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Biology SL
Biology SL
Sample Internal Assessment
Sample Internal Assessment

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Table of content
Research question
Aim
Background information
Hypothesis
Method
Conclusion
Evaluation of method
Instrument uncertainties
Weaknesses and possible improvements

Investigating the Effect of Time on the Plasmolysis of Potatoes

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Table of content

Research question

What causes potatoes to go soft after being stored for a long time?

Aim

The purpose of this experiment is to determine how much water a potato cylinder (4 cm in length) can hold after being exposed to different amounts of time, measured in weeks, in order to better understand how time influences the quantity of water that potatoes lose. Since I believe the amount of water the potato cylinder will absorb by osmosis will be equal to the amount of water lost over time, I will determine this by measuring the change in mass of the cylinder after it has been submerged in water for 1.5 hours.

Background information

Osmosis is the passive movement of water across a cell's membrane that is only partially permeable. Since this transport is passive and not active, no ATP is required for the water to travel through the partially permeable membrane along the concentration gradient. The cytoplasm and plasma membrane are drawn away from the cell wall in plant cells, such as those found in potatoes, as water escapes the tissue of the cell by osmosis. The cell wall, on the other hand, keeps its shape because it is a semi-rigid structure made primarily of cellulose. Plasmolysis is the scientific name for this process of water loss and plasma membrane gathering. Plant cells rely on turgor pressure, which is created when water fills the cytoplasm, for support, as is visible in flowers when they wilt from a lack of water. Additionally, this explains why potatoes soften over time. Water will eventually osmotically exit cells when they are exposed to a hypertonic environment, which means that the cells already contain more water than is present outside. Because of this, potatoes that are left out for a longer period of time will eventually grow turgid because the water will be sucked out of the cells through the partially permeable membrane. The change in mass of the potatoes over time can be used to calculate how much water is lost by the cells through osmosis.

 

Water will migrate up the concentration gradient and into the cell cytoplasm when flaccid or plasmolyzed cells are placed in a hypotonic solution with reduced osmotic pressure. The cell cytoplasm will progressively become turgid again after this movement until an equilibrium point is reached. Because of this, the ability of the potato cells to absorb water when placed in a beaker of water increases as more water is lost from the cells through osmosis. As a result, it is possible to forecast that over time, potatoes will lose bulk as a result of water evaporation through osmosis. However, this water can be reabsorbed when placed in a hypotonic solution.

Hypothesis

My hypothesis is that plasmolysis happens as potatoes age and hence more water is lost from them as they become softer over time. The plasmolyzed potato cells can reabsorb the water that was lost over time and become turgid once more when they are submerged in water by osmosis. As a result of the potato's increased capacity to absorb water, I predict that as the potato ages, its mass will increase. For this reason, I anticipate that the mass of the entire potato will progressively decrease over time while the mass of the potato cylinders will alter when they are submerged in water.

Variables

Dependent variable

Amount of water taken up, which I assume will be the same as the amount of water lost. Measured in:

Change in mass (g) ±0.05g

Independent variable
Storage time of potatoes over a period of 5 weeks, taking a measurement every week.
Controlled Variables

Age of potatoes: all potatoes taken from the same batch, assuming they were harvested at about the same time.

Temperature: I will place the potatoes in an incubator with a set temperature (20°C).

Size of potato cylinder (4cm in length) ±0.1cm

Water solution; by using distilled water.

Light intensity: placing the potatoes in incubators will eliminate alteration through light.

Time of potato cylinders in water (90min) ±0.1min

Uncontrolled·Variables
Biological condition of the potatoes: monitored by taking potatoes of the same age, as well as similar size and mass
Figure 1

Equipment list

  • Potatoes: twenty-five potatoes from the same type and batch, with about the same size and weight.
  • Ruler (mm)
  • Measuring cylinder (20ml)
  • 5 Beakers (500ml)
  • Distilled water
  • Two tiles for cutting
  • Cork borer for cutting potato cylinders
  • Balance (g)
  • Incubator set to 20°C
  • Marker pen
  • Stop-watch (s)

Method

  • Select twenty-five potatoes, with approximately the same size and mass, from the same batch and divide them into five groups. Label them accordingly: potatoesin group 1 with the labels 1a, 1b, 1c, 1d, 1e, potatoes in group 2 with the labels 2a, 2b etc.
  • Weigh each potato and record the mass.
  • Place potatoes in groups 2 to 5 into an incubator and turn the temperature to 25°C.
  • Take the 5 potatoes of group 1 and using a cork borer, cut a cylinder out of the centre of each potato.
  • Place each cylinder onto a tile and label these tiles according to the potato from which the cylinder was cut from. Carefully, using a millimetre ruler to cut each cylinder to a length of 4cm.
  •  Take five 500ml beakers and label them a to e.
  • Fill the beakers with 500ml of distilled water each.
  • Using an mg balance, weigh each cylinder and record the mass in a table.
  • Place each cylinder into one of the prescribed beakers and be careful to start the stopwatch at the same time. 9) After 1.5 hours, take each cylinder out of the beaker and place it on a separate tile, with the prescribed labels a to e.
  • Using a mg scale, weigh the mass again.
  • Record the new results in the table.
  • In intervals of one week, repeat the steps 3 to 10 for the potatoes in each group, so that group two is investigated in week two, group three in week three etc.
  • Make sure to record the mass of the whole potato before cutting the cylinder.
Figure 2 - Table On Data Collected For The Change Of Mass Of Whole Potatoes Over A Period Of 5 Weeks.
Figure 2 - Table On Data Collected For The Change Of Mass Of Whole Potatoes Over A Period Of 5 Weeks.

Since the scale was accurate to the nearest 0.01g, an uncertainty error of ±0.05g must be taken into account when determining the mass of each whole potato.

Figure 3 - Table On Data Collected For The Change Of Mass Of Potato Cylinders, Cut Out Of The Whole Potatoes Measured In Figure 1, When Put Tnto A Beaker Filled With Water For 1.5 Hours.
Figure 3 - Table On Data Collected For The Change Of Mass Of Potato Cylinders, Cut Out Of The Whole Potatoes Measured In Figure 1, When Put Tnto A Beaker Filled With Water For 1.5 Hours.

Since the same scale was used and was precise to the nearest 0.01g, an uncertainty error of 0.05g must be taken into account for the measurements of the entire potatoes.

 

By summing the measurements for each potato or cylinder and dividing the total by the number of measurements taken for each sample, in this case five, the mean average in Figure 1 and 2 was determined.

 

Mean Average \(\frac{al+a2+a3+a4+a5}{5}\)

 

Calculation of Standard Deviation and Relative Percentage Value

Time (weeks)
Mean decrease in mass / (g) ±0.05g
Standard deviation
Relative percentage value (%)
Week 0
0
0
0
Week 1
3.16*
0.98
31.01
Week 2
4.42
0.35
7.92
Week 3
6.78
0.62
9.14
Week 4
8.00
0.26
3.25
Figure 4 - Table On Standard Deviation And Relative Percentage Value For Measurements Taken For The Change Of Mass Of The Whole Potatoes.
Time (weeks)
Mean decrease in mass / (g) ±0.05g
Standard deviation
Relative percentage value (%)
Week 0
0.16
0.015
8.33
Week 1
0.32*
0.075
24.44
Week 2
0.35
0.011
3.14
Week 3
0.47
0.01
2.13
Week 4
0.58
0.027
4.65
Figure 5 - Table On Standard Deviation And Relative Percentage Value For Measurements Taken Forthe Change Of Mass For Each Potato Cylinder.

Excel was used to determine the Standard Deviation, which is listed in Figure 4 and 5. It displays how widely results vary from the mean on average. The relationship between the standard deviation and the mean average is depicted by the relative Percentage value. This percentage is calculated by dividing the standard deviation by the mean average and multiplying the result by 100.

 

Calculation of the Percentage Value for the change of mass.

 

The average mass loss is divided by the average beginning mass measured each week to get the percentage number. It is impossible to compare the increases in mass proportionally since biological factors affect the initial masses of both the whole potatoes and the potato cylinders. However, because the percentage value takes into account the connection between the original mass and the change in mass, it can be seen as a more accurate method of assessment.

Time (weeks)
Mean initial mass / (g) ±0.05g
Mean decrease in mass / (g) ±0.05g
Percentage value for change in mass (%)
Week 0
167.54
0
0
Week 1
189.14
3.16
1.67
Week 2
184
4.42
2.40
Week 3
191.88
6.78
3.53
Week 4
169.6
8.00
4.72
Figure 6 - Table On The Percentage Value For The Mean Change In Mass Of The Whole Potatoes.
Time (weeks)
Mean initial mass / (g) ±0.05g
Mean decrease in mass / (g) ±0.05g
Percentage value for change in mass (%)
Week 0
4.08
0.16
3.92
Week 1
4.18
0.32
7.66
Week 2
4.11
0.35
8.52
Week 3
3.96
0.47
11.87
Week 4
3.99
0.58
14.54
Figure 7 - Table On The Percentage Value For The Mean Change In Mass Of The Potato Cylinders.
Figure 8 - Table On Percentage Value For The Mean Change In Mass Of Whole Potatoes Over A Period Of 5 Weeks.
Figure 8 - Table On Percentage Value For The Mean Change In Mass Of Whole Potatoes Over A Period Of 5 Weeks.
Figure 9 - Percentage Value For The Mean Change In Mass Of Potato Cylinders When Put Into Water For 90 Minutes, Over A Period Of 5 Weeks.
Figure 9 - Percentage Value For The Mean Change In Mass Of Potato Cylinders When Put Into Water For 90 Minutes, Over A Period Of 5 Weeks.

Anomalous results

The data point for the percentage value estimated of both whole potatoes and potato cylinders in the first week does not fit to the general trend and consequently is off the expected line of the graph, as can be seen in figures 8 and 9. Looking back at the initial data gathered for the overall change in mass It is evident that the findings from Potato D in Week 1 do not fit into the typical pattern of results since they are greater. This is true for both potatoes and the cylinders. Tables 4 and 5's standard deviation and relative percentage value amply demonstrate this point.

 

The relative percentage number, which depicts the correlation between the standard deviation and the mean average, allows one to compare the dispersion of the data.Small standard deviations indicate that the values are close together when they are less than 33% of the mean.The relative percentage value for all measurements is lower than 33%, as shown in tables 4 and 5. However, it is apparent that Week 1 calculations yield a higher relative percentage value than estimates for any other week. The data is quite dispersed, as shown in table 4 where a value of 31.01% is just under 33%. The results for the potato cylinders in Week 1 with a percentage value of 23.44% can be stated to be similar.

 

This prompted me to review the initial raw data I had gathered in Week 1 and to take the measurements from Potato D out of the equation. The unusual nature of these data, which should not be taken into account in the calculations, is indicated by the asterisk (*) next to them. The anomalous data from Potato D in week 1 had been omitted in the calculations for the percentage value for the change in mass of the whole potatoes and potato cylinders shown in Tables 10 and 11.

Time (weeks)
Mean initial mass / (g) ±0.05
Mean decrease in mass / (g) ±0.05g
Percentage value for change in mass (%)
Week 1
190.25
2.75
1.45
Figure 10 - Table On The Percentage Value For The Mean Change In Mass Of The Whole Potato In Week 1, Excluding The Anomalous Result {Potato D).
Time (weeks)
Mean initial mass / (g) ±0.05g
Mean increase in mass / (g) ±0.05g
Percentage value for change in mass (%)
Week 1
4.19
0.28
6.68
Figure 11 - Table On The Percentage Value For The Mean Change In Mass Of The Potato Cylinders In Week 1, Excluding The Anomalous Result (Potato D).
Time (weeks)
Mean decrease in mass / (g) ±0.05g
Standard deviation
Relative Percentage Value (%)
Week 1
2.75
0.41
14.91
Figure 12 - Table On Standard Deviation And Relative Percentage Value For Measurements Taken For The Change Of Mass Of The Whole Potatoes In Week 1, Excluding The Anomalous Result (Potato D).
Time (weeks)
Mean decrease in mass / (g) ±0.05g
Standard deviation
Relative Percentage Value (%)
Week 1
0.28
0.022
7.86
Figure 13 - Table On Standard Deviation And Relative Percentage Value For Measurements Taken For The Change Of Mass Of The Potato Cylinders In Week 1, Excluding The Anomalous Result (Potato D).

It is confirmed that Potato D was an unexpected result by the relative percentage values falling from 31.01 to 14.91 for whole potatoes in Week 1 and from 23.44% to 7.89% for cylinders. The newly processed data for Week 1 is much more dependable because it is much closer to the mean.

Figure 14 - Percentage Value For The Mean Change In Mass Of Whole Potatoes, Over A Period Of 5 Weeks, Excluding The Anomalous Result.
Figure 14 - Percentage Value For The Mean Change In Mass Of Whole Potatoes, Over A Period Of 5 Weeks, Excluding The Anomalous Result.
Figure 15 - Percentage Value For The Mean Change In Mass Of The Potato Cylinders When Put Into Water For 90 Minutes, Over A Period Of 5 Weeks, Excluding The Anomalous Result.
Figure 15 - Percentage Value For The Mean Change In Mass Of The Potato Cylinders When Put Into Water For 90 Minutes, Over A Period Of 5 Weeks, Excluding The Anomalous Result.

Conclusion

As shown in Figure 14, the average percentage change in mass of a whole potato over a period of five weeks followed a roughly linear rising trend. Over the course of five weeks, an approximate percentage of 4.72% of the starting weight of 167.54g was lost. This reduction was gradual, as shown on Figure 14.

 

It is evident from Figure 15 that the potato cylinders likewise exhibit a linear rising trend in terms of percentage mass change. However, this graph shows how much weight the potato cylinders added after being submerged in water for 1.5 hours. As can be observed, potatoes left until Week 4 grew 14.54% of their original mass while potatoes examined in Week 0 gained an average of only 3.92%. The progressive increase in percentage change over time suggests that the longer the potatoes stayed in the incubator, the more water the potato cylinders could retain when submerged in a water-filled beaker.

 

It is possible to conclude that as the whole potatoes became lighter, the change in mass of the potato cylinders rose by comparing the data obtained for the change in mass of the whole potatoes to the change in mass of the potato cylinders. The idea of osmosis can be used to explain this. The water progressively passed through the partially permeable membrane of the potato cells along the concentration gradient since the concentration of water in the air was lower than the concentration of water inside the cytoplasm when the potatoes were placed in the incubator. The potato became softer as a result of the cells' progressive flaccidity brought on by the loss of water and turgor pressure. The information gathered supports this because the potato cells' loss of water also resulted in a reduction in mass.

 

The analysis of the potato cylinders' change in mass after being submerged in water for 1.5 hours confirms that the gradual softening of potatoes is caused by the loss of water through osmosis. According to my findings, the potato cylinders' percentage rise in mass increased continuously, indicating that more water was absorbed each week. This shows that because the concentration gradient between the water in the beaker and the inside of the cells was steeper, the potato cells, which were left for a longer amount of time and subsequently lost more water through osmosis, had a greater potential in reabsorbing the water again. The potatoes that were just left for a little period of time, such as those in Week 0, were still turgid; as a result, when they were put into the beaker, less water migrated by osmosis into the cells, leading to an increase of only 3.92%.

 

As previously noted, I deleted an odd result, Potato D in Week 1, during the analysis of the raw data. By calculating the raw data's standard deviation using the relative % value, I was able to see that the data for both the whole potato and the cylinder of Potato D in Week 1 were significantly off the mean. Additionally, it is evident from looking at Figure 8 and 9 that the processed data does not follow the general pattern of the data gathered. Graphs created from the newly processed data after removing the measurements made for this potato show a steady rising trend. This led me to the conclusion that Potato D in Week 1 was unusual because to biological reasons such weakened cell walls.

 

In conclusion, it is clear that potatoes lose water by osmosis and grow lighter and softer by measuring the percentage change in mass of whole potatoes and potato cylinders placed in a beaker of water over a period of five weeks. My hypothesis is supported by the evidence, which shows that water eventually leaves the cytoplasm of potato cells by osmosis, making the cells less rigid and the potato softer.

Evaluation of results

The data gathered during Week 1 was first found to be fairly far from the mean, as shown by the standard deviation, which was calculated for the data for both the whole potatoes and the potato cylinder and recorded in Figure 4 and 5. The measurements of the whole potatoes obtained from Week 1's relative percentage calculation were 31.01% while the measurements of the cylinders were 23.44%. As the other measurements revealed a considerably lesser percentage value, I contemplated going back to the raw data and excluding Potato D as an anomaly, even though these numbers are still below 33%, which is the percentage at which the standard deviation is thought to be substantial. The relative percentage value for the whole potatoes reduced to 14.91% and the relative percentage value for the cylinders decreased to 7.86%, as shown in Figure 12 and 13 , indicating an improvement in reliability.

 

The calculations for the mean were more precise and, consequently, the overall results were more trustworthy after removing the anomalous result and processing the data without Potato D.

Evaluation of method

Controlled variables

Temperature: It was crucial to maintain the temperature at which the potatoes would be stored because a higher temperature would have caused the water to evaporate more quickly, increasing the rate of osmosis through the cell walls and, ultimately, resulting to a greater loss of mass. To achieve this, I put the potatoes in an incubator that was set to 20 °C.

 

Water Solution: Because osmosis depends on water moving along a concentration gradient, different water solutions would have had varying effects on how much water the potato cylinders absorbed. I utilised distilled water to regulate this variable in order to maintain a steady water concentration and improve the accuracy of the results.

 

Light Intensity: Just like temperature, the amount of light the potatoes were exposed to over the course of five weeks would affect how quickly osmosis took place. The intensity of the light would not affect the findings if the potatoes were placed in an incubator that was sealed from light.

Biological uncertainties

Although I chose potatoes that were roughly the same size and weight from the same batch, biological variations made it more challenging to measure precise statistics. Biological variables caused the potatoes' starting weight to vary, despite the fact that I took care to select potatoes of the same size. I made sure to calculate a mean average out of five repeats for each week to prevent this from changing the results. As a result, I was able to identify abnormal results and reject them from the data processing, ensuring that the data I collected was accurate. Through the use of a relative percentage value to calculate the standard deviation of the mean, I was able to pinpoint an unexpected outcome that was brought on by biological uncertainties like weakened cell walls. As a result, even if there were biological differences, I was able to spot erroneous data and discard it because it would have drastically altered the investigation's outcome. As a result, the results derived from the data that had been collected remained accurate.

Instrument uncertainties

Mass of potatoes and potato cylinders

I used the same scale, which measures to the nearest 0.01g, to determine the mass of the potatoes and potato cylinders. Because of this, a 0.05g uncertainty must be taken into account.

Size of potato cylinders

I cut the potato cylinders out of the whole potatoes using a cork borer to maintain a constant diameter. I measured the length with a ruler that was accurate to 0.0lcm. A total uncertainty of 0.1 cm must be taken into account because there is an uncertainty of 0.05 cm at both the cylinder's beginning and finish.

Time of potato cylinders in water

I used a stopwatch that was accurate to within 0.01 minutes, which resulted in an uncertainty of 0.05 minutes for both the stopped and began times. resulting in a ±0.1-minute total uncertainty.

 

Overall, the impact of these instrument uncertainties on the investigation's overall findings is negligible. The measurements are not significantly affected because the time it took to submerge the potato cylinders in water was long 90 minutes compared to the uncertainty of ±0.1 minutes. I also made sure the diameter of the potato cylinders remained constant by using a cork borer. When considering the uncertainty for the mass of the potato cylinders, it is evident that an uncertainty of ±0.05g does indicate that a different scale would have been more appropriate for a more accurate measurement for values between 3.85g and 4.25g. The standard deviation nevertheless suggests that the measurements were accurate, as seen in Figure 5 and later Figure 10.

Weaknesses and possible improvements

I had originally planned to cut the potatoes into cubes rather than cylinders for my research. However, when I was preparing, I discovered that it was challenging to cut cubes precisely 2 cm3 in size. I made the decision to use a cork borer to cut cylinders out of the potato rather than varying the size of the potato cubes, which would have affected the accuracy of the data I had obtained. The potato cylinders were then all the same diameter as a result, and just the length needed to be reduced to 4 cm.

 

The dependability of my mean average was constrained because I only collected data from five repetitions with five distinct potatoes each week. I could have removed more cylinders from each potato and compared the changes in mass to boost the validity of my data. Additionally, I could have collected measurements at various intervals throughout the week to add more data points to the graphs and create a better best-fit line. This would have revealed a graph's trend in a more trustworthy manner.

 

The number of repeats taken for Week 1 was decreased to 4 as I eliminated one abnormal result from the analysed data. If more repeats had been taken, this would also have been more dependable. I should perform more repeats using more potatoes to improve the investigation since I should be conscious of biological uncertainties and potential anomalous results.

 

The uncertainty was the same, 0.05g, when the mass of the cylinders and the whole potatoes were measured on the same scale. Because they had a bigger mass, this did have a significant impact on the accuracy of the mass of the entire potatoes. But since the potato cylinders were lighter and had a lesser mass, a milligramme (mg) scale would have provided more accurate results.

 

The arrangement of the cells within the potato may have had an impact on how much water was lost by the cells through osmosis. Because the cells in the centre of the potato are not subjected to the same degree of concentration gradient, they may have lost less water and hence had a smaller capacity to reabsorb it than the cells on the potato's outermost layer. Even though I always cut the potato cylinders out of the centre of the whole potato, which allowed me to control this factor, I could have improved my experiment by comparing the increase in mass of cylinders taken from the potato's edge to those taken from its centre. This would have more clearly illustrated how water escapes from potatoes and perhaps shown how, over time, potatoes gradually soften from the outside to the interior.