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Membrane Structure and Function Cell Membrane-the plasma membrane is selectively permeable – allows some substances to cross more easily that others-fluid mosaic model: fluid b/c the membrane is held together by weak interactions. Mosaic b/c made of phospholipids, proteins and carbs Early membrane model-sandwich model. Phospholipid bilayer sandwiched between 2 protein layers.-problems: varying chemical composition of membrane, hydrophobic protein parts the freeze-fracture method revealed the structure of the membrane interior phospholipids exist in a bilayer-amphipathic = hydrophilic head, hydrophobic tail-hydrophobic barrier keeps hydrophilic molecules out membrane fluidity-low temps: phospholipids w unsaturated tails (kinks prevent close packing)-cholesterol resists changes: limits fluidity at high temps. Hinders close packing at low temps Membrane Proteins--Integral proteins: embedded in membrane. Determined by freeze-fracture. Transmembrane w hydrophilic heads/tails and hydrophobic middles.--Peripheral proteins: extracellular or cytoplasmic sides of membrane. Not embedded. Held in place by the cytoskeleton or ECM. Provides stronger framework. Functions of membrane proteins include: transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to the cytoskeleton Membrane Carbohydrates-function – cell-cell recognition, developing organisms-glycolipids, glycoproteins-ex: blood transfusions are type-specific-synthesis and sidedness of membranes Selective Permeability-small molecules (polar or nonpolar) cross easily (hydrocarbons, hydrophobic molecules, CO2, O2)-hydrophobic core prevents passage of ions and large polar molecules *Passive transport – NO energy needed*-diffusion down concentration gradient (high concentration à low)-ex: hydrocarbons, CO2, O2, H2O-diffusion until equilibrium is reached -OSMOSIS = diffusion of water. Water moves from lower concentration of solute to higher concentration of solute (high to low water concentration)--external environments can be hypotonic, isotonic, or hypertonic to internal environments of cell---hypo: water into cell. Cell lysed in animal. Turgid/normal in plant.---iso: same in/out movement. Normal in animal cell. Flaccid in plant---hyper: water out of cell. Shriveled in animal. Plasmolyzed in plant. --osmoregulation: ---control solute and water balance---contractile vacuole: bilge pump forces out fresh water as it enters by osmosis (ex: in freshwater protest) -facilitated diffusion--transport proteins (channel or carrier proteins) help hydrophilic substances cross--provide hydrophilic channel or loosely bind/carry molecules across (ions, polar molecules like water and glucose) ex: aquaporin – channel protein that allows passage of waterex: glucose transport protein – carrier protein *Active Transport – REQUIRES energy (ATP)*-proteins transport substances AGAINST concentration gradient (low à hi conc) ex: Na+/K+ pump, proton pump-electrogenic pumps – generate voltage across a membrane Na+/K+ pump: pumps Na+ out and K+ in. nerve transmission.proton pump: push protons (H+) across membrane. Ex: mitochondria in ATP production-cotransport: membrane protein enable ‘downhill’ diffusion of one solute to drive ‘uphill’ transport of other ex: sucrose-H+ cotransporter PASSIVE VS. ACTIVEPassive: little/no energy. High à low concentrations. DOWN concentration gradient. Like diffusion, osmosis, facilitated diffusion (w transport protein)Active: requires energy (ATP). Low à high concentrations. AGAINST concentration gradient. Like pumps, exo.endocytosis. Bulk transport – transport of proteins, polysaccharides, large molecules.-endocytosis: take IN macromolecules, form new vesicles-exocytosis: vesicles fuse w cell membrane, expel contents Types of endo:-phagocytosis: cellular eating. solids-pinocytosis: cellular drinking. fluids-receptor-mediated endocytosis: ligands bind to specific receptors on cell surface
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