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Lecture 2

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University of Toronto St. George
Jane Mitchell

(lecture 2) Ppt21 [Cell membranes-fluid] *phospholipids Rapidly: -lateral diffusion(옆으로 퍼져) -tails flex (bent) and rotate Usually within one leaflet *they rarely ―flip-flop‖ (moving from one leaflet to another leaflet) on their own. -however, phospholipid-translocator (a unique enzyme(protein)) help the flip-flop of lipids for synthesizing a new membrane Ppt22 [3 factors effecting membrane fluidity] -1.temperature (very important) (in low temp-rigid, gell like) By observing the artificial bilayer lipids, we can determine the temperature by the fluidity of the membrane. -2. phospholipid saturation The presence of cis-double bond (unsaturated, has kinks) allows to remain flow easily in low temp -3.length of phospholipids tails Shorter tials increase the fluidity of the membrane in a low temp Clicker question The composition of phospholipids in the plasma membranes of two different single-celled organisms isolated from the ocean are examined. Organism A was isolated from the water surrounding a thermal vent where the temperature was 80oC. Organism B was isolated from the ocean floor where the temperature was 4oC. If you were to compare the membrane phospholipids of the two organisms what do you expect to find? A.Organism B will have more saturated phospholipids that also contain shorter hydrocarbon tails. (B is in lower temp, so it should have unsaturated phospholipids!) B.Organism A will have more saturated phospholipids that also contain longer hydrocarbon tails. C.Organism A will have more unsaturated phospholipids that also contain longer hydrocarbon tails. (A should have less unsaturated since it is in higher temp) D.Organism B will have more unsaturated phospholipids that also contain longer hydrocarbon tails. (B is in low temp so it should have shorter tails!) Ppt23 [Lipid movement to the other leaflet—Flip flop] -enzymes in cell membrane , phospholipid translocators , flip lipids from one leaflet to other -eg. phospholipids synthesized in cytosolic leaflet of endoplasmic reticulum (flip flop is important for membrane synthesizing since it allows the new lipid formed in a cell to be locate outside of the cell) -―phospholipid translocators‖ help to build bilayer by flipping a lipid from one leaflet to another +++ *2 Cytosol: aqueous part outside of the nucleus, does not include the membrane-bound organelles in the cell. But does include the soluble protein ex) lysosome, cytoskeleton *3 Lumen: inside of the organelle (aqueous) Pppt24 *Phospholipids formed in endoplasmic reticulum When the plasma lipids needs to grow, the lipids needs to get to the plasma membrane and Also the material from internal organelles---transported by vesicles *Lipids from endoplasmic reticulum transport as a vesicle, fuse w gogi and bud off from golgi then fuse w plasma membrane *the exocytosis the content inside the vesicle. The cytosolic face remain cytosolic through the whole process---maintain the leaflet membranes Ppt25 [Asymmetry of the Lipid Bilayer] *glycolipids - outer leaflet of plasma membrane - interaction of cell with environment -sugar toward the extracellular space (outside of the cell) to protect from harsh conditions -glycosylation is Synthesized in the lumen of golgi, which later become the exoplasmic face exceptions *specific phospholipids – found on the cytosolic leaflet of Plasma Membrane -they specifically binds to cytosolic proteins and localize the to the membrane -eg. Phosphatidylserine (green heads in the diagram, Carries negative charge)-bound by Protein kinase C -it is important for localizing the protein to the membrane which can interact with the receptor and transmit signals into the cell Ppt27 [membrane proteins] -specific functions -associated with the lipid bilayer in different ways Ppt28 [Transmembrane proteins—spanning(가로지르는) ] *Amphiphilic –hydrophilic domains – aqueous (on both cytosolic and extracelluar faces)) –hydrophobic membrane-spanning domain (span the membrane and interact w phospholipids’ tails, and the portions of the proteins have amino acid side chains that are non-polar(hydrophobic) so they interact w hydrophobic area of bilayer) •Single-pass ① single pass α-helix transmemrane protein •Multipass ② multiple pass α-helices transmembrane protein ③ rolled-up β-sheet (β-barrel)---more rigid and had aqueous pores The amino acid side chain needs to be hydrophobic to interact w hydrophobic tail of phospholipids,,, The back bone of alpha helical structure is hydrophobic so it needs to be shield w amino acid,,,that is why it is helical Ppt30 [Examples of membrane protein functions] ① Is usually receptors. Has extracelluar loops that allows them to interact w environment, and the cytosolic side allows to interact w intercellular signalizing proteins ② Many of them are ion channel—they undergo conformational(구조,형태) changes to regulate the permeability of various ions across the bilayer +single pass α-helix transemebrane protein is not usually found as ion channel b/c one single alpha helix is not able to shield ion from the phospholipid tails in bilayer So ion channel should be multiple pass to create a pore that can create a region an ion to interact w and shield from phospholipid tails. ③ Beta barrel structures also form channels The pore is aqueous,,,more found in bacteria than in mitochondria or chloroplast,,,,rigid (no conformational changes), but regulate the permeability by hiring another protein to sit on the pore Iclicker Porin proteins form large water-filled pores in mitochondrial and bacterial membranes by folding into β barrel structures, such that the inside of the barrel forms the pore. The amino acids that face th
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