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Transcript
CHAPTER 7 MEMBRANE STUCTURE AND FUNCTION AP Biology Study guide #20-27 AP Biology Diffusion 2nd Law of Thermodynamics Governs biological systems! Universe tends to disorder! Diffusion – diffusion movement from high>low concentration of THAT SUBSTANCE! AP Biology Diffusion of 2 Solutes Each substance diffuses down its own concentration gradient, independent of concentration gradient of any other substance. AP Biology Diffusion •PASSIVE TRANSPORT •No energy required •So – How does that happen? Brownian Motion AP Biology Osmosis is the diffusion of water across a membrane Water is very important to life, so we talk about water separately Diffusion of water from high concentration of water to low concentration of water AP Biology across a semi-permeable membrane Concentration of water Direction of osmosis is determined by comparing total solute concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water water AP Biology hypotonic hypertonic net movement of water Osmosis 250 times the volume of cell/second! Osmosis = NET movement of water across a selectively permeable membrane driven by a difference in solute concentration on either side of the memrane. “Free” water moves Slide 2 Less solute = more free water Water flows from low solute [ ] to high solute [ ] Until equilibrium AP Biology osmosis Osmosis AP Biology What determines when and what direction water will move? 3 molecules of albumin 15 molecules of glucose 66,000 mw 180 mw AP Biology Isotonic? Hypertonic? Hypotonic? A & B are isotonic A & B are hypertonic to C CAPisBiology hypotonic to A and B Osmotic Pressure Pressure generated by diffusion of water across a membrane When pressure is equal water flow will stop Called “hydrostatic pressure” – water-stopping pressure Osmolarity- [ ] in terms of # of particles in a volume of liquid 1 osmolar soln = 1 M of osmoltically active particles per liter. AP Biology Red Blood Cells in NaCl solutions 100mOs hypotonic AP Biology 500 mOs isotonic hypertonic Osmosis problems Hydrostatic generator Osmosis problems AP Biology End Diffusion Osmosis AP Biology Cell membrane The Cell Membrane AP Biology 2007-2008 Overview Cell membrane separates living cell from nonliving surroundings thin barrier = 8nm thick Controls traffic in & out of the cell selectively permeable allows some substances to cross more easily than others hydrophobic vs hydrophilic Made of phospholipids, proteins & other macromolecules AP Biology Phosphate Phospholipids Fatty acid tails hydrophobic Phosphate group head hydrophilic Fatty acid Arranged as a bilayer Aaaah, one of those structure–function examples AP Biology Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads AP Biology AP Biology Two generations of membrane models How do we know? phospholipid bilayer -1920’s; 1972; Singer, Nicholson 1930-60’s; models from EM views; Dispersion model ??-Not as hydrophyllic as pure PL’s? Hydrophyllic regions in aqueous D & D - Hydrophyllic Protein sandwich Hydrophobic in hydrophobic PL ??- all membranes identical??? FLUID MOSAIC MODEL! ??-amphipathic proteins? Solubile in H2O Freeze Fracture evidence AP Biology Permanent model???? hydrophobic region in aqueous? More than lipids… In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer It’s like a fluid… It’s like a mosaic… It’s the Fluid Mosaic Model! AP Biology Evidence for the drifting of membrane proteins Other Evidence: microsurgery on cells AP Biology Membranes are fluid- like salad oil! •held in place weak hydrophobic interactions •PL’S drift laterally •rarely flip-flop between layers •larger proteins move slower •some proteins guided by cytoskeleton Fluid Membrane “motors” •some proteins anchored by cytoskeleton AP Biology Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Transmembrane proteins Peripheral protein Cytoplasm AP Biology Filaments of cytoskeleton The fluidity of membranes Maintains and increases/decreases fluidity AP Biology Fluidity influenced by temperature cool - more solid- PL’s closely packed if rich in unsaturated fatty acids - more fluid than those rich in saturated fatty acids kinks prevent tight packing cholesterol steroid- wedged between PL’s of animal cells •warm -limits mvmnt. of PL’s, reduces fluidity •cool - maintains fluidity, prevents tight packing-EX: salmon AP Biology •Must be fluid to work w/ enzymes & be permeable •Cells alter lipid makeup to adjust for temp. changes EX: cold organisms ( winter wheat, salmon, bears) increase % of unsat PL’s in autumn - prevents solidifying membranes AP Biology Membrane fat composition varies Fat composition affects flexibility membrane must be fluid & flexible about as fluid as thick salad oil % unsaturated fatty acids in phospholipids keep membrane less viscous cold-adapted organisms, like winter wheat increase % in autumn AP Biology cholesterol in membrane Why are proteins the perfect molecule to build structures in the cell membrane? AP Biology 2007-2008 Classes of amino acids What do these amino acids have in common? nonpolar & hydrophobic AP Biology Classes of amino acids What do these amino acids have in common? I like the polar ones the best! AP Biology polar & hydrophilic Membrane Proteins Proteins determine membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Membrane proteins: peripheral proteins loosely bound to surface of membrane cell surface identity marker (antigens) integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein transport proteins channels, permeases (pumps) AP Biology 2 major types of membrane proteins. Peripheral proteins - not embedded; bound to inner/outer surface; may be connected to integral proteins Integral proteins - penetrate bilayer; - Transmembrane protein •hydrophobic regions of nonpolar aa’s in contact with bilayer core; often alpha helices •hydrophilic regions of aa’s in contact w/ environment – beta pleated AP Biology - Unilateral – partially through; coupled Protein’s domain anchor molecule Within membrane Polar areas of protein nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane polar amino acids hydrophilic extend into AP Biology extracellular fluid & into cytosol Nonpolar areas of protein H+ Examples Retinal chromophore NH2 water channel in bacteria Porin monomer b-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH H+ Cytoplasm proton pump channel in photosynthetic bacteria AP Biology function through conformational change = shape change Many Functions of Membrane Proteins Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell surface identity marker Cell adhesion Attachment to the cytoskeleton Uniport Symport Antiport AP Biology Membrane carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish one cell from another antigens important in organ & tissue development basis for rejection of foreign cells by immune system AP Biology Some proteins reinforce shape of cell •cytoplasmic side, •some connect to cytoskeleton • exterior side, •some attach to fibers of ECM •ECM = extracellular matrix AP Biology Sidedness of the plasma membrane What makes the endomembrane system work? AP Biology Membranes are bifacial and sided •may differ in lipid composition •proteins have a direction orientation •outer surface has carbo’s •asymmetry begins w/ synthesis in ER •proteins in plasma membrane provide a variety of major cell functions AP Biology The detailed structure of an animal cell’s plasma membrane, in cross section AP Biology Cell-cell recognition = distinguish one type of neighboring cell from another important in cell sorting •organization into tissues and organs during development basis for rejection of foreign cells by immune system •key in on surface molecules, often carbs AP Biology Carbo’s - usually branched oligosaccharides = < 15 monomers •may be covalently bonded to •lipids= glycolipids •proteins = glycoproteins •external OS’s vary from species to species, individual to individual, and even from cell type to cell type w/in same individual - identification marks each cell type as distinct •blood types (A, B, AB, O)-RBCs •Glycocalyx of animal cells – fuzzy! AP Biology End membranes AP Biology Movement across the Cell Membrane AP Biology 2007-2008 Diffusion 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Diffusion AP Biology movement from high low concentration Diffusion Move from HIGH to LOW concentration “passive transport” no energy needed AP Biology diffusion movement of water osmosis Diffusion across cell membrane Cell membrane is the boundary between inside & outside… separates cell from its environment Can it be an impenetrable boundary? NO! OUT IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O AP Biology OUT IN waste ammonia salts CO2 H2O products cell needs materials in & products or waste out Diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids inside cell NH3 What molecules can lipid salt NOT get through directly? polar molecules H 2O outside cell sugar aa H 2O ions salts, ammonia large molecules starches, proteins AP Biology Channels through cell membrane Membrane becomes semi-permeable with protein channels specific channels allow specific material across cell membrane inside cell NH AP Biology 3 salt H 2O aa sugar outside cell Facilitated Diffusion Diffusion through protein channels channels move specific molecules across cell membrane facilitated = with help no energy needed open channel = fast transport high low AP Biology “The Bouncer” Active Transport Cells may need to move molecules against concentration gradient shape change transports solute from one side of membrane to other protein “pump” conformational change “costs” energy = ATP low ATP high AP Biology “The Doorman” Active transport Many models & mechanisms ATP AP Biology ATP antiport symport Getting through cell membrane Passive Transport Simple diffusion diffusion of nonpolar, hydrophobic molecules lipids high low concentration gradient Facilitated transport diffusion of polar, hydrophilic molecules through a protein channel high low concentration gradient Active transport diffusion against concentration gradient low high AP Biology uses a protein pump requires ATP ATP Transport summary simple diffusion facilitated diffusion active transport AP Biology ATP How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” AP Biology exocytosis exocytosis Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by molecular signal AP Biology The Special Case of Water Movement of water across the cell membrane AP Biology 2007-2008 Osmosis is diffusion of water Water is very important to life, so we talk about water separately Diffusion of water from high concentration of water to low concentration of water AP Biology across a semi-permeable membrane Concentration of water Direction of osmosis is determined by comparing total solute concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water water AP Biology hypotonic hypertonic net movement of water Managing water balance Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater Managing water balance Isotonic animal cell immersed in mild salt solution example: blood cells in blood plasma problem: none no net movement of water flows across membrane equally, in both directions volume of cell is stable AP Biology balanced Managing water balance Hypotonic a cell in fresh water example: Paramecium problem: gains water, swells & can burst water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP ATP plant cells turgid AP Biology freshwater Water regulation Contractile vacuole in Paramecium ATP AP Biology Managing water balance Hypertonic a cell in salt water example: shellfish problem: lose water & die solution: take up water or pump out salt plant cells plasmolysis = wilt AP Biology saltwater 1991 | 2003 Aquaporins Water moves rapidly into & out of cells AP Biology evidence that there were water channels Peter Agre Roderick MacKinnon John Hopkins Rockefeller Osmosis… .05 M .03 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell AP Biology Any Questions?? AP Biology