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Movement across the Cell Membrane AP Biology 2007-2008 Diffusion 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 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 Managing water balance Cell survival depends on balancing water uptake & loss AP Biology freshwater balanced saltwater 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 1 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 ATP ATP plant cells turgid = full cell wall protects from bursting AP Biology freshwater Pumping water out Contractile vacuole in Paramecium ATP AP Biology 2 Managing water balance Hypertonic a cell in salt water low concentration of water around cell problem: cell loses water & can die example: shellfish solution: take up water or pump out salt plant cells plasmolysis = wilt can recover AP Biology saltwater 3 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 Water moves from: High Water Potential → Low Water Potential Low Solute Conc. → High Solute Conc. Hypotonic Soln. → Hypertonic Soln. Low Osmotic Poten. → High Osmotic Poten. Osmotic Potential: tendency to H2O to move across a membrane into a solution. AP Biology 1991 | 2003 Aquaporins Water moves rapidly into & out of cells evidence that there were water channels protein channels allowing flow of water across cell membrane AP Biology Peter Agre Roderick MacKinnon John Hopkins Rockefeller Do you understand 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 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 conformational shape change transports solute from one side of membrane to other protein “pump” “costs” energy = ATP LOW conformational change ATP HIGH AP Biology Active transport Many models & mechanisms ATP AP Biology ATP antiport symport Active transport Movement of molecules against the concentration gradient. Requires Energy! AP Biology AP Biology 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 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 Osmoregulation Regulates solute concentrations and balances the gain and loss of water. AP Biology Marine fish are AP Biology hypotonic to sea water They lose water by osmosis and gain salt by diffusion from food They balance water loss by drinking seawater and excreting salts Freshwater animals AP Biology constantly take in water by osmosis from their hyoptonic environment They lose salts by diffision and maintain water balance by excreting large amounts of dilute urine. Salts lost by diffusion are replaced in foods and by uptake across the gills. The kidneys are the principle site of water balance and salt regulation. AP Biology •The Nephron consists of a single long tubule and ball of capillaries called the glomerulus. •Bowman’s capsule surrounds and receives filtrate from the glomerulus. AP Biology The filtrate will flow through three regions of the nephron. 1. 2. 3. AP Biology Proximal Tubule Loop of Henle Distal Tubule Filtration •Blood pressure forces fluid from the blood in the glomerulus into the Bowman's capsule •This is a nonselective process. •The filtrate contains salts, glucose, amino acids, vitamins, nitrogenous wastes and ofher small molecules. AP Biology Reabsorption •Ions, water and nutrients are reabsorbed. •These are actively and passively transported from the filtrate into the interstitial fluid and then capillaries. •Toxic materials are secreted. AP Biology Descending Loop •Reabsorption of water continues through channels formed by aquporin proteins. •Movement is driven by the hypertonic interstitial fluid. •The filtrate becomes increasingly concentrated. AP Biology Ascending Loop •Salt but not water is able to diffuse from the tubule into the interstitial fluid. •The filtrate becomes increasingly dilute. AP Biology Distal Tubule •Regulates the K+ and NaCl concentrations of body fluids. AP Biology Collecting Duct •Water is lost as well as some salt and urea, and the filtrate becomes more concentrated •Urine is hypertonic to the body fluids. AP Biology AP Biology AP Biology AP Biology 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
