Membrane Structure and Function Ch 5 Cell Membrane
Membrane Structure and Function Ch 5
Cell Membrane Aka: Plasma membrane, phosopholipid bilayer basic structure Selectively permeable: allows some substances to cross more easily than others. 07_11 Membrane. Selectivity_A. swf http: //www. ncnr. nist. gov/programs/reflect/rp/biology/cell_membrane_p 2. jpg http: //image. tutorvista. com/content/feed/u 303/lipidbilayer. gif
Components and Structure of the cell membrane • Fluid Mosaic Model – The membrane moves around – Various substances are embedded in the phospholipid bilayer • Lipids: amphipathic (has a hydrophillic and hydrophobic region) • Proteins: move laterally and help with transport of substances • Carbohydrates: extend from surface, docking port
Fluidity of Membranes • Held together by weak hydrophobic interactions • Most lipids & some proteins move laterally (sideways) rarely cross the hydrophobic core • Fluidity is temp dependent: cooler = less fluid • As temperatures decrease eventually the membrane will become rigid – Unsaturated phospholipids stay fluid longer – kinked due to double bonds – Saturated phosphlipids are more closely packed • Cholesterol in Animal cells limits fluidity – Temperature buffer
Fig. 7 -5 b Fluidity of Membranes Fluid Unsaturated hydrocarbon tails with kinks (b) Membrane fluidity Viscous Saturated hydrocarbon tails
Membrane Proteins • Integral – Span all or part of membrane • Hydrophobic: nonpolar a. a. w/ α helices • Hydrophillic: extend to inner & outer surfaces • Peripheral– surface proteins – Held in place by cytoskeleton (inside cell) and extra cellular matrix (ECM) (outside cell)
Carbohydrates and Cell membranes • Cell to cell recognition, surface tags that allow cells to recognize each other (good or bad) • Glycolipids: carb attached to lipid • Glycoprotein: carb attached to protein
Synthesis of Membranes 1. Proteins & lipids made in ER, Carbs are added & modified 2. Golgi: further Carb modification & Carbs are added to lipids = glycolipids 3. The various proteins leave Golgi via vesicles to plasma membrane 4. Vesicles fuse with plasma membrane, this releases secretory proteins and positions carbs of membrane proteins on outside of membrane
Fig. 7 -10 ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 apparatus Vesicle 3 4 Secreted protein Plasma membrane: Cytoplasmic face Extracellular face Transmembrane glycoprotein Membrane glycolipid
Selective Permeability • Passive transport – no energy required – “down” a concentration gradient • Simple Diffusion 07_11 Diffusion_A. swf • Facilitated Diffusion – Channel or Carrier • Osmosis • Active Transport – energy required – against a concentration gradient • Ion Pumps • Cotransport • Bulk Transport – Exocytosis – Endocytosis
Simple vs. Facilitated Diffusion • Simple: • Substances move with the concentration gradient • NO ENERGY REQUIRED – Non polar (CO 2 & Hydrocarbons) can diffuse through the lipid portion of the membrane) – Polar molecules pass through slowly (glucose & water) • Facilitated Diffusion • NO ENERGY REQUIRED • Transport Proteins – Channel proteins, hydrophilic to allow ions to pass through • Ion channels are gated channels – Aquaporins: let water through membrane – Carrier Proteins: change shape & hold on to substance as it passes through the membrane
Osmosis: The diffusion of water • 07_13 Osmosis_A. swf • Diffusion of “free” water • Tonicity – the ability of a cell to gain or lose water – Isotonic – balanced – no net movement of H 2 O • Plants - flacid – Hypertonic – H 2 O moves out • Solute concentration greater outside cell • Plants – plasmolysis (shrivel up) – Hypotonic – H 2 O moves in • Solute concentration greater inside cell • Plants – turgid (swell) • Osmoregulation – the regulation of H 2 O (contractile vacuole in some fresh water organisms)
Active Transport: • REQUIRES ENERGY & Against the conc. gradient • 07_16 Active. Transport_A. swf • Ion Pumps: – Sodium-Potassium (Na+, K+) pump, 3 Na+ out for every 2 K+ in to cell, creates a voltage potential – Proton pump in plants, fungi & bacteria moves H+ out of cytoplasm into extracellular solution and creates a voltage potential Electrochemical gradient is how ions move in and out of cells
Active Transport: • Cotransport: – A substance is actively pumped across the cell membrane using ATP. – The same substance naturally diffuses back across the cell membrane w/ it’s conc. gradient – When it diffuses back, it brings another substance with it, against that substances conc. gradient using the electrochemical gradient that was set up by the first substance. – Ex: Sucrose-H+ cotransport & diarrhea medications
Fig. 7 -19 – + ATP – H+ H+ + Proton pump H+ H+ – H+ + – + Sucrose-H+ cotransporter H+ H+ Diffusion of H+ H+ Sucrose – – + + Sucrose
Bulk Transport: Requires energy • Exocytosis • Transport vesicle from golgi fuses w/ plasma membrane and then the material is exited form the cell • 07_20 Exocytosis_A. swf • Endocytosis: Cell takes in bio material by creating a vesicle from plasma membrane – Phagocytosis: cellular eating – Pinocytosis: cellular drinking – Receptor-mediated endocytosis: binding of ligand to receptor triggers vesicle formation – 07_20 c. Recept. Med. Endocyt_A. swf
Concept Check Transport • 1. Draw three cells in • 2. Describe how the following structure of membrane environments and proteins is important to describe the water the selectively movement between the permeable membrane cell & environment: in cells. Include the following terms: – Cell in hypotonic environment hydrophobic, – Cell in hypertonic hydrophillic, amino environment acids, tertiary protein – Cell in isotonic structure. environment
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