An Investigation to Determine the Water Potential of the Cells in A Potato Tuber.

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Biology Coursework

An Investigation to Determine the Water Potential of the Cells in A Potato Tuber

Aim –

The aim is to investigate the effect of varying concentration of a certain sugar solution on the amount of osmotic activity between the solution and a potato chip of a given size.

Plant cells always have a strong cell wall surrounding them. When they take up water through the roots by osmosis they start to swell, but the cell wall prevents them from bursting.  The roots of the plant are specially adapted root hair cells.  They have large surface areas and thin cell walls to aid absorption by osmosis.  The evaporation of water through stomata on plant leaves causes a transpiration stream.  Therefore the water is taken up through the xylem vessels.  Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises and eventually the internal pressure of the cell is so high that no more water can enter the cell.  These cells are said to be plasmolysed. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what makes the green parts of the plant "stand up" into the sunlight.   A lack of water may lead to the wilting of the plant of possibly death.  

When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid." This is the exact opposite of "turgid". The contents of the potato cells shrinks and pulls away from the cell wall. These cells are said to be plasmolysed.

When plant cells are placed in a solution, which has exactly the same osmotic strength as the cells, they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means, "about to be".  Water potential is defined as “ the tendency of water to renter or leave a cell” Water moves from a region of high water potential to a region of lower water potential.

All plant cells membranes are partially permeable which means they all some substances to enter and leave the cell.  Whether osmosis occurs depends on the balance between external internal solute and water potentials.  If the solutions on each side of the partially permeable membrane are of equal water or solute potential, then there will be no net movement of water molecules across the membrane.  Therefore this is an equilibrium state and the solutions are referred to as being isotonic.

A solution that has a low water potential and therefore more solute particles than another is known as hypertonic.  The molarity is the concentration of solute particles. A substance that is dissolved in water is called a solute, a solvent is a liquid that is able to dissolve another substance and a solution is the solvent and solute mixed together.    

Hypothesis –

Osmosis is defined as the net movement of water or any other solution molecules from a region in which they are highly concentrated to a region in which they are less concentrated. This movement must take place across a partially permeable membrane such as a cell wall, which lets smaller molecules such as water through but does not allow bigger molecules to pass through. The molecules will continue to diffuse until the area in which the molecules are found reaches a state of equilibrium, meaning that the molecules are randomly distributed throughout an object, with no area having a higher or lower concentration than any other.

This is because the water molecules pass from a high concentration, i.e. in the water itself, to a low concentration, i.e. in the potato chip. Pure water has the highest water potential which is zero. Therefore, the chips in higher water concentrations will have a larger mass than in higher sugar concentrations.  The water potential inside the cells would be exceeded by the water potential of the external solution, resulting in a net flow of water molecules into the cells by the process of osmosis.  I predict that that as the solute potential of the external solution is decreased (the solution becomes more concentrated) less water will move by osmosis into the cells and as a result the increases in mass of the potato cores will be smaller.  Until the isotopic point is reached, where the internal and external water particles are equal, and will be visible as no change in mass of the potato cores.   After this point has been reached, the solute potential in the external solution will be less than that of side the cell.  Therefore there will be no net movement of water molecules out of the cell.  This will result in a visible decrease in the mass of the potato cores from their original size.

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It may be expected that factors such as osmosis (the diffusion of water molecules) and diffusion (the random movement of particles from areas of high concentration to low concentration) may affect the rate in which the particles move.  They may speed up or slow down this rate and in turn, affect the rate of osmosis. I also predict that the molarity of the isotonic solution should be the same.

Variables-

For this experiment to be a fair test, certain aspects of the experiment will have to be kept constant whilst one key variable is changed.  The independent variable for his ...

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