Ch. 3: Plasma Membrane Structure and Function Biochemistry: The unique properties of water δ- δ+ Hydrogen bonding is when the partial + charge on Hydrogen is attracted to the partial – charge of another compound. δ+ Water molecules have polar covalent bonds. Properties of Phospholipids Phospholipids P.Membrane (PM) held together by weak hydrophobic interactions – Glycerol with Phosphate Head + 2 Fatty Acid Chains – Amphiphilic (“Both” “lover”) • • Hydrophilic headPhosphate Hydrophobic tail Glycerol – Forms 2 layers in water (negatively charged phosphate head attracted Fatty water Acids molecules) to + end of polar – Makes up cell membranes Fluid Mosaic Model Fluidity: (not rigid) • P.Membrane (PM) held together by weak hydrophobic interactions (bi-lipid tails face each other, away from water) • Lateral drifting ability of lipids • Temperature Dependent Why is this rare? Cholesterol puts gaps between phospholipids, increasing fluidity Unsaturated tails prevents packed phospho-lipid rafts “Mosaic” PM is made up of a mosaic or “collage” of: • Phospholipids Hey Sugar! – • cholesterol Let’s “stick” • integral and peripheral proteins Hey Suga-! • glycolipids glycocalyx • glycoproteins Integral or Transmembrane Proteins • Penetrate hydrophobic core of membrane Surface or Peripheral Proteins • Loosely bound to surface • Some attaches to cyto-skeleton or ECM (Extracellular matrix) Review: What organelles are responsible for creating membrane proteins? Membrane Transport • Cells NEED to be able to: – remove waste – take in necessary nutrients from interstitial fluids – Send out signals to other cells – Receive signals from other cells • Transport Classified as: – Passive – Active Selective Permeability of Plasma Membrane General rule: like dissolves like • Non-polar, hydrophobic solutes dissolve in lipid • Ions, hydrophillic, or polar solutes dissolve in water Selective Permeability of Plasma Membrane Selective Permeability: some substances can pass through lipid core or membrane more easily than others 1. CO2, O2, non-polar molecules, and other lipids, are hydrophobic and can pass hydrophobic lipid membrane core easily 2. Water, sugars, charged ions, or polar molecules cannot pass lipid core easily so must use hydrophillic transport proteins to pass (ex. Aquaporins) 3. Small molecules are more permeable than larger ones Passive Transport • Molecules move down [gradient] (from high to low concentration) until equilibrium is reached • Spontaneous process • No ATP needed; uses Kinetic energy (KE) or Hydrostatic Pressure as E source • Types of Passive Transport: – Simple Diffusion – Facilitated Diffusion – Osmosis – Filtration Simple Diffusion • Diffusion – molecules of any substance moves down [gradient], unassisted • Ex. O2 in blood, CO2 in cells Back to Types of PT Back to Types of PT Facilitated Diffusion • Assisted diffusion of molecules with help from channels or carriers Channels specific for particular molecule, like sugars, amino acids Carriers move substances like ions, water. Selective by size and charge Water always moves from hypotonic to hypertonic Osmosis • Diffusion of WATER across the membrane • Tonicity dependent – Isotonic solution: solution in equilibrium to another solution across the membrane – Hypotonic: solution with less dissolved [solute], higher [water] compared to another solution – Hypertonic: solution with more dissolved [solute], lower [water] compared to another solution Back to Types of PT Filtration • “Forcing” of water and solutes through membrane by hydrostatic pressure • Selective only by SIZE • Ex. only blood cells/proteins too large to pass are held back Active Transport • Molecules move up or against [gradient] (from low to high) to create an electrochemical gradient • Nonspontaneous • Requires ATP as E source • Types: – Primary Active Transport (T) – Secondary Active (T) – Clathrin-coated Vesicular (T) • Endocytosis • Exocytosis Active Transport generates an electrochemical gradient: charge difference (disequilibrium) between both sides of the membrane Back to Types of AT Back to Types of AT Primary Active Transport Uses ATP E directly Ex 1: Sodium-Potassium Pump 3-D overview – Pump keeps Na+ moving out of the cell, against its gradient, building its concentration (disequilibrium) – Pump keeps K+ moving into the cell against its gradient, building its concentration – Na+:K+ ratio 3 out : 2 in – Na+:K+ pump uses high concentration gradients to store PE for future cellular work or for secondary AT • Ex 2: Pumping H+ ions into lysosome to create acidic env’t for cellular digestion Back to Types of AT Secondary Active Transport (Coupled Transport) Symport: twothe • Involves transported transport of a substances move substance in against the samea direction concentration Ex. Hydrogen gradient creating PE ATP ADP + Pi H+ H+ gradient H+ Antiport: two H+ H+ powered transported H+ indirectlymove by substances in an opposite ATP direction (“wave” powered to pump each other) Sucrose transported against gradient into cell, using KE stored from H gradient, falling back down gradient Clathrin: protein coat on PM helps with 1) specifying cargo 2) membrane deformation Endocytosis • The engulfing of substances by pseudopods extensions of the plasma membrane • Three types: – Phagocytosis (cell eating – lg. particles engulfed) – Pinocytosis (cell drinking – sm. ions and liquids engulfed) – Receptor Mediated Endocytosis (use of surface proteins tothat engulf specific substrate) Often hijacked by pathogens mimic aaneeded substance by the cell Exocytosis Fusing of vesicles to the plama membrane, thus releasing its contents Function of Membrane Proteins TRANSPORT SIGNAL TRANSDUCTION INTERCELLULAR JOINING •protein channels or carriers •substrates bind to protein for passive transport •Ex. Gap Junctions, Tight Junctions, surface sendsDesmosome a signal • protein pumps for active within the cell to start atransport chemical chain reaction or •Clathrin-lined membrane for cell response CELL to CELL vesicular transport RECOGNITION ENZYMATIC CYTOSKELETON and ECM •Sugar on glycoproteins or •Catalysis of Chemical Reactions ATTACHMENT glycolipids at the Membrane Surfaceact as “name •Maintenance of Cell Shapetags” for cells. Recognition of invaders, helps with cell communication and coordination End of Slide Show Signal Transduction 3 Stages of Signal Transduction 1) Reception: A ligand or substrate binds to receptor protein. Receptor proteins can be on the cell surface, but not always. Receptor protein changes shape 2) Transduction: Amplifies and sends the signal through chemical relay 3) Cell Response: Specific response is triggered Examples of Signal Transduction Back to Function of Membrane Proteins Why and is this hormone-receptor Steroids Hormones are types of protein found the surface lipids, whichnot can passon through of the plasma membrane? phospholipid membranes easily. Back to Function of Membrane Proteins Cell Junctions Tight Junctions: Gap Junctions: Desmosomes: -“anchoring” junctions -Intermediate filaments extend from disc-shaped plaque to reduce tearing due mechanical stress to prevent separation -Integral proteins communicating of-Disc-shaped neighboring cells fusew/ junction plaque Transports ions, linkersugars, protein simple -prevents Hollow, fibers small molecules leakage btwn transmembrane “zipping” cells into Abundant in ionprotein cylinders, tissues extracellular dependent together to connexons, that space (ex. excitable cells prevent provide Digestive tract) (ex. neurons) separation cytoplasmic channels btwn cells