02B_Leek+Experiment

Biology Report for Leek Experiment Aim: The main aim of the experiment was to estimate the approximate osmotic concentration of the leek stem cells by placing them in substances of differing osmotic concentrations and using the changes in the weight and length of the leeks to gauge the osmotic concentration of the leek stem cells.

Hypothesis: The leeks soaked in distilled water, 1% salt solution, 2% salt solution, 4% salt solution and 7% salt solution will all increase in mass while the leek soaked in 10% salt solution will decrease in mass. This is because substances will travel from a region of higher water potential to a region of lower potential. The distilled water, 1% salt solution, 2% salt solution, 4% salt solution and 7% salt solution have higher water potentials than the substances within the leek cells and hence will enter them, resulting in an increase in weight. The substances within the leek cells have a higher water potential then the 10% salt solution, and hence will leave the leek cells and result in a decrease in weight.

Method: Step 1: Cut the stalk of the leek to obtain a length of 3cm.

Step 2: Cut the strip longitudinally to obtain 4 equal quarters. Repeat till 6 strips have been obtained.

Step 3: Label 6 empty petri dishes: Distilled water, 1% salt solution, 2% salt solution, 4% salt solution, 7% salt solution and 10% salt solution.

Step 4: Place one strip of the leek into each petri dish so as to avoid confusion during measuring and weighing.

Step 5: Weigh each leek on a measuring scale and record the weights as the initial weight.

Step 6: Measure the length of the inner surface of each leek and record the length as the initial length. If the leeks have bent after cutting, use the string provided to measure the inner surface of the leek, then align the string next to a ruler and measure it to obtain the length of the inner surface of the leek. *

Step 7: Fill each petri dish respectively with the substance it is labelled with, distilled water, 1% salt solution, 2% salt solution, 4% salt solution, 7% salt solution and 10% salt solution. *Note: Although the leeks were trimmed down to 3cm at the start of the experiment [step 1], it is still necessary to measure the lengths of each individual strip as there could be slight deviations in the length of each strip due to human error etc. at the start of the experiment. Step 8: Place the respective leeks into each petri dish, making sure that the inner epidermis is facing the liquid.

Step 9: Record observations of the leeks in each substance(eg. bends immediately, etc.)

Step 10: After 20 minutes, remove the leeks from the substances. Discard the leftover liquids in the petri dishes and place the respective leeks back into the labelled petri dishes.

Step 11: Weigh each leek and record their weights.

Step 12: Use the string provided to measure the inner surface of the leek, then align the string next to a ruler and measure it to obtain the length of the inner surface of the leek. Record each length.

Step 13: Draw the results of the experiment into a table and a graph.

Step 14: Record observations of the texture, etc. after the experiment.

Results: ** Leek/Data ** || Starting length (cm) || Starting weight (g) || Length after 20 min(cm) || Weight after 20 min (g) || Percentage difference in length || Percentage difference in weight || Leek 1 (Distilled water) || 2.9 || 0.14 ||  3.2 ||  0.22 ||  +0.10% ||  +0.57% || Leek 2 (1% salt solution) || 3.1 || 0.16 ||  2.9 ||  0.26 ||  -0.065% ||  +0.38% || Leek 3 (2% salt solution) || 3.2 || 0.24 ||  3.1 ||  0.34 ||  -0.031% ||  +0.42% || Leek 4 (4% salt solution) || 3.6 || 0.18 ||  3.1 ||  0.24 ||  -0.14% ||  +0.33% || Leek 5 (7% salt solution) || 2.9 || 0.28 ||  3.1 ||  0.39 ||  +0.069% ||  +0.39% || Leek 6 (10% salt solution) || 3.1 || 0.18 ||  3.3 ||  0.24 ||  +0.065% ||  +0.33% ||

Graph of Percentage Difference in Length and Weight of Leeks

Data analysis The leek placed in distilled water increased both in weight and length. This suggests that the substances within the leek have a lower potential than the distilled water, as substances travel from regions of higher to lower water potential. Therefore, the distilled water entered the leek, causing the cells to become turgid, and hence increase in mass and length.

The leek placed in the 1% salt solution decreased in length but increased in weight. The results suggest that the 1% salt solution has a higher water potential than the substances within the leek cells, as substances travel from regions of higher to lower water potential. This would explain the increase in weight of the leek, as the cells became turgid, causing the leek to increase in weight. However, the length decreased, which could possibly be due to an error in measurement.

The leeks placed in 2% salt solution and 4% salt solution showed similar results as the leek placed in 1% salt solution, decreasing in length but increasing in weight. The increase in weight implies that the substances within the leek are still of lower water potential than then solutions, hence causing them to enter the leek cells and resulting in an increase in weight. However, the decrease in length is once again unaccounted for as it is in contrast with the implications of the increase in weight.

The leeks placed in the 7% salt solution and 10% salt solution showed similar results, increasing in both length and mass. This shows that the substances within the leeks have lower water potential than the solutions,producing an increase in mass as the solutions moved from a region of higher to lower water potential, causing the leek cells to become turgid. The length also increased, also because of the cells turning turgid, and hence making the leek increase in length.

Overall, it can be concluded that the substances within the leek cells have a lower water potential than the distilled water, 1% salt solution, 2% salt solution, 4% salt solution and 7% salt solution. It is isotonic to the 10% salt solution.

Observations

During the course of the experiment, some changes in the various aspects of the leeks placed in different substances were noticed. For instance, the moment the leeks were cut, they bent immediately. Also, the outer surface of the leeks felt rough, while the inner surface felt smooth. Leek 1 (placed in distilled water) bent the most, followed by leek 2( placed in 1% salt solution), leek 3( placed in 2% salt solution), leek 4(placed in 4% salt solution), leek 5( placed in 7% salt solution) and leek 6( placed in 10% salt solution). In conclusion, the higher the percentage of salt in the solution, the less bent the leeks were.

Difficulties encountered

During the experiment, there were some difficulties that I encountered. For one, slicing the leeks longitudinally and equally proved to be quite a challenge as it was hard to get them to equal sizes. Also, many of us were confused as to how much of each solution we should use as it was not stated in the worksheet. Later on, we decided to put in just enough liquid to submerge the enter strip of leek. Another difficulty faced was that the results of the experiment were not very accurate. For the leeks placed in 1%, 2% and 4% salt solution, the lengths decreased, although the weights increased. This made the data contradictory as the increase in weight suggested that the substances within the leek had lower water potential than the solutions, therefore causing the solution to enter the leek cells and become turgid, causing it to increase in weight and length. However, it decreased in length which made the conclusion that the substances within the leek had lower water potential than the solution questionable.

Things done wrong during the experiment
 * 1) We did not cut the leeks to 3cm before cutting them into quarters. Instead, we first cut it into quarters, then trimmed each piece down to 3cm. This made the process more tedious and less accurate as each piece was trimmed to 3cm individually, causing more human error to be made.
 * 2) We did not lay the leeks flat on the tile to cut it into quarters. Instead, we stood it upright, which made it more difficult and dangerous to cut it accurately.
 * 3) During the experiment, we realised that a few of the leeks did not have their inner epidermis facing the solution. This could have made it more difficult for the liquids to be absorbed into the leek, and hence made the results less visible.

Discussion questions The 10% salt solution is isotonic to the leek cells. This is because the overall change in weight and length of the leeks is the least when they were placed in the 10% salt solution. When a solution is isotonic to the leek cells, little amounts of substances should move in and out of the leek cells. Therefore, there will be little change in the weight and length of the leek. Hence, the 10% salt solutions are isotonic to the leek cells as there is the least change in weight and length of the leeks.
 * 1) At what osmotic concentrations are the salt solutions isotonic to the leek cells? Explain how you arrive at your answer.

2. Explain the gain or less in length of the stems. The length of the stems all increased. This could be because the solutions had higher water potentials than the substances within the leek cells, causing the solutions to move from a region of higher to lower water potential, into the leek cells. This would make the leek cells become turgid, and hence cause an increase in the length of the stems. 3. Instead of measuring the change in length of the stem, is there another variable you can measure as an indication to osmosis? The weight of the stems can be used to measure osmosis. If the leeks gain weight, it would suggest that the solutions have entered the leek cells, showing that the solution has a higher water potential than the substances within the leek cells.

4. Suggest a way to improve this experiment. The percentage of salt in the salt solutions could be more varied, (eg. 1%, 7%, 15%, 30%, 60%). This would produce more visible results as there would be a more dramatic difference in the reactions of the leek cells to the solution surrounding it.

Vanessa Yap Wei Shan (29) 315