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Transcript
The Cell Membrane AP Biology 2007-2008 Overview The cell membrane separates a living cell from it’s nonliving surroundings. It’s a thin barrier ~8nm thick. (A nanometer = 1 billionth of a meter) It’s function is to control traffic in & out of the cell. We call this selective permeability. The cell membrane allows some substances to cross in or out of the cell more easily than others. It is made of a phospholipid bilayer, proteins & other macromolecules. Phospholipids Phosphate 2 Fatty acid tails hydrophobic Phosphate head hydrophilic Arranged as a bilayer Fatty acid AP Biology Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads AP Biology A membrane is a collage of proteins & other molecules embedded in the matrix of the lipid bilayer Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Transmembrane proteins Peripheral protein Cytoplasm AP Biology Filaments of cytoskeleton Membrane Proteins Proteins determine membrane’s specific functions cell membrane & organelle membranes each have unique collections of proteins Membrane proteins: peripheral proteins loosely bound to surface of membrane cell surface identity marker (antigens) integral proteins penetrate lipid bilayer, usually across whole membrane transmembrane protein transport proteins channels, permeases (pumps) AP Biology Why do we refer to the cell membrane as a fluid mosaic? AP Biology 2007-2008 Many Functions of Membrane Proteins Outside Plasma membrane Inside AP Biology Transporter Enzyme activity Cell surface receptor Cell surface identity marker Cell adhesion Attachment to the cytoskeleton Membrane carbohydrates Play a key role in cell-cell recognition ability of a cell to distinguish one cell from another antigens important in organ & tissue development basis for rejection of foreign cells by immune system AP Biology Movement across the Cell Membrane AP Biology 2007-2008 Diffusion 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Diffusion AP Biology movement from high low concentration Diffusion Move from HIGH to LOW concentration “passive transport” no energy needed AP Biology diffusion movement of water osmosis Diffusion across cell membrane Cell membrane is the boundary between inside & outside… separates cell from its environment Can it be an impenetrable boundary? NO! OUT IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O AP Biology OUT IN waste ammonia salts CO2 H2O products cell needs materials in & products or waste out 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 “The Bouncer” Channels through cell membrane Membrane becomes semi-permeable with protein channels specific channels allow specific material across cell membrane inside cell NH AP Biology 3 salt H 2O aa sugar outside cell Active Transport Cells may need to move molecules against concentration gradient shape change transports solute from one side of membrane to other protein “pump” conformational change “costs” energy = ATP low ATP high AP Biology “The Doorman” Active transport Many models & mechanisms ATP AP Biology ATP antiport symport 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 protein channel high low concentration gradient Active transport diffusion against concentration gradient low high AP Biology uses a protein pump 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” AP Biology exocytosis exocytosis 2 Types of Endocytosis Pinocytosis AP Biology Pockets form along the cell membrane, fill with liquid, and pinch off to form vacuoles within the cell Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by molecular signal AP Biology The Special Case of Water Movement of water across the cell membrane AP Biology 2007-2008 Osmosis is 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 Isotonic animal cell immersed in mild salt solution example: blood cells in blood plasma problem: none no net movement of water flows across membrane equally, in both directions volume of cell is stable AP Biology balanced Managing water balance Hypotonic a cell in fresh water example: Paramecium problem: gains water, swells & can burst water continually enters Paramecium cell solution: contractile vacuole pumps water out of cell ATP ATP plant cells turgid AP Biology freshwater Water regulation Contractile vacuole in Paramecium ATP AP Biology Managing water balance Hypertonic a cell in salt water example: shellfish problem: lose water & die solution: take up water or pump out salt plant cells plasmolysis = wilt AP Biology saltwater 1991 | 2003 Aquaporins Water moves rapidly into & out of cells AP Biology evidence that there were water channels Peter Agre Roderick MacKinnon John Hopkins Rockefeller Osmosis… .05 M .03 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 Any Questions?? AP Biology Key Vocabulary Solute Substance that is dissolved in a solvent to make a solution Solvent Substance in which a solute is dissolved to form a solution Solution AP Biology Mixture of two or more substances in which the molecules of the substances are evenly distributed Concentration The mass of a solute in a given volume of solution AP Biology Ex. If you dissolved 12 grams of salt in 3 liters of water, the concentration would be 12g/3L or 4g/L. If you dissolved 12 grams of salt in 6 liters of water, the concentration would be 12g/6L or 2g/L. The first solution is twice as concentrated at the second solution. Hypertonic Describes the side of the membrane with the higher concentration AP Biology Hypotonic Describes the side of the membrane with the lower concentration AP Biology Isotonic When both sides of the membrane are in equal concentrations AP Biology Comparison: Hypertonic/Hypotonic AP Biology Equilibrium When the concentration of a solute is equal throughout the solution Once equilibrium has been reached, particles continue to move across the membrane in both directions AP Biology Diffusion The process where molecules in a solution randomly move from an area of higher concentration to an area of lower concentration AP Biology Comparison: Diffusion, Facilitated Diffusion, Active Transport AP Biology