Monday, June 21, 2010

On the Origin of the Double Membrane in Mitochondria and Chloroplasts

I've just finished revising my chapter on lipids and membranes for the 5th edition of my textbook. I decided to insert a short explanation about double membranes in order to clear up some common misconceptions.

Eukaryotic cells are surrounded by a single membrane—the familiar lipid bilayer we learned about in high school. Prokaryotic cells come in two varieties, those that have a single membrane like the gram positive bacteria, and those that have a double membrane, like the gram negative bacteria. A double membrane consists of two lipid bilayers (plasma membrane and outer membrane) with an enclosed intermembrane compartment.1

There are membrane bound compartments within eukaryotic cells. Many of them are surrounded by a single lipid bilayer. Some have a double membrane. The nucleus, for example, is surrounded by a complex double membrane that completely breaks down and is reformed during mitosis and meiosis. Mitochondria and chloroplasts also have double membranes.

Mitochondria and chloroplasts are derived from ancient gram negative bacteria that entered into a symbiotic relationship with primitive eukaryotic cells. The bacteria entered the cytoplasm of the much larger eukaryotic cell where they continued to generate energy by creating a proton gradient across their inner membranes. The protons were temporarily stored in the intermembrane space until they were used to drive ATP synthesis during their return to the cytoplasm. According to chemiosmotic theory, the generation of this protonmotive force in primitive bacterial cells required an intermembrane compartment bounded by two membranes [Ode to Peter Mitchell, Ubiquinone and the Proton Pump].

It's no surprise that mitochondria and chloroplast have a double membrane because their ancestral bacterial cells also had double membranes.

The fact that gram negative bacteria have a double membrane has been known for over half a century. The fact that mitochondria and chloroplasts descend from bacteria has been accepted for almost forty years. The fact that the ancestral bacteria are gram negative bacteria became well established 25 years ago.

In spite of all this evidence, there's still a persistent mythology about the origin of the double membrane in mitochondria and chloroplasts. I was reminded of this when I read the article that won third place in the 3 Quarks Daily 2010 prize for best blog posting about science [The Winners of the 3 Quarks Daily 2010 Prize in Science]. The judge was Richard Dawkins.

First prize was won by Ed Young of Not Exactly Rocket Science for his article on Gut bacteria in Japanese people borrowed sushi-digesting genes from ocean bacteria. Second prize went to Carl Zimmer for Skull Caps and Genomes. Third prize was for an article by Margaret Morgan on The Evolution of Chloroplasts: endosymbiosis and horizontal gene transfer.

Morgan repeats the common myth ...
About 2.7 billion years ago, another remarkable change was occurring: the evolution of eukaryotic cells. This entailed the process of endosymbiosis [Gk: endon "within", syn "together" and biosis "living".] In endosymbiosis, one organism engulfs another and incorporates it into its own body or cells. It's important to remember that this takes place by invagination: think of pushing your finger into the side of an inflated balloon. Your finger is surrounded by both its own external membrane (your skin) as well as the membrane of the balloon itself. Now imagine (and sorry, the metaphor gets a bit gross at this point!) that your finger falls off and the balloon seals itself up again. Now your finger is inside the balloon, wrapped in a double membrane. That endosymbionts evolved by this process is evidenced by the fact that they have a double membrane, including their own original form that resembles the ancestral bacterial surface.
This is very wrong. The original bacteria had a double membrane and that double membrane was an integral part of the energy producing pathway that became so important for the eukaryotic cell. It's simply not true that the double membranes of bacteria and chloroplasts were the result of endocytosis.

Unfortunately, there are a lot of other things about this article that are wrong or misleading. I suppose it's further evidence that Richard Dawkins is not a biochemist!


1. I don't mean to imply that a membrane consists only of lipids. Proteins make up a substantial percentage of all membranes.

Ogura, M. (1963) High resolution electron microscopy on the surface structure of Escherichia coli. Journal of Ultrastructure Research 8:251-263 [doi:10.1016/S0022-5320(63)90006-6 ]

9 comments:

  1. I was surprised to learn that mitochondrial functions are not required for human cellular viability. You can get rid of all the mitochondrial genomes in a human cell, by growing cells in the presence of ethidium bromide for example, to produce so-called "rho-zero" cells with mitochondria but no mitochondrial DNA, including some critical genes for the electron transport chain. The rho-zero cells need to be supplied with exogenous pyruvate, and uridine, and they need their growth medium changed relatively frequently to remove lactic acid, but other than that, they're fine. Dunno if this is in the current edition of the textbook or not, but figured I'd share just in case. :)

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  2. Hum,

    I does not make any sense that, even if the Bacteria were engulfed the way it is described, the "double membrane" would be there to stay. If Bacteria reproduce, why and how would their overall replication machinery also duplicate this second envelope? No reason.

    I am surprised that people once might have taken this engulfing seriously as an origin for a double membrane.

    -G

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  3. Thanks for the pointing out that this story about the origin of the double membrane was incorrect.

    On a tangential note, you said: "The protons were temporarily stored in the intermembrane space until they were used to drive ATP synthesis during their return to the cytoplasm."

    This is not true for the mitochondria: the outer mitochondiral membrane is quite permeable to protons. So it would not be correct to say protons are stored in the intermembrane space of the mitochondria. Is it different in bacteria?

    Finally, a question on a completely unrelated issue: what is it about cellulose that makes it so extraordinarily difficult to break down, such that no animal has yet evolved an enzyme that can do it?

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  4. ^
    One problem is cellulose has a crystalline structure, so it's tough for enzymes to attack it.

    Some animals have cellulases. For example, see http://www.pnas.org/content/early/2010/03/02/0914228107.abstract

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  5. I don't think two membranes are needed for the oxydative phosphorylation, else gram positives and mycobacteria couldn't use it.
    The ancestors of mitochondria possibly entered the cell the way Rickettsia (typhus pathogen) do nowadays, by endocytosis, followed by escape from the phagosome. The third membrane was thus lost. Rickettsia are among the closest relatives of the ancestors of mitochondria.
    For chloroplasts, we know secondary endosymbioses with three or four membranes. Three is again suggestive of membrane loss after endocytosis.

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  6. To the first comment:
    Do not confuse mitochondrial fonctions with respiration alone. It is known that yeasts or trypanosomes or apparently human cells can live without respiration, if they lose the mitochondrial genome. The latter codes for a few essential respiratory proteins. However, these cells still have mitochondria that provide essential biochemical fonctions (e.g. the Krebs cycle!).
    Mitochondrial DNA-less yeasts are known as petite mutants (because of the small colony size on plates) and live happily as long as they can ferment sugars for ATP. However mitochondrial protein import mutants are lethal.
    Indeed no eukaryote is known to be free of mitochondria or mitochondrial remnants.

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  7. ichneum says,

    I don't think two membranes are needed for the oxydative phosphorylation, else gram positives and mycobacteria couldn't use it.

    I assume that gram positive bacteria utilize their thick peptidoglycan cell wall as a substitute for an outer membrane.

    I think the mycobacteria have similar cell walls during most stages of their life cycle.

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  8. ichneum says,

    Do not confuse mitochondrial fonctions with respiration alone. It is known that yeasts or trypanosomes or apparently human cells can live without respiration, if they lose the mitochondrial genome.

    The ability to utilize a protonmotive force to synthesize ATP is extremely important for all eukaryotic species. It's true that some unicellular species can survive without it as long as they are supplied with an abundant source of chemical energy, such as glucose, but that's the exception that proves the rule.

    I can't imagine how any human could survive without ATP synthase or the enzymes associated with membrane-associated electron transport. That's why there are so few genetic diseases associated with this pathway.

    BTW, it's wise to avoid the old fashioned terms "respiration" and "oxidative phosphorylation" because the are so many examples where oxygen is not the terminal electron acceptor.

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  9. Larry Moran said:
    I can't imagine how any human could survive without ATP synthase or the enzymes associated with membrane-associated electron transport.

    I can't either. I was refering to the rho-zero cells of anonymous commenter #1.
    Red blood cells and platelets do actually well for weeks without mitochondria (or nuclei, for that matter).

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