Plasma Membrane 2-1 PDF

Cellular Biology & Homeostasis CELL MEMBRANE PART 2 VP Summer 2023 Clara Camargo, DVM LEARNING OBJECTIVES 1. The concept of membrane flexibility and fluidity 2. Movement of phospholipids 3. Role of cholesterol as a “fluidity buffer” 4. The extracellular matrix and its function in cell phy...

Cellular Biology & Homeostasis CELL MEMBRANE PART 2 VP Summer 2023 Clara Camargo, DVM LEARNING OBJECTIVES 1. The concept of membrane flexibility and fluidity 2. Movement of phospholipids 3. Role of cholesterol as a “fluidity buffer” 4. The extracellular matrix and its function in cell physiology CELL MEMBRANE FLUID MOSAIC MODEL FOR MEMBRANE STRUCTURE RECAP: -Compartmentalization -Properties -Fusion and Fission -Components -Self-healing properties (self-sealing) -Phospholipids -Visualization techniques (electron microscopy) Lehninger- Principles of Biochemistry CELL MEMBRANE - MEMBRANE DYNAMICS Features of all biological membranes: • Flexibility: Ability to change shape without losing integrity or becoming leaky Vesicles: • Fluidity: Ability to flow How? → noncovalent interactions of lipids in the bilayer • Intra- and extra-cellular structures consisting of liquid enclosed by a lipid bilayer • Form naturally during Structure and flexibility of lipid bilayer depends on: • Lipids composition • Changes with temperature Membrane fluidity https://www.youtube.com/watch?v=jM_xePC70Yo • secretion (exocytosis) • uptake (endocytosis) • membrane transport CELL MEMBRANE THE LIPID BILAYER: A TWO-DIMENSIONAL FLUID Around 1970 researchers first recognised that individual lipid molecules can diffuse freely within lipid bilayers • First demonstrations resulted from studies of synthetic bilayers Two types of preparations were effective for these studies: 1. Liposomes (bilayers as closed spherical vesicles), commonly used as model membranes in experimental studies 2. Black membranes (planar bilayers, formed across a hole in a partition beween two aqueous compartments), are used to measure the permeability properties of synthetic membranes CELL MEMBRANE - THE BILAYER Synthetic lipid bilayers form liposomes when in solution Electron micrograph of unfixed, unstained phospholipid vesiclesliposomes CELL MEMBRANE – Phospholipid movement Phospholipids can move!!! CELL MEMBRANE - Phospholipid movement MOVEMENT OF PHOSPHOLIPIDS WITHIN THE LIPID BILAYER Catalysis of transbilayer movement of lipids by: Transversal Diffusion Flip-flop: * • Flippases • Floppases • Scramblases happens rarely (unless the process is catalysed)  special transporter proteins that move Lateral Diffusion phospholipids and other lipids in the membrane O * O diffusion in the plane: happens readily and rapidly DEMONSTRATION of MEMBRANE FLUIDITY and LATERAL DIFFUSION RATES of LIPIDS and PROTEINS Demonstration of lateral diffusion 1. Fluorescence microscopy using two different markers  Fusion of two different membranes FYI 2. FRAP - Fluorescence Recovery After Photobleaching https://www.youtube.com/watch?v=CfRv mtBdZ9I CELL MEMBRANE - Fluidity The fluidity of a lipid bilayer must be precisely regulated;  depends on its composition and temperature Membrane phase transition: the change of Organisms can adapt to environmental a lipid bilayer from a liquid state to a two- temperature fluctuations, adjusting the dimensional rigid crystalline state (gel) at a fatty acid content of their membranes to characteristic temperature (melting point) maintain a relatively constant fluidity. Effects of temperature on the structure and metabolism of cell membranes in fish FYI https://pubmed.ncbi.nlm.nih.gov/6372513/ CELL MEMBRANE - Fluidity Lipid bilayer is stabilized by hydrophobic interactions between lipids’ fatty acid chains Fluidity depends on: 1. Phospholipid content (FA length and saturation) 2. Cholesterol content 3. Temperature  At low temperatures: less lipid movement Lipid bilayer is in paracrystalline state (more rigid state)  At higher temperatures (20-40°C): more lipid movement Lipid bilayer becomes more fluid Liquid-disordered state CELL MEMBRANE – Fluidity/FA Influence of cis-double bonds in hydrocarbon chains in membrane phospholipids:  Unsaturated FA make more difficult to pack the hydrocarbon chains together  lower melting point (liquid at colder temperatures)  Also form a thinner membrane as the phospholipids will be further spread apart Influence of a shorter hydrocarbon chain in membrane phospholipids  Reduces the tendency of hydrocarbon tails to interact with one another → the membrane remains fluid at lower temperatures short-chain FA  lower melting point  Hydrocarbon chain of membrane phospholipid tails can vary from 14-24 C, most are between 18-20 C CELL MEMBRANE – Fluidity/FA Saturated fatty acids: Tend to form paracrystalline structures (less space between phospholipid tails) ↑ saturated fatty acids content of a lipid bilayer ↑ Paracrystalline-to-fluid transition temperature of the membrane (↑ melting point) Unsaturated fatty acids: Cis-double bonds → kinks ( more space between tails) Inhibits the paracrystalline conformation ↑ Unsaturated fatty acids content of a lipid bilayer ↓ Paracrystalline-to-fluid transition temperature of the membrane (↓ melting point) CELL MEMBRANE – Fluidity/cholesterol Eukaryotic plasma membranes can contain very large amounts of cholesterol: up to one molecule for every Cholesterol can inhibits or phospholipid molecule delay phase transitions Cholesterol molecules improve the permeability-barrier properties of the lipid bilayer • Orient themselves in bilayer with their hydroxyl groups close to the polar heads of the phospholipid molecules • The rigid steroid ring can support the hydrocarbon chains and stabilize them CELL MEMBRANE - CHOLESTEROL  at high temperatures Cholesterol helps keep the membrane more stable, stiffening the bilayer and making it less fluid and less permeable  at low temperatures, acts as ‘antifreeze’ Cholesterol also helps the membrane remain fluid by preventing fatty acid tails from interacting with each other and ‘clumping up’ Plasma membrane fluidity https://www.jove.com/embed/player?id=10972&t=1&s=1&fpv=1 What can you “see” in this video? Think about cytoskeleton and plasma membrane lectures https://www.youtube.com/watch?v=HwP4hLeVGoE Amoeba attacks dividing paramecia! CELL MEMBRANE - Extra Cellular Matrix (ECM) Tissues are not made solely of cells  cells are contained in a = Basal Lamina complex and intricate network of macromolecules, known as the: EXTRACELLULAR MATRIX: • Basement membrane and ● fibronectin • Interstitial matrix CELL MEMBRANE - ECM THE CELL AND ITS “SOCIAL“ CONTEXT: THE EXTRACELLULAR MATRIX (ECM) • The extracellular matrix in biology is a collection of extracellular molecules secreted by cells  Structural and biochemical support to surrounding cells  Common functions: cell adhesion, cell-to-cell communication and differentiation Animal ECM:  Interstitial matrix: present between different animal cells (in intercellular spaces) • Gels of polysaccharides and fibrous proteins fill intercellular spaces, act as compression buffer to the ECM  Basement membranes/basal lamina: sheet-like depositions, special type of ECM that lines the basal side of epithelial and endothelial tissues Each type of connective tissue in animals has a different type of ECM:  Bone tissue: consists of collagen fibers and bone mineral  Loose connective tissue: reticular fibers and ground substance  Blood: ECM is blood plasma CELL MEMBRANE- ECM EXTRACELLULAR MATRIX: Macromolecules in the ECM are mainly produced locally by cells in the matrix and secreted via exocytosis. Fibroblasts • Fibroblasts produces and secretes ECM • In connective tissue (collagen fibers) Can differentiate into • Chondroblasts- cartilage • Osteoblasts- bone tissue • Myofibroblasts – muscle tissue CELL MEMBRANE - ECM MAJOR CLASSES OF MACROMOLECULES IN THE ECM (MAMMALS HAVE > 300 MATRIX PROTEINS): 1) Glycosaminoglycans (GAGs, e.g. cartilage): large and highly charged polysaccharides GAG + proteins  proteoglycans GAG + PROTEOGLYCANS = Ground Substance (amourphous gelatinous material, fill the space between fibers and cells) 2) Fibrous proteins like collagen (e.g. skin and bone) 3) Non-collagen fibrous proteins (elastin, fibronectin, laminin) Protein in green GAG in red Structural proteins: fibrous collagen and elastin Adhesive proteins: laminin and fibronectin CELL MEMBRANE - ECM COLLAGEN are fibrous, long, stiff, triple-stranded helical proteins forming fibrils.  Rich in proline and glycine amino acids  The fibrils are glycosylated What is protein glycosylation? Where does it happen? CELL MEMBRANE - ECM  The human genome contains 42 distinct genes coding for different collagen chains CELL MEMBRANE - ECM ELASTIN gives tissues their elasticity (skin, blood vessels, lungs are strong but elastic enough to function properly) Scanning electron micrograph of a dog‘s aorta (low magnification) Network of longitudinally oriented elastin fibers (high magnification) CELL MEMBRANE- ECM Elastin is a hydrophobic protein rich in proline and glycine (like collagen) but is not glycosylated A RUBBER BAND: • The molecules are joined together by covalent bonds to generate a cross-linked network • Each elastin molecule can extend and contract in a manner resembling a random coil, so that the entire assembly can stretch and recoil like a rubber band CELL MEMBRANE- ECM In the ECM there are also glycoproteins with multiple domains each with specific binding sites for other matrix macromolecules and for receptors of the cell surface. Fibronectin (2500 amino acids long!) is a multifunctional adhesive glycoprotein that plays an important role in tissue repair, regulating cell attachment and motility and in embryogenesis CELL MEMBRANE - ECM THE BASAL LAMINA (or basement membrane) is a specialized form of ECM Basal lamina is thin, flexible and tough  essential component of all epithelia CELL MEMBRANE- ECM LAMININ is the primary organizer of the sheet structure of the basal lamina  Composed of three long polypeptide chains held together by disulfide bonds Multidomain protein (each chain 1500 amino acids long), asymmetric molecule 27 CELL MEMBRANE - Integrins Family of transmembrane proteins synthesized in several types of cells • Facilitate cell adhesion (link cytoskeleton filaments with the ECM) • Mediate cellular signals (signal transduction pathways, cell recognition, cell movement) • Can be receptors for certain viruses (adenovirus, hantavirus, foot and mouth disease, polio virus...) HAPPY STUDYING Clara Camargo, DVM [email protected] ©2021 Ross University School of Veterinary Medicine. All rights reserved.