Apparatus
1 x Potato 1 x Scales 1 x Scalpel 1 x Stopwatch
18 x Test Tubes 1 x Ruler 1 x Pipette (10cm3) 1 x Cork Cutter (4mm)
Sucrose Solution 1 x Tile 1 x Tweezers
Variables
I will need to keep all the variables constant apart from the variable that is under investigation, the concentration of the sucrose solution. I will need to keep all the variables as constant as possible to make my experiment as fair as possible and to thus to make my results more valid and my data reliable.
I will need to keep the temperature constant by simply using solutions at room temperature and by not heating them. I need to keep this variable constant otherwise an increase in temperature will cause an increase in the kinetic energy of the water molecules. This will cause osmosis to take place at a faster rate as the molecules are able to diffuse faster, this will lead to a larger weight gain/loss over the 90 minutes.
The size of the concentration gradient also affects the rate of osmosis. I will have to make sure that I use the right concentration by labeling the test tubes, being conscientious when filling up the test tubes with the solution and also washing the pipette after each solution to avoid contamination of the solutions. The concentration gradient can also be controlled by cutting potato chips from the same potato so that the concentration of the sugar and salt in the cell vacuole is roughly the same.
I also need to keep pressure in each experiment constant. A higher pressure will create a greater water potential speeding up the rate of osmosis and creating the same consequences as an increase in the temperature. This can be controlled by conducting the experiment in the same room and conditions, thus keeping pressure constant.
I will have to try and keep the surface area and size of the potatoes the same, because by increasing the surface area the rate of osmosis will also increase. This happens because osmosis can occur in a greater number of cells causing the potato to gain/lose weight faster. This variable can be controlled to an extent by using the same cork cutter.
The amount of solution will need to be kept constant by using a pipette. The amount of solution needs to be kept constant as a variation would cause a variation in the concentration gradients and how long it takes for osmosis to reach an equilibrium, this would affect the rate of osmosis and the overall weight change. The rate of osmosis is affected due to the variation in size of the concentration gradient, the overall weight change is affected by the amount of water molecules that have to diffuse for the solutions to reach equilibrium.
I will need to keep the time the potatoes spend in the sucrose solution constant, otherwise osmosis will take place over a longer/shorter period of time leading to a weight loss that is inaccurate. I will use a timer to control this variable.
I will also need to cut all the potato skins off the potatoes because these are dead cells where osmosis will not occur therefore a smaller weight change than possible will occur. This can be easily controlled by cutting them off with a scalpel.
Safety
I will need to ensure that my experiment is safe, I can do this by taking care when I use the scalpel when cutting the potato, care will also be taken how I hold the potato when using the cork cutter in case it goes straight through the potato. I will also take care with the glassware that I use e.g. test tubes so that breakages will not occur. This can also be avoided by being as organised as possible and only having the necessary equipment on my desk at one time. Lab coats will be worn to avoid spillage of fluids and especially the sucrose solution, as this is very sticky on to my clothes. I shall also be careful and aware of other people’s actions and experiments.
Analysis
From my table of results and the graph that I have drawn from my results I can conclude that my prediction was correct. The potato chip did in fact gain weight when immersed in a distilled solution and it also lost weight when immersed in a solution of sucrose as stated in my prediction. These results can be easily explained by having an understanding of osmosis.
The potato chip gained weight when immersed in distilled water because water molecules diffused into the chip by osmosis across the cell membrane thus causing a weight gain. The water diffused across the cell membrane because there was a higher concentration of sucrose and salt molecules (i.e. lower concentration of water) inside the cell than the water outside the cell creating a concentration gradient. This caused the water molecules to move into the cell to equal out the concentration of water and to reach equilibrium. This intake of water would have caused the cell vacuoles to expand in size exerting pressure on the cell membrane and consequently making it stretch until it reached the cell wall and became fully turgid. This would have caused an increase in the size of the chip however I did not measure the potatoes’ length at the end of the experiment and so this cannot be proven.
The potato chips lost weight when immersed in a sucrose solution because water molecules diffused out of the cells across the cell membrane by osmosis thus causing a weight loss. The water molecules diffused out of the cell because there was a higher concentration of sucrose molecules (i.e. lower concentration of water) outside of the cell than inside the cell creating a concentration gradient. This caused the water molecules to move out of the cell and into the surrounding sucrose solution to make the concentration of water outside the cell the same as the concentration of water inside the cell thus achieving equilibrium. The loss of water would have resulted in the cell vacuole shrinking in size and therefore it would exert a smaller pressure on the cell membrane causing it to shrink and retract away from the cell wall, this is known as plasmolysis. This should have resulted in the chip decreasing in length however I did not measure the potatoes’ length at the end of the experiment and so this cannot be proven.
My graph, shows the average percentage weight change at different concentrations. It shows a curve, being steepest at lower concentrations and slowly reducing the gradient of its curve as the concentration of the solution increases. The difference in gradients can be shown by how there is a 4% weight change in the potato between 80% and 100% sucrose solution and yet between 0% and 20% concentration there is a 20% change in weight. The curve also goes into negative due to the effects of leaving a potato in a sucrose solution.
From my graph I can conclude that at 8% concentration of sucrose solution the cell and solution would be at equilibrium resulting in no weight loss. I can also conclude that the concentration of salts and sugar molecules in the cell is 8%.
I can try to offer an explanation for the graph showing a curve. The curve may be less steep at higher concentrations due to the rate of osmosis at these concentrations is already near its optimal rate due to the limiting factor of the kinetic energy and therefore the speed of the water molecules. Also the effect of a 20% increase in concentration at high concentrations is only a small percentage increase. Therefore an increase in the concentration gradient would not cause that bigger difference. However at lower concentrations a small increase in concentration and concentration gradient would affect the speed and rate of osmosis greater. This is due to a 20% increase in concentration, in percentage terms being much greater than at high concentrations thus causing a greater increase in concentration gradient than a 20% increase at higher concentrations would. Also at lower concentrations the concentration gradient is not very steep and so osmosis is not taking part at a fast rate therefore the limiting factor of the rate of osmosis being the kinetic energy of water does not play a big part.
From my graph I can also deduce that there is a maximum weight loss a cell can lose this is because the curve gets less steep and so it would eventually go straight meaning no more weight loss could take place. However this can’t be seen on my graph for a number of reasons and this is because I might not have left the chips inside the sucrose solution for long enough for this to happen. Another reason might be that there was not enough sucrose solution for osmosis to take place until all the cell’s water content had gone. From the line getting less steep and that we know it will end up straight I can also deduce that the total weight loss of the cell is a long way off 100% weight loss. This is for the quite simple reason that osmosis is the movement of water molecules only therefore the maximum weight loss is once all the water has been removed from the cell. This will still leave structures such as the cell wall and membrane which will still contribute to the weight of the cell thus the cell can’t lose all of its weight through osmosis.
Evaluation
I am pleased with the procedure that I used to conduct my experiment because I think it produced a set of reliable results without any anomalous results. I think my results despite lacking in accuracy were still good enough to provide me with results from which I was able to draw a graph, which produced a near perfect curve. This curve allowed me to draw conclusions from it and enabled me to offer explanations for it. My results also proved my prediction in that the potato did lose and increase its weight under certain conditions. I think my experiment was a good one for the type of results I was acquiring. The level of accuracy in my opinion was accurate enough for a GCSE practical. Under these points I believe my experiment was accurate enough to give me a set of results from which I could draw conclusions. However despite this, my experiment still lacked accuracy in its procedure and measurement taking, also it was unable to measure certain variables from which I had to assume the variable was constant. An example of this would be the water temperature and water pressure. Both of these had to be assumed they were constant throughout. Also I think that the weight of my potato was a source for inaccuracy. This is because at 4cm the potatoes were very light at just 0.3 grams. This caused a change in mass between each concentration gradient to be very small and therefore the percentage mass change would have been less accurate due to the lack of variation the mass of the potato could have been. This could have been overcome by using larger potato chips, using a more sensitive balance or instead of taking average mass taking the total mass of the three chips. In this experiment there was also human error which was brought in, when measuring out the amount of sucrose solution. Although I used a pipette to improve the accuracy of my results there would have been a slight error in me measuring the right amount of solution. Also after each experiment I washed out the pipette to avoid contamination, this itself may have caused contamination by leaving some water droplets in the pipette having the effect of diluting the solution. There was also human error in me cutting the potatoes to the right length this involved me cutting them all to 4cm however there probably would have been a slight variation in their lengths. There was also an error in the weighing of the potato after the experiment because of excess sucrose solution on the potato after being withdrawn from the solution. I tried to eliminate this by leaving the chip on some paper towel however this may have taken too much water off the potato thus affecting the mass. I was also lucky enough not to get any anomalous results I think I can put that down to the accuracy of the procedure and also the use of using repeats. This would prove the results were more reliable and anomalous results would become more apparent. There was also a time delay in some potatoes coming out of the solutions, therefore some would come spend a longer time in the solution than they were meant to. This would have resulted in a larger weight change. This could have been avoided by starting the experiment for different concentrations at different times. However this would not have been practical due to time limitations. If I were to perform this experiment again I would make a few changes and improvements to improve my results and also to increase my investigation. To increase and widen my investigation I would use a different variable or increase to the measurements I took. For example I would have liked to measure the length of the chip after the experiment to see if an increase in size or plasmolysis had occurred. I would also use a different variable for example changing the water temperature this would increase the rate of osmosis and therefore the speed of the water molecules. I would also try to increase the accuracy and reliability of my results by investigating more than one potato, taking more repeats. I would also start my experiments at different times to stop there being a rush at the end. I would also use larger potatoes so that a larger variation in the change in weight could be obtained.
