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Transcript
Lecture 3 Outline (Ch. 7) I. Membrane Structure II. Membrane Proteins III. Permeability IV. Transport Across Membranes A. Passive B. Facilitated C. Active D. Bulk V. Lecture Concepts Membrane structure 1915, knew membrane made of lipids and proteins • Reasoned that membrane = bilayer Where to place proteins? Lipid layer 1 Proteins Lipid layer 2 Membrane structure Experiment to determine membrane fluidity: • marked membrane proteins mixed in hybrid cell Membrane structure Membrane fluidity • phospholipid f.a. “tails”: saturation affects fluidity • cholesterol buffers temperature changes Membrane structure “fluid mosaic model” – 1970s • fluid – phospholipids move around • mosaic – proteins embedded in membrane Membrane structure • freeze fracture • proteins intact, one layer or other • two layers look different Membrane structure • cell membrane – amphipathic - hydrophilic & hydrophobic hydrophilic hydrophobic hydrophilic • membrane proteins inserted, also amphipathic Membrane Proteins Membrane proteins: Integral: inserted in membrane - transmembrane – span mem. Peripheral: next to membrane - inside or outside Membrane Proteins Selectively Permeable Membranes Cell membranes only allow some molecules across w/out help: • Small, non-polar molecules OK ex. hydrocarbons, O2, CO2 • No charged, polar, or large molecules ex. sugars, ions, water* Transport Across Membranes Types of transport: A. Passive transport B. Facilitated diffusion C. Active transport D. Bulk transport • Energy Required? • Directionality? Passive Transport - Simple Diffusion • NO ENERGY required • DOWN concentration gradient • molecules equally distribute across available area - non-polar molecules (hydrocarbons, O2, CO2) Passive Transport - Osmosis • osmosis – movement of water across cell membrane • water moves via special channels • moves into/out of cell until solute concentration is balanced Passive Transport - Osmosis • tonicity – # solutes in solution in relation to cell - hypotonic – fewer solutes in solution - isotonic – equal solutes in solution animal cell - hypertonic – more solutes in solution plant cell Passive Transport - Osmosis Paramecium example • regulate water balance • pond water hypotonic • water into contractile vacuole – water expelled Facilitated Diffusion • NO ENERGY required • DOWN concentration gradient - Large, charged, polar (sugar, ions, water) • Use transport proteins - channel or carrier proteins Active Transport • ENERGY IS required - Usually ions or large molecules (Na+, K+, glucose) • UP/AGAINST concentration gradient • transport carrier proteins a. ion pumps b. co-transporters • Ex. Na-K ion pump - Na+ ions: inside to out - K+ ions: outside to in - net: high Na+ outside, high K+ inside cell Active Transport • Ex. proton (H+) pump • ATP used pump H+ ions out • against concentration and charge gradients *gradients – used by cell for energy potential Bulk Transport • ENERGY IS required • Several or large molecules • Molecules moved IN - endocytosis • phagocytosis – “food” in • pinocytosis – water in • receptor-mediated endocytosis – proteins bind molecules, vesicles inside • Molecules moved OUT - exocytosis Self-Check Type of transport Energy required? Movement direction? Examples: Simple diffusion no Down conc. gradient O2, CO2, nonpolar molecules Osmosis Facilitated diffusion Active transport Bulk transport Lecture 3 concepts - Describe the ‘fluid mosaic model’ including meaning and experimental evidence - List and describe types of membrane proteins - Define amphipathic & explain application to cell membranes - Discuss what is meant by a selectively permeable membrane and how this applies to cell membranes - Name types of transport across cell membranes and describe each one - Given a tonicity of solution, determine direction of water movement and hypothesize result on a cell - Write out a list of new terminology and provide descriptions