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The purpose of mitochondria is to generate ATP, utilising a chemiosmotic gradient across the inner mitochondrial membrane. Protons are pumped out of the mitochondria into the intermembrane space, but because the outer mitochondrial membrane is highly permeable to ions, the pH of the intermembrane space quickly equilibrates with the rest of the cell. As the outer membrane apparently does not contribute to maintaining this chemiosmotic gradient, what is its purpose? It seems to be doubly redundant in chloroplasts as H+ transport doesn't even occur across the inner membrane.

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  • $\begingroup$ In mitochondria, the protons pumped by the respiratory complexes do not end up in the intermembrane space --- that would be futile, since, as you say, the outer membrane is permeable and the protons would quickly dissipate. The respiratory complexes are located in the christae, which are closed-off invaginations of the inner membrane, from which the protons cannot easily escape (other than via ATP synthase or uncoupling proteins). See for example onlinelibrary.wiley.com/doi/10.1016/S0014-5793(03)00633-1/pdf $\endgroup$ – Roland Mar 24 '17 at 19:01
  • $\begingroup$ @tomd, Fair enough. I just wanted to point out that the respiratory chain generally does not pump protons towards the outer membrane (as it is shown in many textbooks). $\endgroup$ – Roland Mar 25 '17 at 20:46
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Structure origins

Originally the mitochondrial outer membrane (MOM) was likely the result of a bacteria that could survive in a vesicle formed of the plasma membrane.

The invagination of an ancient bacteria into an ancestral eukaryotic cell causing the double mitochondrial membrane to form.

This explains why the plasma membrane and the MOM are structurally similar, however, the proteins that are present in either mean that their functions are very different.The MOM has a specialised translocation complex for example.

Functional significance.

The MOM is actually much more complex than an evolutionary hangover and is essential to modern cells by regulating what that mitochondrial inner membrane is exposed to. The MOM plays this role using porins. Indeed, if the porins of the MOM are disrupted, the entire cell dies (Chipuk et al, 2006). Additionally, the MOM allows a physical association between the ER and the mitochondria called the mitochondria-associated ER membrane (MAM). The MAM is essential for $ {Ca}^{2+} $ transfer.

The MAM thus integrates signal transduction with metabolic pathways to regulate the communication and functional interactions between the ER and mitochondrion. (Hayashi et al, 2009)

Answer: The MOM is essential in regulating what the MIM is exposed to, and allows signalling between the ER and mitochondria.

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Not sure I can help much, but as far as I know the outer mitochondrial membrane is associated with the endoplasmic reticulum as well. This help lipids (apart from proteins) to enter the mitochondria since these lipids originate from the ER. More importantly, the outer membrane has integral proteins called porins present which help in the movement of molecules into the mitochondria. The outer membrane is also required since it's permeable to nutrient molecules while the inner membrane is not.

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    $\begingroup$ Please add some references to your answer. $\endgroup$ – another 'Homo sapien' Mar 24 '17 at 12:07

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