-Temperature
-Water potential of potato initially
-Size of potato
-Mass of potato
-Volume of solution potato is in
-Type of potato
-Time left in solution
-Surface area of potato
-Using same balance to measure potato
-Light intensity
-Potato cut at the same time, and left the same time
If I am going to conduct a fair test then I will have to keep these factors the same, this is so that I can achieve reliable results to test out my hypothesis. The temperature will be at a constant room temperature, this is so that it won’t affect the rate of reaction, as a higher the temperature the object is, the molecules inside it are vibrating fast, so is the fact that at a hotter temperature osmosis will happen faster, than at freezing temperatures, so keeping this factor the same will mean then that, the only factor that we are changing will be the sucrose solution’s concentration which is affecting osmosis, which is what we are searching for. The initial water potential of the potato will be kept the same by using the same type of potato, cut at the same time, and left the same time, this is so that these factors are the same for all the experiments, so that changes in the reading will be due to the changes to the sucrose concentration instead of anything else. The size of the potato will be of an uniform shape and size, as so to provide the same surface area, I will cut the potato pieces into 3cm lengths (as it is same thickness already), this is so that the rate of reaction, will not be affected by the amount of surface an reaction is taking place, as the bigger the surface area, the more reactions it will have on average than an a smaller surface area, this again ties in with the fact that we are searching for the changes due to the concentration differences in sucrose, and not the surface area.. The light intensity will be kept the same by having the same light intensity, and no extra light shone on the experiment, nor will any part of it be in the dark, although even though it won’t change the experiment a great deal, we will have to take careful precautions none the less so that we can be sure opf the experiment. The volume of the solution will have to be the same, but will have to cover the potato completely, as otherwise if the volume is different for each of the experiment, even though the concentrations are the same, we won’t be able to discover the cause is due to osmosis or due to the different volume of solution. I will see how many ml of solution I will need in the preliminary tests.
SCIENTIFIC KNOWLEDGE
In a high concentration of water, the amount of solute (in this case sucrose) is low. This could be called a weak or dilute solution. In a low concentration of water, the amount of solute is high. This could be called a strong or concentrated solution. When a semi-permeable membrane divides two such solutions, the water molecules will move from the area of high concentration to the area of low concentration, until both sides have reached equilibrium.
Knowing that osmosis will occur between a semi-permeable membrane whenever there is a difference between the water concentrations on the two sides of the membrane, and when knowing that this happens to cells they will either become turgid if water flows into them, or plasmolysed if water flows out of it, and thus change their mass.
Knowing this we can set an experiment, so that if the concentration of a solution into which a cylinder of potato is placed is greater than a certain level the cylinder will contract, and if the concentration is less then that level, it will expand. This can be shown using pure water (high water molecule concentration), the potato cell in the dilute solution will swell and increase its mass and volume/length. If places in a highly concentrated solution of sucrose, the opposite would happen and the potato will decrease in volume/length and mass.
I predict that 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 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 the water molecules will be zero, therefore no osmosis has occurred.
Using this information, a graph can be made:
At this point, it suggests that no osmosis occurs, therefore suggesting that the concentration of water inside the cell is equal to the solution outside the cell.
PRELIMINARY EXPERIMENTS
In the preliminary experiment, I used:
-Test tubes
-Potato cylinders-3cm
-Sucrose solution-bench solution 1mol
-Scalpel
-Balance-2d.p. (sufficient as other measurements won’t be this accurate)
-Test tube rack
-Syringe-20ml (sufficient as only needs maximum of 5 ml solutioin)
METHOD OF PRELIMINARY EXPERIMENT
I drew up a table of what percentage concentrated solutions I needed to measure and found that since 10-15 minutes is the average time needed for the experiment to work, I decided on 5 readings:
-0%
-20%
-40%
-60%
-80%
-100%
The percentage refers to the percentage of sucrose solution needed to be mixed with the respective amount of water, e.g. for 20% sucrose solution, you need 4ml water and 1ml sucrose.
Example of one reading; 60%: I took a cylinder of potato (already cylinderised and put in glass jar in some of its own juice-as this will be already at an equilibrium-as same concentrations of sugars and water in the potato juice), length measured to 3cm (cut using a scalpel if necessary on the white board), put it in a test tube, on the test tube rack. Took one syringe and measured out 2ml of water and took another clean one and measured out 3ml of sucrose solution, squirted into a beaker-I HAD TO MIX THE SOLUTION FIRST TO MAKE SURE THAT THEY WERE EQUALLY SPREAD OUT in a beaker (as otherwise I had to wait a long time for diffusion to happen), or else the solution won’t be as the calculated concentration, as one part of the solution might have more solute than another part.
Same with the sucrose solution-I mixed the solution before I took some out, as there might be some molecules that haven’t fully dissolved in the solution and sink to the bottom, therefore meaning a higher number of solute per part, than the whole average amount. I poured the beaker’s solution into the test tube and timed it for 10 minutes (I decided on 10 minutes as it was enough so that I could do other experiments while the experiment would be sufficiently accurate with the results). After the 10 minutes was up, I poured the test tube’s content’s onto a filter funnel. I then took the potato tube with care, washed it under running water and rolled it around a paper towel to dry off the excess water and sucrose solution, and then I measured its mass. This is so that I can get rid of the heavier than water sucrose solution on the potato tube (but not in it) which will affect the mass-as the added on heavy sucrose solution will not be the true mass of the potato tubes and then consequently the results. But the water in the potato will be already in the potato membrane, and can’t get out, so therefore the results will be more accurate this way.
For my preliminary experiment however I only got time to do 4 readings. This is summed up in a table.
Result’s table:
This shows that the isotonic level is at some point just above 40% sucrose solution.
I would need to repeat my experiment quite a few times to get reliable results, and to average out some inaccuracies that might have happened. Also I will need to use new dry test tubes after each experiment as to prevent contamination. Also the mixing beaker will be washed using water and dried using paper towels for the same reason, as the contamination, will affect the results, which would then display low accuracy and no repeat values.
SAFETY
I will have to wear goggles to prevent the sucrose liquid getting into my eyes, take care with the sharp scalpel, and to wear aprons to prevent splashes getting on my uniform.
Preliminary work-shapes my understanding
The preliminary work let me decided on some crucial factors:
-Time for experiment: 10 minutes
-Amount of solution: 5ml
-Length of potato: 3cm-make sure no skin is on the potato, as it will block the osmosis happening on that part of the potato, therefore smaller surface area.
-Dry potato after experiment to get rid of excess water-see reasons for this actions above
- Mixing beaker will be washed using water and dried using paper towels-see reasons for this actions above
-Concentrations of sucrose to be used:
-0%
-20%
-40%
-60%
-80%
-100%