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Introduction

Investigating the Effect of Chilling on the Water Potential of Maris Piper Potatoes 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. The cell wall is a fairly rigid structure composed largely of long parallel strands of cellulose, a polysaccharide of � glucose, held together by hydrogen bonds. 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. ...read more.

Middle

This is the most difficult variable to control. * Time of soaking must be constant for all the potato pieces. The rate of osmosis will differ according to the water potential gradient between the potato core and the soaking solution. The greater the difference in water potential, the faster the rate of osmosis. As water diffuses across the plasma membrane from the higher water potential to the lower water potential the difference in water potential will decrease and the rate of osmosis will decrease until equilibrium is established on both sides of the membrane. There is not enough time to carry out pilot investigations to determine the length of time needed for equilibrium so the potato cores will be soaked overnight for 24 hours because this should be plenty of time for equilibrium to occur. * Species and age of potato (since being harvested) - different varieties of potato may have different water potentials so it is important to use all one variety. Many fruits and vegetables undergo changes after they have been harvested so by using potatoes that are bought at the same time from the same shop should reduce this variation but it is impossible to be sure that the potatoes were all picked at the same time or grown under the same conditions. Maris Piper potatoes have been chosen because they are described on the packaging as suitable for chipping. This means they must contain a fairly low reducing sugar content and any increase should therefore be easy to detect. * The volume of the salt solutions will be kept constant using a 25cm3 measuring cylinder. * The concentrations of the salt solution will be controlled by making careful dilutions and using only one batch of salt solution to reduce variation. * The mass of the potato will be measured using an electronic top pan balance, which weighs accurately to 0.01g. ...read more.

Conclusion

If the change in mass is measured, the effect of the residual liquid should not affect the results. Each core will be placed into a separate labelled test tube containing 10 cm3 of sodium chloride with a particular water potential. This will be repeated for each water potential. The potato cores will be left in the solutions for 24 hours. A water bath will be used to maintain a constant temperature. 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. Additional work if there is time To check that any difference in water potential is due to an increase in reducing sugar content, a Benedict's test could be carried out. 10g of tissue from potato stored at 2oC and 24oC will be homogenised in a blender with a minimum volume of water. The mixture will then be filtered and 5cm3 of filtrate will be placed into a test tube. 2 cm3 of Benedict's solution will be added to each and the contents of the test tube boiled for 8 minutes in a heat block. The colour of the two solutions will then be compared. If a reducing sugar is present the colour will change from blue to green then yellow then orange then brick red. The final colour of the solution will depend on the concentration of reducing sugar in the potato tissue sample; the more the concentration of reducing sugar the greater the colour change (Larkcom and Miller, 2001). ...read more.

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