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Transcript
Transport of Substances Across a Cell Membrane Diffusion 2nd Law of Thermodynamics governs biological systems universe tends towards disorder (entropy) Diffusion movement from high low concentration Diffusion Move from HIGH to LOW concentration “passive transport” no energy needed 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? OUT IN food carbohydrates sugars, proteins amino acids lipids salts, O2, H2O NO! OUT IN cell needs materials in & products or waste out waste ammonia salts CO2 H2O products Diffusion through phospholipid bilayer What molecules can get through directly? fats & other lipids What molecules can lipid inside cell salt NH3 NOT get through directly? polar molecules H 2O ions salts, ammonia outside cell sugar aa H2O large molecules starches, proteins Small nonpolar molecules such as fats, O2 and CO2 Diffuse easily across the phospholipid bilayer of a membrane (nonpolar molecules) 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 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 hypotonic hypertonic net movement of water Managing water balance Cell survival depends on balancing water uptake & loss 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 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 freshwater Water regulation Contractile vacuole in Paramecium ATP 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 saltwater 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 Transport proteins may facilitate diffusion across membranes Many kinds of molecules do not diffuse freely across membranes (size, charge, polarity) For these molecules, transport proteins Provide passage across membranes through a process called facilitated diffusion Solute molecule Figure 5.15 Transport protein Channels through cell membrane Membrane becomes semi-permeable with protein channels specific channels allow specific material across cell membrane inside cell NH3 H2O salt aa sugar outside cell 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 “The Bouncer” Facilitated Diffusion Ion Channels allow specific ions to pass through the protein channel. regulated by the cell and are either open or closed to control the passage of substances into the cell Carrier Proteins bind to specific molecules, change shape and then deposit the molecules across the membrane. Once the transaction is complete the proteins return to their original position. 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 “The Doorman” Active transport Many models & mechanisms ATP ATP antiport symport Sodium Potassium Pump Cells expend energy for active transport Transport proteins can move solutes against a concentration gradient Through active transport, which requires ATP Transport protein Solute 1 Solute binding Figure 5.18 ATP P ADP 2 Phosphorylation P Protein changes shape Phosphate detaches 3 Transport 4Protein reversion P 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 uses a protein pump requires ATP ATP Transport summary simple diffusion facilitated diffusion active transport ATP How about large molecules? Moving large molecules into & out of cell through vesicles & vacuoles endocytosis phagocytosis = “cellular eating” pinocytosis = “cellular drinking” exocytosis exocytosis Exocytosis and endocytosis transport large molecules To move large molecules or particles through a membrane A vesicle may fuse with the membrane and expel its contents (exocytosis) Fluid outside cell Insulin, Crying Vesicle Protein Figure 5.19A Cytoplasm Membranes may fold inward Enclosing material from the outside (endocytosis) Vesicle forming Figure 5.19B Endocytosis fuse with lysosome for phagocytosis digestion non-specific pinocytosis process triggered by receptor-mediated endocytosis molecular signal