Investigation to determine the lowest concentration of lead ions that causes cell membranes to lose partial permeability

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Will Pearl

Investigation to determine the lowest concentration of lead ions that causes cell membranes to lose partial permeability

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Plasmolysis is the separation of plant cell cytoplasm from the cell wall as a result of water loss. As moisture leaves the vacuole the total volume of the cytoplasm decreases while the cell itself, being rigid, hardly changes. Plasmolysis is unlikely to occur in the wild except in severe conditions. Plasmolysis is induced in the laboratory by immersing a plant cell in a strongly saline or sugary solution, so that water is lost by osmosis.

If a cell of almost any water plant is soaked in a 15% sugar solution, its cell contents soon separate from the wall and form a mass in the centre of the cell. This occurs because the sugar solution that has passed freely through the cell wall now encounters a selective semi-permeable cell membrane. The large vacuole in the centre of the cell originally contains a dilute solution with much lower osmotic pressure than that of the sugar solution on the other side of the membrane. The vacuole thus loses water and becomes smaller. The space between the cell membrane and the cell wall enlarges. The membrane and the protoplasm within it are eventually crowded into a ball at the centre of the cell. Plasmolysed cells die unless the cell is transferred quickly enough from sugar solution to water. When that is done, it recovers.

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Plant cells have a cell wall as well as a plasma membrane. The cell wall is a strong and rigid structure that is used by the cell to create osmotic pressure within the cell. This pressure can build because of the rigidity of the cell wall. The cells within a plant that contain high water pressure act as the plant supportive structure, helping to give it its shape. With the loss of this osmotic pressure the supportive structure of the plant is lost and the plant becomes limp and shriveled.

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