Week 5 Notes: Membranes
Week 5 Notes: Membranes BIOL 3510
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This 4 page Class Notes was uploaded by Marin Young on Sunday February 21, 2016. The Class Notes belongs to BIOL 3510 at University of North Texas taught by Dr. Chapman in Spring 2016. Since its upload, it has received 74 views. For similar materials see Cell Biology in Biological Sciences at University of North Texas.
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Date Created: 02/21/16
Week 5: Chapter 11 | BIOL 3510 Notes by Marin Young Sunday, February 21, 2016 5:07 PM Membrane Structure and Function • Membranes are semi-permeable,amphiphilic, and fluid due to a few important chemical characteristics ○ Membranes are made of lipid molecules,loosely defined by Dr. Chapman as "things that do not dissolve well in water" In general, lipids are fairly nonpolar, or hydrophobic Lipids in membranes have polar/hydrophilic regions (as in, one side of the moleculeis polar) □ You probably remember"hydrophilic phosphate heads and hydrophobic fatty acid tails" from talking about phospholipids in earlier bio classes Membrane lipids are called amphiphilic or amphipathic because they're part hydrophobic and part hydrophilic □ This is comparable to a cat who hates getting wet but likes to try to catch running water with its paw ○ Lipids' chemical structure includes hydrocarbon fatty acids, mostlyalkyl groups: Alkyl (alkane) groups with no double bonds are called saturated, meaning they have as many hydrogen atomsas possible Some fatty acids contain one or more cis double bonds □ These fatty acids are unsaturated because they could have more hydrogen atoms(two per carbon-carbon double bond) □ Cis double bonds cause a bend or kink in the fatty acid chain This increases fluidity by reducing the density and strength of intermolecular forces More double bonds correspond with a lower melting point (can get colder without solidifying) □ All this chemistry can be hard to rememberwithout getting something backwards, so instead, think of oil and Crisco/shortening You probably already know that oil has more unsaturated fats and Crisco has more saturated fat, and I'm sure you know that oil is liquid at room temp. and Crisco is fairly solid So, unsaturated = more fluidity at a given temp = lower melting point (can get colder without becoming too solid) □ Example: arachidonic acid, a 20-carbon fatty acid chain seen in signaling molecules called prostaglandins, has four double bonds Note that trans double bonds are rare, except in processed foods ("partially hydrogenated" oils) ○ Membrane fluidity follows the chemical principles of melting points (technically, instead of liquid and solid, the phases are liquid crystalline and gel) Amount of unsaturation is one factor Another is the degree of variety: a larger number of different chemicals increases fluidity and lowers melting point □ This is due to colligative properties from gen chem: adding a solute to water lowers its freezing point by breaking up the interactions of identical molecules □ For this reason, there are about 5,000different lipid moleculesthat can be found in membranes We know of hundreds of thousands of lipid moleculesin total Lipid mobility includes tail flexion (bending due to rotating of single bonds), lateral diffusion, rotation, and flip-flop (from one layer/leafletto the other, in a bilayer membrane) □ The technique Fluorescence RecoveryAfter Photobleaching (FRAP) evaluates mobilityof lipids by lateral diffusion (sliding past each other) Tag a region of lipids with a fluorescent dye, bleach it with a laser, and measure amount of fluorescence in the bleached area over time Fluorescence in that area increases due to diffusion of lipids as equilibrium (in this case, even/random distribution of bleached lipids around the whole membrane)is approached Faster recovery= more mobility Faster recovery= more mobility The density of packing is also related to fluidity; it depends on variety in fatty acids and shapes of phospholipids, plus cholesterolcontent • There are three structural classes of lipids seen in membranes ○ Phospholipids consist of an amino acid (or other polar molecular tag), a phosphate group, a glycerol backbone, and two fatty acid tails, which are usually different from each other The amino acid or other "tag" (such as choline) and the phosphate group make up the polar region The size and chemical nature of the "tag" matters for signaling Chemical classification: a "class" of molecules would be the group of all phosphatidyl serines (phospholipids whose amino acid is serine), while a species is a very specific molecule,such as the particular phosphatidyl serine with fatty acid X and fatty acid Y for its tails Phospholipids form bilayers spontaneously □ Spherical vesicles (when made naturally) or liposomes (when phospholipids in the lab are placed in water, for applications like drug delivery or cosmetics)have a full bilayer surrounding a polar interior □ This is different from micelles or droplets, which have a single layer with the fatty acid tails facing a hydrophobic interior The phospholipid bilayer is selectivelypermeable: only small, uncharged molecules can pass through □ Larger moleculesand any charged moleculesneed a transport protein Attracting ions to the membrane near a transport protein is another use for amino acid tags on phospholipids--the amino acid side groups can attract oppositelycharged ions □ The bilayer itself is about 5 nm, 50 atoms, or 20 amino acid residues thick ○ Glycolipidshave a sugar (monosaccharideor oligosaccharide) attached to a glycogen or ceramide (which is involved in signaling and can form hydrogen bonds) backbone with two fatty acid tails Sphingolipids are a type of glycolipid found in lipid rafts, areas of a membrane with lots of sphingolipids and cholesterolin one ordered region The sugars in glycolipids are very polar because sugars have so many -OH groups (think of glucose, C 6 12) 6 Gangliosides (modified glycolipids in neurons) have many polar sugars Glycolipids can only form their own bilayers under very special conditions ○ Sterols are fused-ring systemswith an attached -OH group for the polar region Plants and somefungi have phytosterols (phyt- means plant) Sterols are too big and nonpolar to form bilayers on their own; like glycolipids, they must be integrated into phospholipid bilayer membranes Cholesterolin particular is important for buffering the effects of temperature on membrane fluidity in animal cells • Membranes perform five important functions: receiving information, transport of molecules,movement of the cell or organelle, compartmentation (to create microenvironmentssimilar to "biochemicalneighborhoods"), and energy transduction involving concentrationgradients ○ Mitochondrial membranesprovide a crucial proton concentrationgradient for aerobic respiration, which generates most of the cell's ATP supply The mitochondrial membranealso organizes (compartmentalizes)the many redox reactions involved in respiration ○ Lipid distribution is variable and asymmetric:the two leaflets (layers) have different compositionsfor functional reasons Glycolipids should face outward since they're used for signaling (example:phosphatidylinositol) Flippases and scramblases (no kidding) are responsible for changing lipid composition □ The Golgi apparatus has specific flippases that help precisely fine-tune lipid composition When vesicles enter or exit the cell, the inside of the vesicle corresponds to the external environment;the cytoplasmicside is always the cytoplasmicside Related: the fluid mosaic model describes the permeability and mobility of membranelipids and their many associated components ○ Many functions of membranesare closely related to membrane proteins, which can be integral (in the membrane)or peripheral (on the membrane) These proteins include transporters, linkers (like integrins, which link the cytoskeletonto the extracellular matrix), receptors, and enzymes Protein structure is asymmetricallike lipid composition □ Disulfide bridges (-S-S-) are uncommonin the cytosolbecause the reducing environment would cleave them (-SH H-S-) □ Positiveinside rule: basic/positivelycharged side chains are more commoninside the cytoplasm than outside the cell Peripheral membrane proteins bind (via weak interactions like hydrogen-bonding and electrostatic attractions) to integral proteins □ These can be isolated by "salting out": add an ionic compound to the sample to disrupt weak bonds and separate the peripheral protein from the membrane Integral membrane proteins can be embedded deep in or all the way through the membrane via hydrophobic interactions, which just means hydrophobic things stick together to hide from water □ The part of a protein "meant" to touch the fatty acid tails is nonpolar, and the parts that go outside the bilayer into the aqueous environmentare polar o □ This is seen in a hydropathy plot, a graph of hydrophilicity in the amino acid sequence (1 structure) of a protein Groups of about twenty hydrophilic amino acid residues indicate transmembrane segments Hydropathy plots are good for seeing the number of times a protein crosses through the membrane (seven in Bacteriorhodopsin,which uses retinal on a Lys to capture light energy, create a proton gradient, and generate ATP) ◊ This plot shows two transmembrane alpha helices, each about 20 amino acid residues long (the two black regions above the middle line) Thickness has to match that of the lipid bilayer ◊ The green polar regions are likely on the cytoplasmicor extracellular surfaces of the membrane They can be any length □ Transport proteins often have alpha-helix channels with polar side chains facing the inside of the channel, like a polar tunnel Porins in mitochondrial membraneshave beta-barrels □ An integral protein can also be outside the membranebut covalentlybound to a membrane lipid G proteins are an example: tethered to membrane □ Isolating integral membrane proteins is hard because they lose their structure if separated from the membrane Detergent (like soap) can help break up a membrane and free a protein, but it can also destroy protein structure by changing the environmenttoo drastically ◊ This is a smaller problem with non-ionic (polar uncharged) detergents ◊ Mixed micelles:detergent tails bind to and stabilize hydrophobic regions ◊ This requires very carefully optimizing the type and concentrationof detergent • Many interesting examples of the diversity of life are related to membraneproteins • Many interesting examples of the diversity of life are related to membraneproteins ○ Photosyntheticreaction center in Rhodopseudomonas viridis: 4 polypeptides (H, M, L, and cytochrome),first membrane proteins to be structurally analyzed There are lots of alpha helices; the L and M subunits (multi-pass transmembraneproteins) have five each Cytochromeis peripheral, or protein-attached ○ Red blood cells' donut-like shape is held by proteins: remember"Actin Anchors the Spectrin Skeleton" Spectin extends from one bridge junction to the next Actin attaches to bridge junctions and other cellular structures Know that transmembrane glycophorins are integral, while spectrin, attachmentproteins, and actin are peripheral ○ Vesicle budding/exocytosisuses membraneproteins to gather moleculesfor export (at a coated pit) ○ Lectins (membrane proteins on endothelial cells lining blood vessels) grab neutrophils to roll them along vessel wall and help them slide into infected tissue They bind neutrophils by recognizing a glycoprotein
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