Chapter 7
Chapter 7: Membrane Structure and FunctionSelective permeability: Allows some substances to cross more easily than othersI. Membrane StructureAmphipathic molecule: A molecule with both a hydrophilic and hydrophobic region (phospholipid)Fluid mosaic model: Membrane is a fluid structure with various proteins embedded in or attached to a phospholipid bilayer Membrane models1. Charles Overton (1895): Membranes are made of lipidsa. Substances that dissolve in lipids enter cells faster than substances that are insoluble in lipidsb. 20 years later: RBC membranes isolated and chemically posed of lipids and proteins2. Irving Langmuir (1917): Made artificial membranesa. Added phospholipids dissolved in benzene (organic solvent) to waterb. Benzene evaporated1. Film of lipids covered surface of water2. Hydrophilic heads immersed in water3. Gorter and Grendel (1925): Phospholipid bilayers a. Molecular arrangement shelters hydrophobic tails from water b. Exposes hydrophilic headsc. Measured phospholipid content of membranes isolated from RBC1.Just enough to cover cells with 2 layers4. Davson and Danielli (1935): Phospholipid bilayer between 2 layers of globular proteinsSandwich modelb. By end of 1960s: 2 problems1. Not all membranes are alike a. Differences in thickness and appearanceb. Different number of proteinsc. Differences in types of lipids and phospholipids2.Hydrophobic parts of proteins exposed to aqueous environment 5.Singer and Nicolson: Fluid mosaic modela. Placed proteins in a location compatible with their amphipathic characterb. Dispersed and individually insertedc. Hydrophilic regions protruding6.Freeze-fracture: a. Phospholipid bi-layer is splitb. Halves viewed (EM)c. Protein particles interspersed in a smooth matrixB. Membranes are fluid1. Membranesheld together by hydrophobic attractions2. Most lipids and some of the proteins drift laterally a. Rare for molecules to flip-flop from one layer to the other1. Hydrophilic part of molecule would have to cross the hydrophobic core3. Phospholipids move rapidly (2 um/sec)a. Proteins: Much larger, move more slowlySome drifta. Human cell fused to mouse cell (less than an hour)2.Some move in a highly directed mannera. Driven along cytoskeletal fibers by motor proteins 3. Some proteins are immobile: Attached to cytoskeleton5. Membrane remains fluid as temp decreases At some critical temp it solidifies 1.Phospholipids become tightly packedb. Critical temp depends types of lipids membrane is made of1.Rich in phospholipids with unsaturated hydrocarbon tails a. Remains fluid to lower tempsb. Kinks where double bonds are locatedc. Do not pack together as closely as saturated hydrocarbons 6. Cholesterol is wedged between the phospholipid molecules in the plasma membranes of animal cellsa. Different effects on membrane fluidity at different temps1. 37oC : Makes membrane less fluid by restraining movement of phospholipids2. Lowers the temp required for membrane to solidify 7. Membranes must be fluid to work properly a. When it solidifies: Permeability changes, enzymatic proteins may become inactiveb. Some cells alter lipid composition as an adjustment to changing temp1. Plants that tolerate extreme colda. Winter wheatb. % of unsaturated phospholipids increases in autumnc. Keeps membrane from solidifying in winterC. Membranes as Mosaics of Structure and Function1. Proteins determine membranes specific functions2. Plasma membrane and membranes of organelles have their own unique proteins3. 2 Major populations of proteinsa. Integral proteins: Penetrate hydrophobic core of lipid bilayer1. Transmembrane proteins: Completely span membrane2. Some are unilateral: Reach partway across membraneb. Peripheral proteins: Not imbedded in bilayer1. Appendages attached to the surface 2. Cytoplasmic side: Peripheral proteins and their integral partners may be held in place by attaching to the cytoskeleton4.Exterior: Membrane proteins are attached to fibers of the ECM (Integrins)4. Membranes are bifacial: Inside and outside facesa. Each protein has directional orientationb. Carbohydrates restricted to exterior surfacec. Molecules that start on inside face of ER end up on outside face of plasma membraned. Vesicle fusion with plasma membrane1. Enlarges membranes, secretes products2. Extra-cellular surface carbohydrates 1.Synthesized in ER 2.Modified in GolgiD. Membrane Carbohydrates and Cell-Cell Recognition: 1. Sorting of cells into tissues and organs in an animal embryo2. Basis for rejection of foreign cells by the immune system3. Cells recognize each other by keying on surface molecules (often carbohydrates)a. Branched oligosaccharides: b. Some: Covalently bonded to lipids: Glycolipidsc. Most: Covalently bonded to a protein: Glycoprotein1. Four blood groups2. Variation in oligosaccharides on surfaces of RBCsII. The Traffic of Small MoleculesA. Selective Permeability1. Permeability of the Lipid Bilayera. Hydrophobic core impedes transport of ions and polar molecules (Hydrophilic)b. Hydrophobic molecules:c. Hydrophobic core impedes transport of ions and polar molecules1. Larger, uncharged polar molecules (glucose, other sugars)2. All ions3. H2O cannot pass through easily2. Transport Proteins: A protein that spans the membrane providing hydrophilic channel across the membrane, that is selective for a particular solutea. Hydrophylic channel that certain molecules or atomic ions can tunnel through b. Some bind to their passengers and physically move them across the membraneinto the cellc. Specific for the substance it translocates1. Glucose carried to liver by blood a. Specific transport proteinsb. So selective: Rejects fructosed. Diffusion determines the direction of the trafficB. Passive Transport: Diffusion1.Diffusion: The tendency for molecules of any substance to spread out into the available spacea. KE: Thermal motion (heat)b. Simple law: In the absence of other forces a substance will diffuse from where it is more concentrated to where it is less concentratedc. Concentration gradient: Moving from high to low concentrations across plasma membrane1. Each substance diffuses down its own concentration gradienta. Unaffected by concentrations of other substances2. Example: Uptake of O2 by a cell during cellular respirationa. Dissolved O2 diffuses into cell across plasma membraneb. Cellular respiration consumes O2c. Maintains concentration gradient in that direction3. Passive Transport: Diffusion of a substance across a membraneC. Osmosis; the passive transport of water1. Hypertonic: Solution with higher concentration of solute2. Hypotonic: Solution with lower solute concentrationa. Tap water is hypertonic to distilled water3. Isotonic: Solutions of equal solute concentration4. Osmosis: Diffusion of water across selectively permeable membranea. Sea water: b. Isotonic solutions: Water moves across membrane at equal rates in both directionsD. Balancing water uptake and loss1. Water Balance of Cells without wallsa. Animal cell immersed in an environment that is isotonic to cell1. No net movement across membrane2. Volume is stableb. Environment hypertonic to cell (salt)1. Cell shrivelsc. Environment Hypotonic to cell1. Cell bursts or lysesd. No cell walls: Cannot tolerate excessive water uptake or loss of watere. Isotonic environment1. Seawater to many marine invertebrates2. Extra-cellular fluid of terrestrial animals f. Osmoregulation: The control of water balance1. Paramecium: a. Pond water is hypotonic to the cellb. Plasma membrane that is much less permeable to water than the membranes of most other cellsc. Contractile vacuole: Functions as a pump to force water out of the cell as fast as it enters by osmosis2. Water Balance of Cells with wallsa. Plant cell in hypotonic solution (rainwater)1. Swells as water enters by osmosis2. Wall expands until it exerts back pressure3. Opposes further water uptake 4. Wall pressure exerts a force equal and opposite to the osmotic pressure of the cella. Turgid: Very firmb. Mechanical support (non-woody)c. Cells must be hypertonic to solutiond. If isototic: Water doesn’t enter1. Flaccid: (limp)2. Over fertilizing b. Hypertonic environment: Cell wall is of no advantage1. Plasmolysis: As plant cell shrivels; plasma membrane pulls away from cell wallE. Specific proteins facilitate passive transport of water and solutes1. Facilitated diffusion: Diffusion with the help of transport proteins that span the membrane2. Transport protein: Many of the properties of an enzymea. Proteins are specialized for the solute it transports1. Specific binding siteb. Transport proteins can be saturatedc. Can be inhibited by molecules that imitated. Do not catalyze chemical reactions1. Catalyze the process of transporting molecules 3. Channel proteins: Provide corridors for specific molecule or ion to cross1. Hydrophilic passage waya. Water molecules, small ionsb. Aquaporins: Water channel protein2. Gated channels: Stimulus causes them to open or closea. Chemical: 4. Some helps molecule across by changing shape1. Triggered by the binding and release of transported molecules3. Inherited diseases: Specific transport systems are defective or missinga. Cysitnuria: Absence of protein that carries cystine and other aa across membranes of kidney cellsb. Reabsorb aa from urine and return them to bloodPainful stones accumulate and crystallize F. Active Transport1. Active transport: Moving a molecule against concentration gradienta. Animal cells have a higher concentration of K+ than surroundingsb. Lower concentration of Na+c. Pumps Na+ out and K+ INd. Specific proteins, ATP supplies energy1. Transfers terminal phosphate group to transport protein1. Induces conformation change2. Translocates a solute across the membrane3. Sodium-potassium pump: Transport system which exchanges Na+ for K+ across animal cellsG. Ion Pumps Generate voltage (electrical PE) across membranes1. Membrane potential: Voltage across membrane1.-50 to –200 mvolts2. Neg sign: Inside cell3. Favors passive transport of cations into the cell and anions out2. 2 forces drive diffusion 1. Chemical force: Ion’s concentration gradient2. Electrical force: Effect of membrane potential 3. Electrochemical gradient: Combination of chemical and electrical force on movement of ions across membranes4. Ex: Resting nerve cell1. Na+ inside cell is much lower than outside2. When cell is stimulated: Gated channels that facilitate Na+ diffusion openf. Na+ K+ pump1. Pumps 3 Na+ out and 2 K+ in2. Electrogenic pump: Transport protein that generates voltage across membranea. Stores energy3.Main pump of animal cellsg. Proton pump: Main electrogenic pump of plants, bacteria and fungi1. Actively transports H+ ions (protons) out H. Cotransport: A single ATP powered pump that transports specific solute can indirectly drive active transport of several other solutes1. Protein couples downhill diffusion of one substance with uphill transport of a second against its concentration gradienta. Plant cell: Uses gradient of H+ to drive active transport of aa, sugars and nutrients into cell1. Transport protein couples return of H+ and transport of sucrosea. Sucrose into cell against concentration gradientb. Must travel with a H+I. Exocytosis and endocytosis transport large molecules1. Exocytosis: Cell secretes macromolecules by fusion of vesicles with plasma membrane2. Endocytosis: Cell takes in macromolecules and particulate matter by forming new vesicles from plasma membranea. Small area of plasma membrane sinks inward to form pocketb. Deepens, pinches; forming a vesicle 1.Contains material that had been outside cellc. 3 types1. Phagocytosis: (Cellular eating)a. Cell engulfs particle by wrapping pseudopodia around itb. Packaging it within a membrane-enclosed sac (food vacuole)c. Particle digested after food vacuole fuses with lysosome1.Hydrolytic enzymes2.Pinocytosis: (Cellular drinking)a. Cell takes in droplets of extracellular fluid into tiny vesiclesb. Dissolved solutes: Unspecific transporte. Receptor-mediated endocytosis: Specific endocytosis1. Proteins imbedded in membrane exposed to extracellular fluid2. Ligands: Any molecule that binds specifically to a receptor site of another moleculea. Ligare: to bind3. Receptor proteins clustered in coated pitsa. Lined on cytoplasmic side with a fuzzy layer of proteinb. Help deepen pit and form vesicle4. Enables cell to acquire bulk quantities of specific substancesa. Human cells take in cholesterol for synthesis of membranes and precursor for synthesis of other steroids1.Travels in blood in particles: Low-density lipoproteins (LDLs)a. Complexes of lipids and proteinsb. Bind to LDL receptors on membranes1.Enter by endocytosis2.Hypercholesterolemia: High level of cholesterol in blooda. Defective LDL receptor proteinsb. LDL particles cannot enter cellsc. Cholesterol accumulates in blood: atherosclerosis 1.Build up of fats on blood vessel liningsf. Exo and endocytosis occur continually 1.Amount of plasma membrane in non-growing cell remains constant ................
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