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Osmoregulation in Cells Ch. 7 AP Biology Ms. Haut Water Balance of Cells Without Walls • Behavior of cell in a solution depends on: – Solute concentration – Permeability of membrane http://static.flickr.com/46/178037251_5dbba5a33c_o.jpg Water Balance • Cells without walls cannot tolerate excessive uptake or loss of water – Most terrestrial organisms are bathed in extracellular fluid that is isotonic to the cells – Organisms in hypertonic/hypotonic environments have adaptations Hypotonic solution Isotonic solution Hypertonic solution Animal cell H2O Lysed H2O H2O Normal H2O Shriveled Water Balance • Paramecium—protist that lives in pond water • Environment is hypotonic to cell • Contractile vacuole pumps out excess water to prevent lysis http://www.daviddarling.info/images/contracti le_vacuole.gif Water Balance http://cache.eb.com/eb/image?id=6541&rendTypeId=4 Water Balance of Cells With Walls • Wall aids in maintaining water balance • Wall expands just so much before it exerts a back pressure that opposes further water uptake Hypotonic solution Isotonic solution Hypertonic solution Animal cell H2O Lysed Hypotonic solution Animal cell Plant cell H2O H2O H2O Normal Isotonic solution H2O H2O H2O H2O Shriveled Hypertonic solution H2O H2O H2O H2O Turgid (normal) Flaccid Plasmolyzed Lysed Normal Shriveled Water Balance http://www.uic.edu/classes/bios/bios100/mike/spring2003/plasmolysis.jpg How Solutes and Pressure Affect Water Potential • Both pressure and solute concentration affect water potential • The solute potential of a solution is proportional to the number of dissolved molecules • Pressure potential is the physical pressure on a solution Water Potential () • The free energy of water that is a consequence of solute concentration and applied pressure – Water will always move across the membrane from the solution with the higher water potential to the one with lower water potential. Effect of solutes and pressure on water potential – Pure water has a water potential of zero, and addition of solutes lowers water potential into the negative range. – Increased pressure raises the water potential into the positive range. – A negative pressure, may also move water across a membrane; this bulk flow (movement of water due to pressure differences) is usually faster than movement caused by different solute concentrations. Water Potential () = P + S P = Pressure potential S = Solute potential or osmotic potential Quantitative Analysis of Water Potential • The addition of solutes reduces water potential Addition of solutes 0.1 M solution Pure water H2O = 0 MPa P = 0 S = –0.23 = –0.23 MPa Quantitative Analysis of Water Potential • Physical pressure increases water potential Applying physical pressure Applying physical pressure H2O H2O = 0 MPa P = 0.23 S = –0.23 = 0 MPa = 0 MPa P = 0.30 S = –0.23 = +0.07 MPa Quantitative Analysis of Water Potential • Negative pressure decreases water potential Negative pressure H2O P = –0.30 S = 0 = –0.30 MPa P = 0 S = –0.23 = –0.23 MPa • Water potential affects uptake and loss of water by plant cells • If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and become plasmolyzed Plasmolyzed cell at osmotic equilibrium with its surroundings P = 0 S = –0.9 = –0.9 MPa 0.4 M sucrose solution: P = 0 S = –0.9 = –0.9 MPa Initial flaccid cell: P = 0 S = –0.7 = –0.7 MPa Distilled water: P = 0 S = 0 = 0 MPa Turgid cell at osmotic equilibrium With its surroundings P = 0.7 S = –0.7 = 0 MPa Example • 0.1M solution has a S of –0.23MPa; in the absence of pressure, the water potential is –0.23 MPa. = P + S = 0 + (-0.23 MPa) = -0.23MPa • So, if this solution is separated from pure water ( = 0 MPa) by a selectively permeable membrane, water will diffuse into the solution Water Balance • Turgor loss in plants causes wilting, which can be reversed when the plant is watered