7/24/2016 Plasma (Cell) Membrane Cell membrane separates living cell from nonliving surroundings • thin barrier = 8nm thick • controls traffic in & out of the cell • selectively permeable Chapter 7 • allows some substances to cross more easily than others Membrane Structure and Function • hydrophobic vs hydrophilic • made of phospholipids, proteins & other macromolecules Sandwich Model Fluid Mosaic Model Davson/Danielli (1935) S.J. Singer & G. Nicolson (1972) Membrane proteins are inserted into the phospholipid bilayer Phospholipid bilayer between 2 protein layers Fluid: membrane held together by weak hydrophobic interactions Mosaic : phospholipids, proteins, carbs Problems : Varying chemical composition of membrane due to function (Proteins are amphipathic called FLUID MOSAIC because: hydrophilic & hydrophobic) lipids and protein are liquid in nature and can move around each other membrane is DYNAMIC (always changing) Phospholipid Bilayer • Amphipathic : hydrophilic head, hydrophobic tail • Selectively permeable • Small molecules cross easily (hydrocarbons, hydrophobic mols, CO2, O2) • Hydrophobic core prevents passage of ions, large polar molecules animation – fluid mosaic 1
7/24/2016 Polar vs Non-Polar Amino Acids Phospholipid Bilayer Structure Nonpolar and hydrophobic AA Polar and hydrophilic AA • • Interior of membrane Outer surfaces Freeze Fracture • Anchors protein into membrane • Extend into extracellular fluid and into cytosol polar area nonpolar areas Membrane Fluidity Synthesis and Sidedness of Membranes Low temps : phospholipids 2 sides of membrane differ in specific lipid & protein composition w/unsaturated tails (kinks Membrane is built by ER and Golgi apparatus prevent close packing) Cholesterol resists changes by: limit fluidity at high temps hinder close packing at low temps Adaptations: winter wheat unsaturated phospholipids • fat composition affects flexibility (like thick salad oil) Membrane Proteins Transmembrane Protein Structure Integral Proteins Peripheral Proteins Embedded in membrane Extracellular or cytoplasmic sides of membrane Determined by freeze fracture NOT embedded Transmembrane with hydrophilic heads/tails and hydrophobic Held in place by the cytoskeleton or ECM middles (integrins) HYDROPHOBIC HYDROPHILIC Provides stronger framework INTERIOR ENDS Act as identity markers (antigens) (alpha helix) 2
7/24/2016 1. Transport Functions of Membrane Proteins transport proteins : - involved in facilitated diffusion - go through the entire membrane outside - hydrophilic channel, hydrophobic outside - aquaporins : channel proteins increase water movement Plasma membrane 2 types inside channel : gated pores move water and ions freely in and out of cell carrier : substrate binds to site on protein 2. Cell-Cell Recognition 3. Signal Transduction recognition proteins : receptor proteins : binds to specific molecules (like tips of icebergs emerging from ocean surface) - specific shape binding site fits chemical messenger shape (neurotransmitters, blood antigens, hormones) - Contain carbohydrate antennas (glycoproteins) - initiates cell response - Used as chemical ID markers to differential cell types ***cell/cell recognition (immune response) ***embryo cells tissues organ systems *** A,B,O blood groups- carb antennae are different *** important in drug treatments 4. Enzymatic Activity 5. Intercellular Joining Membrane proteins of adjacent cells hook together in various enzymes : embedded in cell membrane to catalyze biochemical kinds of junctions reactions in cell - gap junctions: connects cytoplasm of cells (cell- cell communicaiton) - tight junctions: fusion of adjacent cell membranes (seal intercellular space, prevent free passage of substances) 3
7/24/2016 Cell Transport- Passive Transport 6. Attachment to Cytoskeleton and ECM 1. Diffusion Microfilaments of cytoskeleton- non-covalently bound to membrane - movement of molecules of a solute from areas of proteins high to low concentration ( down a concentration - maintains cell shape gradient ) until equilibrium is reached - stabilizes location of certain membrane proteins ex: hycrocarbons, CO2, Os, H2O - can coordinate extracellular and intracellular changes Cell Transport-Passive Transport Effects of Osmotic Solutions 2. Osmosis - movement of water from areas of high to low concentration until equilibrium is reached - direction of movement depends on CONCENTRATION OF WATER on either side of the concentration gradient ** differences in FREE WATER concentration is important Osmoregulation: organisms lack rigid cell walls must control water and solute balance ** tonicity : ability of a surrounding ex: contractile vacuole solution to cause a cell to gain water Effects of solutes and pressure on water Water Potential potential ( ψ ) and water movement Water potential ( ψ ) - free energy of water (potential energy) - predicts direction of water flow (solute concentration + physical pressure) H 2 O moves from high ψ low ψ without any barrier to flow - - Megapascal ( Mpa) or Bars of pressure : unit of measurement Ψ of pure water = 0 bars animation 4
7/24/2016 Pressure potential ( ψ P ) - Physical pressure on a solution from pressure Solute Potential ( ψ S ) inside or outside the cell - osmotic pressure (outward pressure of water in cell/ inward pressure of water - pressure that must be applied to a solution to prevent the inward outside cell) flow of water through a semi-permeable membrane - Can be positive or negative to atmospheric pressure - Solute always pulls water towards it atmospheric pressure = 0 bars ψ P - Inverse relationship with ψ (increase in solutes decreases free ψ P of open beaker of water = 0 bars water, therefore reduces water potential) - always expressed as 0 or a negative number ψ S pure water = 0 bars animation Calculating Solute Potential ( ψ S ) Water Potential Equation ψ S = -iCRT Water potential equation: ψ = ψ S + ψ P i = ionization constant (how much particles ionize) always 1-2 Water potential ( ψ ) = free energy of water C = molar concentration Solute potential ( ψ S ) = solute concentration (osmotic potential) R = pressure constant (0.0831 liter bars/mole K) Pressure potential ( ψ P ) = physical pressure on solution; turgor T = temperature in K (273 + 0 C) pressure (plants) bars = unit of measurement Pure water: ψ P = 0 Mpa or bars The addition of solute to water lowers the solute potential ( more Plant cells: ψ P = 1 Mpa or bars negative ) and therefore decreases the water potential animation Example Which way will water move? 1. Which chamber has a lower water potential? 2. Which chamber has a lower solute potential? From an area of: 3. In which direction will osmosis occur? higher ψ lower ψ (more negative ψ ) 4. If one chamber has a Ψ of -2000 kPa, and the other -1000 kPa, which is the chamber that has the higher Ψ? low solute concentration high solute concentration high pressure low pressure 5
7/24/2016 How Does Water Move Up Plants? Sample Problem 1. Calculate the solute potential of a 0.1M NaCl solution at 25°C. 2. If the concentration of NaCl inside the plant cell is 0.15M, which way will the water diffuse if the cell is placed in the 0.1M NaCl solution? Cell Transport- Passive Transport Facilitated diffusion- gated ion channels ion channel : transport protein that span - influenced by ions charge – the more 3. Facilitated Diffusion- carrier protein negative the charge, more likely to move the thickness of the membrane with a out and vice versa polar pore through which ions can pass - type of passive transport - movement of a substance from areas of high to low - some gates always open - allows ion to move thru membrane without touching non polar lipid interior concentration (down gradient) - some gates stimulated by : - with the aid of a carrier protein (driven by diffusion) - form of passive transport: ions move electrical charge down concentration gradient ex: glucose or amino acids into RBC stretching of membrane binding of specific molecules Active Transport Facilitated diffusion- channel protein Aquaporin : channel protein that allows passage of H 2 O - movement of substances through a membrane against (up) a concentration gradient (low high concentration) - requires energy (from ATP) ex: membrane pumps Na+/K+ pump proton (H+) pumps ex: bulk transport endo/exocytosis 6
7/24/2016 Bulk Transport Electrogenic Pumps: generate voltage across membranes Na + /K + Pump Endocytosis Proton Pump Exocytosis process where cells engulf Push protons (H + ) across membrane process of removing large substances out - Pump Na + out, K + into cell substances too large to enter by of cell - mitochondria (ATP production) passing through membrane - Nerve transmission Ex: cells manufacture proteins vesicles fuse with cell Types membrane and dump contents - phagocytosis : cells engulf solid out of cell particles too large to pass thru membrane - removal of cell debris, - pinocytosis : cells engulf liquid bacteria/viruses, old organelle substances - receptor mediated: animation ligands bind to animation specific receptors on cell surface animation Passive vs Active Transport Passive Active No ATP Requires ATP High low concentration Low high concentratin Down concentration gradient Against concentration gradient 7
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