The Cell Membrane
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Go to the “Free Response Help” link on my website to check out the Unit 2 FRQs!!! AP Biology Pre-Lab Preparation!!! Go to “Labs & Lab Notebook” link on my website. Click on “LabBench” link. Click on “Lab 1: Diffusion & Osmosis.” Read through Concepts #1-5, Design Exercises #1-2, and Self-Quiz ?s #1-3 of the Pre-Lab. Answer the general questions COMPLETELY that are listed on your Lab Notebook Guidelines sheet. Remember to ignore the “Water Potential” section and calculations AP Biology Due on Friday!!! The Cell Membrane AP Biology Arranged as a Phospholipid bilayer Serves as a cellular barrier / border polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads AP Biology impermeable to polar molecules Cell membrane defines cell Cell membrane separates living cell from aqueous environment But…separation CANNOT be complete, or the cell would die! Controls traffic in & out of the cell allows some substances to cross more easily than others hydrophobic (nonpolar) particles can cross more easily than vs. hydrophilic (polar) particles AP Biology Permeability to polar molecules? Membrane becomes “semi-permeable” via protein channels specific channels allow specific material across cell membrane inside cell NH AP Biology 3 salt H 2O aa sugar outside cell Cell membrane is more than lipids… Transmembrane proteins embedded in phospholipid bilayer create semi-permeable channels lipid bilayer membrane AP Biology protein channels in lipid bilyer membrane Classes of amino acids nonpolar & hydrophobic AP Biology Classes of amino acids AP Biology polar & hydrophilic Membrane Proteins Within membrane Polar areas of protein nonpolar amino acids hydrophobic anchors protein into membrane On outer surfaces of membrane in fluid polar amino acids hydrophilic extend into AP Biology extracellular fluid & into cytosol Nonpolar areas of protein + H H+ Examples Retinal chromophore NH2 aquaporin = water channel Porin monomer H 2O b-pleated sheets Bacterial outer membrane Nonpolar (hydrophobic) a-helices in the cell membrane COOH H++ H proton pump channel H O AP Biology 2 Cytoplasm Many Functions of Membrane Proteins “Channel” Outside Plasma membrane Inside Transporter Enzyme activity Cell surface receptor Cell adhesion Attachment to the cytoskeleton “Antigen” AP Biology Cell surface identity marker Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Peripheral protein AP Biology Transmembrane proteins Cytoplasm Fluid Mosaic Model Filaments of cytoskeleton Parts of the “mosaic”… Fat composition affects flexibility membrane must be fluid & flexible % unsaturated fatty acids in phospholipids keep membrane more fluid Cold weather plants = increased unsaturated fatty acids (“winter wheat”) cholesterol in membrane prevents membrane from “freezing” AP Biology Membrane carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish one cell from another antigens AP Biology basis for rejection of foreign cells by immune system Movement across the Cell Membrane AP Biology Diffusion 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Diffusion AP Biology movement from HIGH LOW concentration Simple Diffusion Move from HIGH to LOW concentration “passive transport” no energy needed AP Biology diffusion movement of water osmosis (a) Diffusion of one solute. The membrane Molecules of dye Membrane (cross section) has pores large enough for molecules of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side WATER with more molecules. The dye diffuses from where it is more concentrated to where it is less concentrated (called diffusing down a concentration gradient). This leads to a dynamic Net diffusion Net diffusion equilibrium: The solute molecules continue to cross the membrane, but at equal rates in both directions. Equilibrium (b) Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both. Each dye diffuses down its own concentration gradient. There will be a net diffusion of the purple dye toward the left, even though the total solute concentration was initially greater on the left side. Net Osmosis diffusion Net diffusion AP Biology Net diffusion Net diffusion Equilibrium Equilibrium Facilitated Diffusion Diffusion through protein channels channels move specific molecules across cell membrane facilitated = with help no energy needed open channel = fast transport HIGH LOW AP Biology Active Transport Cells may need to move molecules against concentration gradient AP Biology conformational shape change transports solute from one side of membrane to other protein “pump” conformational change “costs” energy = ATP Active transport ATP AP Biology Getting through cell membrane Passive Transport Simple diffusion diffusion of nonpolar, hydrophobic molecules lipids HIGH LOW concentration gradient Facilitated transport diffusion of polar, hydrophilic molecules through a (hydrophilic) protein channel HIGH LOW concentration gradient Active transport diffusion against concentration gradient LOW HIGH uses a transport protein A.K.A. “protein pump” AP Biology requires ATP ATP Transport summary simple diffusion facilitated diffusion active transport AP Biology ATP How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” Exocytosis Example: Pancreas cell secreting insulin into bloodstream AP Biology exocytosis Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific (takes in all molecules in extra-cellular fluid) receptor-mediated endocytosis AP Biology triggered by molecular signal The Special Case of Water Movement of water across the cell membrane AP Biology 2007-2008 Osmosis is just diffusion of water Water is very important to life, so we talk about water separately Diffusion of water from HIGH concentration of water to LOW concentration of water AP Biology across a semi-permeable membrane Concentration of water Direction of osmosis is determined by comparing total solute concentrations Hypertonic - more solute, less water Hypotonic - less solute, more water Isotonic - equal solute, equal water water AP Biology hypotonic hypertonic net movement of water Managing water balance Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater Managing water balance Hypotonic a cell in fresh water high concentration of water around cell problem: cell gains water, swells & can burst example: Paramecium ex: water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell “osmoregulation” plant cells turgid = full cell wall protects from bursting AP Biology freshwater Pumping water out Contractile vacuole in Paramecium ATP AP Biology Managing water balance Hypertonic a cell in salt water low concentration of water around cell problem: cell loses water & can die example: salinity increases in lake plant cells plasmolysis = wilt can recover if conditions change AP Biology saltwater Managing water balance Isotonic animal cell immersed in mild salt solution no difference in concentration of water between cell & environment problem: none no net movement of water flows across membrane equally, in both directions cell in equilibrium volume of cell is stable example: blood cells in blood plasma slightly salty IV solution in hospital AP Biology balanced Aquaporins AP Biology protein channels allowing rapid flow of water across cell membrane explains efficient nature of osmosis Do you understand Osmosis… .05 M .02 M Cell (compared to beaker) hypertonic or hypotonic Beaker (compared to cell) hypertonic or hypotonic Which way does the water flow? in or out of cell AP Biology
