Investigate and find the water potential of baking potatoes and sweet potatoes in (N/mm2) using various strength solutions of sucrose (mol).

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16th March

Biology Coursework

Aim

        To investigate and find the water potential of baking potatoes and sweet potatoes in (N/mm2) using various strength solutions of sucrose (mol)


Hypothesis

        I believe that the baking potatoes will have a less negative water potential. In the experiment they will not increase in mass at a lower of sucrose molarity than the sweet potatoes.

Baking potatoes contain more water than sweet potatoes. This is clearly obvious, cut a baking potato into cubes and it releases a lot of water, more so than sweet potatoes.

        Water potential determines the rate of osmosis. Water movement form a high concentration to a low one is the means for testing the aim. Distilled water and other strength solutions will flow into the sweet potato more readily than into the baking potato. This is because baking potatoes have more water, and thus a less negative water potential. Sweet potatoes have less water content and will therefore have a more negative water potential.

        Both potatoes will have a lower water potential then the distilled water and other sugar solutions. As water diffuses into their cells they will increase in mass. The weight difference will be converted into a percentage so that the water potential can be determined.

When plotted on a graph, percentage change of mass against solution molarity, the line of best fit will cross the x-axis. At this point the potatoes will not have increase in mass, their mechanical pressure will offset the solute potential. At this point there would be no diffusion gradient and osmosis would cease. The potatoes would have the same water potential as their surroundings.

Using tables I can determine what the water potential is in kPa or N/mm2 by means of comparison using the graph obtained by this experiment. (See appendices i)

Obviously the sweet potato and the baking potato would have different water potentials. Their lines of best fit would cross at different points. Baking potatoes would be closer to the origin, as its water potential is less negative and will correspond to a less negative water potential in the sucrose solution.

Background Information

        Osmosis is a special case of diffusion, specifically related to water molecules acting as a solvent with a solute, in this experiment the solute would be sucrose.

        Osmosis is the diffusion of water molecules from a region of high concentration to a region of low concentration through a partially permeable membrane, in our case a lipid bi-layer.

        To explain osmosis, we will take the common experiment of concentrated sucrose solution in dialysis tubing, suspended in distilled water.

        

Different liquids containing water have a measure called water potential (Ψ psi) to show the tendency for water molecules to enter or leave that solution by osmosis.

Pure water has the highest water potential, set at zero kPa. If you were to dissolve a solute into distilled water, you would lower its water potential. As a result solutions at atmospheric pressure have negative values set in kPa. Water diffuses from a more positive water potential to a more negative one, for example 0 to –10kPa, -5 to –70kPa, or 2 to-5kPa.

Water potential is determined by the presence of dissolved solutes (solute potential) and the actual pressure acting on the water (pressure potential) such as when a cell is turgid. Water potential is calculated using the formula:

Water Potential = Solute Potential – Pressure Potential

Ψ        =        Ψs        -        Ψp

        

Solute potential is negative, since the solute dissolved lowers the potential below zero, as already explained.

Pressure potential however is positive. If a pressure is applied to distilled water, its pressure potential increases. Hydrostatic pressure (pressure potential) usually is positive but can be negative such as in xylem where the water is under tension so the pressure potential returns a negative value. In plant cells the force of the water pushing against the cell wall creates a positive pressure potential causing the cell to become turgid.

If we return to the sucrose experiment and let it continue, the bag will eventually become turgid.

If the bag is suspended in fresh distilled water, it will increase in mass as it gains water. Because the bag acts like a cell wall, it cannot stretch further and pressure begins to build up eventually rising out to cancel out the solute potential. Osmosis “ceases” since both inside and outside the bag have the same water potential, but in actual fact water molecules are entering and exiting the bag at the same rate.

Every cell in a plant has a lipid bi-layer surrounding its content. It’s hydrophobic/hydrophilic nature makes it theoretically impermeable to water molecules. So how does water get into a cell to affect its water potential?

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It is generally accepted that the cell membrane is a “fluid mosaic”, that is the lipid molecules are free to orient as they like. Water can sometimes find gaps in-between various molecules embedded in the surface such as glycoproteins.

Also cells have protein lined pores that selectively allow some compounds through. These water channels occur in huge quantities all over the cell membrane’s surface.

When a cell is in water of the same water potential as itself, there is no net water movement, even though water molecules are moving in and out of the cell. The two sets of water ...

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