the meaning of Mr.niger. d
It Reply
Now Mr. Niger Dac M;movement R;respiration N;nutrition G;growth E;excretion R;reproduction D;death A;adaptation C;competition
Odion
Sorry.... I;irritability
Odion
GOOD MORNING MY NAME IS MESUMBE PRECIOUS WHAT IS YOUR NAME
Mesumbe Reply
ccc
Peace
hi
Emmanuel
Emmanuel Daniel
Emmanuel
what is the meaning of Mr Niger?
Emmanuel
the meaning of Mr Niger, movement , reproduction , nutrition, inspiration , growth, excretion ,reproduction
Shonde
Emmanuel Daniel it's Mr NIGER D
debby
and d is for death
Shonde
Brown Jones ug
Anguyo
state all the elements and their symbols
Taiwo Reply
what types of muscles are found in the heart
Diamond Reply
Cardiac muscle
Divya
cardiac or myocardia muscle
onuoha
cardiac muscles
Elvis
cardiac muscle
Jemima
Cardiac muscle
Dr
Thank you dear!
yimam
please can you tell me the meaning of Mr Niger?
Emmanuel
It is an initial to represent life processes of organisms. M means movement, R for reproduction, N stands for nutrition, I for irritability or sensitivity,G means growth , E for excretion and R stands for respiration.
Quartey
cardiac muscle
Stanisla
what is digestion
Dolla Reply
it is the chemical break down of insoluble food substances such as fatty acid to soluble substances which are then used for body processes
Lawerence
does human being sperm performed in the ovary of goat
sadeeq Reply
levels of ecological study
sadeeq
no
Dolla
human sperms contains acid
Dolla
what is an artery
Dolla
It is one of the organs of the circulatory system that carries oxygenated blood from the heart to other parts of the body.
Quartey
artery is type of blood vessel which carries blood away from heart...
shams
how liver destroy red blood cells?
shams
big bang theory was discovered by
Sweety Reply
Georges Lemaître
Mr

Get the best Biology course in your pocket!





Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Biology' conversation and receive update notifications?

Ask