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Dalhousie University
BIOL 2020

Membranes Membrane structure SEE SLIDE 2 (CELL 3 MEMBRANES) 2 layers - bilayer (almost all membranes are bilayers) 2 layers held together because water on one side reacts with hydrofilaments Fatty acid tails on inside We know that this is a plasma membrane because of the sugar Integral proteins: span the membrane Bitrophic: when the protein spans both layers Polytrophic: when the protein spans the membrane more than once Monotrophic: when the protein spans only one leaflet Peripheral protein: protein that is loosely attached and doesn't go into the leaflet All biological membranes are bilayers of phospholipid, sugars and proteins SEE SLIDE 3 (CELL 3 MEMBRANES) Given names E face (ectoplasmic face - exterior) and the P face (protoplasmic face or cytoplasmic face) The 2 leaflets are NOT the same Every lipid bilayer is asymmetrical - each leaflet is unique Membrane Functions ONE: Compartmentalization Cell membranes are continuous bilayers and unbroken Plasma membrane encloses the entire cell Allows for independently regulated processes in each compartment (SEE SLIDE 7 - CELL 3 MEMBRANES) The thickness of a membrane is somewhere between 5-10nm - on average it is 7-8nm Can change in thickness depending on what lipids they use and where those lipids are found - heterogeneous Can take membrane liquids, add water and shake them up - will form a liposome Some phospholipids will form micelle - found in soap January 15th (Lecture 4) (SEE SLIDE 8 - CELL 3 MEMBRANES) Most commonly used to study membranes is the red blood cell (don't have organelles or nuclei - in humans) On cilia the membrane goes around it Membrane functions TWO: Selectively permeable barrier Barrier from the outside world - regulating the passage of molecules Regulation of passage of specific molecules Receptors or channels for transporting substances Pure phospholipid bilayer membranes are impermeable to all proteins and to most small molecules SEE SLIDE 10 (CELL 3 MEMBRANES) If it's tiny and polar (uncharged) they will go through the membrane (e.g. gases) Anything with a charge cannot pass through the membrane Water is complicated -- it's polar (hydrogen is slightly positive and the oxygen is slightly negative) but because of its tiny size it can cross the membrane Lipid bilayers don't mix with water so why can it go through the membrane? Movement of solute molecules across biological membranes SEE SLIDE 11 (CELL 3 MEMBRANES) Indirect - ATP isn't used directly but it sets off the gradient Aquaporins Actual channel for water molecules Tiny proteins Have a shape like an hourglass where water molecules enter the top and go single file through the pore and this explains why water molecules can go across biological membranes A billion water molecules can go through in 1 second Membrane functions THREE:Response to external environment Signal transduction Receptors - ligands (receptors imbedded into the membrane) FOUR: Intercellular interactions Cell-cell recognition Cell-cell signalling Cell-cell adherence (SEE SLIDE 14 - CELL 3 MEMBRANES) Polymers: proteins are polymers of individual amino acids Polysaccharides: polymers of monosaccharides Property that all of them have in order to be called a fat/lipid is that they all have hydrophobic parts to them Polar head group is hydrophilic Long-chain fatty acids SEE SLIDE 15 (CELL 3 MEMBRANES) 6 to 30 CH2 residues If they have the maximal amount of hydrogen they are said to be saturated Oleate: has a double bond in the middle - that double bond is in the cis position (like most of them) Trans is rare - hydrogen on different sides Polyunsaturated: 2 or 3 double bonds Triacylglycerol SEE SLIDE 16 (CELL 3 MEMBRANES) There is always an even number of carbon Can attach a fatty acid tail to its carboxyl group to an alcohol (found on glycerol molecule) Fat that we store can move in blood stream We store fat for long term energy storage Can compact themselves and form sold fat if they have a lot of double bonds We don't do this but plants do Store glycogen You can change linseed oil and convert them by eating them with a catalyst into solids (margarine) Converting cis into trans and making vegetable oil instead of liquid into shortening - tastes better January 17th (Lecture 5) Typical phospholipid SEE SLIDE 18 (CELL 3 MEMBRANES) Lipids are amphipathic meaning they have a hydrophobic part (nonpolar tail) and a hydrophilic part (polar head group) Have a glycerol backbone Attached to them is the long chain fatty acid -- linked up to an alcohol Ester linkage Plasmalogen: has an ether linkage - stronger lipid - resists oxidation Phosphate is negatively charged A lot of phospholipids give a negative chafe onto the membrane (SEE SLIDE 19 - CELL 3 MEMBRANES) Phosphoglycerides get their name because they have a glyceride backbone Head group is what gives it the name Cardiolipin is found in mitochondria and the heart - inhibits protons from going through the membrane Sphingolipids: Rare - do not have glycerol as the backbone Attached to an amino acid (Ceramide) All the sugars have a different backbone than phospholipids Ganglioside: found in myelin and if you produce too much of it,it's bad Blood group antigens - glycolipids SEE SLIDE 21 (CELL 3 MEMBRANES) Have sugars attached A blood has an extra enzyme (GalNAc) If B you add Gal If A you add GalNAc If AB you add both People with O blood-type have less problems with malaria and have antibodies to A and B Steroids All have this structure of 4 rings and are referred to as being planar Cholesterol is amphipathic Cholesterol forms testosterone and estrogen Without it you wouldn't be a man or a woman Cholesterol SEE SLIDE 23 (CELL 3 MEMBRANES) An important and amphipathic constituent of mammalian plasma membranes Don't need to know this pathway in FULL detail There are a lot of people who make or eat to much cholesterol If you have high levels of cholesterol you have high levels of LDL = obese Statins: prescribed drug for lowering cholesterol HDL - high density liquid protein (maybe, idk) - if you have HDLs you will be skinny and happy - (: - and you'll live longer -- poop out cholesterol (SEE SLIDE 24 - CELL 3 MEMBRANES) Cholesterol's usefulness: attaches itself (non- covalently) to hydrogen bonds - tail has a kink Cholesterol has properties that increases fluidity When we go outside and our skin gets cold, instead of your cells cracking and dying, cholesterol keeps the membranes filled with fluid Also holds the membrane together in hot situations (SEE SLIDE 25 - CELL 3 MEMBRANES) SM has 22 CH2s Phospholipids involved in membrane curvature - when the membrane has to make a big curve you will find many PEs not PCs Lipid compositions of some biological membranes SEE SLIDE 26 (CELL 3 MEMBRANES) Human myelin has a higher concentration of glycolipids Cardiolipin is high in mitochondria and low in other tissues Asymmetry The 2 leaflets of a bilayer membrane are often different in their lipid composition Sphingolipids are mainly in the outer leaflet (exoplasmic) of the plasma membrane Phosphatidylserine and phosphatidylethanolamine are mainly in the cytosolic leaflet of the plasma membrane Every cellular membrane has an exoplasmic and a cytosolic face SEE SLIDES 29, 30 (CELL 3 MEMBRANES) Black in on the outside, red is on the inside A lot of the organelles (the ER, golgi, lysosomes, vesicles) all have a cytoplasmic face on the inside and not the outside Sometimes the inside is actually the outside (Slide 30): Make a vesicle - now the grey is on the inside (the outside in the inside) (SEE SLIDE 31 - CELL 3 MEMBRANES) Can take cells or membranes and freeze them (liquid nitrogen is most common) - use a very cold knife and shatter the cell and it will split between the 2 lipid bilayers After you break it you can study each membrane by using antibodies to it or by using freeze fracture Membrane fluidity SEE SLIDE 34 (CELL 3 MEMBRANES) If the phospholipids have double bonds in them they are more fluid Eicosapentanoic acid (EPA) is polyunsaturated and is one of the essential fatty acids. The other one is is DHA Omega has a double bond going backwards January 20th (Lecture 6) Membrane fluidity Increasing the double bonds makes the membrane more fluid Another thing that affects membrane fluidity is cholesterol - at high temperatures it holds the membrane together Homeoviscous adaptation An organism can change their membrane fluidity - bacteria does this all the time At high temperatures it has an enzyme that puts in more double bonds Controversial whether or not animals can do this (SEE SLIDE 36 - CELL 3 MEMBRANES) Unsaturated membranes are thinner than saturated membranes If we have no double bonds the lipids are densely packed - need a very high temperature The longer the acyl tail, the higher the melting temperature Fluid mosaic model Membranes are fluid Proteins penetrate the bilayer and exist as a mosaic Asymmetry to membrane (lipid, protein and sugar) The lipid bilayer is the core of the model Membranes can move Sugars are always on the exoplasmic face (the P face) Factors affecting fluidity Length of the fatty acid side chains Presence of double bonds in the fatty acid chains (degree of saturation) Cholesterol SEE SLIDE 40 (CELL 3 MEMBRANES) Lipids can diffuse rapidly in the bilayer, but with the exception of cholesterol, lipids do not flip from one leaflet to the other Membranes are self sealing (repairing) - when one microinjects into a cell, the membrane seals automatically when the needle is withdrawn Phospholipids spontaneously for bilayers in water Lipids are always synthesized on the P face - how do they get to the E face? Why can membranes seal? Lipids show lateral diffusion In bacteria one phospholipid can move from one end to the other in 1 second -- in our cells this is closer to 1 minute They are also rotating Why does it take so long for 1 phospholipid to go from 1 membrane to the other? The charges Need to put a charge to a hydrophobic group (cytosol end) which is very difficult Phospholipid synthesis SEE SLIDE 43 (CELL 3 MEMBRANES) Don't need to know the pathway or structure The initial part of assembling a phospholipid starts with acetyl CoA The enzyme that builds phospholipids are integral proteins which are hydrophobic Fatty acid comes in Flippase: the enzyme that takes a phospholipid from the cytoplasmic side to the exoplasmic side Flopase: enzyme that will take a phospholipid from the exoplasmic side to the cytoplasmic side Scramblase: enzymes that do both When we put them
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