Osmosis in Potato Cells


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Osmosis in Potato Cells

Plan

Introduction

Given that osmosis is the movement of water molecules by diffusion
from a place where the water molecules are in a higher concentration
(high water potential) to a place where the water molecules are in a
lower concentration (low water potential) through a partially
permeable membrane, the experiment to test osmosis in potato cells was
set up. The experiment would prove the theory of osmosis and determine
whether the concentration of water in the solution surrounding the
potato was higher or lower than that of the water concentration inside
the potato. This would cause the potato to become turgid or
plasmolysed.

Diagram of apparatus

Variables and Method

Above all other factors, for this experiment to work it needed to be a
fair test. For this to be possible, all variables had to be controlled
throughout.

Seven separate concentrations of water and sucrose solutions were used
in this experiment with a maximum of 20% sucrose. These were
distributed accurately with the use of measuring cylinders as the
concentrations of water and sucrose held great sway on the dependant
variables, such as the mass or length of the potato at the end. Each
solution had a volume of 20cm and both the temperature and pH were
recorded.

It was necessary that all seven samples of potato had to be the same
length, width and therefore surface area before the experiment began.
This was so equal amounts of solution could make contact with each
potato and that there was the same amount of water inside each potato
to begin with. For accuracy a cylinder shaped cork borer of size five
was used and once all the samples had been pulled out of the potato a
metal rule marked 5cm of length on the potato and the left over were
cut off with a sharp blade. The mass of each potato was then recorded
to two decimal places using the balance so these could be compared
with the end results.

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Once all the samples of potato were ready and the different solutions
were marked clearly on the test tubes, the stopwatch was started and
each potato was put in at 30 second intervals so they would all be in
the solution for the same period of time, therefore making it a fair
test. Cellophane was then placed over the top of the test tubes to
prevent any of the solution evaporating and possibly altering results.

Prediction

The prediction for this experiment is quite simple as it would not be
accurate to estimate the changes in mass in the potatoes. One can say
that the higher the concentration of sucrose in the solution, the more
water would diffuse out of the potato making it plasmolyse and
therefore lose mass. Plasmolysis occurs when water is diffusing out of
the vacuole causing it to shrink and leaves it flaccid.

However, a weak solution of sucrose would cause the water to diffuse
into the potato by osmosis, making it turgid and therefore gains mass.
Turgor occurs when water is diffusing into the vacuole making it
expand and become turgid.

Finally, if the concentration of water is equal to that of the water
inside the potato there would be a state of equilibrium and the mass
of the potato would remain exactly the same. Osmosis would still be
occurring but there would be no net movement of water.

The direction of the movement of water is to do with the water
potential. The water will always diffuse down a concentration gradient
from where there is a high water potential to where there is a low
water potential. The molecules are continuously colliding due to their
kinetic energy, which results in collisions with the partially
permeable membrane. As the water molecules are so small they can fit
through the tiny gaps in the membrane and so if they collide in the
right place they can pass through, unlike the larger sucrose
molecules. This is the collision theory.

Method continued

Once all the potatoes had been in the solutions for 3 hours 50 minutes
they were poured out of the test tube, dabbed twice with a paper towel
(to remove the same amount of loose water) and the mass was
immediately recorded on the balance. The length of the potatoes was
then measured and all the results recorded. Both the pH and the
temperature of the solution were noted as at the beginning and the
practical part of the experiment was complete.

Preliminary experiment

Prior to the final experiment, groups of three were set the task of
carrying out a preliminary study to find out what the most suitable
fixed variables should be, for example how many different
concentrations, what the concentrations should be and the size of the
potato samples. Using the one set of results from this smaller
experiment made the prediction of the final experiment much easier and
made it clear how the final experiment should be set out. Because two
other people had the same fixed variables as me, I could use these
results later to compare with my own as there was not enough time to
do a repeat. Repetition means more reliable results and having three
sets instead of two means that if one is completely wrong you can tell
which as it stands out from the other two, whereas with just two sets
of results, there would be nothing else to compare them to and
therefore no way of telling which was correct.

A volume of 20cm, temperature of 23 and a pH of 6 was constant
throughout.

Sucrose / %

Water / %

Mass of potato before / g

Mass of potato after / g

20

0

3.05

3.02

15

5

3.04

3.03

10

10

3.01

3.07

5

15

2.81

2.88

0

20

2.93

3.01

These results were not surprising and the only one that I was not sure
whether it would lose or gain mass was the potato in the solution of
10cm of sucrose and water because, as I am unaware of the point of
equilibrium, the solution in the middle or with equal percentages
would be my estimate closest point of equilibrium

Observations

Safety

For this experiment to be safe, all the usual requirements were met.
Lab coats were worn and done up correctly, safety goggles were used
and all hair was tied back.

The test tubes were balanced in racks to hold them up straight and all
beakers were washed every time a different solution was in them to
prevent mixing.

Potatoes were cut on tiles so as to not make dents in the work bench
and when the experiment was to be left, all equipment still in use was
placed neatly at the side of the benches to not obstruct any other
work that went on between the completion of setting up the experiment
and the recording of results.

Hands were washed after handling the potato, and all apparatus used
was cleaned thoroughly at the end so it would remain neutral for the
next time of use.

Modifications

As there were no anomalies in the average % change in mass and only
two in the length of the potatoes, there was no need to make any
modifications to the table of results.

However, one observation that I did make was in fact the potatoes in
20% and 18% sucrose solution actually floated at the beginning and the
theory for this will be in the analysis.

Results

The results from the experiment have been collected and it was decided
that the clearest, most suitable way to display them was using a
table.

Averages have been taken from the three separate tests to get the most
accurate results.

It is clear on the table as mass and length before and after have been
recorded as well as the percentage changes, both separate and average.

Analysis

Trend and conclusion

The trends of both of these graphs are quite similar to the prediction
graph, excluding the two anomalous points in the percentage change of
length. The middle sections in both graphs have a slightly more gentle
gradient, where the amount of water separated by the partially
permeable membrane is more equal and therefore fewer molecule diffuse
into or out of the potato.

The first and last sections both have substantial differences in the
percentage change between each point as the amount of water inside and
surrounding the potato is so varied. Therefore more diffusion is
occurring, changing the length and mass of the potato a lot.

However, the prediction graph would not be expected if it was
representing just one sample of potato in a constant solution due to
lack of time. The experiment would take much longer to reach the point
of equilibrium. Osmosis would be continually occurring but with the
absence of any comparisons of the rate, the prediction graph would
still look very similar, but in the space of time we were given for
the experiment the real graph would much more likely look like this:

Therefore, using varied solutions of sucrose in the prediction graph
makes it much easier to get those results in a shorter amount of time.

The results were very accurate in respect to the written prediction
and it is clear from the results table that the more sucrose there was
in the solution, the more length and mass was then was lost in the
potato. This was due to the diffusion of water (osmosis) from the
potato (high water potential) to the highly concentrated sucrose
solution (low water potential). These potatoes became plasmolysed and
by the end of the experiment had shrunk and become bendy.

Obviously, at the other end of the experiment, the solutions with a
weak concentration of sucrose had water diffusing into the potatoes.
These samples became turgid and were longer than at the start and far
harder to bend, unlike the flaccid ones.

The potatoes that were in solutions between the extremes had changed
very little by the end and it was only the small changes in mass and
length that were noticeable.

The two potatoes in the strongest solutions of sucrose had floated at
the beginning. This is because the water in them was less dense than
the high levels of sucrose in the solutions. As the water diffused out
of the potato it slowly sunk as the solution became less dense.

This experiment has successfully proved the theory of osmosis and the
collision theory. As said in the introduction osmosis is the movement
of water molecules by diffusion from a place where the water molecules
are in a higher concentration (high water potential) to a place where
the water molecules are in a lower concentration (low water potential)
through a partially permeable membrane. However, this process would
not be possible without movement of particles using kinetic energy.
The collision theory is such that as the particles are moving around
with this energy, they are constantly colliding with other molecules
and the partially permeable membrane. This membrane has tiny gaps that
only water molecules can fit through and if they make contact with the
right part of the membrane, they can pass through and therefore
diffuse.

I do not know what the point of equilibrium was for this experiment as
none of the potatoes remained exactly the same due to the ongoing
process of osmosis, but the solution with the potato that had an
average % change in mass of only -1.86 was 10cm sucrose, 10cm water.
This was part of my prediction.

Evaluation

Accuracy of results

The accuracy of results of this experiment was very good as the
difference between them and the prediction was only slight. Given that
three experiments were carried out to find the same results and any
incorrect measurements did not affect the average greatly, it was much
more reliable than just one or two sets of results. Two can be used
for comparisons but a third is necessary to check which one, if not
both, of the other two is correct. On the lines of best fit most of
the points were drawn through as they had very similar patterns to
that of the line drawn in the prediction graph.

In the average percentage change in mass graph, it was clear that not
one anomaly was recorded. However the average percentage change in
length graph contains two points that do not go through the line of
best fit and are therefore counted as anomalies.

Improvements to the Method

Had the time been available, more repeats would have been carried out
to make any incorrect measurements less of a difference. If the
temperature of the room could have been controlled, then perhaps the
results would be different and slightly more accurate but there no
water bath available to do this.

Although this experiment cannot have been 100% accurate, I believe
that with the equipment, resources and time available, this method was
the best it could have been.

Further work

Of course many other fixed variable could have been altered to find
out more information about what other factors affect osmosis. For
example, varying the temperature with a constant solution would prove
whether or not heat or coldness slows or speeds up the rate of
osmosis. A water bath could have been used to monitor the temperature
and raise or lower it when it was necessary. The mass of potato should
be measured at the beginning and at set periods throughout the
experiment. Each potato has the same concentration of solution but are
set at different temperatures. Once removed from the water bath the
mass should be recorded and the percentage change measured and then
compared with the other samples. This process should be repeated three
times for reliability.

A change in volume could clarify differences that were very small with
only 20cm of liquid and could erase anomalies.

Clearly, a lot of extra work can be done with osmosis in potato cells
but the prediction of the conclusion was proved. The higher the
concentration of the sucrose in the solution, the more mass or length
is lost in the potato, or, the higher the concentration of water in
the solution, the more mass or length is gained in the potato.

Biology Coursework

Osmosis in Potato Cells


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