Membrane Structure & Function

Jobs of the cell membrane Isolate the cytoplasm from the external environment Regulate the exchange of substances Communicate with other cells Identification

Membrane structure Semi-permeable barrier Phospholipid bilayer few molecules are allowed across while keeping the majority of produced chemicals inside the cell Phospholipid bilayer phospholipids aligned tail to tail Amphipathic hydrophobic region forms between the hydrophilic heads on the inner and outer surfaces http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/orient2.swf

Fluid-mosaic model~ Proteins (the mosaic) are embedded in lipids (the fluid) Phospholipids are not bonded to each other, which makes the double layer fluid http://telstar.ote.cmu.edu/Hughes/tutorial/cellmembranes/bil.swf

Membrane structure Phospholipids~ membrane fluidity Cholesterol~ membrane stabilization Membrane carbohydrates ~ cell to cell recognition Oligosaccharides (cell markers) Glycolipids Glycoproteins Membrane proteins ~ transport / communication

Membrane Proteins 2 types: Integral proteins~ transmembrane proteins Peripheral proteins~ surface of membrane

Membrane protein function: Transport Proteins – regulate movement of substance Carrier Proteins- "grabs" certain molecules and pulls them into the cell Gated Channels – similar to carrier proteins, not always "open" Enzyme activity – control metabolic pathways Receptor Proteins – (Signal Transduction) set off cell responses (release of hormones or opening of channel proteins) Recognition Proteins – idenitfy cells to the immune system http://trc.ucdavis.edu/biosci10v/bis10v/media/ch03/membrane_proteins_v2.html

Transport Proteins: Uniport - one molecule in one direction. Symport - Two different molecules in one direction. Antiport - Two molecules in opposite directions. http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

Transport Across Membrane 2 types: Passive transport No energy Active transport energy http://www.northland.cc.mn.us/biology/Biology1111/animations/transport1.html

Traffic of molecules across the membrane Pass freely: Hydrophobic molecules (Hydrocarbons and O2) Small polar uncharged molecules (H2O and CO2) Pass with help: Ions ( H+, Na+, Cl-). Can’t pass: Large polar uncharged molecules (sugar)

Passive Transport Diffusion of a substance across a biological membrane Diffusion ~ molecules move from high concentration to low concentration down a concentration gradient Facilitation Diffusion ~ assisted by proteins (channel or carrier proteins) Osmosis~ the diffusion of water across a selectively permeable membrane http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf http://www.bbc.co.uk/education/asguru/biology/01cellbiology/05pathways/07passivefacilitated/index.shtml http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

A substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient. Each substance diffuses down its own concentration gradient, independent of the concentration gradients of other substances. The concentration gradient represents potential energy and drives diffusion. Diffusion Movie

Water balance Osmoregulation~ control of water balance Solution~ mix of solute (dissolved) and a solvent (dissolving) Hypertonic~ solution with higher concentration of solutes Hypotonic~ solution with lower concentration of solutes IMPORTANT: The hypertonic solution has a lower water concentration than the hypotonic solution Isotonic~ equal concentrations of solutes

Water balance in living cells Cells with Walls in environment: Hypotonic = Turgid (very firm) Isotonic = Flaccid (limp) Hypertonic = Plasmolysis~ plasma membrane pulls away from cell wall Cells without Walls in environment: Hypotonic = Lyse (explode) Isotonic = Normal Hypertonic = Crenate (Shrivel) http://www.linkpublishing.com/video-transport.htm#Osmosis_-_Red_Onion http://ccollege.hccs.edu/instru/Biology/AllStudyPages/Diffusion_Osmosis/Elodeagif.swf

Specialized Transport Active transport~ movement of a substance against its concentration gradient with the help of cellular energy Low  High

Types of Active Transport Sodium-potassium pump Actively maintains the gradient of Na+ and K+ ions across the membrane Antiport energized by ATP Neurons use sodium-potassium pumps Proton Pump transports Hydrogen ions Chloroplasts and Mitochondria http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#

Endocytosis and Exocytosis http://multimedia.mcb.harvard.edu/media.html Endocytosis and Exocytosis Exocytosis~ secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane Phagocytosis (solid) Pinocytosis (liquid) receptor-mediated endocytosis (specific materials) http://www.johnkyrk.com/cellmembrane.html http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter6/animations.html#