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Biological Membranes.docx

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Department
Biology
Course
BIOL 130
Professor
Heidi Engelhardt
Semester
Fall

Description
Biological Membranes  membrane structure o membrane lipids (ref: Unit 2b Biomolecules Part 3, slides 19-36) o membrane proteins  functions, peripheral vs integral – transmembrane helices, pores o membrane carbohydrates (sugars)  glycocalyx  membrane transport o What drives movement of solutes? diffusion, osmosis, tonicity, turgor pressure o What determines what can cross a membrane? size, polarity, charge  for substances that can’t diffuse through bilayer:  membrane transporter proteins o carriers versus channels o passive versus active transport  ion channels – chemical & electrical gradients, selectivity, gating  carrier proteins  down gradient - facilitated diffusion  against gradient - active transport; Na+/K+-ATPase  coupled transport  membrane proteins and gene expression MEMBRANE STRUCTURE  all cells have a plasma membrane o encloses contents of entire cell  eukaryotic cells have membrane-bound organelles o nuclear ‘envelope’ o double membranes of mitochondria and chloroplasts o endoplasmic reticulum o Golgi apparatus o lysosomes / vacuoles o transport vesicles o et al.  Functions o scaffold for biochemical activities o provide a selectively permeable barrier • prevent unrestricted exchange of molecules o transport solutes • exchange of molecules across the membrane o respond to external signals - signal transduction • signals travelling from a distance or from nearby cells o energy transduction - conversion of one form of energy into another o compartmentalization (eukaryotes) • create separate environments for different activities  Phospholipids o Polar head group + phosphate group + glycerol + 2 fatty acid chains o Named by head group, not length of chain or saturation of chain (phosphatidyl choline (PC))  Membrane Fluidity o membrane fluidity : how easily lipid molecules move within a membrane leaflet o alignment of phospholipid tails • tightly packed tails  membrane more viscous, less fluid • freely moving tails  higher fluidity o influenced by: • length of fatty acids • from 14-24 carbons, 18-20 carbons most common • degree of saturation of fatty acids  # double bonds • typically one saturated fatty acid and one with one or more double bonds o all membrane lipids are amphipathic o cholesterol • under physiological conditions, cholesterol makes membrane stiffer – less fluid • cholesterol can make up to 50% of plasma membrane lipid in some animal cells o temperature • transition temp = temp at which membrane ‘gels • at and above ‘room temperature’ phospholipids in membranes are fluid, and move freely • as temperature drops, fluidity (and permeability) decreases • at very low temps, hydrophobic tails pack together and membrane ‘gels’ (solidifies) o fluid state must be maintained for normal cell function • change composition of membranes • alter phospholipids • desaturate fatty acids (to deal with cold) • change length of FA chains (yeast, bacteria) • adjust amounts of cholesterol (animals) • these mechanisms have been demonstrated in: • pond fish dealing with dramatic day / night temp differences • cold-resistant plants • extremophile bacteria living in hot springs • winter wheat preparing for autumn ↑ polyunsaturated FAs • sperm reduce their cholesterol just before fertilization …  Asymmetry of Membrane Lipids o Flipping is rare and controlled o the appearance of PS (lipid that is usually on cytosolic side) in outer ‘leaflet’ of membrane usually indicates that the red blood cell is going to die (through apoptosis) o Red Blood Cells and model organisms • best understood plasma membrane! • cells are inexpensive and available in large numbers • already present in single cell suspension • simple - no nucleus, no ER, no mitochondria, no lysosomes • very pure preps of plasma membranes • purified intact plasma membranes can be prepared by producing red blood cell ‘ghosts’ o Asymmetry is preserved during membrane transport (see diagram)  Membrane Proteins o Integral (transmembrane) • Go across whole membrane • Single pass transmembrane protein • Multi pass transmembrane protein o Peripheral • Lipid-linked • Protein-linked o Helices • Hydrophilic channels can be formed from several alpha-helices o Pores • Proteins folded into pleated sheets can form pores o Cells can restrict the movement of membrane proteins • Proteins holding membrane • Membrane proteins lock together (build tissues) • Tight junctions • Prevent different proteins from crossing (creates protein domains) • Creates seal between cells (prevents ‘leaking’)  Membrane carbohydrates o Eukaryotic cells are coated with sugars • Glycol calyx • Sugar on inside of organelles therefore sugars on outside of cell membrane MEMBRANE TRANSPORT  need to allow passage of certain substances in / out of cell o gases, ions, nutrients / waste products  lipid bilayers tend to block passage of polar (water-soluble) molecules  substances can enter a cell by … o passing directly through lipid bilayer o being transported across bilayer by membrane proteins acting as carriers or channels o being engulfed by the cell, avoiding passing through the membrane  What drives movement of solvents o Diffusion • dissolved solutes (molecules / ions in solution) are in constant, random motion • solutes will spontaneously ‘spread out’ (↑ entropy) until concentrations on all regions are equal • at that point – no NET flux (movement continues- don’t stop and stay still) • Dynamic equilibrium- random motion continues o Osmosis • diffusion of water across a semi-permeable membrane down its concentration gradient (toward a higher solute concentration) • once ‘water concentration’ equal on both sides, no net movement of water • water concentration depends on total concentration of osmotically active particles (solutes)- doesn’t matter what solutes they are (can be a mixture) • all ions, molecules dissolved in fluid • water is constantly moving through cell membrane in both directions • ideally, ‘osmotic tone’ (concentration of osmotically active substances) is equal inside and outside cell • intracellular and extracellular fluid are isotonic • if total solute concentration changes on either side, net movement of water will toward fluid with higher concentration of solutes
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