lect5 - Bio 1A. January 30, 2009 Lecture 5. Wilt Biological...

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Bio 1A. January 30, 2009 Lecture 5. Wilt Biological Membranes I. Components of the membrane A. Lipid. The membrane is formed of a bi-layer of lipids (7nm), hydrophobic tails in the center ( no H 2 O) and polar “heads” in the aqueous environment. Once a bilayer forms, the hydrophobic-hydrophilic axis is fairly stable. 1. The lipid bilayer is called a fluid because the individual lipid molecules do not interact strongly with one another, and hence, can move within the plane of the bilayer. 2. Unsaturated fatty acids and cholesterol perturb what little lateral inter-lipid interactions that exist, and thereby raise the fluidity of the membrane. 3. Stained transmission electron micrographs (TEM) show the membrane as a structure with two lines, each due to one of the component layers of the bilayer. This lipid bilayer is, however, a single membrane. B. Protein 1. At the time of their synthesis, some proteins with a hydrophobic N-terminus (sometimes called a "signal" sequence, usually 15-60 amino acids long) can be inserted into, or through, the lipid bilayers of the endoplasmic reticulum, and may eventually be cycled to the plasma membrane of the cell. Usually this "signal" sequence is cleaved off and is not retained in the mature protein. 2. The proteins are either excreted by exocytosis , or may remain tethered in the membrane due to the compatibility of their hydrophobic regions with the lipid bilayer. Some integral membrane proteins may traverse the membrane several times. Others traverse only once, and then possess external facing peptide regions as well as a tail of protein that remains in the cytoplasmic phase. 3. Integral membrane proteins can and do move within the plane of the lipid, like rafts
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lect5 - Bio 1A. January 30, 2009 Lecture 5. Wilt Biological...

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