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Transcript
The Cell Membrane AP Biology AP Biology Helpful links as you go through this chapter… http://student.ccbcmd.edu/courses/bio1 41/lecguide/unit3/eustruct/cmeu.html Cell membrane defines the perimeter of the cell Cell membrane separates living cell from aqueous environment thin barrier = 8nm thick Controls traffic in & out of the cell allows some substances to cross more easily than others Movement 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 OUT IN waste ammonia salts CO2 H2O products cell needs materials in & products or waste out Selective Permeability Barrier allows SOME substances to cross more easily than others Also called SEMI-Permeable Key aspect of membrane structure and function Remember: selectively permeable membranes are important in many organelles as well as the plasma membrane of the cell itself. Membrane Structure Primary components Lipids Foundation molecules Primarily PHOSPHOLIPIDS Proteins Embedded within the lipids of the membrane Important in carrying out most membrane functions Remember: It’s the PROTEINS that do all the WORK! Phospholipids Phosphate head “attracted to water” hydrophilic Fatty acid tails Phosphate hydrophobic Arranged as a bilayer Fatty acid “repelled by water” Aaaah, one of those structure–function examples Why the bilayer? Water is attracted to hydrophilic heads and NOT to hydrophobic tails Water pushes tails away and they end up pointed toward each other and AWAY from water. Results in Bilayer •Animation – Click HERE Bilayer as Selective Barrier Because of the close placement and polarity of the phosphate heads, there are some molecules that cannot penetrate and some that can. What CANNOT penetrate Phospholipid Bilayer? What CAN penetrate Phospholipid Bilayer? • Anything NONPOLAR •Lipids •Oxygen •Carbon dioxide NONPOLAR molecules do NOT get attracted and Stuck to polar heads; thus They pass through. • POLAR / Charged things •Ions (H+, Cl-, Na+) •Water •Glucose •Polar amino acids Ions get large hydration Shells in aqueous Solution which makes them Too large to fit b/t Phospholipids Also, polar/charged molecues Become attracted and stuck to polar heads ______ attracted to nonpolar center of bilayer Arranged as a Phospholipid bilayer Serves as a cellular barrier / border sugar H 2O salt polar hydrophilic heads nonpolar hydrophobic tails impermeable to polar molecules polar hydrophilic heads waste lipids How, then, can these polar, but necessary molecules pass through the membrane? PROTEINS Embedded in the bilayer Permeability to polar molecules? Membrane becomes semi-permeable via protein channels specific channels allow specific material across cell membrane Thus control is also maintained. Not just anybody can get in. inside cell NH3 salt H 2O aa sugar outside cell Aquaporin – A Protein Channel Used to be thought that HI water Conc. water could penetrate the phospholipid bilayer unaided Now we know that a protein channel called aquaporin is found in nearly all cell membranes in large amounts This is the channel that allows the passage of water. Remember, water is polar and would NOT be attracted past the polar heads LO water Conc. Cell membrane is more than lipids… Transmembrane proteins embedded in phospholipid bilayer create semi-permeabe channels protein channels in lipid bilyer membrane AP Biology 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 Classes of amino acids What do these amino acids have in common? I like the polar ones the best! polar & hydrophilic Protein’s regions anchor molecule Within membrane Polar areas of protein nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane in fluid polar amino acids hydrophilic extend into extracellular fluid & into cytosol Nonpolar areas of protein H+ Examples Retinal chromophore NH2 aquaporin = water channel Porin monomer H 2O b-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH H HH+++ Cytoplasm proton pump channel in photosynthetic bacteria H 2O function through conformational change = protein changes shape Membrane Proteins do LOTS of different jobs!! “Channel” Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell adhesion Attachment to the cytoskeleton “Antigen” Cell surface identity marker Membrane Proteins Proteins determine membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Classes of membrane proteins: peripheral proteins loosely bound to surface of membrane ex: cell surface identity marker (antigens) integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein ex: transport proteins channels, permeases (pumps) Cell membrane must be more than lipids… In 1972, S.J. Singer & G. Nicolson proposed that membrane proteins are inserted into the phospholipid bilayer FLUID MOSAIC MODEL Made up of lots of little pieces – like a mosaic Pieces can slide past each other – like a soap bubble – like a fluid Click here for an animation Don’t forget Cholesterol Embedded in membrane b/t phospholipids Keeps membrane firm yet fluid Important for cell membrane structure Even though we often think of cholesterol as being “bad”… Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein Transmembrane proteins Cytoplasm Filaments of cytoskeleton 1972, S.J. Singer & G. Nicolson proposed Fluid Mosaic Model 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 Getting through cell membrane Passive Transport Simple diffusion diffusion of nonpolar, hydrophobic molecules lipids HIGH LOW concentration gradient Facilitated diffusion diffusion of polar, hydrophilic molecules through a protein channel (“door”) HIGH LOW concentration gradient Active transport pump against concentration gradient LOW HIGH uses a protein pump requires ATP ATP Transport summary HI HI LO LO LO HI simple diffusion facilitated diffusion active transport NONPOLAR cmpds. Lipids; oxygen; Carbon dioxide POLAR cmpds. Water; Ions – Na+, Cl-; Glucose, etc. ATP REQUIRED How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles Endocytosis – taking in phagocytosis = “cellular eating” - solids pinocytosis = “cellular drinking” - liquids Exocytosis Expelling / secreting substances exocytosis Endocytosis Solids fuse with lysosome for digestion phagocytosis Liquids pinocytosis Both are non-specific processes Cholesterol is taken Into the cell This way. receptor-mediated endocytosis LIGAND – substance that fits receptor SPECIFIC The Special Case of Water Movement of water across the cell membrane AP Biology 2007-2008 Osmosis is just 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 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 moves from a HYPOTONIC (HIGH Concentration of water) solution to a HYPERTONIC (LOW Concentration of water) solution LESS water MORE solute MORE water LESS solute Yellow dot = water SOLUTE Blue = WATER hypotonic hypertonic net movement of water Managing water balance Cell survival depends on balancing water uptake & loss freshwater balanced saltwater 1 Managing water balance Hypotonic a cell in fresh water high concentration of water around cell problem: cell gains water, swells & can burst (lysis) KABOOM! example: Paramecium ex: water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP ATP plant cells No problem, here – I love it! turgid = full cell wall protects cell from bursting freshwater Pumping water out Contractile vacuole in Paramecium ATP 2 Managing water balance Hypertonic I’m shrinking, I’m shrinking! a cell in salt water low concentration of water around cell problem: cell loses water & can die example: shellfish solution: take up water or pump out salt plant cells Water… plasmolysis = wilt = flaccid cells Cell membrane collapses away from cell wall can recover, but may die Salt on roadsides after snow… saltwater 3 Managing water balance Isotonic That’s perfect! animal cell immersed in mild salt solution no difference in concentration of water between cell & environment problem: none no net movement of water flows across membrane equally, in both directions OK..but I could cell in equilibrium be better… volume of cell is stable example: blood cells in blood plasma slightly salty IV solution in hospital balanced 1991 | 2003 Aquaporins Water moves rapidly into & out of cells evidence that there were water channels protein channels allowing flow of water across cell membrane Peter Agre Roderick MacKinnon John Hopkins Rockefeller Do you understand Osmosis… Remember: It’s all RELATIVE .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 ‘PUMPS’: Special Active Transport Sodium Potassium Pump Na+/K+ Pump Classic example Important in nerve cell and nerve impulse transmission (among others) Generates voltage difference across the membrane of the neuron For animations click Here or Here Cotransport Membrane proteins couple the transport of one solute to another A combination of both active transport and facilitated diffusion (passive). Pump (active) Transport channel (passive) Cotransport – Example – in plants Proton Pumps pump H+ against the concentration gradient and OUT of the cell. Creates a very hi concentration of H+ outside the cell. ACTIVE transport That means, if given an opportunity, H+ would flow WITH its concentration gradient back INSIDE the cell. Cotransport – Example – in plants In plants, sucrose is in HIGH concentration inside the cells, but plants need to get even even MORE sucrose into cells for storage. Thus, sucrose must move into a plant cell AGAINST its concentration gradient. Cotransport – Example – in plants “Downhill” (WITH concentration) movement of H+ is coupled with the “uphill” (AGAINST concentration) movement of sucrose into the cell. Sucrose MUST bind to an H+ ion. “Rides” with the H+ as H+ flows INTO the cell through the cotransport channel WITH it’s concentration gradient. Remember, the H+ concentration gradient is maintained by the proton pump The only way that sucrose can enter is if it is bound to H+. Then the very specific cotransport channel will allow both to “diffuse” through. PUMP to maintain H+ conc. = ACTIVE Cotransport channel = facilitated diffusion = PASSIVE