Equilibrium occurs when there is an equal amount of a substance either side. (In this case water). The arrows show the direction of the movement of the particles from high concentration to a lower concentration. The water particles do actually move in the other way as well but because more travel in the other direction this not noticeable. Even when equilibrium occurs the particles keep crossing but they are moving equally each way.
Variables
There are many things that could be changed in this investigation but we are interested in the concentration of the cell sap to do this I will change the molarity of the solution from 1M to 0 (180g dissolves in 1 litre of water to make 1M solution). Then when we have a result when there is no percentage change we have found the concentration because we already have equilibrium so the concentration is the independent variable.
The dependant variable will be the mass, as this will change depending on the concentration as shown in my preliminary work with the distilled water and the 1M solution of glucose. This is the variable I am interested in for my results.
In this experiment there are other variables such as room temperature, the initial mass of the potato may effect it as well and the type of potato or size of the potato.
Fair test
For a fair test I will use the same potato and all the chunks will be roughly the same size. This will stop there being any variable depending on the size or type of potato. All of the tests and solutions will be in the same room in the same area so they will be at nearly the same temperature. I will also use two tests and work with the average. This will give me a better result, as I am doing it more then once giving me an average rather then relying on one set of data. This will mean in both tests I’m going to use the same solutions so they are not different.
Safety
In this experiment I will have to be careful when using the scalpel and the borer on the potato.
What results will I take?
For more accuracy I will take 5 results, which should give me a better graph. I will also do the experiment twice and I will use an average for my graph. I want to get the most accurate results because this will give me a better result and by being accurate there will not be as many mistakes.
Prediction
My own Prediction is that when the potato is in the water solution will take on more water this is shown in my preliminary work and this is because there is a higher water potential in the water so the water will be taken across into the potato by osmosis. This also means that the Potato will lose weight in the glucose solution as the water in the potato will move across the membrane into the glucose solution as there is a lower water potential there. I also believe the answer will be closer to 0M then 1M because in my preliminary work there was a smaller percentage change on the distilled water then the 1m solution. I also think my results will give me a straight line graph as they are directly proportional until the potato cant take on any more which is when we use distilled water.
Results
Averages
To work out the average of the results I added the results for the two results for both the experiments together and divided by two.
For example
9.89 + 13.83 = 23.72 divide by 2 = 11.86 so the average percentage increase for a piece of potato in a 0M solution of glucose is 11.86%.
An average is much more accurate then the results of one test as it incorporates more then one answer giving a much more accurate answer reducing the impact of any anomalies that could of occurred.
I am going to display this data on a scatter graph and then add a line of best fit, as this will roughly give me the answer to the concentration of water in the cell sap.
Analysis
The answer to my question what is the concentration of cell sap of a potato is where the line of best fit crosses the X axis (when Y=0) this is because there is no change in mass. This means that the concentration of the solution must be exactly the same as the concentration in the potato as osmosis is occurring but equally. I can find this exactly by working out the equation for the line and re arranging it.
Y = Mx + C C=11.86 as this is where it crosses the Y axis (X=0) and M is the gradient –54.216
So Y = -54.216x + 11.86
So by re arranging that formula when Y=0 X=0.218755
So the exact concentration of the cell sap is 0.218755 M in the potato I examined. In other potatoes this number could vary. Also I measured the R squared value shown at the bottom of my graph reading 0.9906 If the Number is close to 0 it means there is no correlation –1 a perfect negative correlation or 1 a perfect positive correlation. This is very close to a 1 correlation showing there is a very good correlation and almost a straight line.
Conclusion
These results show that my prediction is correct; as there is more glucose there is more mass lost and that the answer is closer to zero then one. There is a very good correlation here proving this. This is caused by the water in the cell sap moving across the semi permeable barrier to equal the outside concentration. So if there is a high concentration of glucose out side then the water moves across the barrier making the mass of the potato lower and vice versa.
Evaluating
I feel my results are pretty accurate they all fit very close to the line of best fit there are only very small variations. The method I used was a simple step method that gave me the results I needed and wasn’t very complicated. I feel that the results are strong enough to support the conclusion I have written. They follow a strong positive correlation and nearly all of the mass changes are within 5 grams of each other, and they are nearly all within 5 % of each other on the percentage change column, which is accurate enough to support my initial prediction. There was one anomaly in my results in the 0.4M solution where both the mass and percentage change is more than 5g and % this could have been caused by a different size chunk of potato. I could of repeated the experiment more times to get a better average and that would be able to help get rid of any anomalies also to of improved the experiment or if I was to do it again I could of used a more accurate weighing scale or a sharper scalpel as the one I used was quite blunt and made it hard to cut exactly the right size.
Taking it further
If I was to take this experiment further I could look at different initial masses, surface areas or different varieties of potatoes.
By Naomi Colcomb
