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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Famous quotes containing the word outer:
“The outer world, from which we cower into our houses, seemed after all a gentle habitable place; and night after night a mans bed, it seemed, was laid and waiting for him in the fields, where God keeps an open house.”
—Robert Louis Stevenson (18501894)