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:
“No one knows better than children how much they need the authority that protects, that sets the outer limits of behavior with known and prescribed consequences. As one little boy expressed it to his mother, You care what I do.”
—Leontine Young (20th century)