Osmosis is the net diffusion of water molecules from a region of high water potential to a region of lower water potential across a partially permeable membrane. Deionised water has the highest water potential with a value of 0 kPa.
Solutions have a water potential below this and the greater the concentration of solutes the lower the water potential (Toole and Toole, 1995). Plant cells are bounded by a partially permeable plasma membrane, which is surrounded by a fully permeable cellulose cell wall.The cytoplasm and vacuole of a plant cell contains water with many solutes such as glucose, mineral ions and enzymes, all of which contribute to a low water potential. Soil water generally has a higher water potential than plant cells, which allows water to be absorbed down a diffusion gradient.
Plant cells will continue to absorb water in this way until either the water potential is equal on both sides of the membrane or the plasma membrane is pushed right up against the cell wall.This rigidity prevents the cell from bursting. The cell will become increasingly turgid until a point is reached where the pressure exerted by the cell wall on the membrane prevents further uptake of water by the cell. At this point the cell is said to be fully turgid. In whole plants it is the turgidity of their cells that helps hold them upright and spreads the leaves out to the sun so that they can photosynthesise efficiently (Jones and Jones 2000).Animal cells are not protected by a cell wall so if the fluid surrounding an animal cell has a higher water potential than the cytoplasm, the cell will continue to absorb water until either an equilibrium is reached or the plasma membrane bursts.
If a plant cell is placed into a solution of lower water potential, osmosis will occur in the reverse direction; the net movement of water molecules will be into the surrounding solution and the cell will become increasingly flaccid as pressure exerted by the membrane on the cell wall decreases.If this process continues, the membrane will start to pull away from the cell wall as the contents of the cell are steadily depleted. This process is known as plasmolysis. If a plant cell is placed into a solution with a water potential that equals its own, water molecules will still move between the external solution and the contents of the cell, but the system will be in equilibrium as the net movements into and out of the cell will be the same. The cell will therefore neither swell up nor shrink (Adds, Larkcom and Miller, 2001).
Root vegetables store carbohydrate in the form of starch. Starch is a mixture of two polymers of ? glucose; unbranched amylose and branched amylopectin. Both these molecules are coiled for compact storage, have a large molecular mass and are insoluble. This means that they can be stored within plant cells without affecting the water potential (Jones and Jones, 2000).
Freezing can cause severe damage to cells because the volume of water increases when it freezes to ice. This bursts plant cells.To protect themselves from this type of damage many fruits and vegetables will respond to frost by breaking down the insoluble starch in their cells into soluble maltose, this will increase the soluble sugar content of their cells (Stark, Olsen, Kleinkopf and Love). The soluble sugar therefore decreases the freezing point of the cell contents. It should be possible to detect the increase in sugar content of vegetable cells by measuring the reduction in water potential after exposure to low temperatures. Hypothesis The potatoes that are stored in the lower temperature of 2oC will have a lower water potential than the potatoes stored at 24oC.
This temperature is slightly above room temperature so it can easily be controlled in an incubator. Outline method Fourteen Maris Piper potatoes of roughly the same size and age will be used. Seven will be stored at 24oC for one week and seven will be chilled to 2oC for one week. Potato cores will be removed from each potato, dipped into one of the salt solutions, dried in a systematic way, weighed on a top pan balance and placed into test tubes containing 10cm3 of a range of sodium chloride solutions with different water potentials. The potato cores will be left in the solutions for 24 hours.
They will then be removed, dried systematically and reweighed. The mean % change in mass of potato will be calculated for each different salt solution. The mean % change in mass will be plotted on a graph against the water potential of the salt solution. Where the line crosses the x-axis will represent the approximate water potential of the potato tissue.
This method has been chosen because it will measure any change in solute concentration i. e. both reducing and non-reducing sugars as well as other possible breakdown products such as amino acids.
