Investigate the water potential of potato.

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Isabel Murtough        Biology coursework        MR

Aim: to investigate the water potential of potato.

Research:

Osmosis in plant cells

Osmosis is the net movement of water molecules from a region of their higher concentration to a region of their lower concentration through a partially permeable membrane.

If the concentration of water molecules is the same in the plant cell and the surrounding liquid, there will be no net movement of water into or out of the cell; and if the concentration of water molecules is greater in the cell than the surrounding liquid, there will be a net movement of water from the plant cell to the surrounding solution.  Osmosis will occur whenever a partially permeable membrane separates two solutions containing different concentrations of water molecules.  If the cell is surrounded by pure water, or by a solution whose solute concentration is lower, and water potential higher, than that of the cell’s contents, water flows into the cell by osmosis and swells up.  The external solution is said to be hypotonic to the solution in the cell.  This means the solute concentration is lower than that of the cell.  If the cell is surrounded by a solution whose solute concentration is higher and water potential lower than that of the cell, then water will flow out of the cell causing the cell to shrink.  When the external solution is of higher concentration it is said to be hypertonic to the solution in the cell.  If the cell has the same solute concentration as the surrounding solution there will be no net flow of water in or out of the cell, in this instance the external solution is said to be isotonic.  

Mary Jones and Geoff Jones Biology

The effect of osmosis on plant cells

Plant cells generally have a lower water potential than that of their surroundings, this is because of their high concentration of solutes in their vacuole.  The plasma membrane and the tonoplast are both partially permeable, letting water through but not solutes.  The cell wall however is fully permeable to both water and solutes.  When a plant cell is put into a pure water solution whose water potential is higher than that of the cell, the net movement of water is into the cell causing it to swell.  However, due to the cell wall it does not burst causing hydrostatic pressure to develop.  This is called pressure potential which opposes the continued uptake of water into the cell through osmosis.  The pressure potential reaches a maximum and the cell becomes fully turgid.  Biology third edition, Campell. If the cell is out into a solution with a lower water potential, the net movement will be from the cell to the surrounding solution.  This would cause the cell to collapse, when the cytoplasm withdrawals from the cell wall plasmolysis occurs.

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Results of an experiment to determine the water potential of potato tuber cells.  Samples of potato tissue were placed in a series of sucrose solutions of different molarities, and the change in mass was measured.  As you can see from the graph, the concentration of sucrose causing no change in mass has a molarity of 0.27M.

Water potential

The net direction of water movement in a cell depends on the water potential of the cell and whether it is more negative or positive than the water potential of the surrounding solution.  

C.J Clegg

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