sample lab report- potato osmosis

Potato Osmosis Biology SL - ATh

INVESTIGATION

“Potato Osmosis”

INTRODUCTION

Osmosis is a process that occurs at a cellular level that entails the spontaneous net

movement of water through a semi-permeable membrane from a region of low solute concentration

to an area of high solute concentration in order to equalize the level of water in each region.

Involved in this process are hypotonic, hypertonic and isotonic solutions. A hypotonic solution is one

with a lower osmotic pressure, indicating that the net movement of water moves into the said

solution whereas a hypertonic solution is one with a higher osmotic pressure, thus the net

movement of water will be leaving the hypertonic solution. Lastly, an isotonic solution entails no net

movement of water across a semi-permeable membrane as the two substances involved display

osmotic equilibrium.

AIM

To observe the effect of solutions different levels of NaCl concentration on potatoes, considering the

process of osmosis

METHOD

(see ‘Potato Osmosis’ – exercise document)

Table of NaCl-H2O Dilutions

NaCl Solution / ml ±1 Distilled H2O/ ml ±1 NaCl Concentration/M

100 0 1.00

75 25 0.75

50 50 0.50

25 75 0.25

0 100 0.00

Official Timeframe of Experiment

Insertion Time: 29 November, 2007 – 8:54 am

Extraction Time: 30 November, 2007 – 9:40 am

Legend for Potato Differentiation

DATA COLLECTION

Potato Mark

Potato 1 Vertical line

Potato 2 Horizontal Line

Potato 3 Notch

(Fig. 1) Diagram demonstrating the

correct preparation of a potato –

cylindrical strips marked according to

the legend


Raw Data Table – Mass and Lengths of Potato Strips – Pre- & Post-Experiment

NaCl Concentration/M

Potato Samples

Initial Mass/ g±0.01

Mass after Solution / g±0.01

Initial Length /mm±1

Length after Solution/ mm±1

1.00 Potato 1 1.93 1.42 40 35

Potato 2 1.88 1.31 40 34

Potato 3 2.02 1.56 40 38

0.75 Potato 1 1.98 1.42 40 36

Potato 2 2.09 1.46 40 37

Potato 3 2.01 1.36 40 37

0.50 Potato 1 2.02 1.38 40 38

Potato 2 2.00 1.47 40 36

Potato 3 2.00 1.45 40 38

0.25 Potato 1 2.01 1.98 40 40

Potato 2 1.98 1.86 40 39

Potato 3 2.01 1.98 40 41

0.00 Potato 1 2.04 2.48 40 44

Potato 2 1.98 2.46 40 45

Potato 3 1.98 2.17 40 44

Observations – Pre-Insertion

- Generally rigid in structure

although slightly bendy

- Pale yellow in colour

- Moist

- All strips appear the same/similar in

structure and size at this point

Observations – Post-Extraction

- Strips immersed in 1.0M NaCl

Solution are very soggy, soft and

appear shrunken

- Strips immersed in 100% H2O are

very rigid, swollen, turgid and

appear larger/longer - they are

slightly bent and cannot be

straightened due to their rigidity

- Strips become progressively soggier

as the solutions they are immersed

in are higher in concentration of

NaCl

(Fig. 2) Potato strips from the same potato arranged in

descending order of concentration to demonstrate the

differences in structure post-extraction


DATA PROCESSING

Calculating Percentage Change

*Required in order to calculate percentage change in mass and in length

Processed Data Table – Average Percentage Change in Mass and Length of Potato Strips


% Change in Mass/%

Average % change/%

% Change in Length/%

Average % change/%

1.0

-26.42 -26.51

-12.50 -10.83 -30.32 -15.00

-22.77 -5.00

0.75

-28.28 -30.25

-10.00 -8.33

-30.14 -7.50

-32.14 -7.50

0.50

-31.68 -28.56

-5.00 -6.67 -26.50 -10.00

-27.50 -5.00

0.25

-1.49 -3.02

0.00 0.00 -6.06 -2.50

-1.49 2.50

0.00

21.57 18.47

10.00 10.83 24.24 12.50

9.60 10.00

Processed Data Table – Difference between % Change in Mass and Length and Average % Change


% Change in Mass

Average % Change - Mass Difference

% Change in Length

Average % Chang - Length Difference

1.0 -26.42 -26.51

0.09 -12.50 -10.83

-1.67

-30.32 -3.81 -15.00 -4.17

-22.77 3.74 -5.00 5.83

0.75 -28.28 -30.25

1.97 -10.00 -8.33

-1.67

-30.14 0.11 -7.50 0.83

-32.14 -1.89 -7.50 0.83

0.50 -31.68 -28.56

-3.12 -5.00 -6.67

1.67

-26.5 2.06 -10.00 -3.33

-27.5 1.06 -5.00 1.67

0.25 -1.49 -3.02

1.53 0.00 0.00

0

-6.06 -3.04 -2.50 -2.5

-1.49 1.53 2.50 2.5

0.00 21.57 18.47

3.1 10.00 10.83

-0.83

24.24 5.77 12.50 1.67

9.60 -8.87 10.00 -0.83


Average Percentage Change in Mass and Length of Potato Strips at each NaCl Concentration

CONCLUSION & EVALUATION

As can be seen from the above graph and observations, we can ascertain that as NaCl

concentration in the solution decreases, the mass and the length exemplified after an approximate

24-hour period increases. Thus, we can state that there appears to be a negative correlation

between NaCl concentration and the mass and length of the potato strips, clearly evident in the

above graph which shows an exponential decrease in both mass and length. This can also be initially

seen in the post-extraction observations where it is evident that the potato strips immersed in lower

NaCl concentration were far more turgid than those immersed in 100% NaCl solution which were

flacid and fragile (see strip-comparison in Fig. 2).

This occurrence can be explained through the process of osmosis. As mentioned in the

introduction, a hypertonic solution is one with higher osmotic pressure meaning that the net

movement of water leaves the solution. This would explain the physical changes – the increase in

mass and length as well as the increase in turgidity - in the potato strips immersed in 100% H2O

solutions or low NaCl-concentration solutions. Since the solution it is submerged in is higher in

concentration in water molecules, or hypertonic, the water molecules will diffuse into the area of

lower H2O-concentration (the potato strip) in order to achieve equilibrium. Alternatively, the

decrease in mass and length in the potato strips submerged in highly concentrated NaCl solutions

can be explained by its immersion in a hypotonic solution. Hypertonic solutions, as mentioned

R² = 0.978

R² = 0.989

-40

-30

-20

-10

0

10

20

30

0 0.25 0.5 0.75 1 1.25

Ave

rage

Pe

rce

nta

ge C

han

ge/%


Average Percentage Change in Mass and Length of Potato Strips at each NaCl Concentration

% change in mass

% change in length


before, are described as those with lower osmotic pressure, indicating that the net movement of

water moves into the solution. Therefore, as NaCl solution is less concentrated in H2O molecules

than the potato strips, the decrease in mass and length and loss of turgidity results from the net

movement of water leaving the potato strips, which is higher in osmotic pressure, and diffusing into

the solution.

Nevertheless, there are several possible sources of error that could have greatly or

negligibly affected the outcome of the experiment. First, we must note the varying external factors

resulting from an uncontrolled environment – the biology classroom. Primarily, these would include

varying temperatures and humidity which could potentially affect the rate of osmosis as increased

temperature results in increased diffusion while increased humidity results in an increased number

of water molecules. Secondly, we must note the human errors involved, for example, miscalculations

in experimental preparations. These would include the miscalculation of solutions leading to an

inaccurate concentration of NaCl as well as the possibility of impurities in the NaCl concoction in the

first place while imprecise cutting of the potato strips could’ve affected the surface area and thus

the rate of osmosis. This leads us to the errors resulting from variances in the substances used. As

already discussed previously, differences in surface area of each potato strip caused by imprecise

cutting as well as the marks (lines and notches) imprinted would’ve affected the rate of osmosis

while the concentration gradient between each potato strip is likely to differ as well. This stems from

the differences in water content of each potato, as, for example, a potato with high water

concentration in highly concentrated NaCl solution would have a faster rate of erosion. Further

affecting factors could include barriers to diffusion such as the size of pores which would also

determine the rate of osmosis. All the mentioned errors above hold the possibility of skewing the

data.

Subsequently, such errors could have an effect on the reliability of the results. The level of

accuracy which has been used throughout this investigation would come into question as a

combination of these errors would not permit such precision. Values of percentage change have

been taken at two decimal places corresponding with the actual values of mass and length, however,

this could be seen as far too precise. A better option would have been to take percentage change as

whole numbers or at one decimal place. Nevertheless, we attempted to reduce the potential errors

through several measures. With surface area, a cork borer was used in order to uniform the size of

the potato strips while the varying concentration gradients were controlled through the completion

of several trials (three trials with three potatoes) in order to limit error. Furthermore, to control the

effects of the external environment, foil was secured over the beaker containing the submerged

potato strips. However, if we refer to the graph, we can see the minimum and maximum spread for

each data-point is generally close-set while the R2 value, which calculates the spread of the data-

points from the line of best fit, are both relatively high – both around 0.9. This demonstrable trend

indicates a limiting of the amount of error, and thus fairly reliable results despite possible errors.

Overall, the results ultimately seem reliable although it might’ve been even more reliable by

reducing the level of precision (decimal places) when recording it.

Ultimately, potential improvements will stem from attempting to reduce the amount of

error in this investigation, particularly involving controlling the external environment and the

miscalculations. To control the external affecting factors, the solution containing the potato strips

can be kept overnight instead in a controlled environment with consistent temperatures and

humidity. Limiting the human error would be difficult and time-consuming as this would involve


highly-precise instruments or even more focus dedication from the experimenter during preparation.

Finally, nothing can be done to uniform the response of the materials used, thus the completion of

even more trials limits the potential error and allows the formation of generalizations. Despite the

improvements proposed, those relating to limiting human error and completing more trials may

prove to be futile as they are not only time-consuming, but the demonstrable trends resulting from

this experiment indicate that no further improvements are necessary to reach the desired

conclusion.

Having established that there is no real need to pursue drastic improvements for the initial

experiment, we can now proceed to discuss possible extensions to the investigation. While we

already know the results of osmosis on a potato, we may now wish to better understand it. This can

be done by recording the progress of the potato’s transformation either (a) over a period of time

(perhaps 24 hours) or (b) until it has reached the point of equilibrium. The mapping of this progress

would involve the periodic removal of the samples in order to measure its mass and length, after

which it can be compiled into a graph to chart the transformation under osmosis. Alternatively, we

could compare the progress of a potato to another type of vegetable or fruit in order to ascertain

water content of each. Lastly, the submerged potato strips may be subjected to different kinds of

environment, particularly, varying humidity and temperature, without the protection of a foil cap.

This would reveal how much of an impact environmental factors would have on the osmotic process

and how would the effects manifest. In relation to the question of the sailor, this could represent the

life-span one would expect when trapped in certain climates.

sample lab report- potato osmosis

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