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Transcript
Cell Membranes & Movement Across Them 2006-2007 Cell (plasma) membrane Cells need an inside & an outside… separate cell from its environment cell membrane is the boundary IN OUT food - sugars - proteins - fats salts O2 H2 O waste - ammonia - salts - CO2 - H2O products - proteins cell needs materials in & products or waste out Fluid Mosaic Model The membrane contains many types of molecules Lipids Proteins Carbohydrates A “mosaic” of molecules Molecules are always shifting and moving They are “fluid” Thus… the “fluid mosaic model”! Building a membrane How do you build a barrier that keeps the watery contents of the cell separate from the watery environment? FATS LIPIDS Remember: oil & water don’t mix!! What substance do you know that doesn’t mix with water? Lipids of cell membrane Membrane is made of special kind of lipid phospholipids “split personality” Membrane is a double layer phospholipid bilayer Hydrophilic (“water loving”) phosphate inside cell Fatty acids outside cell Hydrophobic “water fearing” Proteins in the membrane Transport proteins Carrier proteins Transport specific substances Channel proteins “Holes” that molecules can pass through Other proteins Marker/recognition proteins Cell “fingerprints” for immune system Receptor proteins Receive signals from outside (example: hormones) Selectively permeable membrane Cell membrane controls what gets in or out Need to allow some materials — but not all — to pass through the membrane Selectively permeable only some material can get in or out So what needs to get across the membrane? sugar lipids aa O2 H 2O salt waste Crossing the cell membrane What molecules can get through the cell membrane directly? fats and oils can pass directly through inside cell waste outside cell lipid salt sugar aa H 2O but… what about other stuff? How do you build a semi-permeable cell membrane? Channels are made of proteins proteins both “like” water & “like” lipids bi-lipid membrane protein channels in bi-lipid membrane Cell membrane channels Need to make “doors” through membrane protein channels allow substances in & out specific channels allow specific material in & out H2O channel, salt channel, sugar channel, etc. inside cell waste salt H 2O aa sugar outside cell Movement through the channel Why do molecules move through membrane if you give them a channel? HIGH ? LOW ? Molecules move from high to low Diffusion move from HIGH to LOW concentration Diffusion Move from HIGH to LOW concentration passive transport no energy needed diffusion diffusion of water osmosis Simple vs. facilitated diffusion simple diffusion inside cell lipid facilitated diffusion inside cell H 2O protein channel outside cell outside cell H 2O Simple Diffusion Move from HIGH to LOW fat inside cell LOW fat fat fat fat fat Which way will fat move? HIGH outside cell fat fat fat fat fat fat fat fat Facilitated Diffusion Move from HIGH to LOW through a channel sugar sugar sugar sugar inside cell sugar sugar LOW Which way will sugar move? HIGH outside cell sugar sugar sugar sugar sugar sugar sugar More facilitated diffusion Carrier proteins Molecule binds on outside Protein changes shape Molecule passes through Example: glucose Ion channels Let charged particles through Examples: Sodium (Na+), Potassium (K+), Calcium (Ca+2), Chlorine (Cl-) Important in muscle contraction and brain signaling Glucose channel Diffusion summary Move from HIGH to LOW concentration directly through membrane simple diffusion no energy needed help through a protein channel facilitated diffusion (with help) no energy needed Factors affecting HIGH diffusion Size of molecule Type of molecule Type of membrane LOW Osmosis Movement of Water Across Cell Membrane 2006-2007 Osmosis Water is very important, so we talk about water separately Osmosis diffusion of water from HIGH concentration of water to LOW concentration of water across a semi-permeable membrane Keeping water balance Cell survival depends on balancing water uptake & water loss Hypotonic (freshwater) Isotonic (balanced) Hypertonic (salt water) Keeping right amount of water in cell Hypotonic KABOOM! freshwater a cell in fresh water high concentration of water around cell cell gains water problem: cells gain water, swell & can burst Animal cells: cytolysis No problem, here Plant cells: turgor pressure example: Paramecium solution: contractile vacuole pumps water out of cell Controlling water Contractile vacuole in Paramecium Keeping right amount of water in cell Hypertonic I’m shrinking, I’m shrinking! a cell in salt water low concentration of water around cell cell loses water example: shellfish problem: cell loses water in plants: plasmolysis in animals: shrinking cell solution: take up water I will survive! saltwater Keeping right amount of water in cell Isotonic That’s better! no difference in concentration of water between cell & environment cell in equilibrium example: blood problem: none water flows across membrane equally, in both directions volume of cell doesn’t change I could be better… balanced Active transport Cells may need molecules to move against concentration “hill” need to pump “uphill” from LOW to HIGH using energy protein pump requires energy ATP Examples: amino acids, sodiumpotassium pump in neurons ATP Transport summary simple diffusion facilitated diffusion active transport ATP Endocytosis and exocytosis Some molecules too big for proteins Food Waste Cell “eats” the molecules Endocytosis Cell expels the molecules Exocytosis Endocytosis Cell membrane surrounds substance and forms a vesicle Can ingest: Large particles or other cells Phagocytosis Examples: white blood cells, amoebas Liquids Pinocytosis Exocytosis Vesicles join with the cell membrane Release contents outside the cell Can expel: Waste Proteins Other molecules Examples: hormones, neurotransmitters