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 ]
