BIOL 1020 – CHAPTER 7 LECTURE NOTESChapter 7: Membrane structure and functionI. Roles of biological membranesA. membranes separate aqueous environments, so that differences can be maintained1. the plasma membrane surrounds the cell and separates the interior of the cell from the external environment2. membrane-bound organelles have their interior region separated from the rest of the cellB. passage of substances across membranes is generally regulated, helping to establish and maintain appropriate environments in the cell even as the outside environment changesC. membranes provide a surface on which many chemical events can occur1. enzymes embedded in membranes catalyze many chemical reactions, and the locations of reactants and products on oneside or the other of the membrane is often used to help control reaction rates2. proteins and glycoproteins embedded in membranes are used for chemical recognition and signalingII. Physical properties of cell membranes: the lipid bilayer and the fluid mosaic modelA. biological membranes are lipid bilayers with associated proteins and glycoproteins1. most of the lipids involved are phospholipids, although others like cholesterol and various glycolipids are also presentB. phospholipids molecules spontaneously form bilayers in aqueous environments due to their amphipathic nature and overallcylindrical structure1. amphipathic molecules have distinct hydrophobic and hydrophilic regions2. recall the hydrophilic “head” and hydrophobic “tails” of phospholipids- tails come from two chains of fatty acids linked to glycerol- head comes from a polar organic molecule linked via a phosphate group to the glycerol backbone3. the two tails combine with the head to give a roughly cylindrical shape to the phospholipids molecule, a shape that favors the formation of lipid bilayers over lipid spheres4. there are other amphipathic molecules, such as detergents (soaps, etc.), that come to a point at their single hydrophobictail, thus tending to form spheres instead of bilayers5. detergents can “solubilize” lipids to varying degrees; high enough concentrations of detergents will disrupt cell membranesC. the fluid mosaic model describes the structure and properties of cell membranes1. while a structural model including a lipid bilayer was proposed in the 1930s, early models sandwiched the lipid bilayer with membrane-associated proteins2. EM data after the 1950s showed that membrane bilayers are uniformly about 8 nm thick, too thin for the sandwich model; also, isolated membrane proteins were often found to have a globular nature that did not fit the sandwich model3. in 1972, the fluid mosaic model was proposed where some proteins are imbedded in lipid bilayers that act as two-dimensional fluids; this model explained the existing data and made two key predications that have been verified:- materials, including embedded proteins, can be moved along the membrane due to its fluid properties- digestion of certain “transmembrane” proteins applied to one side of a membrane will produce protein fragments that differ from those found if digestion is done only on the other side4. biological membranes act as two-dimensional fluids, or liquid crystals- free to move in two dimensions, but not in the third, the molecules of the membrane can rotate or move laterally- molecules rarely “flip” from one side of the membrane to the other (that would be movement in the third dimension)- the fluidity of a membrane is a function of both temperature and the molecules in the membrane cells need membranes to be within a reasonable range of fluidity – too fluid and they are too weak, too viscous and they are more like solid gels at a given temperature, phospholipids with saturated fats are less fluid than those with unsaturated fats in an unsaturated fat, a carbon-carbon double bond produces a “bend” that causes the phospholipids to be spaced further away from its neighbors, thus retaining more freedom of motion the upshot is: at colder temperatures, unsaturated fats are preferred in cell membranes; at higher temperatures,saturated fats are preferred other lipids, such as cholesterol, can stabilize membrane fluidity- organisms control membrane fluidity by several means by regulating their temperature by changing the fatty acid profile of their membranes by adding fluidity modifiers or stabilizers like cholesterol5. biological membranes resist having open ends- a lipid bilayer will spontaneously “self-seal”- usually, this results in nearly spherical vesicles with an internal, aqueous lumen the spherical tendency can be modified with structural elements, such as structural proteins1 of 3BIOL 1020 – CHAPTER 7 LECTURE NOTES winding membrane surfaces must be kept far enough apart and structurally supported to prevent them from self-sealing vesicle formation takes advantage of self-sealing as regions of membrane are pinched off by protein contractilerings- fusion of membrane surfaces can occur when they are in close proximity fusion is common between vesicles and various organelles contents of two separate membrane-bound lumens are mixed when fusion occurs fusion of vesicles with the plasma membrane delivers the material in the vesicle lumen to the outside of the cellD. membrane-associated proteins1. membrane proteins are classified as either integral or peripheral- integral proteins are amphipathic proteins that are firmly bound to the membrane, and can only be released from the membrane by detergents- some integral proteins are transmembrane proteins, extending completely across the membrane hydrophobic alpha-helices are common in the membrane spanning domains of transmembrane proteins some wind back-and-forth across the membrane, but most only span the membrane once- peripheral proteins are not embedded in the membrane; they are usually bound ionically or by hydrogen bonds to a hydrophilic portion of an integral protein2. the protein profile of one membrane side typically differs from that of the other side- many more proteins are on the cytoplasmic side of the plasma membrane, as revealed by freeze-fracturing plasma membranes- the types of processing that a protein receives differs depending on the target side, or if it is integral3. membrane proteins perform several functions, including acting as enzymes, regulating transport across the membrane, and in cell signalingIII. Transport and transfer across cell