BCHM 3010 Membranes
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This 5 page Class Notes was uploaded by Morgan Dimery on Sunday February 21, 2016. The Class Notes belongs to 3010 at a university taught by Dr. Cheryl Ingram-Smith in Spring 2016. Since its upload, it has received 19 views.
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Date Created: 02/21/16
Membranes • Membranes provide a barrier between a cell and its environment, and in eukaryotes they divide the inside of the cell into compartments o Selectively permeable-‐ regulates what goes in and out of the cell • Polar lipids, proteins, and carbohydrates are in membranes o Lipids are what cause the membrane to form • Lipids form structures to hide the hydrophobic regions-‐ it also depends on the type of lipid and the concentration • They can form micelles, liposomes, and bilayers (membranes) • Micelles form in solutions of amphipathic molecules-‐ the head is bigger than the tail o There are a few thousand lipid molecules for each micelle o The head is pointed out and the tail is in-‐ units are wedge shaped o Different lipids have a different critical micelle concentration that they start to aggregate into micelles at • Vesicles (liposomes) are double layers of lipids o Small bilayers spontaneously become vesicles o The central aqueous cavity is able to enclose dissolved molecules-‐ this makes them good for carrying things such as drugs o They can fuse with each other or to cell membranes o The head groups are on the outside, but also in the very middle of the ball o Whatever is inside the cavity will get released inside the cell • Bilayers are two lipid monolayer leaflets-‐ essentially just layering lipids together o Heads (hydrophilic) are outside, and tails (hydrophobic) are inside o They separate the inside from the outside and control what goes in and out of the cell-‐ not many things can go through o They are flexible, but proteins make them more rigid o Proteins help charged things get through the hydrophobic tails and are also involved with signaling on the outside o Membranes import nutrients into the cell, export waste and toxins out of the cell, and retain other things inside of the cell • The fluid mosaic model shows how the viscous solvent formed by lipids is like an ocean-‐ membranes are not simple things, there are many things in them!! • The composition of membranes varies a lot o Different amounts of proteins o Different phospholipids o Different sterols-‐ cholesterol is found in some places but not in others o Sometimes protein can be about 75% of the membrane for some cells Organelle membrane composition varies-‐ even the inner and outer membranes of mitochondria are not the same • Bilayers are asymmetric o Cells regulate what is in each leaflet so they vary from each other o The outer leaflet is more positive o Phosphatidyserine is usually more inner, if it is on the outside then it is signaling something-‐ sometimes it means that the cell needs to die o Different sides of bilayers have different composition-‐ some things are only found on one side (inner/outer) • Proteins have many functions in membranes: o Receptors-‐ detecting signals from the outside o Channel, gates, and pumps-‐ these are transport proteins, they help things move across the membrane o Enzymes-‐ if enzymes are found here it is because they are more efficient this way • There are 3 types of membrane proteins 1. Peripheral 2. Integral 3. Amphitrophic-‐ can be touching the membrane, or not touching it • Peripheral membrane proteins associate with the polar head group of the membrane o They are soluble proteins, only loosely associated with the membrane o Can be removed by messing up ionic interactions or changing salt concentration/pH o Once they are purified off they are no longer associated with any lipids o They are just sitting on the membrane, not stuck on there • Integral membrane proteins are tightly associated with the membrane-‐ they will not fall off easily o Hydrophobic regions in protein interact with hydrophobic regions of lipids o Detergents can be used to take the membrane apart, but the protein must be associated with detergent or a lipid o They can be made up of alpha helices or beta sheets, but never both at the same time • Hydropathy plots help you to predict if you will have 20 residues or more in a row that are all hydrophobic-‐ this is based off of alpha helices • Integral proteins vary in structure o Even if most of the protein is outside of the membrane, it is still considered integral because if won’t come off easily o Helices from different proteins can associate with each other-‐ this way the inner part of the complex can be hydrophilic, and all of the outside of the complex is hydrophobic. This is how channels are formed in membranes o Beta sheets can behave similarly-‐ sides can be different on beta sheets. Beta sheets can form circles together with the outside being hydrophobic, and the inside being hydrophilic-‐ this forms a hole in the membrane that acts as a channel • Amino acid type varies throughout the protein o Nonpolar residues are mostly found on the inside o Tyr and Trp (aromatic) are found primarily where nonpolar and polar regions meet each other o Charged resides are found closer to the outside where there is an aqueous environment, or surrounded on the very inside of the bilayer • Some membrane proteins are covalently linked by a lipid-‐ they anchor the protein to the membrane o Some examples are sterols and GPI anchors § GPI anchors are attached to the membrane by a lipid but they also have a carbohydrate coming off of the other side of the protein. They can help pathogens hide from the immune system • Organisms are able to adjust the composition of their membranes to maintain proper fluidity-‐ fatty acid composition mostly determines this o You don’t want the membrane to get too rigid o When there is a lot of fluid (lower temperature) you need more unsaturated fatty acids so it doesn’t get too fluid o At higher temperatures you need more saturated fatty acids so they can pack tighter and become more fluid o An organism always needs some amount of packing in their membranes so that they stay uniform • Membranes contain many sterols and hopanols that play roles in membrane fluidity o Eukaryotes have sterols such as cholesterol, and prokaryotes have hopanols o Cholesterol doesn’t pack well with saturated fatty acids because it is too bulky, but it can help with packing of unsaturated fatty acids o At lower concentrations is helps the fluidity by preventing tails from packing too tightly together o At higher concentrations it interacts with tails to keep them from moving so that they can pack together with each other • Lateral diffusion is when lipids diffuse within the same leaflet-‐ very fast! • Transverse diffusion is when lipids diffuse from one leaflet to another-‐ rare and very slow! • In membrane diffusion there are enzymes that catalyze transverse diffusion o Flippase: outside to inside-‐ needs energy! o Floppase: inside to outside-‐ needs energy! o Scramblase: randomizes the membrane, can only go down the gradient-‐ doesn’t need energy! • Lipid rafts are areas in the membrane that do not behave the same as the rest of the membrane o Contain clusters of glycosphingolipids with longer than usual tails o More ordered o Higher in cholesterol o May have some kind of signaling going on here o Quite prevalent in membranes o Caveolin is here and it makes the membrane curve and gives it certain properties Only small, nonpolar molecules are able to cross the membrane themselves-‐ other things need to be able to get across too and proteins help this along!!!! • There are different types of membrane transport: o Simple diffusion o Facilitated (passive) diffusion o Primary active transport o Secondary active transport o Ion channels § Electrochemical gradient is used for the movement of ions-‐ it takes charge into consideration-‐ you want there to be the same charge on the inside and outside o Ionophores • In simple diffusion solutes move down the concentration gradient to make the concentrations equal on both sides (high to low concentration) o Only small, nonpolar molecules can do this • In facilitated diffusion polar solutes can move through the membrane down the concentration gradient with the help of transporters (permeases) o They are able to help because polar things like to hydrogen bond, the protein (permease) makes hydrogen bonds with the solute to replace the ones it had with water-‐ this lowers the energy barrier • Transporters and channels are different from one another: o Transporters are much more specific and slower-‐ they’re saturable, they can only go so fast-‐ glucose is moved through in this way o There are two doors, one on each side of the membrane-‐ the first one opens and lets the solute inside, and this one must close before the other door can open and let the solute all the way through the membrane o Channels have a much broader range of things that they can let through-‐ it is much faster, almost as fast as diffusion, and not saturable o An electrochemical gradient is used in channels o There is only one door-‐ solute will flow until it is the same on the inside and the outside In passive transport, molecules can only move down a concentration gradient. No energy is required! In active transport molecules can move against the gradient, but energy is required! • There are two types of active transport: o Primary: uses energy from a chemical reaction-‐ ATP is common for this. You already have a lot on one side, but you want there to be even more! o Secondary: you have something that you want to move that can’t use ATP by itself, so you use the ATP to move something else-‐ the gradient (built up energy) from this this other transport provides the energy to move the solute that you originally wanted to use § Uniport-‐ when you are moving one thing § Cotransport-‐ when you are moving two things • Symport-‐ both things move in the same direction • Antiport-‐ things move in opposite directions • Glucose is helped through the membrane by alpha helices and beta sheets-‐ they can have a polar side and a nonpolar side o This can allow for hydrophilic parts to be on the inside and this will keep the polar molecule happy throughout transport • Ionophores are very unregulated o Down a concentration gradient o Like making a hole in the membrane o Destroy ion gradients-‐ they’re poison! o This is a way for bacteria to protect themselves • Aquaporins allow ONLY water to pass through membranes very rapidly + o They are positively charged to keep things like H O o3t o Different types of cells have different types of aquaporins If the concentration gradient is ever destroyed, the cell will die!!!!
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