GCSE Biology What effects the rate of Osmosis

Prediction

        I predict that as the concentration of the solution increases, the mass of the chip will decrease and therefore the chip will eventually become more plasmolysed.

        I believe the mass will decrease with the concentration increasing because osmosis occurs as water molecules move from a hypotonic solution to a hypertonic solution. The hypertonic solution would be in the solution around the chip and so water will move out of the selectively permeable membrane of the chip and to the solution with an increased concentration.

Risk assessment

        I risk injury while cutting the chips into smaller pieces for making them suitable for the investigation. I will try to minimise the risk by cutting as far away as possible from our fingers and by using a chopping board. I will also obviously wear goggles and a lab coat to protect myself and my clothes from the solution and any juices that may emerge from the chips while slicing them. I will wear these throughout the experiment.  

Equipment

• 3 Large potatoes
• Knife – for cutting potatoes
• Ceramic work surface/ mat /Chopping board
• Pencil and Ruler
• 6 Petri dishes with lids
• Digital balance – with readings to 3.d.p
• 6 solutions – with different concentrations
• Measuring cylinder
• Stopwatch

Diagram

Method

• Set up equipment as in diagram
• Cut out 18 chips from potatoes
• Weigh a chip and then record as original mass
• Place chip in the solution
• Leave the chip in the solution for 3 minutes
• Take out chip and record new mass
• Repeat 2 more times in the same solution
• Change solution
• Repeat method with all other chips.

Preliminary experiment

        I will do a preliminary experiment to consider whether any aspects of the investigation need to be changed. This will include how long each chip can stay in the solution and more importantly whether I have suitable time to complete the experiment. I need to verify how many times we can repeat the experiment to get accurate results and also whether the ranges for the variables are suitable for the whole of the investigation.

Preliminary experiment results

        My preliminary results seem to outline that a concentration of 0.00 Molars (distilled water) increases the mass of the chip, whereas with a concentration of 1.50 Molars, the mass of the chip decreases. I followed my original diagram and method although found many limitations to them both.    

After the preliminary experiment

         After the preliminary experiment I realised that some changes needed to be made. I found that a chip needed to remain in the solution for at least 10 minutes for effective results. If this was to occur then I would need more time to complete the experiment. Therefore I have decided to change the apparatus as shown in the improved diagram; I will now use a large beaker instead of a Petri dish, this will allow us to take 3 readings at the same if I attach 3 chips to cocktail sticks and place this in the solution, this would give me more accurate results as repeats would be done at the same time. A suitable chip size is 50mm by 5mm by 5mm. I have also decided to use an extra input variable which is 0.00M (pure water) and to have an extra chip as the control. Finally, I now understand that I no longer have to cut the chips as this will be done for us before the lesson. Although I may want to finish off the cutting as I presume that all the chips will not be cut to equal dimensions.  

Equipment needed                 

• Potato chips
• Digital scales
• Stopwatch
• Tweezers
• Paper towel
• An extra independent variable reading (pure water)
• Measuring beaker
• Cocktail sticks
• Knife
• Chopping board

Equipment no longer needed

• Petri dish
• Measuring cylinder

Improved diagram

Revised method

• Set up equipment as in diagram
• Weigh 3 chips and record as original mass.
• Attach same 3 chips of equal size onto 2 cocktail sticks so repeats would be done at the same time; this is also shown in diagram.
• Place in solution.
• Leave chips in solution for 10 minutes.
• Take out chips with tweezers and dry using paper towels.
• Record new mass of chips.
• Change solution.
• Repeat method for next 3 chips.

Results table

Results in line graph presenting percentage (%) change in mass

Analysis

        I have used a wide range of concentrations ranging from 0.00 Molars (distilled water) to 1.50 Molars. In relation to my results table, the highest value (increase in mass) was 0.10g, with the lowest (decrease in mass) being -0.42g.

         My results tell me that the mass decreases when the concentration of salt in the water increases. This is shown in my results table as with distilled water; the percentage change in mass is 4.098 whereas with 1.50 Molars, the percentage change in mass is -16.960.

        We can see specifically looking at the line graph that the line goes rapidly and reasonably consistent down to 0.75 Molars with it decreasing as the X axis increases, the line then curves and moves off to 1.50 Molars; where the experiment ended. At 0.00 Molars, the change in mass is not at 0% but at 4.908%, with the information that the line does not pass through the origin and knowing that the line is not entirely straight; we can deduce that the concentration of salt and the mass of the potato are not proportional to each other. Although we do see a distinct pattern on the graph between the 2 variables. As the independent variable (concentration of solution) increases, the dependent variable (mass of chip) decreases. I can also conclude that the rate of osmosis was high up to 0.75 Molars where it then rapidly slowed down and would seem to eventually stop, or continue to slow down onto an inadequate and unnecessary rate.

        The gradient remains reasonably constant up to 0.75 Molars; where it rapidly decreases. It then decreases until the gradient is near to a 0% representation in mass change. This makes the line essentially parallel to the X axis. The line may be in this form because at the point where the gradient becomes close to 0, the chip must have become fully plasmolysed as at the lower concentration the chip had a constant mass.

        If we look carefully at my line graph with the inclusion of the line of best fit, you can clearly see the consistent pattern. As the concentration increases on the x- axis you can clearly see the percentage mass decreases at the same time as shown by the y-axis. This proves that my initial prediction is correct. “As the concentration of the solution increases, the mass of the chip will decrease”.

As earlier stated, osmosis is the diffusion of water molecules from a hypotonic solution to a hypertonic solution through a selectively permeable membrane. This is also typical with this experiment, as the hypotonic solution is in the chip and the hypertonic solution is in the actual solution. The chip increased in mass with distilled water proving that at this point, the Molar concentration in the chip was greater than in the solution. The isotonic point must be at 0.20 Molars according to the graph. As this is where the change in mass is at 0%. The mass continues to decrease showing that the Molar concentration in the solution is now greater than in the chip. Therefore the chip continues to become plasmolysed until the point where plasmolysis cannot occur anymore. This reduced speed of osmosis occurs at 0.75 Molars on continues until there is a stop at 1.25 M up to 1.50 M as indicated by the graph. By this point the change in mass is diminutive considering the potato chip has become fully plasmolysed.

        The results undoubtedly agree with my original prediction made before any results were taken which stated: “As the concentration of the solution increases, the mass of the chip will decrease.”

        My results prove this theory correct although the later part of my prediction needed to be questioned. The later part of my hypothesis stated that: “The chip would become more plasmolysed.”

        Theoretically this is correct although I wasn’t able to fully test this prediction as the change in mass was so minuscule; therefore it seemed that the chip was turgid throughout the whole investigation although really the chip had become more or less plasmolysed, maybe having larger sample pieces or having left the potato chips for an increased amount of time would have tested this prediction further.  

        I believe that my results are reliable considering that all the points are close, if not on the line of best fit. On the results table, we see that all of the readings are fairly close to the average. Although there was one anomalous reading found during the experiment recorded on the table, whereas this didn’t lead to any changes on the actual graph, the potential reasons for having got this anomalous result are explained in the evaluation.

Evaluation

        I believe the experiment was fundamentally easy to accomplish considering the limitations to equipment and more importantly time. It would always be useful to have more time to carry out experiments so that extra readings could be taken thus increasing the reliability of my results. Although for this experiment time wasn’t really a problem as all the results were taken before the end of the lesson. There was one problem we encountered which was to cut the potato chips so they all had an equal surface area to volume ratio. Maybe if this experiment was done at a later date, I would have had a machine to cut the potato chips as because of this slight inequality, the rate of osmosis may have changed and therefore the results may have been slightly altered.

        There were quite a few elements of my original plan that needed to be revised; after completing the preliminary experiment I created a revised method and an improved diagram (shown in the plan). I found that the potato chip needed to remain in the solution for 10 minutes instead of the earlier stated 3 minutes for really effective results. If this change was to occur than I would either need to insert the chips into the solution simultaneously or I would need more time to complete the experiment. As an increased time would be unavailable and there wouldn’t be enough beakers, I have decided to change the apparatus as shown in the improved diagram; I will now use a large beaker instead of a test tube, this will allow me to take 3 readings at the same time if we attach 3 chips to cocktail sticks and place this in the solution, this would also give me more accurate results as repeats would be done at the same time. A suitable chip size is 50mm by 5mm by 5mm. We have also finally decided to use an extra value for the input variable which is 0.00M (pure water). I had decided not to use an extra chip for a control as it would overall prove useless to the experiment.

I believe I did take a sufficient amount of results in the short period we had to complete the experiment. Looking specifically at the graph I didn’t seem to see any missing gaps which therefore also leads to the range being suitable for the experiment. Although there is one potential gap which would probably be at the end, possibly, it would have been useful to have had another value for the input variable (1.75M). If it was possible to attain this concentration, my prediction would have been proved right even further (the theory complete plasmolysis).

I believe the results were accurate as they were all situated near, if not on the line of best fit. The results were also accurate taking into consideration that the concentration was to 2.d.p and my percentage increase/ decrease was to 3.d.p. Although initially one fault in the accuracy would have to be that the mass was only measured on the digital scales to 2.d.p and I believe that it needed to be more precise. Although this couldn’t be changed as the only scales we had available were to 3.d.p. The accuracy was also increased as we had one large measuring cylinder to decrease the chance of taking the wrong volume of the solution.

        There was one anomalous result reported in my results table, although as this was one of the readings taken before the average, my graph had no anomalous points. There are a variety of reasons for why I had an anomalous reading:

• The potato chip was not cut accurately.
• Whilst drying the potatoes, more moisture could have been taken from one chip than the other as the only material we had to dry the surface of the chip was a paper towel, we used them efficiently though may have left one chip longer on the paper towel than another. Inversely, maybe water was absorbed from the chips while taking them out of their beakers.
• Something may have occurred to the solution such as a defect or a slight miscalculation of the volume of a solution.
• Human error and limitation to resources.
• The chips have originally had minor differences in characteristics such as one may have had a greater density than another or possibly the structure of the cell membrane of the chip may have been inimitable to some extent.
• Having to have used the same measuring cylinder to measure the solutions accurately due to the limitation of equipment.
• Use of cocktail sticks was used and could not be changed, as I did not have enough time to repeat the experiment. This changed the surface area of the potato chips.

        The results were reliable apart from that one anomalous result as all other readings were very close to their respective average. Therefore I believe it wasn’t really essential to have taken more readings for the same result although I do agree that this would slightly increase the reliability of my results. The results were consistent with each other apart from the anomaly which wasn’t ignored although didn’t disrupt the structure of the graph. If I did want to further increase the reliability of my results, I could have obviously taken more readings to find more averages; this would mean needing more time. It would also be extremely useful to have more group members so there would be awareness of what is actually happening to the chip and more attention to specific concentrations. I furthermore had to rely on one large measuring cylinder which meant we had to rinse and dry the measuring cylinder after each reading was taken with it. It would be useful next time to have the freedom of having more measuring cylinders. Having previously cut the potato chips out or to have a machine cut the potatoes out more accurately would’ve definitely increased the consistency. Even leaving the potato chips in the solution even longer would have increased the reliability of our results. Finally it would be beneficial to have an alternative method of drying the chip so to minimise the risk of absorbing water from the chip or leaving water on the surface of the chip.

        If I wanted to extend my investigation there are a number of elements I could change such as using temperature as a variable that would be changed to see how hot or cold concentrations affect the rate of osmosis. Or we could use the size (surface area to volume

ratio) as a changeable variable to see how that affects osmosis. One other factor that would be interesting to test would be how a different substance that is soluble to the solution affects the rate of osmosis, we would presume that it would be extremely similar to that of the experiment held in this investigation, although there may be other factors involved.    

        The experiment overall was extremely successful in that the practical was completed in time and I had acquired reliable and accurate results which is proved on my line graph. I took all the necessary precautions outlined in the risk assessment of my plan. Overall I can again succeeding outline that through the experiment I have accomplished my aim and fulfilled the entire investigation.