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Transcript
The Cell Membrane AP Biology 2007-2008 The Structure of A Cell ALL cells have a cell membrane A thin, flexible barrier around the cell Separates the inside of the cell from the outside AP Biology 2 Structure of a Cell Controls traffic in & out of the cell Acts like a “gate” selectively permeable allows some substances to cross more easily than others hydrophobic vs hydrophilic Made of phospholipids, proteins & other macromolecules AP Biology The Cell Membrane Structure 2 layers of PhosphoLipids = Phosphate + lipid 2 fats (lipids) as the tail that “hate water” – hydrophobic 1 phosphate molecule as the head that “loves water” – hydrophilic Faces the water inside the cell This is referred to as a “Lipid Bilayer” Proteins AP Biology 4 Phosphate Phospholipids Fatty acid tails hydrophobic Phosphate group head hydrophilic Fatty acid Arranged as a bilayer Aaaah, one of those structure–function examples AP Biology Phospholipid bilayer polar hydrophilic heads nonpolar hydrophobic tails polar hydrophilic heads AP Biology Chapter 4 – Membrane Structure & Function Plasma membrane Structure & Function Phopholipids, Glycolipids, Proteins, Glycoproteins, Cholesterol AP Biology Membrane is also proteins, cholesterol. Glycoprotein Extracellular fluid Glycolipid Phospholipids Cholesterol Transmembrane proteins Peripheral protein Cytoplasm AP Biology Filaments of cytoskeleton Membrane 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, pumps AP Biology 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 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 aids lipids salts, O2, H2O AP Biology OUT IN waste ammonia salts CO2 H2O products cell needs materials in & products or waste out Diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids inside cell NH3 What molecules can lipid salt NOT get through directly? polar molecules H 2O outside cell sugar aa H 2O ions salts, ammonia large molecules starches, proteins AP Biology 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 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 Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater 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 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 STOP HERE!!!! AP Biology 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” 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” 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 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 Endocytosis phagocytosis fuse with lysosome for digestion pinocytosis non-specific process receptor-mediated endocytosis triggered by molecular signal AP Biology