Biology IA - Osmolarity of Potato Cells PDF

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IB Biology practical on Osmolarity of Potato Cells YEAR 1...

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Hayley Yeung 12E – Biology: Osmosis Practical

Practical #1: Estimation of Osmolarity in Plant Tissue via the Change in Mass of Potato Cylinders Submerged in a Range of Concentrations of Sodium Chloride Solution Hayley Yeung

1.Research Question How can we estimate the osmolarity of potato cells by submerging them in NaCl solutions of various concentrations and calculating the percentage change in mass?

2.Introduction The transportation of materials in and out of cells is essential for all living cells and can be done in two ways: active transport or passive transport. Passive transport occurs spontaneously due to the inherent Kinetic energy all cells have. Osmosis is one type of passive transport and is the diffusion of water through a selectively permeable membrane from an area of high concentration to an area of lower concentration. Osmosis is different from diffusion as only small molecules such as water molecules are able to diffuse through the selectively permeable membrane while larger molecules such as proteins are unable to do so. Under this idea, osmolarity is the measure of solute concentration, as defined by the number of solute particles per litre solution. In this case, how much NaCl is dissolved in one litre of filtered water. In this experiment, we are able to show the process of osmosis and calculate the osmolarity of a potato since potato cells and plant cells have selectively permeable membranes. In general, plant cells have a higher solute concentration than distilled water (Osmolarity = 0), making distilled water a hypotonic solution with a higher concentration of water than potato cells. This then cases water molecules to move into the cell via osmosis, making the cell turgid and causing and increase in mass. However, the reverse can be true with hypertonic solutions where there is a higher concentration of water inside the cell, causing water to move of out the cell via osmosis making the cell flaccid and decrease in mass. By submerging potato cells into Sodium Chloride solutions of various concentration, we are hoping to find the isotonic point where the water concentration is the same inside and outside of the cell causing no net movement of water and no change in mass. The concentration of the isotonic solution would then give us the estimated osmolarity of potato cells. Hypothesis:  A potato cell is a plant cell meaning that in its normal state it would be turgid in an hypotonic solution. However, the cell wall prevents the cell from bursting while the pressure inside the cell rises until its internal pressure is equal to the outside preventing any further net intake of water and cell lysis. Since potatoes are a root it has higher starch content that normal plant cells. This means that potato cells will not have an osmolarity of 0 mol/dm-3 but will have an osmolarity above it that depends on the amount of starch in its cytoplasm. The concentration for the isotonic point will give us the estimated osmolarity of potato cells which I hypothesize to be between 0.2 and 0.4 mol/dm-3. Variables:  Independent variable: The Concentration of Sodium Chloride Solution  Dependent variable: The percentage change in mass o Formula used to calculate the dependent variable:

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Hayley Yeung 12E – Biology: Osmosis Practical

Controlled Variables

How it is kept controlled

Type of Solution

The same Sodium Chloride solution was used throughout the experiment, only diluted at different molarities for our independent variable

Volume of NaCl solution

A measuring cylinder was used to measure the volume of 1M NaCl solution required to dilute it to each given concentration. The solution was then transferred to a volumetric flask and filled to the line so each concentration had 50cm3 of solution.

Immersion of the potato cylinder

All potato cylinders were put into aluminium containers of the same size and volume and were all fully immersed in the solutions.

Time of immersion in the solution

All potato cylinders were put the NaCl Solutions at the same time and taken out 2 days later at the same time.

Temperature of the environment

The aluminium plates used were stored in an air-conditioned room that remained at a constant temperature throughout the experiment.

Diameter of potato cylinder

Each potato cylinder is bored out of a potato with a cork borer of the same diameter ensuring that each potato cylinder the same surface area to volume ratio.

3. Data Collection and Processing Table #1: Raw Quantitative and Qualitative Data Table: Initial Mass / g Concentration of NaCl Solution / mol dm-3

Final Mass / g

Trial 1

Trial 2

Trial 3

Qualitative / Description of Potato Cylinders

Trial 1

Trial 2

Trial 3

Qualitative / Description of Potato Cylinders

0.0

2.7

2.07 2.92

Firm + Rigid with Smooth outer surface

2.46 2.33 2.54

Still relatively firm + rigid w/ smooth outer surface

0.1

2.73 2.48 2.36

Firm + Rigid with Smooth outer surface

2.53 2.37 2.02

Slightly less rigid than initial observation

0.2

2.17 2.07 1.95

Potato is firm

2.08 1.90 1.88

Slightly less

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Hayley Yeung 12E – Biology: Osmosis Practical

+ smooth

rigid and smoother

0.3

1.67 1.68 1.67

Firm w/ smooth texture

1.27 1.27 1.28

0.4

2.32 2.63 2.68

Firm w/ smooth exterior

1.64 1.86 1.80 Flimsy and soft but still has a smooth exterior

0.5

1.59 1.58 1.63

Firm w/ smooth outer surface

1.12

1.19

Softer, smoother w/ slightly frayed edges

0.6

2.59 2.63 2.59

Firm w/ smooth outer surface

1.92 1.96 1.92

Soggy w/ smooth outer layer

0.7

2.49 2.35 1.89

Firm, stiff + rigid w/ smooth exterior

1.82 1.72 1.36

Flimsy, Soft +Soggy

0.8

2.13 1.83 1.98

Firm, stiff + rigid w/ smooth exterior

1.60 1.40 1.50

Flimsy, Soft and Soggy

0.9

2.37 2.28 2.56

Firm and rigid w/ smooth outer surface

1.82 1.72 1.93

Greyish colour, squishable and bendy

1.0

1.52 1.54 1.51

Smooth outer layer w/ firm structure

1.14 1.17 1.18

Softer

1.11

Smoother slimier texture, less rigid and more flexible

Table #2: Processed Qualitative Data Tables Concentration of NaCl Solution / mol dm-3

0.0

Change in Mass / g Trial 1

Trial 2

Trial 3

-0.24

+0.26

-0.38

Percentage Change in Mass Trial 1

-8.89%

Trial 2

Trial 3

Average (Mean) Percentage Change in Mass Average

Standard Deviation

-3.11%

0.14

+12.56% -13.01%

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Hayley Yeung 12E – Biology: Osmosis Practical

0.1

-0.20

-0.11

-0.34

-7.33%

-4.44%

-14.41%

-8.72%

0.05

0.2

-0.09

-0.17

-0.07

-4.15%

-8.21%

-3.59%

-5.32%

0.03

0.3

-0.33

-0.41

-0.39

-23.95% -24.40% -23.35%

-23.90%

0.01

0.4

-0.68

-0.77

-0.88

-29.31% -29.28% -32.84%

-30.47%

0.02

0.5

-0.47

-0.47

-0.44

-29.56% -29.75% -26.99%

-28.77%

0.02

0.6

-0.67

-0.67

-0.67

-25.87% -25.48% -25.87%

-25.74%

0.00

0.7

-0.67

-0.63

-0.53

-26.91% -26.81% -28.04%

-27.25%

0.01

0.8

-0.53

-0.43

-0.48

-24.88% -23.50% -24.24%

-24.21%

0.01

0.9

-0.55

-0.56

-0.63

-23.21% -24.56% -24.61%

-24.13%

0.01

1.0

-0.38

-0.37

-0.33

-25.00% -24.03% 21.85%

-23.63%

0.27

4. Data Processing: In order to calculate the change in mass of the potato cylinders I used Excel to programme a chart that would do the equation

Change ∈ mass=Final mass−Initial mass for all values, ensuring that the positive and negative values would correlate with a loss in mass or gain in mass. To calculate the percentage change in mass I then used this equation:

Percentage change∈mass=

Final mass−Initial mass x 100 % Initial mass

I then calculated the average percentage change in mass with this equation:

Average Percentage Change∈mass=

(% Change∈mass of Trial 1+ Trial 2+ Trial 3) 3

Lastly, I used Excel to calculate the standard deviation of our results given by this formula:

5. Graph Page 4

Hayley Yeung 12E – Biology: Osmosis Practical

Graph 1. Mean and Standard Deviation Scatter Plot Graph

Graph 2. Mean Percentage Change Line Graph with Linear Line of Best Fit

7. Conclusion

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Hayley Yeung 12E – Biology: Osmosis Practical As seen in the Graph 1, our results did not have a high degree of accuracy and is unreliable due to the large standard deviation as shown in the error bars and did not have a clearly identifiable trend. A large standard deviation is especially prominent for our results from the concentrations 0 mol/dm-3 and 1 mol/dm-3 highlighting anomalies from those two data sets indicating a large range of results making them very inaccurate. In addition, Graph 2 clearly shows that there was a decrease in mass for all concentration which disagrees with my initial hypothesis that concentrations 0.1 – 0.2 mol/dm-3 would be hypertonic and causing osmosis to transport water into the cell and increase its mass. I hypothesized that the isotonic point where there was not net mass gain or loss (ie. The xintercept) would give us an estimate for the osmolarity, however, as seen from our graph, there is no x-intercept. More anomalies can be seen in the data point for 0.1/mol-dm3 as it indicates a higher water concentration than 0.2/mol-dm3 which should, in theory, translate to less mass loss. However, in the graphs we can see that there is more mass loss for 0.1/mol-3 than 0.2/mold2. The reliability of these results can also be questioned at the value for 0.4mol/dm-3 where we see the largest decrease of mass indicating an anomaly. According to the theory of Osmosis, the steeper the concentration gradient or the larger the difference in water concentration the faster the water would move via Osmosis. Therefore, it would only make sense for the 1.0mol / dm-3 solution to have the largest mass loss since from our graph we can see that the smallest mass loss occurs at 0.0 mol/dm-3. Overall, we are unable to make any conclusions or estimates about the osmolarity of potato cells since we were unable to find the isotonic point where the line of best fit crosses the xaxis. This investigation was inconclusive.

8. Evaluation Procedural errors of this experiment

Effect on Investigation

How to correct mistake

Soaking up some of the liquid after taking the potato cylinders out of the solutions

By using a paper towel to dry off the potato cylinders we may have accidently absorbed some of the water inside the cylinder than moved in via osmosis and affected the change in mass.

Do not use a paper towel to absorb the excess water from the potato strip and let it air drink by itself for a minute or two.

Time the potato cylinders was in the solutions

Since each concentration was done by a different group of our results may be unreliable since the potato cylinders were put in the solutions and submerged for different periods of time

Ensure that all groups put the potato cylinders in at the same time and retrieve them at the same time.

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Hayley Yeung 12E – Biology: Osmosis Practical Division of labour

The Size of the Potato Cylinders

Freshness of the potatoes used

Limitations/Weaknesses of the investigation Human Error

Only doing three repeats

Each group was responsible for one doing three repeats on one concentration of NaCl solution allowing for a large margin for human error leading inaccurate results. Since we only chose to do three repeats this means that our results were also quite unreliable. Although we all used a cork borer of the same size each piece of potato had a different length which may have affected its Surface Area to Volume ration making our results less accurate. The potatoes we used were not fresh and had previously been used by other classes which may have affected our results.

Effect on Investigation Since there was a large group of people collecting data for this experiment and as mentioned each group was in charge of a specific concentration, many human errors may have occurred causing our results to be unreliable. For example, potato cylinders were not all submerged at the same time, different balances were used and different people diluted each concentration allowing room for error. Makes the data collected less reliable as three repeats is insufficient to fully eliminate any anomalies when calculating the mean average.

Make each group do one of each concentration and compare all results to calculate a mean value for all concentrations to increase repeats and make results more accurate and reliable.

Ensure that the same size cork borer is used and all cylinders are cut to the same length (i.e. 4 cm in length)

Use fresh potatoes that do not have any punctures.

Suggestions for improvement Ensure that each group does the experiment for all concentrations so that the mean average for each concentration would eliminate anomalies caused by human error.

Do more repeats – at least 5 to ensure that average is reliable.

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Hayley Yeung 12E – Biology: Osmosis Practical What potato/which part of the potato was used

Different potatoes and different parts of the potato may have slightly different consistency and density affecting its water concentration

Ensure that the potato cylinders are bored from the same part of the same potato. If the potato is not big enough then use the smallest no. of potatoes possible to achieve this.

Temperature of the surroundings/solution

Since the temperature was not actively monitored and only left in an airconditioned room, there could have been changes to the temperature as different groups of students used the room.

Put the solutions in a water bath set to a certain temperature (ie.25 degrees Celsius) and leave it there for the duration of the experiment.

Overall, our results were inconclusive and unreliable as proven by large standard deviation for certain concentrations (0.1 mol/dm-3 and 1 mol/dm-3) which then affected the results of our experiment as a whole. Although we did use specialised equipment such as volumetric flask to dilute the concentrations which has a tolerance of 0.2% since it is calibrated to a certain volume to ensure that our measurements were accurate. The main way we could have improved and fixed our error was by dividing the experiment into trials instead of concentration with all groups doing this experiment once for all volumes, then using the mean average of those values.

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