Osmotic Pressure:
In a hypotonic solution, water moves into the cell, and as the water moves into the cell, osmotic pressure increases. The reason why plant cells do not burst due to this pressure is that they have a strong cell wall made of cellulose that holds the cell together.
Functions of Cell Wall in Plants:
- They provide rigidity to the cells, maintaining their shape.
- They protect against pathogens.
- They prevent the cell from bursting when enough water enters the cell.
- They are a store of carbohydrates for the plant as they are made of the carbohydrate polymer called cellulose.
Factors That Can Affect The Experiment:
- Temperature – As the temperature increases, the particles will gain energy, and thus move about more causing the rate of osmosis to increase.
- Surface Area – If the potato pieces were smaller, together they would have a larger surface area, assuming the total mass stayed the same, and thus the area for osmosis to take place would increase, and therefore the rate of osmosis would increase as well.
Preliminary Results:
For our preliminaries, we chose two solutions of sugar and salt each, with different concentrations of solute. We also conducted one test with just a solution of plain water. Although we found a general trend that mass increases for sugar, and decreases for salt, I believe that these results were not very accurate, as we did not control the surface are of the potato pieces, and instead concentrated on the mass. However, this experiment gave us an idea of what to expect for our main experiment, and we were able to make a prediction. Moreover, from our preliminaries, we were able to make the decision to use salt in our main experiment. We decided this as we found that it was a lot easier to weigh and handle the salt, rather than sugar, thus it would give us a better chance to obtain reliable and consistent results.
Prediction:
My prediction is that for the lower concentrations, the mass of the potato will increase as the conc. inside the cell will be greater, however, I believe that as we increase the concentrations of the solute, the mass of the potatoes will decrease.
List Of Apparatus:
- Potatoes
- Core borer
- Salt
- Sugar (for preliminaries)
- Distilled water
- Weighing scales
- Scalpel
- Spatula
- Measuring cylinders
- Beakers
- Ruler
- Weighing boats
- Test tube holders
- Stirring Rod
Method:
- First we prepared the potato pieces by using a core borer to produce pieces of the same circumference.
- Then using a ruler and a scalpel, we cut the pieces to the same length so as to ensure the same surface area.
- We then placed the pieces in weighing boats and measured their mass on weighing scales.
- Next we prepared the solutions by inserting the right amount of solute into 100cm³ of distilled water in a measuring cylinder.
- We then stirred it using a stirring rod in a beaker.
- After that we measured out 25cm³of the solutions using a measuring cylinder.
- Then we poured 25cm³ of the solutions into the test tubes.
- Next we placed the potato pieces in the test tubes containing the solutions.
- We placed the test tubes in holders, and left them for 6 hours.
- Then we extracted the potato pieces, dried them and measured their mass.
- Lastly, we recorded the results on a table, and calculated the average mass change for each concentration.
Reliability:
- We used the same weighing scale for all measurements so as to ensure that there was no discrepancy in the recordings.
- We used the same core borer to make sure that the pieces had the same circumference.
- We properly dried the potato pieces to ensure that any water on the surface of the potatoes would not affect the results.
- We used the same type of test tubes so as to make sure that the surface area in contact remained the same.
- We also made sure that the setup of the apparatus was the same for every test so that it did not affect the results.
Safety:
- Roll up sleeves and any loose clothing.
- Be careful when using the scalpel.
- Keep the desk clear of bags, books and other items.
- Wear goggles.
OBTAINING EVIDENCE
Through this experiment, I have found out that the point where the solution is isotonic is between 0.8% and 1.2% concentration of salt; I will be able to predict the exact point by drawing a graph of the results. The results show that as we go below this point, the mass of the potatoes increases, due to water being taken in, and when we go above this point, the mass of the potatoes start to decrease as water is being lost.
Graph:
The graph that I have drawn of Average Mass Change against % Concentration Of Salt clearly shows the effect concentration has on the mass of the potato pieces. The linear relationship between the variables is quite strong, the potato pieces in the solution of plain water show the greatest increase, and after that, as the concentration increases, the mass change of the potatoes decrease, until around 1.0% where the solution is said to be isotonic. As we increase the concentrations past 1%, the potato pieces lose more and more mass. The graph has a strong negative correlation. For the data, I used averages of the results for each concentration, and the graph shows that the value for 1.6% may be anomalous. I will evaluate whether this is the case, and if so what caused it in my evaluation.
Conclusion:
After studying the results, I was able to ascertain that my prediction was indeed right, because it was evident that the experiment followed the pattern I predicted. The mass of the potatoes did go up for the lower concentrations that were below the point where the solution would be isotonic, as the concentration of water was higher in the solution and water entered the plant cells. The mass of the potatoes did decrease for the higher concentrations, where water left the cell due to the concentration of water being higher inside the cell.
Evaluation
I believe that my results are fairly reliable, as I have done them to the best of my ability with the equipment available to me, controlled the variables carefully, and double checked all measurements and also made sure that the apparatus was properly set up every time. Through my graph, I could identify two results that might possibly be anomalous, as it did not completely conform with my line of best fit. The results for 1.6% showed a mass decrease of 0.0466 g, when it should’ve been around 0.09g, and the results for 0.8% showed a mass increase of 0.0566g, when it should’ve been around 0.03g I believe that this could have come about due to us not drying the beaker and the test tubes properly, and thus causing the solution to become more diluted, meaning that the diffusion gradient would be decreased, and less water would be lost by the plant. Therefore, if I were to repeat the experiment, I would take greater care in drying the apparatus, or maybe even use fresh beakers and test tubes for each test.
