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Biology Lab Report Aim: To find the isotonic concentration with respect to the potato cells.

Introduction: Water potential can be defined as difference between the free energy of water molecules in pure water and free energy of water molecules in a solution. It’s a fundamental concept in order to understand water movement. Water potential is a relative term. Water potential of pure water (that is free of any impurity) is taken standard (arbitrarily zero). If differences between two regions exist and is separated by a membrane, water moves from the region of higher water potential to the region of lower water potential. Water movement is due to the virtue of osmosis, which is diffusion of water. It’s always negative except for pure water. Water potential is the sum of Solute potential and Pressure potential.

(https://courses.agron.iastate.edu/tools/water-document/water-potential) Here the Symbol for water potential is Psi. Potato cells can shrink, swell or remain as it is depending upon the concentrations of the surrounding sucrose solutions. Movement of the drop will be on the basis of density of the solution. Due to water movement there will be a change in density at different sucrose concentrations. Thus, the drop can either move up, down or not move at all.

Hypothesis When the drop neither sinks nor does it float up, it is then, when the concentration of potato tissue is the same as the sucrose solution. Hence, both are Isotonic.

Variables  

The concentrations of sucrose Size of potato



Time taken for movement of water.

Material Required S.N O 1 2 3 4

Material

Quantity

Potato Distilled Water 0.5 M Sucrose Solution Test Tubes

Half or one 50 cm3 50 cm3 10

5 6

Coloured Dye Stopwatch

Few cm3 1

7

Pipettes

2

8

Paper Towel

few 3

9

Measuring cylinders of vol. 25 cm

2

10 11

Beakers Knife or cork borer

5 1

Method: 1. Using the table below, dilutions were carried out and different concentrations of sucrose were obtained. Volume of 0.5 mol/dm3

Volume of water

Final concentration

sucrose 20 15 10 5

(cm3) 5 10 15 20

(mol/dm3) 0.4 0.3 0.2 0.1

2. 4 Test tubes were labeled for each of the prepared concentration and the 10 cm3 of respective volume was added to each test tube. 10 cm3 of sucrose solution was added using a measuring cylinder. 3. Using a cork borer, 4 potato cores of the same thickness were cut and their lengths were made identical and equal to 4 cm. 4. Skin is impermeable so it was removed and potato cores were kept in moist paper towel until needed. 5. Then, each test tube was added with a potato core and left for 30 minutes. 6. In the mean time, second set of test tubes were prepared and labeled. 7. 10 cm3 of sucrose solution was added to the second set of test tubes. 8. Each test tube was marked half way from down. 9. Potato cores were removed after completion of 30 minutes and a few drops of dye were added to the solutions left in the test tube. This solution was then mixed well. 10. Using a dropping pipette, one drop from the dye-solution mixture was added carefully into the second test tube. It is important that the drop was added exactly at the half line made on the test tube. 11. As the drop was added, Stopwatch was turned on and distance covered by the drop was measured and noted over 10 seconds. 12. This was repeated several times and three values were chosen. These chosen three values displayed the best trend.

Safety Precautions:    

Cork borer or Knife should be used with caution, in order to avoid cuts. Lab coats should be worn at all times performing the experiment in order to prevent spills over clothes. If allergic, wear Nitrile gloves. Some contents may cause irritation. Hence, Gloves will prevent direct contact with the substances. Overall it is a low-risk experiment.

Results Obtained: Concentration(m)

DISTANCE AT t

DISTANCE AT t

DIFFERENCE IN

AVERAGE

=0s (cm) 3.5

=10s 1.4 1.2 1.4 2.9 3.0 3.0

DISTANCE (cm) -2.1 -2.3 -2.1 -0.6 -0.5 -0.5

(cm)

0.1

0.2 3.5 0.3 3.6 0.4 3.5 0.5 3.5

Graph-1

(cm)

2.3 2.5

-1.3 -1.1

2.4 3.5 3.2 3.2 2.1 2.4

-1.2 0 -0.3 -0.3 -1.4 -1.1

2.2

-1.3

-2.617

-0.533

-1.20

-0.20

-1.26

Difference in distance in 10 seconds 0 0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

Distance in cm

-0.5 -1 -1.5 -2 -2.5 -3 Concentration ( Molar)

Results expected: Resource A Resource A is a reliable source of results. The results we obtained didn’t follow a particular trend, moreover we were not able to find out the isotonic point by performing this experiment. Using results from resource-A, we can explain trends and evaluate deviations from the actual result data.

Concentration(m)

0.1

0.2

0.3

0.4

DIFFERENCE IN

AVERAGE

DISTANCE (cm) -4.5 -4.7 -4.6 -2.5 -2.0 -2.2

(cm)

-4.6

-2.23

+0.5 -0.2 -+0.8 +3.2

+1.1

+2.5 +2.8 +4.8 +4.4

0.5

+2.83

+4.6

+4.5 Graph-2

Difference in distance in 10 seconds 6 4.6

Distance in cm

4

2.83

2 0 0.05

1.1

0.1

0.2

0.25

0.3

0.35

0.4

0.45

0.5

0.55

-2.23

-2 -4

0.15

-4.6

-6

concentration ( molar)

Discussion: 



If we have different concentrations of sucrose solutions we can measure the accurate water potential using Dye method. It depends upon change in density after the potato cells are added to it. Density will change due movement of water from or to the cell depending upon its water potential and causing change in volume of water in the solution, hence altering the water potential. When the outside water potential is less negative than inside, water will move inwards, making the solution more thickened and dense. As the density increases the drop sinks.











Similarly, when water potential of the outside solution is more negative than the potato cell, it will cause water from the cells to move outward and the solution will become less dense and the added drop will float. As concentration of sucrose increased from 0.1M to 0.2M, we can still find the drop to sink but the difference in the distance is lesser. This is due to the fact that the sucrose concentration is increased but it is still hypotonic as compared to the cell concentration. The drop hence sinks. In between 0.2M to 0.3M, at 0.28 molar concentration isotonic point was obtained. At this point the drop showed no upward or downward movement. At this point water potential inside the cell is same as that of the solution. After this point, when concentration is increased, the solution becomes hypertonic as compared to the cell concentration. This means the water moves from inside of the cell to outside. Dye will move upward. A linear graph should be obtained beyond the isotonic point for increasing concentrations.

Evaluation: We fail to obtain appropriate values in order to explain the experiment correctly. The errors that led to deviated results are:   

      

Only three reading were taken: if more number of readings were taken it would have reduced the chance of errors. Errors while performing dilutions: incorrect measurements while performing dilutions might have lead to incorrect values. Incorrect timing and alignment of point: Candidates had to observe the drop for 10 seconds. Manual error while measuring the time and then marking the correct point would have resulted in deviated results. Sucrose solution could have been contaminated by various elements like, not using distilled water. Also the Dye might have altered the concentration of sucrose. Only one drop of Dye was to be added. However, if more concentration of dye was added it would have affected the solution. Different sizes of potato chores are obtained when knife is used. It was essential to store potato chips in paper towel so that the net movement of water is not affected. After a while the drop was not visible as the solution was completely colored. Intrinsic Variability: Biological molecules tend to vary in properties. In this particular case, Osmolality, especially if potato chores were derived from two different source potatoes. Dye had to be added at a very particular point and not more than one drop was supposed to be added. Judgmental error might have occurred while placing the drop at the specific point.

Improvements in the method:    

Placing a white sheet in order to observe the drop clearly. Experiment should be repeated more than thrice to get rid of anomalies. Other dyes like methylene blue can be used to obtain a more visible drop. There are many other easier experiments to determine the water potential of potato. In this particular method sources of error are very high. Example: Calculating percentage mass increase or decrease.

Conclusion:    

Isotonic concentration is the concentration when no osmotic movement takes place. For Resource A, This point was at 0.28 Molar concentration. At Lower concentrations, the solution is dilute. Water movement takes place from the solution to the cell. In this case, Cell swells. Density of the solution increases and the drops sinks down. At higher concentrations such as 0.5 Molar, water moves from cell to the solution. Density decreases and the drop of dye will float. This trend could be observed through the graph-2, as the concentration was increasing, distance covered by the drop towards above direction was increasing. Hence, drop was floating above the line. More the concentration, more is the distance of the drop from the line....