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Cell Membranes lecture notes.doc

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Kathleen Gilmour

Cell Membranes -Fluid-Mosaic Model proposed in 1972 by Singer and Nicholson Functions of Cell Membranes -important for defining the boundaries of cells and organelles (in eukaryotes) due to their structure and ability to be a permeability barrier -the interior environment can be different from the exterior environment because of the cell membrane -membrane proteins regulate what crosses the membrane in and out -important for organizing enzymes and other structures within the cell -detects and responds to signals -important for cell to cell recognition and forming attachments (i.e.: cytoplasmic links via gap junctions and plasmodesmata) -the structure of the membrane reflects it function -e.g.: inner mitochondrial membrane has a 76% protein and 24% lipid composition (protein-rich membrane for the electron transport chain) while the plasma membrane of a Schwann cell is 18% protein and 82% lipid (for insulation and sultatory conduction) Fluid-Mosaic Model -postulates that the membrane is a lipid bilayer with proteins studded into it -the lipid bilayer acts as the permeability barrier due to its hydrophobic core and the proteins serve specific functions for the membrane and cell -the components of the membrane rely on non-covalent, flexible interactions -it is a very fluid structure and things can move around in it -around 10 nm thick -Frye and Edidin, 1920: -take separate cultures of human and mouse cells and label the proteins of each membrane with specific fluorescent markers -then they took individual cells and fused them together using a virus -the pattern of fluorescence mixed in the hybrid cell surface -this indicated that the labelled proteins were able to move freely throughout the cell membrane; evidence of a fluid, mobile membrane -FRAP (Fluorescence Recovery After Photobleaching) -used to show the mobility of protein / lipids 1) visualize the molecule using fluorescent dye 2) photobleach (bleach out fluorescence) one part of the cell 3) the spot should fill in with fluorescent markers if the entity is mobile; if the entity is not mobile, the bleached spot will not recover 4) estimate fluidity based on its recovery 5) generate recovery curves The Lipid Bilayer -idea of lipid bilayer is attributed to Gortel and Grended in 1925 -take cells and measured area they occupied, broke up the cells to get the lipids and measured the area they covered; they found that the area covered by the lipids was twice as great as the area of the cells, suggesting the bilayer -theoretically: -lipids are often hydrophobic and dissolve in non-polar solvents -they want to gather so that they are not exposed to water -the main component of the bilayer are phosphoglycerides (amphipathic) and therefore will naturally arrange themselves into a double layer with the polar head groups facing the aqueous environment and the hydrophobic tails clustered together inside the membrane and away from the aqueous environment -this is thermodynamically favourable Composition: -phosphoglycerides: -have 4 hydrophilic head groups (serine, choline, ethanolamine, inositol) -typically16-18 C long fatty acid chains -1 chain is saturated and 1 chain is unsaturated which allows for the right degree of membrane fluidity -glycolipids: -lipids that have sugar groups attached to them (i.e.: monosaccharides or oligosaccharides) -typically found in the outer membrane and pointing toward the external environment -involved with cell to cell recognition -e.g.: the use of glycolipids to determine ABO blood groups -sterols: -based 4 hydrogen carbon rings -mildly amphipathic -the -OH group is polar and clusters up to the polar head groups of the phosphoglycerides and allows the hydrophobic region of the sterol to be inserted amongst the fatty acids -the type of sterols in protists vary (i.e.: cholesterol etc.) -important in determining membrane fluidity and building blocks for steroids Properties of the Membrane 1) Membrane Asymmetry -the lipid bilayer has 2 different leaflets that differ in composition -phosphatidylcholine is more prevalent in the outer leaflet -phosphatidylserine, phosphatidylethanolamine, phospatidylinositol are more common on the inner leaflet of the membrane -established during biogenesis when the membrane -maintained because they don’t 'flip-flop' from one leaflet to the other; the hydriphilic heads of one leaflet cannot pass through the hydrophobic core of the membrane and is rare for a phosphoglyceride to move from one leaflet to the other (usually aided by enzymes called flipases) -the membrane components can spin / rotate and move within their leaflet (lateral diffusion) -the sterols are relatively equally distributed -glycolipids are more abundant in external layer but account for less than 1% of lipids 2) Fluidity -i.e.: membranes become to rigid when it is too cold because sensory receptors are not functioning; the membrane proteins are not fluid enough for sensory receptors to function -if cell membranes become too fluid, they no longer function as a permeability barrier -degree of fluidity is regulated by: a) the properties of the tails of the fatty acids -if the fatty acid tails pack tightly, it is rigid / not fluid -if the fatty acid tails are less tightly packed, there is more fluidity b) length and degree of saturation of fatty acid chains -important in determining how well the lipids pack together -long fatty acid chains that are saturated (straight) pack together tightly and give less fluid membranes -a mix of long and short fatty acid chains pack less -unsaturated fatty acids (kinked) pack less c) head group -less polar head groups (i.e.: phosphatidylcholine) pack tightly = less fluidity -highly polar head groups (i.e.: phosphatidylethanolamine) pack less = more fluidity d) sterols -interactions with temperature -low temperatures cause fatty acid chains to pack together = less fluidity -the rigid four- ring structure of sterols / cholesterol will disrupt this packing at low temperatures and makes the membranes relatively more fluid -high temperatures cause fatty acid chains to pack less = more fluidity -because cholesterol / sterols have a rigid structure, it creates less fluidity in the membrane at high temperatures -sterols are said to have a buffering effect; they buffer the effects of temperature on membrane fluidity Homeoviscous Adaptation -most organisms don't maintain a constant body temperature and their membrane function is determined by the temperature of their environment -many organisms adjust the lipid composition of their membrane to match the temperature in order to achieve the right amount of membrane fluidity -homeoviscous adaptation: altering the composition of the membrane to maintain fluidity as temperature changes -if temperature drops, the membranes will tend to become less fluid because of temperature -organisms will activate desaturase enzymes to increase the degree of unsaturation of fatty acids (inserts more double bonds), making them more fluid -as environmental temperature drops, the degree of unsaturation increases -the ratio of PC : PE decreases (more PE than PC); the more polar PE creates more fluidity (since head groups repel each other) -if temperature increases: -membranes become more fluid -this is combated by moving to a less polar head group; the ratio of phosphatidylcholine (PC) : phosphatidylethanolamine (PE) increases Dietary Adjustments of Fluidity -semi-palmated sandpiper -non-stop trans-atlantic flight from the Bay of Fundy to South Africa -eats mudshrimps which are rich in omega-3 polyunsatruated fatty acids (PUFA) -this increases the expression of PUFA in membranes of flight muscles; they have PUFA rich flight membranes -they use fatty acids for endurance -membranes that are rich in PUFA makes it easier to use fatty acids during flight; their endurance is fuelled by fat -they also have an increase in citrate-synthase -this controls the entry of substrate of the citric acid cycle -controls the rate of ATP production -allows for the ability to generate ATP while flying -Bob-White Quails (lazy bird) -What happens if you feed it PUFA? -increase in membrane PUFA -increase in citrate-synthase activity / enzyme activity -prepares for activity simply by diet -the adjustment of fatty acid composition signals cells to increase enzyme activity to prepare for activity Membrane Proteins -the "mosaic" component of the fluid mosaic involves the proteins -membrane proteins are less mobile than lipids -are anchored to other intracellular and extracellular molecules -are larger than lipids and is difficult for them to diffuse in the membrane -polarized cells: involve a protein who's protein complement on 1 side of the cell is different than the protein complement on the other side of the cell (i.e.: cells in the epithelium or cells inside the gut) -proteins function as transporters, enzymes (proteins are organized on the membrane to create certain enzymes), receptors in signal transduction pathways, recognition and attachment (of cells and substrates) -integral (transmembrane) proteins -proteins that are embedded within the lipid bilayer -they cross the lipid bilayer (have a transmembrane domain) -a standard transmembrane domain is a sequence of amino acids that have non-polar head groups and form an a-helix which anchors the protein within the membrane -hydrophilic regions of the protein extend into the cytoplasm or extracellular environment -are amphipathic (the hydrophobic regions are through the core of the membrane and the hydrophilic regions are on either side) -most transporter proteins fall in this category -hard to remove these proteins from the membrane -peripheral membrane proteins -linked to the membrane on either side -have a dynamic relationship with the membrane -are associated with one side or the other through non-covalent interactions -easy to remove from the membrane (by adjust pH or ionic strength of the membrane) -lipid-anchored membrane proteins -linked to the membrane on either side -unique pro
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