BIOL-L 312 Cell Biology (Mehta) Lecture Notes - Week 3
BIOL-L 312 Cell Biology (Mehta) Lecture Notes - Week 3 BIOL-L 312
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This 6 page Class Notes was uploaded by Ifeoma O'Gonuwe on Friday January 30, 2015. The Class Notes belongs to BIOL-L 312 at Indiana University taught by Sapna Mehta in Spring2015. Since its upload, it has received 72 views. For similar materials see Cell Biology in Biology at Indiana University.
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Date Created: 01/30/15
127 Notes LECTURE FIVE MEMBRANE STRUCTURE Review 0 Temperature cooler temperatures would make the phospholipid membranes to switch from a fluid state to a more solid state allows them to pack more closely together and vice versa The specific temperature depends on the characteristics of the lipids within that particular membrane 0 Saturationunsaturation Unsaturation causes more fluidity this leads to more double bonds which creates kinks so those kinks are what prevents the molecules from packing as close as they would if they didn t have the kinks o Membranes with greater unsaturated fatty acid composition are more fluid kinks prevent tight packing The Big Reveal 0 Fatty acid HC chain length the longer the tail the less fluid the membrane 0 Why Because you can maximize interaction of fatty acids hydrophobic interactions If you were a cell and you needed to regulate membrane fluidity what would you do 0 Desaturate fatty acids 0 Produce more unsaturated fatty acids 0 Change tail length Cholesterol and Membrane Fluidity 0 Unique fluidity buffer 0 Distinct effects 0 Body temperature 0 Interferes with movement of PL fatty acid chains reducing permeability to small molecules 0 Lower temperatures opposite effect The interference prevents membranes from freezing and maintains fluidity 0 Cholesterol is a steroid It is also amphipathic What it does to the fluidity of the membrane depends on the membrane this is a fluidity buffer It alters the properties of the membrane Cholesterol will do the opposite thing doing what the membrane consists of For example if you have a membrane with a lot of unsaturated fatty acids it would take a very high temperature to make it frozen It adjusts the temperature that makes something fluid or not Membrane fluidity model refined o Lipid rafts model the proteins aren t all floating around there are actually proteins that have a function to make some sort of raft Membranes Exhibit Asymmetry o Asymmetry the composition of the inner layer of the membrane can be dramatically different than the composition of the outer layer All membranes will have this concept of asymmetry The reason why cells have this is so that signaling can happen Most cells that do have a net negative charge will have to be on the inside If it does it will then flip and the signals will be able to tell that the cell is actually dead because it is abnormal for a cell to have its net negative charge on the outside Membrane Proteins o It is the proteins that give the membranes its functions and properties Three Basic Categories of Membrane Proteins Integral proteins will have both hydrophilic and hydrophobic components Peripheral membrane proteins are associated with the membrane Lipid anchored is where you modified the membrane with a lipid by attaching something that is hydrophobic o What do you predict about the R groups That they would be nonpolar Because they are the ones that are within the membrane 0 You can form an alpha helix a beta barrel or a poretransporter Carbohydrates are on the outside of the plasma membrane because they are mostly polar 129 Notes LECTURE SIX MEMBRANE STRUCTURE MEMBRANE TRANSPORT How do we study membrane lipids membrane proteins and membrane function Membrane Systems 0 1 Source of cells or tissues 0 2 Subject cells to technique that ruptures the cell homogenization o 3 Lysate that has many different membranes in it o 4 Centrifugation separate components 0 Nucleus low speed ER vesiclehigh speed 0 Membranes buoyant densities differ 0 Separate by density 0 Simplest membrane in animal 0 RBC of an animal 0 A RBCall of the intracellular organelles are expelled and at least in mammals lacks intracellular membranes 0 So ideal source for studying just constituents of plasma membrane Classic Experiment Demonstrated that lipids are organized as bilayer o Langmuir Trough Measures the surface area that hydrophobic molecules spread out to on air water interface 0 Gorter and Grendal First Experiment 0 Found that area spreadcovered by the lipids was twice the area they calculated of the surface area of their cells by looking at the shape 0 Possible explanation o Lipids are arranged in a bilayer Artifical Membrane Systems 0 Liposomes look like cell membranes allows you to ask questions about the phospholipids 0 Cell membranes are made of both lipids and proteins 0 For the longest time there was no model describing how proteins were involved with lipids 0 Example old model was sandwich model protein on exterior bilayer protein I Problem proteins needed to be hydrophilic for the sandwich model I Researchers started to realize that proteins are in fact amphipathic therefore the model they were using was incorrect FreezeFracture Technique 0 1 Strike a frozen cell with a knife 0 2 Fracture slipids the lipid bilayer preparing the cell surface for scanning electron microscopy 0 On one surface you would see mounds membrane protein 0 On other surface you would see pits contact between the membrane protein and the membrane 0 The freezefracture technique gave evidence to support the fluid mosaic model not the sandwich model Detergents 0 Why are detergents used to disrupt membranes because of their ability to tear apart the membrane can separate membrane proteins 0 Do not use an ionic detergent because if it is a strong enough detergent it can denature the protein because the ionic portion of the detergent will get into the secondary structure of the protein and unravel it How detergents work 0 Low concentration detergents molecules partition the bilayer 0 As concentration increases the bilayer is saturated with detergent molecules and breaks apart 0 Get proteindetergent complexes detergent and detergentlipid micelles Detergents can Solubilize Membrane Proteins 0 Once solubilized how do you study the function of the membrane protein if it is not in its native environment 0 Insert a protein into an artificial membrane liposome o Liposomes containing a single type of membrane protein are useful in studying the function of specific proteins 1 Disrupt membrane 2 Solubilize protein with detergents 3 Mix proteindetergent complex with phospholipids 4 Dialyzedilute to remove detergent 0000 o 5 Perform studies to understand the function of the protein Membrane Proteins are Able to Move Laterally 0 Edidan and Fry Experiment 0 They postulated that if they took two different cells mouse and human cell with different surface proteins to create a heterocaryon fusion cell They would then mark the proteins and see if over time the proteins would distribute all over the cell Proving that the proteins would move within the bilayer I Marked the cells using antibodies labeled with different dyes o FRAP would give you the same information Membrane Proteins can be Restricted to Certain Domains 0 Four ways to restrict the lateral mobility of plasma membrane proteins 0 Lipids rafts are examples of restricted domains for proteins 0 Tight junction separates proteins from the apical plasma membrane and basal plasma membrane 0 Four ways to restrict the lateral mobility of plasma membrane proteins 0 1 Proteins selfassemble into large aggregates o 2 Tethered by assemblies of macromolecules outside the cell 0 3 Tethering inside the cell 0 4 Interactions with another cell I Example tight junction Red Blood Cells 0 What gives red blood cells their characteristic shape 0 What supports thin fragile membranes 0 Have be pliable to be able to squeeze through capillaries etc 0 Answer Spectrin a log rod like protein 100nm in length that forms a filamentous meshwork and makes contacts with membrane proteins at various junctions within the cell 0 Because of the shape size and length of spectrin there is a flexible link between domains in spectrin allowing for flexibility of the membrane without breaking Biological Membranes are Semipermeable 0 Pure phospholipid bilayer membrane permeability only allows for hydrophobic molecules 02 C02 N2 and steroid hormones by diffusion and small uncharged polar molecules 0 Ions Hi Na39HCO39 K Ca2 Cl39 Mgz cannot ever go through the membrane Movement of Water Itself o 1 Start with more solute on one side of the lipid bilayer than the other using molecules that can t cross the selectively permeable membrane 0 2 Water undergoes a net movement from the region of low concentration of solute high concentration of water to high concentration of solute low concentration of water 0 The above two steps describe osmosis and explain why placing RBCs in water 9 lyses the cell Each Cell Membrane Transports Specific Molecules 0 Proteins are specific for different molecules to transport them into the cell 0 The specificity for ion transport proteins are specific enough to the actual ion I Sodium pumps will not transport potassium Three Main Classes of Transport Proteins o 1 Pump require energy because it transports against the concentration gradient 0 2 Carrier transporters passive transport strong interaction with the solute 9 conformational change of the protein to allow the solute to enter the cell 0 3 Channel passive transport weak interactions with the solute it is more like an aqueous pump in which control of openingclosing the pump is given Pump Carrier Channel Specificity Absolute Intermediate Only 1020x Rate ionss 100 lt1000 106 Gradient Uphill Downhill gtmight do Downhill uphill solutes Energy input Required No No IonsConformational 1 1 Many change Passive Transport Channels and Some Transporters 0 Passive transport is driven by concentration gradients and electrical forces facilitated diffusion 0 Simple diffusion uses no proteins and the molecule will just diffuse down the concentration gradient into the cell 0 Concentration gradient ions move across a membrane from high to low concentrations 0 Electrochemical gradient ions move across a membrane from either positive or negative concentration 0 Both electrochemical and concentration gradient drive transport of ions 0 Voltage across the membrane electrochemical gradient 0 High for sodium low for potassium Active Transport Pumps 0 Passive transport is driven by concentration gradients and electrical forces 0 Active transport pump is driven by energy 0 Why going against the concentration gradient 0 Channels are always passive but transporters can be both passive and active