Method
- Appropriate apparatus was collected. (Stated previously)
- The percentage sucrose solutions, 0%, 10%, 20%, 40%, 60%, 80%, and 100% were created using 1M sucrose solution distilled water. It was important pure water was used as its water potential was zero kilopascals, this would then not have any affect on the final results.
- Before the solutions were made, two pipettes were labelled. One, ‘sucrose’ and the other ‘distilled water’. Thus, preventing contamination when measuring and mixing solutions.
- The 0% sucrose solution was simply 30ml of distilled water, measured and poured out three times into a petri dish, as the pipette was 10ml.
- Diluting the 1M sucrose solution 10 times, 3ml of sucrose and 27ml of distilled water created the 10% sucrose solution. These were measured using the assigned pipettes and poured into another petri dish and mixed. The dish was sealed and swirled thoroughly fro approx. 30 seconds to ensure the mixture was evenly concentrated for the addition of the potato samples.
- The same procedure was carried out to create the remaining solutions, with the volumes stated in the previous table.
- Each time the solutions were poured into the petri dishes the corresponding label was written on it, i.e. ‘0%’, ‘10%’…
- With a borer, seven cylinders were cut from the same potato.
- Then with a scalpel each cylinder was measured and cut by a ruler, to 24mm in length. They were cut further into 6 discs, 4mm in length and kept in their group. The groups were then taken to be weighed.
- A set of 6 at a time they were put into a weighing boat, which was prior set to zero by pressing the ‘T’ button. The first set was weighed and recorded and put into an allocated petri dish, which was also recorded (as to which mass was in which dish). The time at which they were placed in was also taken.
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The 2nd group was then weighed in the same way, with the boat set to a mass of zero grams. The potato sample was recorded and again put into a dish of diluted sucrose solution and noted for future reference. This was carried out for the remaining five cylinders and they were left for an hour (approx.).
- During the hour the equipment was cleaned and put away.
- After the hour the solutions were poured away and with tweezers the discs were picked out of the petri dishes and grouped as they were in the dishes.
- They were then placed on a paper towel for the excess fluid to run off, then taken to the weighing scales.
- Like before, the weighing boat was placed on the scales and set to zero. The discs were placed in their groups in the weighing boat and weighed just as before with their change in mass recorded alongside their ‘start mass’. Each time a group was weighed and replaced for another set of discs the boat was dried to prevent any excess fluid being added to the next weighing.
- All the results were annotated into a table and the change in mass was calculated. Due to the different ‘start’ and ‘finish’ masses they needed to be standardised. This was done by calculating the percentage change, rather than just leaving the initial mass change.
- The percentages were then plotted onto a graph and the point of which no change occurred was pointed out. To make it a bit more obvious where no change occurred the start and end mass was plotted, and where they intersected was the point of equilibrium. This percentage solution was then taken from the graph and compared to a given table for conversion, from percentage concentration solution to water potential. This was the estimated water potential of the plant cell.
Results
Discussion
Provided are two types of graphs, presenting the data collected. A duplicate is also provided of each, created by the computer for easier viewing. Graph one, the percentage change in mass shows a very clear set of results forming a trend. The line of best fit seemed to begin to rise in the positive direction, but reached a maximum and slowly but then rapidly fell, overlapping the x-axis (the point of no mass change), then negative values were obtained. A minimum value was reached; the line began to rise. The isotonic value taken from this graph was 0.38M sucrose solution, as labelled. On this graph it was where the Y-axis was zero the point of no mass change occurred, equilibrium. Here, the value from the X-axis was taken, called the isotonic solution.
The second graph given is of the conversion chart. Simply, the table provided was plotted onto a set of axis because when the isotonic solution was converted via the table alone an extremely vague answer is given. The better and more accurate approach to this was to plot the table, which would extend the trend across the values between the relative 0.2 to 0.4M concentration. The line of best fit gradually declined in a negative direction. As the concentration of the sucrose solution increased, the water potential decreased. However, on this occasion the trend wasn’t highly relative to what it was needed for. The isotonic value (0.38M) was plotted along the X-axis and a vertical line was drawn down until it met the line of best fit. A horizontal line was then drawn towards the Y-axis i.e. the water potential values, and here an accurate value was taken for the potato cell. The value found was –980kPa.
Throughout the process of the investigation steps were taken to ensure maximum accuracy and precision. The reasoning behind the use of a petri dish came about as it was the most suitable container to hold the solutions, and potato discs, and it could be sealed with a lid, less chance of spillage. Controlling the environment to the best of my ability was quite important as it limited the possible factor, temperature. This was relatively important because although the samples were in at an ambient temperature I found it sensible to contain the environment because I knew factors such as other pupils walking past or breathing over the samples would affect some and not others. This experiment had to be as fair as possible for all samples otherwise the varied factor (concentration), was pointless because we wouldn’t know what exactly caused or influenced the results, some results could also be false and misleading, accuracy was the key. Initially, the total volume of the sucrose and water solution was 20ml. However, after a practise experiment it was noted that this volume was too little and didn’t saturate/cover all the surface area of the potato discs. After much thought and calculations a new volume was created along with new sucrose and water ratios but with the same percentages. By doing this the potato discs were totally surrounded by the solutions, therefore in each petri dish the total surface area covered was the same and in turn fair.
One step taken into consideration whilst measuring the volume of liquids was the judging of the meniscus. This had to be judged exactly the same for each measurement, otherwise inaccurate results were taken. The readings were always taken where the base of the meniscus touched the line. It was vitally important this principle was carried throughout the experiment. Not just for the readings but for using the same borer, which kept the width and therefore surface area the same for each cylinder. As explained earlier in the investigation, surface area is a variable of osmosis, which in this situation was not required and needed to be kept under control. Likewise, the length of the cylinders were precisely measured with a ruler and cut with a scalpel, 24mm long, and then 4mm. The discs were all the same length for a purpose, to keep the surface area of the samples the same.
Prior to weighing the saturated potato lumps they were picked from the petri dishes and placed on a paper towel, reason being the excess fluid needed to be drained/soaked so it didn’t interfere with the final readings. From both the results and the way the investigation was carried out it seemed
If this experiment were to be carried out once again I would consider placing the potato and solutions into a more controlled environment, for example control the temperature. Another suitable suggestion would have been to leave the specimens much longer, and therefore more detailed and accurate results. On that note, a further experiment would be to create sucrose solutions with the concentrations to two decimal places, between the found concentrations 0.2 and 0.4 (the area of equilibrium), such as 0.31, 0.32, 0.33, 0.34, 0.36, 0.37, 0.38, 0.39 and 0.40M sucrose solutions. With these values the graph would show a more precise isotonic figure and in turn a more accurate figure for the water potential of a potato cell.