Investigate the different water potentials of a fruit and a vegetable.

Tim Pickworth L6H

Objective:

The objective of this experiment is to determine the water potential of both apple cells and potato cells and to see how they differ from one another. We will do this by placing the apple and potato chips in different molarity solutions for a period of time and recording the changes in mass.

Hypothesis:

The theory for this investigation, which is to be proved or disproved, is that as the molarity of the solution increases the average percentage difference in mass will decrease of both the apple and the potato cells. This means in a lower molarity the chips may increase in mass slightly but in a higher molarity the mass of the chips will decrease.

Background information:

The whole idea of the investigation revolves around the theory of osmosis. The definition of this term, is “the movement of water molecules from an area of high water potential to an area of lower water potential, through a semi permeable membrane until equal distribution is achieved”

Water potential is the measure of the free kinetic energy of the water molecules in the solution and it is the term given to the tendency for water molecules to enter or leave a solution by osmosis. The highest water potential is in distilled water and is said to be 0. Solutions containing a solute therefore have lower water potentials so as the molarity increases the water potential in the solution decreases.

The apple chips will contain fructose and sucrose making them more soluble than the potato chips, which contain starch, an insoluble substance.

In plant cells such as the ones we are using the water potential in the cells will be lower than that of the water potential in distilled water. In higher molarity solutions the cells will have a higher water potential than that of the surrounding solution and this is when the cell becomes plasmolysed, which is when the cell membrane shrinks away from its cell wall (the cell becomes flaccid).

Whether water enters a plant cell through osmosis or not depends on the water potential inside and outside of the cell. When the water potential or concentrations are the same both inside and out of the cell no osmosis will take place and there will be no net movement of water across the membrane. This is known as an equilibrium state and the solutions are referred to as isotonic. When there is a higher water potential outside of the cell it is referred to as a hypotonic solution and there is net movement into the cells. The cytoplasm becomes inflated by water uptake and if this exerts pressure against the cell wall the cell is described as turgid.

Finally the there are factors that effect the rate of osmosis and these must be considered when carrying out the final experiment. Firstly the surface area will effect the rate of osmosis, as a larger surface area will mean more osmosis occurs. Next the temperature may have an affect as well as the different water potentials outside of the chip. Time will also have an effect, the longer the chips are left in the solution the more time they will have to perform osmosis.

Apparatus:

Also a King Edward potato and a Golden Delicious apple is needed to create the chips

Method:

Firstly the solutions need to be made. Prepare 30cm³ of each of the following solutions from the 1.0molar salt solution and the distilled water: 1.0m, 0.8m, 0.6m, 0.4m, 0.2m and 0.0m. Use the syringe to get the accurate amount of each solution needed and refer to the dilutions table to see how much of each solution is needed for each molarity. Label the Petri dishes with the molarity of the solution it contains, there should be 12 ...

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Apparatus:

Also a King Edward potato and a Golden Delicious apple is needed to create the chips

Method:

Next you need to gather 18 potato chips and 18 apple chips and using the tile, ruler and knife cut each chip to exactly the same length, 4cm. You then need to place the 18 apple and the 18 potato chips into 6 groups of 5 making sure the different types of chips are kept separate. Now each group of 5 chips needs to weighed on the electronic balance and recorded. You must remember which set of 5 chips goes in which solution otherwise the results will be inaccurate.

Once each weight is recorded put the correct 5 chips into the correct solutions and leave for 1hour making sure none of the 5 chips are touching each other. Obviously it isn’t possible to put each set of 5 chips into the correct solution at the same time so instead put them in in intervals using the stopwatch to decide and measure these intervals. After the hour is up take out the chips using the same intervals so each chip has been in the solution for the same amount of time. Dry off the excess water and reweigh each set of 5 chips again and record your results in a suitable results table making sure once again that you note which set of chips came from which solutions. When plotting a graph to display the information we will use molarity on the x-axis and average percentage change on the y-axis.

Dilutions Table:

Fair Test:

To make this a fair test and ultimately make the results reliable there must be only one variable and obviously here it is the change in molarity of each solution.

There are many factors which must be kept constant to ensure a fair test for example there must be no skin left on any of the chips as this will prevent osmosis occurring in places, to prevent this from happening all the chips will be checked before hand. Also the chips must be separated once in the solutions so they are not touching and the surface area is not affected. If they were touching osmosis levels would fall as the surface area of the chips touching would fall, once in the Petri dishes the chips will be spread out evenly to prevent this. In order to keep the temperature the same throughout the test no windows should be opened to keep the room temperature exactly the same again so osmosis is not affected.

Every single chip must have identical dimensions and each chip should have its excess water removed equally before reweighing, this will mean all the chips surface areas will be the same and also equal amounts of water will be removed from each chip before reweighing to ensure some chips don’t lose more water than others.

Finally the chips should be taken from the same type of potato or apple for example the potato chips from a King Edward potato and the apple chips from a Golden Delicious apple. This is because different types of potato and apples could have slightly different structures and so could lose or gain water quicker. The chips should also be made from the same area of the potato or apple for example the core.

Predictions:

I can firstly predict by using my preliminary work and my background information that at low molarities the chips will gain in mass and in higher molarites the chips will reduce in mass, with the biggest reductions coming from the highest molarity solutions.

This is because where the molarity is low there will be a higher water potential in the solution and a lower water potential in the chips. Therefore I believe osmosis will occur and water will enter the chips causing the vacuole to fill and expand and adjacent cells will press against each other causing the chips to become turgid and the masses to increase.

At higher molarities I believe the chips will lose mass because there will be a higher water potential inside the chips and a lower water potential in the solution meaning the net movement of water will leave the chips and move into the solutions.

I also believe that the potato will have a higher water potential than the apple as the potato contains starch, which is insoluble and therefore wont affect the water potential. The potato contains this starch as it is a storage organ and is left in the ground over winter (perrenating). The starch is easier to store than the carbohydrates, which it needs to grow from in the spring. The apple however contains fructose and sucrose, which are soluble and therefore should cause the water potential to decrease. Using this information I predict the graph to look similar to the one I have constructed below.

Risk Assessment:

The only real hazard in this experiment is that of the knife, which you must take care with when cutting the potato and apple chips to the correct length. Make sure your hands are dry when doing this and use a tile to help prevent the chips from slipping and to prevent damage being inflicted on the bench.

Analysis:

Depending on which solutions the chips were placed in some chips gained in length and others decreased in length. This is because of osmosis, which is the movement of water molecules across a selectively permeable membrane from an area of higher water potential to an area of lower water potential.

At low molarities the chips gained in length, because the cells in the chips are becoming turgid and exerting pressure on the membrane. For example the potato chips on average in a 0.0M solution gained 6.9% and the apple chips in the same molarity solution increased by an average length of 10.3%. In higher molarity solutions the chips reduce in length as the cells in the chips become flaccid and don’t exert pressure on the membrane. For example in the 1M solutions the potato chip lost an average length of 11.5% and the apple chips lost an average length of 7.2%.

At point A on my graph where the molarity is low there is a higher water potential in the solution and a lower water potential in the chips. Because of this osmosis will occur and water will enter the potato chip. This causes the vacuole to fill and expand and adjacent cells will press against each other causing the potato chip to become turgid. This water therefore increases the length of each chip.

At point B on my graph, where the lines cross the x-axis, there is no change in length and the potato chips length stays the same.

The water potential inside the chips and in the solution are equal. Therefore there is no net osmosis occurring and the chips neither gain nor lose water and so their length remains the same.

The estimated water potential therefore of the potato chips is:

0.4 x (-3.51x10³) = -1404 kpa

The estimated water potential of the apple chips is:

0.5 x (-3.51x10³) = -1755 kpa

At point C on my graph the chips lose length. There is a higher water potential inside the chips and a lower water potential in the solution. Water therefore leaves the chips and moves into the solution. Vacuoles get smaller and don’t exert pressure against the cell walls meaning the cell is called flaccid. The chips therefore lose length.

The two sets of results show similar trends and the lines of best fit run parallel to each other.

In lower molarity solutions the change in length is more significant in the apple chips and in the higher molarity solutions the potato chips have the more significant change in length. This therefore means that the apple chips have a lower estimated water potential. The reasoning behind this is that potato is made up of starch, as it needs to store energy through the winter months. Starch contains amylose and amylapectin, which are only slightly soluble in water and so less water will be taken up by the potato chips in distilled water. Apples however contain sucrose and fructose which are far more soluble in water and so more water can be taken into the cells thus meaning apples have a lower water potential than potatoes. In higher molarity solutions the apple cells don’t decrease in length as much as the potato cells because they have a higher molarity content inside them which means less osmosis occurs to form an equilibrium so less water is taken out of the cells.

Evaluation:

During this experiment there were a number of problems with the procedure which affected the accuracy of the results. Some affected the results more than others.

On the whole however the techniques used were suitable to obtain results accurate enough to base a firm conclusion.

The only anomaly found was in the results of the 0.5M solution for the apple results. Here the results showed a significant percentage decrease in the length. The decrease was -7.2% which was the same percentage decrease as shown by the apple chips in the 1.0M solutions. This anomaly can clearly be seen on the graph as the Golden Delicious line goes way under the line of best fit with a fairly steep decline from the 0.2M result and then a sharp incline to the 0.6M result.

I believe the main sources of error in this experiment were with the dilutions, time limit and with the variations of lengths of each chip at the start. There were other factors which I felt were less significant in affecting the results.

The biggest problem was with the variations in length of the chips at the beginning. If a particular chip was longer it would have a larger surface area and so during the time period more osmosis would occur as more of the chip is in contact with the solutions and so a bigger change would occur.

This could be prevented if electric vernier callipers were used to cut out the lengths of the chips ensuring all the chips had exactly the same length at the beginning instead of cutting them ourselves and measuring them out with a ruler of a fairly low accuracy (1.0mm)

The second biggest problem was with the dilutions and the accuracy at which we made them. When using the syringes to collect the required amount of solution it was difficult to get exactly the required amount and the readings off the syringes were left open to human error. Also the accuracy of the syringes was not particularly high and so measurements may have been slightly inaccurate here and there. If too much water was put into the petri dish the molarity would decrease and the % change in length would be affected as less water would leave the chip. In order to improve this apparatus with a higher degree of accuracy must be used to gather each solution and place it into the petri dishes, this would ensure that exactly the right amount of each solution was in each petri dish and each molarity was what it was meant to be.

The third biggest problem with the procedure was the time limit the chips had in the solutions.