<< Chapter < Page Chapter >> Page >
Part a shows red algae with lettuce-like leaves. Part b shows four oval green algae cells stacked next to each other. The cyanobacteria are about 2 µm across and 10 µm long.
(a) Red algae and (b) green algae (visualized by light microscopy) share similar DNA sequences with photosynthetic cyanobacteria. Scientists speculate that, in a process called endosymbiosis, an ancestral prokaryote engulfed a photosynthetic cyanobacterium that evolved into modern-day chloroplasts. (credit a: modification of work by Ed Bierman; credit b: modification of work by G. Fahnenstiel, NOAA; scale-bar data from Matt Russell)

Art connection

The illustration shows steps that, according to the endosymbiotic theory, gave rise to eukaryotic organisms. In step 1, infoldings in the plasma membrane of an ancestral prokaryote gave rise to endomembrane components, including a nucleus and endoplasmic reticulum. In step 2, the first endosymbiotic event occurred: The ancestral eukaryote consumed aerobic bacteria that evolved into mitochondria. In a second endosymbiotic event, the early eukaryote consumed photosynthetic bacteria that evolved into chloroplasts.
The first eukaryote may have originated from an ancestral prokaryote that had undergone membrane proliferation, compartmentalization of cellular function (into a nucleus, lysosomes, and an endoplasmic reticulum), and the establishment of endosymbiotic relationships with an aerobic prokaryote, and, in some cases, a photosynthetic prokaryote, to form mitochondria and chloroplasts, respectively.

What evidence is there that mitochondria were incorporated into the ancestral eukaryotic cell before chloroplasts?

Evolution connection

Secondary endosymbiosis in chlorarachniophytes

Endosymbiosis involves one cell engulfing another to produce, over time, a coevolved relationship in which neither cell could survive alone. The chloroplasts of red and green algae, for instance, are derived from the engulfment of a photosynthetic cyanobacterium by an early prokaryote.

This leads to the question of the possibility of a cell containing an endosymbiont to itself become engulfed, resulting in a secondary endosymbiosis. Molecular and morphological evidence suggest that the chlorarachniophyte protists are derived from a secondary endosymbiotic event. Chlorarachniophytes are rare algae indigenous to tropical seas and sand that can be classified into the rhizarian supergroup. Chlorarachniophytes extend thin cytoplasmic strands, interconnecting themselves with other chlorarachniophytes, in a cytoplasmic network. These protists are thought to have originated when a eukaryote engulfed a green alga, the latter of which had already established an endosymbiotic relationship with a photosynthetic cyanobacterium ( [link] ).

According to the secondary endosymbiosis theory, plastids in modern chlorarachniophytes arose via two endosymbiotic events. In the first event, a cyanobacterium was engulfed by a heterotrophic eukaryote. Cyanobacteria have two membranes and the endosymbiosis event gave rise to a third membrane. One of these membranes was lost. Then, in a second endosymbiotic event, the cell was engulfed by another cell. The first cell became a plastid, an organelle with a vestigial nucleus and an organelle membrane inside it; thus, the plastid has the appearance of a cell within a cell.
The hypothesized process of endosymbiotic events leading to the evolution of chlorarachniophytes is shown. In a primary endosymbiotic event, a heterotrophic eukaryote consumed a cyanobacterium. In a secondary endosymbiotic event, the cell resulting from primary endosymbiosis was consumed by a second cell. The resulting organelle became a plastid in modern chlorarachniophytes.

Several lines of evidence support that chlorarachniophytes evolved from secondary endosymbiosis. The chloroplasts contained within the green algal endosymbionts still are capable of photosynthesis, making chlorarachniophytes photosynthetic. The green algal endosymbiont also exhibits a stunted vestigial nucleus. In fact, it appears that chlorarachniophytes are the products of an evolutionarily recent secondary endosymbiotic event. The plastids of chlorarachniophytes are surrounded by four membranes: The first two correspond to the inner and outer membranes of the photosynthetic cyanobacterium, the third corresponds to the green alga, and the fourth corresponds to the vacuole that surrounded the green alga when it was engulfed by the chlorarachniophyte ancestor. In other lineages that involved secondary endosymbiosis, only three membranes can be identified around plastids. This is currently rectified as a sequential loss of a membrane during the course of evolution.

The process of secondary endosymbiosis is not unique to chlorarachniophytes. In fact, secondary endosymbiosis of green algae also led to euglenid protists, whereas secondary endosymbiosis of red algae led to the evolution of dinoflagellates, apicomplexans, and stramenopiles.

Section summary

The oldest fossil evidence of eukaryotes is about 2 billion years old. Fossils older than this all appear to be prokaryotes. It is probable that today’s eukaryotes are descended from an ancestor that had a prokaryotic organization. The last common ancestor of today’s Eukarya had several characteristics, including cells with nuclei that divided mitotically and contained linear chromosomes where the DNA was associated with histones, a cytoskeleton and endomembrane system, and the ability to make cilia/flagella during at least part of its life cycle. It was aerobic because it had mitochondria that were the result of an aerobic alpha-proteobacterium that lived inside a host cell. Whether this host had a nucleus at the time of the initial symbiosis remains unknown. The last common ancestor may have had a cell wall for at least part of its life cycle, but more data are needed to confirm this hypothesis. Today’s eukaryotes are very diverse in their shapes, organization, life cycles, and number of cells per individual.

Art connections

[link] What evidence is there that mitochondria were incorporated into the ancestral eukaryotic cell before chloroplasts?

[link] All eukaryotic cells have mitochondria, but not all eukaryotic cells have chloroplasts.

Got questions? Get instant answers now!

Questions & Answers

what is genetic engineering
Eveline Reply
what is the meaning of term mitosis
Lwitiko Reply
outline the significance of mitosis to organisms
Lwitiko
name the resources to be conserved
Oreva Reply
define natural resources
Oreva
name the agencies responsible for the conservation of natural resources
Oreva
land,water,forests,
Kiiza
nema,uwa
Kiiza
wassup guyz
Peace
land ,water
Erika
explain the importance of carbon dioxide in the body
Kiiza Reply
how does that works
SAMUEL
diagram of Prokaryotic cells
Magreth Reply
where is it?
Yazi
ʜɪ
Malikie
ʜɪ
Malikie
where is the diagram?
Yazi
waiting.....
Yazi
what
Malikie
good morning guyz
Joelia
gd morning
Hannalyn
morning how are u doing
Paul
doing all fyn en u
Joelia
👍👍👍
Gruxz
morning everyone.. by is Grace we have be saved... Ephesians 2:8....
Cosmo
hi
Kisa
hey guys am new
Ellie
u a most wlcm
Joelia
🍑🐕
Ken
Hello guys im new
Sulaiman
u are welcome
Peace
thanks
Ellie
..amitabatha..
Ken
He guys happy New month
Peace
hello everyone,I'm new here!!
Reine
welcome
James
thanks so much!!
Reine
how re u Reine Balla
James
Happy New mnth to guys
Peace
HAPPY new month to u
James
Thanks and same to u
Peace
welcome one more peace
James
hi guys
Tafadzwa
Define the term Biology element atom
James
my name is rons am asking the question what is blood compatibility
Rons
Do you really no the one you text with.
Israel
I'm fine James, just boredom want to finish me!!
Reine
what is a cell of a bacteria called
Sahfe Reply
what is genotype
James
genotype: this is the combination of alleles an organism has for a given characteristic..
Cosmo
what is maiotic
Gabriel Reply
what is sexual reproduction
Gabriel
what are the effects of concentration gradient in the uptake of water by plant's
Harleen Reply
ls it ok if you have sex during pregnancy
Kags Reply
no
Bernard
yes!!!
Yazi
Yes
Babie
hello
Cabdi
Babie Maseuse .. Hello .. how are you ?
kf
yes it make the baby's strength stay longer
REAL
Who are you?
Babie
why not it's nicer and more enjoyable more than ok for the woman
Lamin
danso from the Gambia and you why do you ask
Lamin
as long as the woman is comfortable and in any possible position good for her
Lamin
unless medically advised to stop or hault and those conditions are also applicable to certain problem in pregancies. Pls any further comment pls never hesitate to ask
Lamin
how tell us
Tony
an assignment for you pls
Lamin
it's recommended
Nyakato
Hello
george
hi how are you?
Lamin
Fine
george
hello everyone
Cosmo
Good night to everyone am from Zambia may the Grace of the Lord be with you all
Cosmo
u too hv gd dreams
Joelia
thanks colleagues and wish you all the best insha Allah
Lamin
InshaAllah
Joelia
Marsha Allah
Lamin
yes
James
What is the meaning of organ
Ronald Reply
tissue combined to form organ
zameer
join
Elishs
orga- these are tissues join together to perform a specific functions.
Elishs
then what are tissues
Kags
group of cells working together to perform a particular function
Harleen
tissue are group of cells put together to perform a certain goal
Bernard
What will happen when read blood cell placed in 0.5 % Nacl solution
zameer Reply
what is biology
Tariro Reply
biology is the study of living organisms
Bernard
examply is the study of.
Nantamu
biology is the study of life.
Nantamu
what is biology
Francis Reply
kidny with large and and many glomeruli are characcteristic feathure of ?
zameer
Can you please put up the notes for antibiotics
Jesus Reply
I can't
Francis
we should first know what is antibiotics.what is antibiotics.
Francis
A cell is the basic functional unit of life.
Jael
what is a cell ?
Rabecca Reply
A cell is the basic functional unit of all organisms
Glandwell
A cell is a basic functional and structural unit of life.
Mavis

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