Chapter 7: Membrane Structure & Function - Los Angeles Mission College

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10/21/2015 Chapter 7: Membrane Structure & Function 1. Membrane Structure 2. Transport Across Membranes 1. Membrane Structure Chapter Reading – pp. 125-129 What are Biological Membranes? Hydrophilic head Hydrophobic tail WATER They’re basically a 2-layered sheet of phospholipids with some proteins & cholesterol. WATER Why phospholipids? Because they are amphipathic – i.e., part is hydrophilic and part is hydrophobic. They self-assemble spontaneously into a variety of organized structures, one of which is a lipid bilayer. 1

10/21/2015 Phospholipids phospholipids have a variety of polar head groups WATER Hydrophilic head Hydrophobic tail WATER Membrane Viscosity Viscous Fluid TEMPERATURE higher temp lower viscosity SATURATION Unsaturated hydrocarbon tails Saturated hydrocarbon tails more saturation of HC tails more viscosity (a) Unsaturated versus saturated hydrocarbon tails CHOLESTEROL (b) Cholesterol within the animal cell membrane increases viscosity at higher temp, prevents hardening at lower temp Cholesterol Membrane Proteins N-terminus EXTRACELLULAR SIDE Membrane proteins may penetrate the interior of the membrane (integral) or interact with it externally (peripheral). the portions of a membrane protein that interact with the hydrophobic interior contain non-polar R groups C-terminus Helix FREEZE FRACTURE TECHNIQUE CYTOPLASM RESULTS Extracellular layer Knife Plasma membrane Proteins Inside of extracellular layer Cytoplasmic layer Inside of cytoplasmic layer 2

10/21/2015 The Fluid Mosaic Model This model (hypothesis) proposes that proteins are scattered within a membrane and can move freely within the plane of the membrane. supported by the experiment shown below RESULTS Membrane proteins Mixed proteins after 1 hour Mouse cell Human cell Hybrid cell ***some proteins are “fixed” by attachment to cytoskeleton or ECM*** Model of Membrane Structure Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments of cytoskeleton Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Main Roles of Membrane Proteins Glycoprotein (a) Cell-cell recognition (b) Intercellular joining (c) Attachment to the cytoskeleton & ECM (extracellular matrix) These are the most common, though there are many other functions for membrane proteins. Signaling molecule Enzymes ATP (d) Transport Receptor Signal transduction (e) Enzymatic activity (f) Signal transduction 3

10/21/2015 Membrane Orientation Transmembrane glycoproteins Secretory protein Golgi apparatus Vesicle ER ER lumen Glycolipid Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Secreted protein Membrane glycolipid Membrane “Faces” are Different While phospholipids move freely and rapidly within a given layer or “face”, they rarely switch layers. phospholipid composition of cytoplasmic vs extracellular face is different & is set up in the ER Lateral movement occurs 107 times per second. Flip-flopping across the membrane is rare ( once per month). 2. Transport Across Membranes Chapter Reading – pp. 129-138 4

10/21/2015 Diffusion Diffusion is the net or overall movement of a substance from higher to lower concentration molecules dissolved in liquid move randomly over time the net effect is equal dispersion of the molecules (provided there is no barrier) aka “moving down concentration gradient” Molecules of dye Diffusion across a permeable barrier Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium As long as there is nothing to block the passage of solutes, the 2 compartments will reach equilibrium (b) Diffusion of two solutes Lower concentration of solute (sugar) Higher concentration of sugar H2O Selectively permeable membrane OSMOSIS is the diffusion of water across a selectively permeable membrane Osmosis Same concentration of sugar Diffusion across a selectively permeable barrier The barrier allows water (solvent) to pass freely while the sugar (solute) cannot pass the net flow of water from [high] to [low] creates osmotic pressure on one side of the barrier 5

10/21/2015 Osmosis and Cells Hypotonic solution H2O Isotonic solution Hypertonic solution H2O H2O H2O (a) Animal cell Lysed Normal H2O Shriveled H2O H2O H2O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed Combating Osmotic Pressure 50 µm Filling vacuole Unicellular freshwater organisms that lack cell walls (such as many protozoa) are vulnerable to osmotic lysis (osmolysis). Paramecium (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole Contractile vacuoles provide protection by taking on excess water and releasing it externally by exocytosis. (b) When full, the vacuole and canals contract, expelling fluid from the cell. “Small-scale” Transport Cells accomplish membrane transport on a “small scale” (molecule by molecule) in 3 basic ways: 1) passive transport (simple diffusion) diffusion directly through the membrane bilayer Passive transport Active transport 2) facilitated diffusion diffusion with the help of specific membrane proteins 3) active transport movement from low to high concentration ATP Diffusion Facilitated diffusion requires special membrane proteins and energy 6

10/21/2015 EXTRACELLULAR FLUID Facilitated Diffusion Channel protein CHANNEL PROTEINS, some of which are gated, allow the passive transport of small molecules such as ions Solute CYTOPLASM (a) A channel protein CARRIER PROTEINS bind to specific solutes and change conformation to release the solute on the opposite side Solute Carrier protein works both directions with overall movement from [high] to [low] (b) A carrier protein The Sodium-Potassium Pump EXTRACELLULAR [Na ] high FLUID [K ] low Na Na Na Na Na Na Na Na CYTOPLASM Na 1 [Na ] low [K ] high P ADP 2 ATP P 3 K K 6 K K K K 5 4 P Pi Ion Transport & Membrane Potential * *cells normally have a negative membrane potential Ion diffusion is driven by differences in concentration & charge across a membrane (electrochemical gradient) i.e., electrochemical gradient 7

10/21/2015 Cotransport Active Transport can be fueled by ATP or other energyrich molecules, OR by the cotransport of another molecule down its concentration gradient – this example shows how plants carry out the active transport of sucrose into vascular cells for distribution to the rest of the plant H ATP H – H Proton pump H – H H – H Diffusion of H Sucrose-H cotransporter H Sucrose – – ATP is still required for this process since it is used to set up the proton gradient sucrose transport depends on Sucrose “Large-scale” Transport Cells accomplish membrane transport on a “large scale” (in bulk) in 2 basic ways: 1) exocytosis release of material packaged in membrane vesicles to the outside of a cell 2) endocytosis ingestion of large objects or large amounts of material by enclosing within a membrane vesicle: PINOCYTOSIS PHAGOCYTOSIS RECEPTOR-MEDIATED ENDOCYTOSIS Exocytosis Fluid outside cell Vesicle Protein Cytoplasm A general process for releasing material from a cell. e.g., neurotransmitters into a synapse, water from a contractile vacuole, antibodies from a B cell 8

10/21/2015 Phagocytosis (“cell eating”) Capture of large extracellular particles in vesicles. PHAGOCYTOSIS EXTRACELLULAR FLUID 1 µm CYTOPLASM Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) how many single-celled organisms feed (e.g., amoeba) how cells of the immune system destroy invaders Pinocytosis (“cell drinking”) PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle Capture of extracellular fluid in vesicles. a non-specific process of capturing solutes in the fluid immediately surrounding a cell Receptor-mediated Endocytosis RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle receptors on the cell surface bind specific substances (receptor ligand) Coated pit Ligand A coated pit and a coated vesicle formed during receptormediated endocytosis (TEMs) Coat protein Plasma membrane 0.25 µm A highly specific process of capturing substances in vesicles. this triggers the formation of a coated pit which ultimately forms a vesicle transporting the receptor-ligand complex inside the cell 9

10/21/2015 Key Terms for Chapter 7 integral vs peripheral proteins, freeze fracture amphipathic, fluid mosaic model cytoplasmic & exoplasmic faces of membranes diffusion, osmosis, isotonic, hypertonic, hypotonic osmotic pressure, osmolysis, contractile vacuole passive transport, active transport, cotransport facilitated diffusion, electrochemical gradient carrier proteins, protein channels, pumps Relevant Chapter Questions 1-6 exocytosis, endocytosis, receptor-mediated end. vesicle, pinocytosis, phagocytosis 10

Capture of large extracellular particles in vesicles. how many single-celled organisms feed (e.g., amoeba) how cells of the immune system destroy invaders PINOCYTOSIS Plasma membrane Vesicle 0.5 µm Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Pinocytosis ("cell drinking")

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