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Free energy and water potential.

Extracts from this document...

Introduction

Introduction Free energy and water potential Free energy is defined as the maximum energy available (excluding temperature change) to do work. The free energy per mole is the chemical potential (????The water potential is the chemical potential of a water solution in a system minus the chemical potential of pure water at atmospheric pressure and at the same temperature. Water potential is a measure of the tendency of water to move from high free energy to lower free energy. The water potential of a system is also the ability to do work compared with the ability of the same quantity of pure water at atmospheric pressure and at the same temperature. Despite it may seem more logical to express water potential in terms of energy, pressure units (e.g. KPa and MPa) which are considered to be simpler to measure. Due to convention the water potential of water is set equal to zero. Therefore the water potential of an aqueous solution will be a negative number. The components of water potential are pressure potential (?p) and osmotic potential???s). At constant temperatures the water potential results from the combined opposing actions of pressure and osmotic potential: ?????p????s It is these two factors which work against each other to determine the direction of net water movement into or out of cells. Plasmolysis When a plant tissue is placed in a hypertonic solution, having lower water potential than that of the cells, the net flow of water is from the solution to the cells, down a water potential gradient, through partially permeable membranes. Consequently the protoplasts of the plant cells pull way from the cell wall. This process is called plasmolysis. The turgor pressure of the cells decreases, causing the cells to become flaccid. The damage is irreversible when a cell becomes plasmolysed. Incipient plasmolysis occurs in tissue in which about half of the cells are just beginning to plasmolyse, representing an internal pressure of 0. ...read more.

Middle

The flow of water also results in the protoplasts of the cells shrinking until no pressure is exerted at all on the cell wall. At this point as the pressure potential is equal to zero, the water potential is equal to the solute potential. The decrease in turgor pressure results in the flaccidity of the cells. Consequently the tuber cylinders lose their rigidity. The process of plasmolysis occurs as the protoplasts shrink to an extent that they begin to pull away from the cell wall. The cells therefore become plasmolysed. With the progression of osmosis the sap solution inside the protoplasts becomes more concentrated, developing a more negative osmotic potential. Hence the water potential of the cells decreases. Meanwhile the concentration of the sucrose solution decreases, increasing the osmotic potential and therefore also the water potential. These notable changes in water potential occur until the cells are isotonic with the sucrose solution. At this point there will be no net flow of water by osmosis as there would be no apparent water potential gradient. At a concentration between 0M and 1M there would be expected to only slight or no net flow of water by osmosis between the tuber cells and the sucrose solution down a water potential gradient. If the sap in the tuber tissue is in osmotic equilibrium (i.e. isotonic) with the outside surrounding sucrose solution and no pressure or tension existed within the tissue, then the osmotic potential of the sap would be equal to the osmotic potential of the surrounding solution. The problem with such a measurement is to obtain zero pressure within the tissue without changing other osmotic properties any more than necessary. This is the method of measuring osmotic potential by incipient plasmolysis. Incipient plasmolysis is the stage when half of the cells of a tissue are just beginning to plasmolyse (i.e. when protoplasts are just beginning to pull away from the cell wall). ...read more.

Conclusion

10 8 6 4 2 0 Volume of distilled water (cm3) 0 2 4 6 8 10 6) The cylinders will then be placed into their test tubes and left overnight (for approximately 24 hours). 7) The cylinders will be removed from their test tubes. Each cylinder will then be dried on paper towel by being lightly rolled 5 times for 5 seconds. Their final masses will be determined by weighing to the nearest 0.01g. These masses will then be recorded in the table. 8) The % change in mass for each cylinder will be calculated by using the following equation: [Final mass - (Original mass / original mass)] x 100 Diagram Results Potato 1st experiment Concentration of sucrose solution (moles/litre) Original mass of cylinder (g) Final mass of cylinder (g) Percentage change in mass (%) 1 1.45 0.93 -35.9 0.8 1.45 0.93 -35.9 0.6 1.37 0.96 -29.9 0.4 1.46 1.24 -15.1 0.2 1.36 1.46 +7.4 0.0 1.44 1.71 +18.8 2nd experiment Concentration of sucrose solution (moles/litre) Original mass of cylinder (g) Final mass of cylinder (g) Percentage change in mass (%) 1 1.35 0.89 -34.1 0.8 1.38 0.92 -33.3 0.6 1.45 1.09 -24.8 0.4 1.35 1.18 -12.6 0.2 1.32 1.42 +7.6 0.0 1.54 1.84 +19.5 Averages Concentration of sucrose solution (moles/litre) Percentage change in mass (%) 1 -35.0 0.8 -34.6 0.6 -27.4 0.4 -13.9 0.2 +7.36 0.0 +19.1 Swede 1st experiment Concentration of sucrose solution (moles/litre) Original mass of cylinder (g) Final mass of cylinder (g) Percentage change in mass (%) 1 1.17 1.06 -9.4 0.8 1.15 1.11 -3.5 0.6 1.24 1.25 +0.8 0.4 1.17 1.25 +6.6 0.2 1.24 1.40 +12.9 0.0 1.16 1.45 +25.4 2nd experiment Concentration of sucrose solution (moles/litre) Original mass of cylinder (g) Final mass of cylinder (g) Percentage change in mass (%) 1 1.11 1.02 -8.4 0.8 1.12 1.07 -4.5 0.6 1.15 1.07 -7.0 0.4 1.13 1.22 +8.1 0.2 1.08 1.23 +13.9 0.0 1.14 1.46 +28.1 Averages Concentration of sucrose solution (moles/litre) Percentage change in mass (%) 1 -8.9 0.8 -4.0 0.6 -3.1 0.4 +7.4 0.2 13.4 0.0 26. ...read more.

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