Eukaryotes – Or, why we love bacteria part II

Ok, so following on from the post on photosynthesis, this post will be about eukaryotes and endosymbiosis.  As with photosynthesis, this is a HUGE topic that I could happily write 5 posts on, but will try to keep it to one, shortish post.

The next 10 or so posts will be inspired by the series which led me into Biology.  David Attenborough’s “Life on Earth” (Which is what inspired me to get up at 1am to write the photosynthesis post), basically I will take particularly awesome organisms and moments from the series, and go into greater depth on them, with diagrams, links to videos etc.  The aim of this will be to a) get people to watch the awesome series as I will mention the episode each post is inspired by, and link to the video, and b) to give more detail on each one, and hopefully show why I am so excited by the natural world.   By the way, David Attenborough got me into university, and through the first 2 years! I did not go to college, but got into university with a letter of motivation, and what I learned from Attenborough, and the research online that it led me to do, got me through the first 2 years of courses, without it, I would have felt more lost than I did, with having a 16 year gap between school and university, all the maths, physics and chemistry can seem scary!

Finally, before we jump into endosymbiosis, I would like to share two thoughts for the day that have occupied my mind on my 1 hour train journey each way to uni and back today to pick up my essay exam question:

1) Never mind made of star stuff….There are atoms in your body which have most likely been part of the sort of animals you see in fossils…there may be part of me that was once part of a carnivore, and another which was part of an animal which it preyed on, and another which was part of a plant eaten by that animal!….part of me may once have been one of the sulfur bacteria I wrote about last time!  Thinking about the journey that molecules and atoms made to go into me sitting here today just blows my mind.

2) I am more closely related to some archaea than they are to bacteria, despite them both being prokaryotes, and often thought of as one group, and I am more related to fungi than some bacteria are to each other!

Bacteria are blue, Archaea are green, Eukaryotes are red

Ok, so, onto todays post: How eukaryotes arose, and a bit about endosymbiotic theory.

First we need to define some terms which will be used constantly through this post:

Prokaryotes are organisms which do not have a nucleus in their cell. These are found today in bacteria, and single-celled organisms known as Archaea.

Eukaryotes are organisms with nuclei, that is, a membrane surrounds the centre of the cell, containing the genetic material.  This is found in all organisms apart from archaea and bacteria.  We are eukaryotes.

Comparison of Eukaryote and Prokaryote cells

Image is from  which also has a nice comparison of eukaryotes and prokaryotes.

Mitochondria: These are organelles, which exist outside of the nucleus of the cell.  They perform some important functions in the cell, including providing ATP which is vital to the processes within the cell. They also control things such as cell growth.  The important thing to note is that although I just wrote that the genetic material is inside the nucleus, mitochondria have their own DNA.  You may be familiar with the term Mitochondrial DNA, which is used for tracing genetic heritage, this is only inherited from your mother, as the mtDNA from sperm is destroyed during fertilization of the egg.

Chloroplasts: This is the region in a plant cell where photosynthesis occurs.  As with mitochondria, these exist outside of the nucleus, but contain their own DNA.

Structure of a plant cell. The mitochondria and chloroplast can be seen outside of the nucleus

Endosymbiosis: When one organism lives within another without damaging the host, and becomes incorporated into it.  The most well-known type of endosymbiosis for most people is the algae which live inside corals, where they provide through photosynthesis, the energy required for the coral.  They become surrounded by a membrane, and thus become part of the coral itself.

The modern theory of endosymbiosis suggests that mitochondria and chloroplasts originated by the joining of two individual organisms. This is similar to the way that coral and zooxanthellae have a symbiotic relationship, although zooanxthelles are able to be ejected from their host if the conditions become unfavourable. (As an aside, zooxanthellae are a form of algae known as Dinoflagellates…and are totally awesome in their own right, their shapes are almost as amazing as diatoms)

This modern theory was put forward by Lynn Margulis in 1967, and is based on the structure of eukaryotic cells, as well as genetic studies of chloroplasts and mitochondria.

It is thought that the original endosymbionts for the chloroplast were cyanobacteria, and the host was a protist.  The cyanobacteria provided sugar to the host from photosynthesis, and the host provided carbon dioxide and nutrients for the cyanobacteria to perform photosynthesis.  As part of this union, chloroplasts (and mitochondria) lost some of their own genetic material, and proteins known as chaperone proteins transport material from the nucleus across to the chloroplast.  This made some of the genetic material in the chloroplast redundant, and it became more dependent on the hosts own genetic material. Chloroplasts today have less than 10% of the genetic material of free-living cyanobacteria.

The exact origin of endosymbiosis to form eukaryotes is still debated (This does not mean scientists do not agree whether it happened, they are discussing the HOW), some think that it was predation that led to the first endosymbiont.  This does happen in nature today, there is a species of comb jelly which preys on jellyfish and incorporates their stings into its own body for defenses, and there is one organism which “eats” photosynthetic green algae, but does not digest them, but uses their photosynthesis to form sugars. (Paramecium bursaria)

For more information on the origins of eukaryotes, this site has good descriptions

There is further evidence for this endosymbiosis in the way that mitochondria reproduce.  Mitochondria reproduce in the same way as bacteria, by a process of binary fission. This differs from the way eukaryote cells usually divide, which is a process called mitosis (This process in itself is fascinating, especially now we have microscopes to show it)

Mitochondria, and chloroplasts both exist outside of the nucleus, in membranes of their own.  Chloroplasts have two membranes surrounding them, reflecting the inner and outer membrane of prokaryotes, some chloroplasts have 3 or 4 membranes surrounding them, suggesting a secondary endosymbiosis of an organism which already contained a chloroplast.

Which happened first? Mitochondria or Chloroplasts? All eukaryotes have mitochondria, but only plants have chloroplasts, so the incorporation of the mitochondria must have happened before the divergence into plants, fungi and animals.

How many times did this occur? phylogenetics suggests that this occurred at least 6 separate times (phylogenetics is the study of relationships between organisms based on molecular data and physical similarities) for chloroplasts.  The initial cyanobacteria endosymbiosis was then repeated with chloroplasts from later organisms, such as red algae.

So, once again, we have those little microscopic organisms we spend so much time trying to eradicate to thank for me sitting here rambling, and you being patient and reading this.

Suggested sites: There are numerous websites which go into detail on this, as well as a large number of scientific papers. If you want to read about the early life on earth, I recommend “Life on a Young Planet” by Andrew Knoll, it covers the first 3 billion years of life on the planet in an easy to read format. For websites, the ones I have linked (including Wikipedia) are good places to start, but also see:


2 thoughts on “Eukaryotes – Or, why we love bacteria part II

  1. Pingback: Starting small – Dinoflagellates « skepticalsquirrel

  2. Pingback: Recap « skepticalsquirrel

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