Investigation to find the Water Potential of Apple Tissue

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Investigation to find the Water Potential of Apple Tissue

Introduction

The water molecules move in and out of the cell through the partially permeable cell membrane by the process of Osmosis. The definition of osmosis as given in "Understanding Biology For A-level, Fourth Edition" by "Susan and Glenn Toole" is as follows, "the passage of water from a region where it is highly concentrated to a region where its concentration is lower, through a partially permeable membrane."

Diagram of Osmosis

This investigation is to establish the exact water potential of apple tissue. The definition of water potential, as written in "Biology 1" endorsed by OCR

"Water potential is the tendency of a solution to lose water; water moves from a solution with high water potential to one with low water potential. Water potential is decreased by the addition of a solute, and increased by the application of pressure. Symbol is ?"

This is called moving down a water potential gradient. When the water potential in both regions is equal both areas are in equilibrium and there is not further net movement of molecules. The water potential of a cell is determined by two factors: the solute potential in the cell (?s), and the pressure potential (?p).

* The solute potential (?s) is a measure of the reduction in water potential due to the presence of solute molecules. It is the negative component of water potential, sometimes referred to as the osmotic potential or osmotic pressure.

* The pressure potential (?p) is the hydrostatic pressure to which water is subjected. The pressure potential is usually positive. It is sometimes called turgor or wall pressure. Therefore:

?cell = ?s + ?p

When the environment water potential and the cell water potential are the same the two systems are said to be isotonic. If the external water potential is more negative than the cell water potential the solution is hypertonic. This means the water leaves the cell and becomes flaccid (?p=0) and the cell membrane pulls away from the cell wall. This is called plasmolysis. If full plasmolysis occurs the cell can never recover, even if more water is taken on. If the external water potential is less negative than the water potential of the cell or the cell is placed in a hypotonic solution, then water begins to move into the cell, causing it to swell and increasing the ?p. ?p continues to rise until it offsets ?s and the water potential equals zero. This also stops water from entering the cell, which is now said to be turgid.
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If I can determine the point at which the pressure potential equals zero then I can find the water potential be knowing the solute potential. During previous questions done in class, I know that by drawing a concentration against percentage change in mass and length graph and find the point at which the curve crosses the x-axis I can determine the solute potential of the tissue being tested. The graph usually looks like this:

The concentration at which the curve crosses the axis is the concentration of the solute in the tissue being tested. By using a ...

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