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WSU BIOLOGY 251 - Cell Membrane

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BIO 251 1nd Edition Lecture 3Outline of Last Lecture I. PerturbationsII. DNAIII. Tay Sachs DiseaseIV. Cell energeticsV. Oxidative DamageVI. OrgansVII. FunctionsVIII. Organization of body-functionsIX. Homeostasis X. CytoplasmXI. OrganellesXII. Cytosol XIII. CytoskeletonXIV. MicrofilamentsXV. Oxidation of glucoseOutline of Current Lecture I. Membrane composition and functionII. Membrane adhesions between different cellsThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.III. Membrane transportIV. Electrochemical gradientsCurrent LectureII. Membrane Composition and FunctionA. Composition (Figs 2.15b and 2.16)1. Lipid Bilayer (Note: water is polar; see Review Box on page 31)a) Phospholipids are primary membrane component(1) polar (hydrophillic) heads on outside (electrically charged)(2) non-polar (hydrophobic) tails on inside (electrically neutral)(3) are fluid; they twirl and moveb) Cholesterol molecules tucked among phospholipids; increase membrane fluidity2. Proteinsa) Some stud outside, some stud insideb) Some span membrane, with polar regions at ends, and non-polar regions buriedwithin lipid bilayer.c) Float like icebergs in a sea of phospholipids.3. Carbohydratesa) Only a small amountb) Glycoproteins (carbo + protein)c) Glycolipids (carbo + lipid)4. Fluid Mosaic Model of Membrane Structurea) Membrane is fluid, like cooking oilb) Mosaic pattern of proteins embedded within lipid bilayerB. Most Important Functions1. Lipid Bilayera) Primary structure of membraneb) Barrier to diffusion(1) physically hinders molecules from crossing(2) hydrophobic interior barrier to passage of water soluble substancesbetween ICF and ECF.c) Is responsible for fluidity of membrane2. Membrane Proteinsa) channels for water & small water soluble moleculesb) carrier molecules that transfer specific substancesc) receptor sites on outer surface that recognize and bind specific molecules in the3. Membrane protein channel regulationa) Many membrane protein channels are like “gates” that open & closeb) Opened or closed by(1) binding of chemical messenger(2) change in electrical currentc) Crucial to how electrical currents are propagatedIII. Membrane Adhesions Between Different CellsA. Extracellular matrix1. meshwork of fiber proteins and carbohydrates2. acts as an extracelluar glue to hold cells together within a tissueB. Cell junctions1. tight junctions (Fig 2.27)a) impermeable junctions that join lateral edges of epithelial cells2. desmosomes (Fig 2.28)a) place where filaments tie cells together3. gap junctions (Fig 2.28)a) gaps between cells joined by tunnel called connexonsb) small molecules can pass through “tunnel” without ever entering the ECFIV. Membrane TransportA. Overview1. Membrane is selectively permeable; some molecules can pass through and some molecules can’t2. Factors that affect permeability of a moleculea) its solubility in lipid (i.e., ability to dissolve in phospholipid bilayer and hence pass through it)(1) uncharged (= non-polar) molecules (e.g., O2, CO2, fatty acids) are lipid soluble and so can pass through lipid bilayer(2) ions (e.g., Na+, K+) and polar molecules (glucose and proteins) are not lipid soluble and can NOT pass through lipid bilayerb) its size(1) small molecules cross membrane more easily than large molecules3. Methods of Crossinga) passive (no energy required)b) active (requires energy)B. Passive Diffusion (Figs 4.6 & 4.7)1. Solutes “try” to distribute evenly in a solutiona) This happens because all molecules are in continuous random motion when the temperature is greater than absolute zero (-459° F)b) difference in concentration of a material between two adjacent areas is called a concentration gradientc) Net movement of solutes is from area of high concentration to low concentration until concentrations are equal (Fig 4.7)2. Factors that affect rate of diffusion of a solute across a membranea) magnitude of concentration gradient (bigger = faster)b) permeability of membrane to solute (more permeable = faster)d) molecular weight of substance (smaller = faster)e) distance over which diffusion occurs (shorter = faster)C. Osmosis (= passive diffusion of water)1. Simply the net diffusion of water down its concentration gradientD. Carrier Mediated Transport1. Carrier proteins span membrane2. Factors that influence kind & amount of substance transporteda) Specificity for substanceb) Saturation: limit to amount of substance that can be transported per unittimec) Competition between different substances for use of same carrier3. Two typesa) facilitated diffusionb) active transportE. Facilitated diffusion1. No energy required for transport2. Carrier assists movement of substance down its concentration gradienta) Glucose is important example (Fig 4.10)(1) Higher in ECF (i.e., plasma) than in ICF(2) But can’t cross membrane because glucose is polar(3) Glucose carrier molecule allows glucose to passsively flow down itsconcentration gradient from ECF into cell3. Small channels can allow movement of ions across membrane (Fig 4.12)F. Active Transport1. Pump protein requires energy (ATP) to move substance across membrane2. Can move molecules against their concentration gradient (Fig 4.13)a) Na+ K+ ATPase pump keeps Na+ high in the ECF and K+ high in the ICFG. Vesicular transport1. Transport in or out of cell in membrane bound vesicleV. Electrochemical gradientsA. Molecules with charges affected by1. concentration gradients, as described above2. electrical gradientsa) Opposite charges attractb) ions will “try” to distribute themselves so that negative and positive charges are distributed equally (Fig 4.3)c) difference in charge between two adjacent area is electrical gradient 3. Simultaneous existence of concentration gradient and electrical gradient is called electrochemical gradient (Fig


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