Hypothesis
Based on my preliminary experiment and background research I have done, I predict that as the concentration of the sucrose solution increases, the loss in mass of the potato cylinder will increase. When the concentration of sucrose solution is the same as the concentration of the cell sap the potato segment will neither gain nor lose mass. By using a range of sucrose concentrations I will be able to deduce the isotonic solution and therefore the concentration of the cell sap.
Equipment
Ceramic Tile
Scalpel
12 Potato Sections
150 ml sucrose solution
150 ml of distilled water
Balance
6 small plastic dishes (big enough to hold 50 ml of solution)
Ruler
Two 50 ml Syringes
Waterproof Pen
Waterproof Partition
The independent variable that I have chosen to change is the concentration of the sucrose solution. The dependant variable I will measure will be the mass of the potato sections. Other variables that I need to control are temperature because I know that this could affect the rate of osmosis and the surface area of the potato because this affects how many molecules can pass into and out of the cell in a second. I will control the temperature by leaving all the samples in their respective concentrations in the same room. Even if the temperature in that room changes (which is unlikely), all the sections will be exposed to it. I will attempt to control the surface area by careful measuring with a ruler however I admit that it will be very hard to make them exactly the same.
This experiment will be a fair test as the temperature, equipment and size of the potato sections will be the same for each solution. I will keep all the variables the same except for the concentration of the sucrose solution. It is important to make sure that the test is fair because if it wasn’t results would be unreliable as other factors could have influenced them. If we know that there is only one variable effecting the results, we can be sure that the results are accurate.
To make sure my experiment is done safely I will ensure that hair is tied back so it doesn’t get caught in anything and overalls are worn to protect clothes. The plastic cups, beakers etc will be placed far from the edge to avoid spillage and kept in a secure room in case of interference.
I will use 6 different solutions in the ratios of (sucrose solution:water) 50ml:0ml, 40ml:10ml, 30ml:20ml, 20ml:10ml, 10ml:40ml and 0ml:50ml. This will give me concentrations of 1 molar, 0.8 molar, 0.6 molar, 0.4 molar, 0.2 molar and 0 molar.
I will use two potato segments in each solution and then take an average mass from them. This will give me more accurate results whilst keeping things fair. I will also take 3 readings (as well as at the start) of the mass on three days (at 24 hour intervals) to increase my chance of getting good, solid results. I will select 1 of the day’s results to draw conclusions from. I will record my results (which will be in the form of mass) in a table and then put them on a graph that will allow me to draw easy conclusions and also gives a clearer indication of any emerging patterns.
Method
- 12 potato segments were cut from an ordinary tuber. Using a ruler and scalpel they were measured and cut to a length of 3.5 cm and a width and depth of 1 cm on a ceramic tile.
- 6 sucrose solutions were made up in plastic cups which held approximately 60 ml using two 50 ml syringes (one for sucrose and one for distilled water) to concentrations of 1, 0.8, 0.6, 0.4, 0.2 and 0 molar). There was an equal measurement of 50 ml sucrose solution in each beaker.
- Plastic sheeting was cut into 3x6 cm strips and these were put inside the plastic cups (see diagram). Each side of the sheeting was marked A or B with a waterproof pen. After their individual masses were carefully recorded from a balance, two potato sections were placed in each beaker, one on each side of the divide.
- On that day at midday, the potato sections were taken out of the solutions and weighed on a balance. They were taken out individually so as not to mix up the sections and lose accuracy. The masses were recorded and the percentage change calculated.
- The cylinders were then put back in the same solutions and on the same sides of the divide as before.
- This was repeated twice at midday on the following days.
- I then looked at my results, comparing them with my background information and saw that Tuesday’s readings were most suitable so I used them
Diagram Showing A Birds Eye View Of One Pot
Potato Cylinder A
Plastic Divide
Sucrose Solution
Plastic Cup
Potato Cylinder B
OBSERVATION–An Investigation To Find Out The Concentration Of The Cell Sap Of Potato Tuber Cells
N.B. all results to 2 decimal places
The highlighted (yellow) column shows the results that I will take the averages from to use to analyse and find the concentration of the cell sap. I will work out the percentage change and plot these results on a graph (see table below). Tuesday’s results seem the most reliable as none of the positive or negative signs differ greatly and it therefore shows what must have initially occurred.
ANALYSIS–An Investigation To Find Out The Concentration Of The Cell Sap Of Potato Tuber Cells
The graph on p7 shows the results for the average percentage decreases for the samples from each solution. The strong negative correlation on the graph clearly shows that as the concentration of the sucrose solution increases, the percentage change of the potato segments mass decreases. This evidence supports the hypthesis I made and shows that the potato cells increase mass in solutions with a high water concentration and decrease in mass in solutions with a low water concentration. My scientific knowledge of osmosis which supports my evidence tells me that this is because the concentration gradient is steeper (when the concentration of the sucrose solution is higher) as the plant cells have a much lower concentration of sucrose solution than their surroundings and so there is a greater change in mass. The greater the concentration of water in the external solution the greater the amount of water that enters the cell by osmosis. The smaller the concentration of water in the external solution the greater the amount of water that leaves the cell.
However I need to find out the concentration of the cell sap of potato tuber cells. I know there will be a point where the concentrations of water inside and outside the potato cells are equal (isotonic). At this point there will be no change in the length, volume and mass of the potato, as the net movement of water will be zero so no osmosis has occurred. Looking at my graph I can see that the line crosses 0% at 0.42 molar. As the potato segment has neither gained nor lost mass it is clear that the solution has the same concentration as the cell sap and so the two are isotonic. Therefore we can say that a rough estimation of the concentration of the cell sap of a potato tuber is 0.42 molar.
The gradient of the line of best fit is –35.5 (to 1 dp) and it intercepts the y-axis at 17. By using the equation y=mx+c we can see that the equation of the line is –35.5x+17. This allows us to approximate the percentage change in mass for any given concentration of sucrose solution and so we can suggest that if the potato segment were placed in a sucrose solution of 0.3 molar would have a mass percentage change of approximately 6.35%.
Looking at my graph I can see that there were no really significant or huge anomalies suggesting that my results are reliable. There are no readings, which seem to be out of line, which backs up my opinion that the experiment was carried out accurately and that it was successful. Any anomalous results that were found would probably be down to human error such as recording the wrong reasons, getting the samples mixed up etc.
EVALUATION–An Investigation To Find Out The Concentration Of The Cell Sap Of Potato Tuber Cells
Whilst doing the experiment I did not face too many problems. However, I do feel that my method of separating the two potato sections in each cup was slightly unstable. Next time I would look to a more suitable and secure partition possibly making slits in the material that could slot over the sides of the cups. This would make me even more certain in my results. However I know that my results are reliable as I kept all the other variables (temperature, surface area, surrounding conditions) the same. I am very confident that my results are reliable and correct because they form a very strong line on the graph with no anomalies. I had no anomalies and this was because my experiment was done well and fairly with all variables being kept constant. All samples were exposed to the same conditions so the investigation would be fair. Therefore I wouldn’t need to drastically change my experiment if I repeated the experiment.
I tried to make accurate readings keeping all results to 3 decimal places until a final figure was made. Each solution was measured carefully by measuring cylinders or the syringes that they were put into the beakers by. However changing the units I measured in to milligrams to allow for even more precision could make more accurate readings.
Other ways to look at osmosis would be to see how other factors affected it. I could investigate temperature, surface area or the surrounding concentrations and investigate their influence on the process. This would be relevant as it would widen my knowledge of osmosis.
One experiment I could do would be to see how surface area affects osmosis. To do this I would use potato segments like the ones used for this experiment. I would set up the experiment similarly to this one by putting two tubules in each cup. However the concentration of the sucrose solution would be kept constant and the surface area would change. I would cut the segments with a scalpel measuring carefully with a ruler raising adding on 4cm² each time. The mass would be recorded for each segment and then they would be left in their solutions for a day before the masses of each segment were recorded again. The percentage decrease in mass would be calculated and then the segments left again. From the results I would be able to see how the surface area of a potato tuber affects the rate of osmosis.