The outer mitochondrial membrane, which encloses the entire organelle, has a protein-to-phospholipid ratio similar to the eukaryotic plasma membrane (about 1:1 by weight). It contains numerous integral proteins called porins, which contain a relatively large internal channel (about 2-3 nm) that is permeable to all molecules of 5000 daltons or less. Larger molecules can only traverse the outer membrane by active transport through mitochondrial membrane transport proteins, and mitochondrial preproteins are imported through specialised translocase complexes. The outer membrane also contains enzymes involved in such diverse activities as the elongation of fatty acids, oxidation of epinephrine (adrenaline), and the degradation of tryptophan.
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“Once conform, once do what other people do because they do it, and a lethargy steals over all the finer nerves and faculties of the soul. She becomes all outer show and inward emptiness; dull, callous, and indifferent.”
—Virginia Woolf (18821941)