BIO 3465 - Laboratory 11 - Osmosis _ Water potential

BIO 3465 - Laboratory 11 - Osmosis _ Water potential - BIO...

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BIO 3465 Plant Physiology Laboratory 11 OSMOSIS & WATER POTENTIAL Goal : Biological membranes are selectively permeable. These membranes allow water movement through the membrane due to osmosis. In osmosis the direction and the rate of water movement are determined by the sum of water potential and the pressure potential of the cell. Goal of this lab is to determine the water potential of plant cells using two different simple techniques. INTRODUCTION: Water potential determines in which direction diffusion (or osmosis) will occur. Water potential is a property of a solution which is essentially equal to the relative free energy (chemical potential) of the water in that solution. The second law of thermodynamics indicates that energy tends to flow along an energy gradient from areas of high to areas of low energy. Accordingly, water will tend to diffuse across a differentially permeable membrane from areas of high water potential to areas of low water potential. Because the turgor pressure of plant cells is often only slightly positive and the osmotic potential is often a larger negative number, water usually diffuses from areas of negative osmotic potential to areas of more negative osmotic potential. If the water potentials of two solutions which are separated by a membrane are equal, no net diffusion will result. These principles will be used to determine water potential of plant storage tissue in this laboratory exercise. The osmotic potential of a solution is that component of the water potential which arises as a result of the presence of solute particles dissolved in the solvent (usually water in living systems). As solute is dissolved in water, the ability of water to interact with its surroundings (i.e., its activity) is decreased. There is a decrease of the relative free energy of that water, and thus there develops an increased tendency for adjacent water at higher free energies to diffuse into the first solution. PROCEDURE: 1) Determination of plant cell water potential A) Tissue volume method (constant volume method) Using a cork borer (about 1.5 to 2.0 cm in diameter), cut a couple of cylinders from potato tuber. Then cut each cylinder into equal size pieces about 3 mm in thickness. Blot each piece with a paper towel toremove any liquid, and weigh them carefully to the nearest 0.01 gm.
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Place potato tissues in about 50 75 ml of the following sucrose solutions in glass containers: (a) distilled water; (b) 0.2 M; (c) 0.4 M; (d) 0.6 M; (e) 0.8 M; (f)
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BIO 3465 - Laboratory 11 - Osmosis _ Water potential - BIO...

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