Membrane Structure PDF

Membrane Structure Session Learning Outcomes (SLOs): 1. Outline the components of biological membranes. 2. Describe the properties of the lipid bilayers that compose biomembrane 3. Describe the roles of phospholipids, cholesterol in cell surface membranes fluidity 4. Explain the fluid state o...

Membrane Structure Session Learning Outcomes (SLOs): 1. Outline the components of biological membranes. 2. Describe the properties of the lipid bilayers that compose biomembrane 3. Describe the roles of phospholipids, cholesterol in cell surface membranes fluidity 4. Explain the fluid state of membranes and the movement of its lipids 5. Describe the impact of temperature, fatty acid composition, and cholesterol presence on membrane fluidity. 6. Outline the types of membrane proteins. 7. Explain the different ways proteins can be associated with a membrane 8. Explain the movement of the cell membrane proteins. 9. Describe the roles of glycoproteins in cell surface membranes and proved examples of its importance. Fluid Mosaic Model of Biomembrane JIN ow epic IAIN JIM Fluid Mosaic Model 1- PHOSPHOLIPIDs 2. CHOLESTROL Glycoprotein 3. GLYCOLIPIDs Integral protein peripheral protein 4. Cell Membrane PROTEINs 5 CARBOHYDRATEs Plasma membrane 1. Affect shape and function 2. Anchor protein to the membrane St 3. Modify membrane protein activities 4. Transducing signals to the cytoplasm Do “A living cell is a self-reproducing system of molecules held inside a container – “the plasma membrane” Membrane comprised of lipid sheet (5 nm thick) Primary purpose - barrier to prevent cell contents spilling out BUT, must be selective barrier  Eukaryotic membrane are dynamic, fluid structures and most of their molecules move in the plan of the membrane Stacked membranes of Golgi complex  General Structure All biological membranes have a common structure Thin film of lipid and protein molecules held stabilize the cell together by noncovalent interactions membranestructure 1- PHOSPHOLIPIDs The most abundant membrane lipids are the phospholipids o Phospholipid have a polar head (H2O loving) group and two hydrophobic hydrocarbon tails (H2O hating) The tails are usually fatty acids, and they can differ in length. One tail usually has one or more cis-double bonds, while the other does not.  Amphipathic Molecules Contain both a hydrophilic and a hydrophobic portion to the molecule Other molecules are amphipathic – Steroids – Glycolipids – lipid with a sugar attached rather than a phosphate group Hydrophilic and hydrophobic molecules interact differently with water Hydrophilic molecules can dissolve in H2O due to the polarity of both of these molecules – H bonds and other non-covalent interactions may aid in this  so Hydrophobic molecules will be “caged” by the polar molecules – requires energy Why when fats or oils are placed in water that they usually sit as a glob on the surface action Thief  The Lipid Bilayer Membrane lipids are amphipathic molecules, most of which spontaneously form bilayers Phospholipid bilayers: two molecule thick, formed the cell membrane Two important of lipid bilayers: 1. Hydrophobic core: prevent the diffusion of water-soluble molecules. It regulated by jygwy. specific.it membrane proteins. Ectenteiatrh 2. Stability by hydrophobic and van der Waals interactions Entail between the lipid chains. if it action a The spontaneous closure of a phospholipid bilayer to form a sealed 6 In compartment son my si in The formation of a sealed compartment is fundamental to the creation of a living cell, and this behavior follows directly from the shape and amphipathic nature of the phospholipid molecule The shape and amphipathic nature of the lipid molecules cause them to form bilayers spontaneouly in aqueous environments. min hole hydrolytic cancarrywater solublemolecules drugs Due to the amphipathic nature of phospholipids, these molecules spontaneously assemble to form closed bilayers Packing arrangements of lipid molecules in an aqueous environment Ida Is Lipid molecules spontaneously aggregate to bury their hydrophobic tails in the interior and expose their hydrophilic heads to water. Being I cylindrical, phospholipid molecules spontaneously form bilayers in aqueous environments  Yim t.si Membrane proteins can be solubilized and purified in detergents i we Detergents: Used to remove the proteins from the membrane isolate Amphipathic molecules Have a single hydrocarbon tail Jumo Form small clusters in aqueous solutions called micelles SDS and Triton X-100 common in the laboratory Structure and function of detergent micelles  if Solubilizing membrane proteins with a mild detergent distrub  of The detegrent have same component the phospholipids The use of mild detergents for solubilizing, purifying, and reconstituting functional membrane systems lyses Tim study the to of the function pumb Formation of and study of it pure phopholipid bilayer j.bg me6 Phospholipid spontaneously is form bilayers - Liposomes can be used to carry membrane impermeable substances into cells. in 1- Water-soluble substances (e.g., proteins, nucleic acids, drugs) can be encapsulated into liposomes. 2- Liposomes can fuse with cell plasma membranes (a lipid bilayer), releasing substances into cells (can be used as drug delivery tools). 3-Liposomes are used as model systems to study membrane permeability (or membrane protein reconstitution). whydo notfuse liposomes withoneanother spontaneously when suspendedin aqueous environment becausethehydrate shellofpolarhead groupsofthe lipids needsto beremov  Arrangement of Phospholipids in Membrane cells Amphipathic molecules have both components so the hydrophilic head molecules interact with the aqueous solution and the hydrophobic tails will interact with each other Outside of cell Carbohydrate Proteins chains phospholipid bilayer Transport Phospholipids Protein Inside of cell (cytoplasm) Lipid composition and structural organization Three major classes of lipids are found in the lipid bilayer: 1. Phospholipids 2. Cholesterol 3. Glycolipids 2. CHOLESTROL control the fluidity Eukaryotic plasma membranes contain large amounts of cholesterol It is found in the cell membranes of animals but not plants. It affects the fluidity of the membrane.  Cholesterol in the Membrane Cholesterol is added to areas that have lots of unsaturated lipids to help fill in the gaps between the tails tomake it harder Helps to stiffen and stabilize the bilayer – Less fluid – Less permeable saturated nogaps between lipids unsaturated morefluidity gaps Figure 11-16b Essential Cell Biology (© Garland Science 2010) 3. GLYCOLIPIDs Glycolipids are found on the surface of all plasma membranes Glycolipids are found only on the non-cytosolic surface – Sugar added in the Golgi Apparatus. themembraneofGolgi Apparatus braneofthe in xibit.it am 1  I 2layers have differentcompisition of Lipid bilayer asymmetry phospholipids Two layers of bilayer have different compositions - Different phospholipid/glycolipid inside vs outside - Membrane proteins embedded into membrane with specific orientation glycolipid Phosphatidylcholine (PC) l Sphingomyelin (SM) l Phosphatidylserine (PS) Phosphatidylinositol (PI) l l Phosphatidylethanolamine (PE) l l The asymmetry of the lipid bilayer is functionally important  Lipid bilayer asymmetry - Membrane synthesis occurs in endoplasmic reticulum (ER) New membrane exported to other membranes by vesicles (budding and fusion) wit 61 ima - Lipid asymmetry occurs during manufacture - To permit membrane growth, newly synthesized membrane must be evenly distributed between both monolayers Ida 5101 mm So w̅ - Flippases selectively transfer specific phospholipids ⇒asymmetric distribution in each monolayer 5 565  interactions less vanderwaals weak hydrophopic allows interaction intheinterior ofmembrane Phospholipid mobility lipidsandproteins tomovelaterally Cell membrane move The two leaflets of a bilayer membrane tend to differ in their lipid composition. Flip-flop of lipids (from one half of a bilayer to the other) is normally very slow. if Flip-flop would require the polar head-group of a lipid to traverse the hydrophobic core of the membrane. Need large energy Some membranes contain enzymes that actively transport particular lipids from one monolayer to the other. Flippases catalyze flip-flop in membranes where lipid synthesis occurs. Lipid bilayer asymmetry To Faces of cellular membranes are conserved during membrane budding and fusion Endocytosis: red membrane is face to cytosol Exocytosis: red membrane is also face to cytosol (exoplasmic face)  phospholipids Inositol phospholipids are only on the cytosolic surface – Functions to relay signals on cytosolic surface that pass through the membrane Hormones neurotransmitters T Signaling functions of inositol phospholipids in the cytosolic leaflet of the plasma membrane y i Lipid rafts Ian joke Lipid rafts are small specialized areas in membranes where some lipids (primarily sphingolipids and cholesterol) and proteins are concentrated signals Plasma membrane contains lipid rafts that are enriched in sphingolipids, cholesterol, and some membrane proteins Lipid rafts connect Cholesterol and sphingolipids cluster with specific proteins in membrane microdomain to am Lipid rafts are resistant to detergent solubilization, which has facilitated their isolation and characterization. Lipid rafts are enriched for may receptors, signaling proteins  Membrane fluidity Why does membrane need to be fluid? - Enables rapid diffusion of membrane proteins within plane of bilayer and permits interaction (important for cell signaling) - Facilitates distribution of membrane lipids and proteins from insertion site (following synthesis) to other regions of cell phagocytosis excostyosis what need to make Vesicles - Allows membranes to fuse and mix molecules - Ensures even distribution of membrane molecules between daughter cells following cell division Mobility of a given membrane components depends on: - Phospholipid composition and its interactions with other molecules depends if it saturated or unsaturated - Temperature increase or decrease fluidity - Lipid composition (tails, cholesterol)  phospholipid composition same Close packing of hydrocarbon tails ⇒less fluidity (increased viscosity) io e Length and unsaturation determine closeness of packing Length varies from 14-24 C atoms; shorter chain length ⇒less interaction ⇒increased fluidity One tail of molecule has one or more double bonds - unsaturated (H atoms); other tail has no double bonds - saturated Wiema Double bonds ⇒kinks ⇒less packing  couldent if it covalent non Agreater proportion it will be solid phospholipids of unsaturated no fluidity increases membranefluidity Van der Waals interactions between fatty acyl chains are the main determinants of acyl chain mobility. Double bonds reduce the number of potential van der Walls interactions between fatty acyl chains noncovalent interaction interaction weakhydrophopic  more fluidity The membrane becomes more difficult to freeze if the hydrocarbon chains are short or have double bonds, so that the membrane remains fluid at lower temperatures Temperature increas temperature distrub van more fluidity der Waals bounds Gel and fluid forms of the phospholipid bilayer ji Heat disorders the nonpolar tail and induces a transition form gel to fluid Cholesterol less fluidity solid Cholesterol is important in maintaining the fluidity Cholesterol Functions in 3 ways: 1. It can weakly bind to hydrocarbon tails making it more difficult for smaller molecules to cross membrane. one tailsto eac awayfromoth 2. It maintains the fluidity and increases the stability of the membrane. If the Iii phospholipids are saturated, it prevents them from being packed too closely, making the membrane more fluid 3. If the phospholipids are unsaturated there are kinks in the tails where the cholesterol molecules can fill in and anchor them making the membrane less fluid. 4- Cell Membrane PROTEINs Membrane proteins can be associated with the lipid bilayer in various ways: attachedwith ehby attached onlyonelayer it b n protein lipid toprotein pass through membrane form porel channel lipid Transmen 87 Peripheral Protein Figure 10-17. Molecular Biology of the cell 6e (Garland Science 2015) Peripheral Proteins Interaction with transmembrane Transmembrane Proteins proteins noncovalen weakattached isspan the entire membrane Also called integral membrane Linked by lipids on either surface of proteins covgiiig.gs the membrane  gy Membrane Proteins Membranes contain specific proteins to mediate their distinctive functions. forintegrination signals Na K interact with cytoskeleton transmition Figure 11-20 Essential Cell Biology (© Garland Science 2010) of signals Carry out the functions of the membrane – Transporters – Na+ pump to move Na+ across – Linkers – integrins to link intercellular components to extracellular ones – Receptors – to bind a compound that sends a signal to the rest of the cell – Enzymes – perform chemical reactions in the membrane  Peripheral Membrane Proteins Proteins that are attached to either surface of the bilayer very strongly Those attached to lipids are covalently linked Those that interact with other transmembrane proteins are attached by noncovalent interactions Leak interaction  Protein Movement disturbation move Fner is Proteins can move through the layer of the membrane similar to the lipids Can’t flip from one side to the other Many membrane proteins diffuse in the plane of the membrane we my moved  fining swimi www.I.mn Sent Prn Fluorescence recovery after photobleaching (FRAP) Ñ5 protein movement www.mi IN III Fluorescence loss in photobleaching  Restriction by Location proteinsmove laterally in MY attachment of d'm thecell Apical side – facing opening Basal side – bottom of the cell Lateral sides – side surfaces  Membrane Domains Cells can restrict the movement of proteins by: éw̅ Cell cortex attachment Interaction with Cytoskeleton Extracellular attachment Attachment to other cells Four ways of restricting the lateral mobility of specific plasma membrane proteins  important Proteins with eggs r binding Three domains in the plasma membrane of a sperm cell Many membrane proteins are glycosylated and have intrachain or interchain disulfide bonds peripheral protein Integralprotein  5- CARBOHYDRATEs Some phospholipids have carbohydrates attached to them – GLYCOLIPIDS Some of the proteins have carbohydrates attached to them – GLYCOPROTEINS Many of the plasma membrane proteins have sugars attached to them short the depends in – Short oligosaccharides – glycoproteins length oftheprotein – Long polysaccharides - proteoglycans I cell helps in cell to Sugars on the surface make up the glycocalyx communication Ii interaction 1.11 – Keeps cells moist and slippery II – Used as cell recognition (lectins) and adhesion molecules A simplified diagram of the cell coat (glycocalyx) lipids  The role of glycoproteins They may help in the recognition of, and interaction with, other cells. They may also play a part in the recognition of hormones and foreign molecules. The cell coat, or glycocalyx is the carbohydrate-rich zone on the cell surface Likely functions – protect cells against mechanical and chemical damage keep foreign objects and other cells at a distance 1- Transport proteins Channel proteins: function by having a hydrophilic channel that certain polar molecules or ions Carrier proteins: use as a tunnel function by holding onto their passengers and change shape in a way that shuttles them across the membrane 2- Attachment to the cytoskeleton and extracellular matrix. 3- Receptor site-Ex. Insulin never goes into a cell but binds to a receptor site on the cell membrane 4- Enzyme-Some proteins in the membrane may expose their active site to speed up a chemical reaction. 5- Intercellular joining (adhesion) 6- Cell to cell recognition (glycoproteins).  Summary 1. Cell membranes have a basic structure composed of a PHOSPHOLIPID BILAYER. 2. Phospholipds have HYDROPHOBIC (non-polar) tails and HYDROPHILIC (polar) heads. 3. The best model of the cell membrane is called the FLUID MOSAIC MODEL. 4. The fatty acid tails of phospholipids can be SATURATED (straight) or UNSATURATED (bent). 5. Proteins can float or be fixed and also have hydrophobic and hydrophilic portions. 6. Some proteins and phospholipids have carbohydrates attached to them to form GLYCOPROTEINS and GLYCOLIPIDS. 7. Phospholipids form the bilayer, act as barrier to most water soluble substances. 8. Cholesterol regulates the fluidity of the membrane, gives mechanical stability and help to prevent ions from passing through the membrane. 9. Proteins act as transport proteins to act as channels for substances to move into or out of the cell. Some act as membrane enzymes and some have important roles in membranes of organelles. 10. Glycolipids and Glycoproteins help to stabilise membrane structure, some act as receptor molecules eg for hormones and neurotransmitters or as antigens for other cells to recognise them.