Factors Affecting Osmosis
Length: 5898 words (16.9 double-spaced pages)
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The aim of this experiment is to investigate the factors affecting
osmosis. I have chosen to investigate the effect varying concentration
of sucrose solution has on the amount of osmotic activity between the
sucrose solution and a plant cell. The plant cell I have chosen to use
is a potato tuber from which I plan to cut potato chips of equal
length which I will place in test tubes filled with varying
concentrations at equal volumes of sucrose solution. After a set
amount of time I will remove the potato chips and record the change in
High water potential is a solution which has a high concentration of
water and a low concentration of solute (e.g. sugar, sucrose or salt)
or no solute at all. Therefore it can either be a very dilute solution
of something like sucrose or pure water, however in each case there is
a lot of water. Whereas low water potential is the opposite of high
water potential being a solution of low concentration of water and
high concentration of solute (sugar, sucrose or salt). Hence it can be
a concentrated solution of something like sucrose, however in this
case there is much less water.
When to solutions one of high water potential and one of low water
potential are divided by a semi-permeable membrane water molecules
will move from the solution of high water potential through the
partially permeable membrane to the solution of low water potential.
This will continue until both sides have reached a state of
equilibrium or isotonic point, which is when both solutions wither
side of the partially permeable membrane have equal water potential
neither having higher or lower potential, they are said to be
isotonic. When this occurs there is no net movement of water across
the membrane only random diffusion of particles.
What I have discussed in the previous paragraph is the process of
osmosis, however in short osmosis is defined as the net movement of
water molecules from a high water potential through a partially
permeable membrane to a low water potential.
As mentioned earlier the net movement of water must take place across
a partially permeable membrane (e.g. cell wall) which is a very thin
layer of material which allows small molecules such as Oxygen, water,
Carbon Dioxide, Ammonia, Glucose and amino-acids to pass through. But
does not allow larger molecules such as Sucrose, Starch and protein to
HIGH WATER POTENTIAL
(Dilute solution or pure water)
LOW WATER POTENTIAL
· Hypertonic solutions are solutions which contain a high
concentration of solute which means they contain low concentration of
· Hypotonic solutions are solutions which contain a low concentration
of solute which means they contain a high concentration of water.
Osmosis is essential for the survival of living things, plants absorb
most of their water by osmosis, while in animals osmosis helps
regulate the flow of water and nutrients between body fluids and
cells. Therefore osmosis occurs in both plant and animal cells.
OSMOSIS IN PLANT CELLS
When a plant cell is placed in a hypertonic solution (e.g. a
concentrated sugar solution), the water potential outside the cell is
lower than inside the cell causing the plant cell to loose water and
become flaccid (shrunken and soft) or plasmolysed. When what I have
stated above occurs the plant cell decreases in mass because water has
left the cell. An everyday example of this involves ocean water which
generally contains higher concentrations of solutes (roughly 3.5 grams
of salt for every litre of water), therefore most land plants when
placed in ocean water will effectively dehydrate because cells in the
plant (high water potential) will loose water to the surrounding salt
water (low water potential).
However when a plant cell is placed in a hypertonic solution (e.g. a
dilute sugar solution) or pure water the water potential outside the
cell is higher than inside the cell causing water to move into the
cell so that it becomes turgid (swollen and hard). Turgidity in plant
cells is very important to plants because this is what makes the plant
"stand up" into the sunlight. When water moves into the plant cell the
pressure inside the cell rises, eventually the internal pressure of
the cell is so high that no more water can enter the cell. When this
occurs the cell is prevented from bursting by the strong cell wall
which surrounds the plant cell of the plant cell. When what I have
discussed above occurs the plant cell increases in mass as there is
water moving into it.
When plant cells are placed in solution which has exactly the same
water potential as the plant cells only random diffusion of molecules
takes place and the plant cells are in a state between turgidity and
flaccidity. This can be called incipient plasmolysis, incipient
meaning about to be.
OSMOSIS IN ANIMAL CELLS
Osmosis in animal cells is irrelevant to my investigation but
nethertheless I will explain briefly what occurs in animal cells
When an animal cell is placed in a hypertonic solution (e.g. a
concentrated sugar solution) the same thing happens as with plant
cells. The plant cell looses water and becomes flaccid (shrunken and
soft) because the water potential outside the cell is lower than
inside the cell.
The majour difference between plant cells and animal cells is that
plant cells have no cell walls which is why they behave differently
when placed in hypotonic solutions. When a animal cell is placed in a
hypotonic solution (e.g. a dilute sugar solution) the water potential
outside the cell is higher than inside the cell causing water to move
into the cell, however animal cells do not become turgid instead they
burst because there is no cell wall to support the cell membrane.
I have already mentioned that osmosis is essential for animals and
plants, however osmosis is also important to humans because industries
use reverse osmosis for such purposes as water purification and food
preservation. Reverse osmosis is the opposite of naturally occurring
osmosis and can be explained in the following way: when osmosis occurs
the flow of water molecules from a high water potential through a
partially permeable membrane to low water potential produces a
measurable pressure which is called osmotic pressure. If pressure is
applied to the solution of low water potential, and if that pressure
exceeds the osmotic pressure water flows through the partially
permeable membrane from the solution of low water potential to the
solution of high water potential.
I carried out a pilot, which is a preliminary experiment to test my
method and prediction and see if they are both correct and accurate,
if not I made modifications.
PREDICTION FOR PILOT
I predict that as the concentration of the sucrose solution increases
the length of the potato cylinders will decrease. I can predict this
because during osmosis water moves down a diffusion gradient from high
water potential through a partially permeable membrane to a low water
potential. Therefore the more concentrated the sucrose solution the
lower the water potential hence the water will move from high water
potential in the potato cylinder through a partially permeable
membrane to low water potential in the sucrose solution. I can predict
that the largest decrease in length will be in the potato cylinder in
the most concentration sucrose solution (2M) because the potato will
have to loose the largest volume of water so that both sides can
reached a state of equilibrium, which is when the water potential is
the same in both the potato cylinder and in the sucrose solution. I
can further predict that the potato chip in 0.0M sucrose solution
which is distilled water will not decrease in length but increase in
length because the distilled water has a higher water potential than
the potato. Hence the water will be moving from a high water potential
in the 0.0M sucrose solution (distilled water) through a partially
permeable membrane to low water potential in the potato chip. I can
quantify my prediction by saying that, as concentration doubles the
decrease in length will also double until a limiting factor operates.
METHOD FOR PILOT
1. Take an averaged sized potato which is hard and healthy, and on a
cutting board cut one end of the potato off using a scalpel.
2. Next with care push a cork borer (size 5) down vertically through
the potato, using a glass rod to push the potato cylinder out. Repeat
this until you have bored out five potato cylinders.
3. Now cut all the ends of the potato cylinders square and using
plastic covered graph paper measure the cylinders to the nearest mm so
that all five are exactly 2cm/20mm in length cutting any excess potato
off on a cutting board and using a scalpel.
4. Now place each potato cylinder in a separate test tube.
5. Then using a 5cm3 syringe cover each chip in different
concentrations of sucrose solution (0.0M; 1.5M; 1.0M; 1.5M and 2M).
6. Cover each test tube with cling film and leave for 30 minutes
timing the time with a stopwatch.
7. After 30 minutes remove the potato cylinders from the test tubes
wiping away any excess water using a paper towel.
8. Measure the potato cylinders to the nearest mm using plastic
covered graph paper and record any change in length in a table.
RESULTS FROM PILOT
Concentration of sucrose solution (M)
Original length of potato cylinders (mm)
Final Length of potato cylinders (mm)
Difference in length of the potato cylinders (mm)
% Change in length of the potato cylinders
0.0 (distilled water)
ANALYSIS OF PILOT
I obtained 2 anomalous results from my pilot experiment; the two
anomalous results were for the 1.0M and 0.5M sucrose solutions. Even
thought the potato cylinders in these two sucrose solutions were left
in the sucrose solutions for the same amount of time as the other 3
potato cylinders, they did not change in length like the other 3
potato cylinders by remained the same length. Theoretically speaking
the two potato cylinders in 0.5M and 1M sucrose solution should have
decreased in length which is why I decided they were anomalous
results. There are two possible reasons to why these anomalous results
were gained, the first reason could be that the potato cylinders
weren't left in the sucrose solution for long enough for osmosis to
take place, or that if some osmosis did occur it was too little to
show a actual change in length. The second reason could be that these
potato cylinders did not increase in length but in width, but I was
not measuring width.
By looking at the 3 remaining results I can still see that my
prediction was correct, because even from the results I have it is
visible that as concentration increases the length of the potatoes
decreases. This is because for 1.5M sucrose solution the potato
cylinder decreased in length by 1mm. Then when the concentration
increased by 0.5M to 2.0M the potato decreased by 2mm which is 1mm
more than the potato cylinder in the previous concentration. My
prediction is further proven when I look at the result for the potato
cylinder in 0.0M sucrose solution (distilled water). This is because I
predicted that the potato cylinder in 0.0M sucrose solution (distilled
water) would increase in length and it did by 1mm and may have
increased by more if the potato cylinder had been left in the sucrose
solution longer. However in my prediction I also predicted that the
potato cylinder in 0.0M sucrose solution (distilled water) would be
the only potato cylinder that increased in mass. Unfortunately my
pilot is unable to prove or deny this because I obtained no results
for 0.5M and 1.0M, thus I don't know if the potato cylinders in these
concentrations would have increased or decreased in length. My
qauntative prediction in which I stated 'as concentration doubles the
decrease in length will also double' also cannot be proven due to
obtaining no results for 0.5M and 1.0M sucrose solution.
After carrying out my pilot experiment I can see by the results and
they way my experiment was carried out that there are certain
modifications I would like do to improve my experiment further.
Firstly I would like to change the time the potato cylinders are left
in the solution for from 30 minutes to 50 minutes. This is because my
results show very little change in length either decreasing or
increasing and two results for 0.5 and 1.0 molar sucrose solutions
show no change in length which is surprising as theoretically speaking
there should be a decreased in length. Therefore I have drawn the
conclusion that the potato cylinders were not left in the sucrose
solutions for long enough for enough osmosis to take place. The second
modification I think that I must carry out is to instead of measuring
the length of the potato in mm I will measure the mass of the potato
in grams by weighting it on a top pan balance. This is because
measuring length is not good form of measuring for this experiment
because it does not include width of the potato cylinder which is
vital too because during osmosis the potato chip can not only get
longer lengthways but also wider. This could also have been the reason
why the two potato cylinders in my pilot showed no change in length
and were categorised as anomalous, because maybe they didn't increase
in length like the remaining potato cylinder but increased in width,
which I didn't measure. Therefore measuring length is inaccurate so to
increase the accuracy as I said earlier it would me more sensible to
weigh the potato chips using a top pan balance because weight is a
uniform measurement and a top pan balance is accurate to 2dp.
PREDICTION FOR MAIN INVESTIGATION (very similar to prediction for
For this investigation I predict that as the concentration of the
sucrose solution increases the mass of the potatoes cylinders will
decrease. I can predict this because during osmosis water moves down a
diffusion gradient from high water potential through a partially
permeable membrane to a low water potential. Therefore the more
concentrated the sucrose solution the lower the water potential hence
the water will move from high water potential in the potato cylinder
through a partially permeable membrane to low water potential in the
sucrose solution. The potato cylinder therefore decreases in mass
because it has lost water by osmosis
I can predict that the largest decrease in mass will be in the potato
cylinder in the most concentration sucrose solution (2M) because the
potato will have to loose the largest volume of water so that both
sides can reached a state of equilibrium, which is when the water
potential is the same in both the potato cylinder and in the sucrose
I can further predict that the potato chip in 0.0M sucrose solution
which is distilled water will not decrease in mass but increase in
mass because the distilled water has a higher water potential than the
potato. Hence the water will be moving from a high water potential in
the 0.0M sucrose solution (distilled water) through a partially
permeable membrane to low water potential in the potato cylinder.
Therefore the potato cylinder will increase in mass because it is
gained water by osmosis.
I can quantify my prediction by saying that, as concentration doubles
the decrease in mass will also double until a limiting factor
operates. The limiting factor being that each potato chip has only a
certain volume of water to loose, however I am not sure if the
limiting factor will operate during my experiment because I may not be
using a large enough range of sucrose solution concentrations.
GRAPH TO SUPPORT MY PREDICTION
Change in mass of potato cylinders (g)
Above is a graph to support my prediction it is also the graph I
expect my results to yield. The graph shows that the potato cylinder
in distilled water (0.0m sucrose solution) increases in mass while all
the remaining potato cylinders steadily decrease in mass as the
concentration steadily increases. However if the limiting factor 'each
potato chip has only a certain volume of water to loose' does begin to
operate in my experiment the graph will be that same as the one above
but will curve at the end. The dotted line drawn on my prediction
graph represents this curve which may or may not be obtained in the
graph my results will yield.
For a fair test I must only vary 1 variable and that is the thing I am
investigating (independent variable), all the remaining variables must
be kept the same. I have to make sure this experiment is a fair test
because if not it will affect my results causing them to be inaccurate
meaning I will not be able to compare my results fairly to draw and
accurate conclusion that will support my prediction.
The independent variable
The independent variable is the variable I change and in this
investigation it is the 5 different concentrations of sucrose
solution. The concentrations of sucrose solution are at intervals of
0.5Mand the concentrations I am using are: 0.0M (distilled water);
0.5M; 1.0M; 1.5M and 2.0M.
The dependant variable
The dependant variable is the variable I measure and in this
investigation it is the change in mass of the potato chips after
osmosis has occurred, allowing me to see whether osmosis has taken
place and to what extent. I will measure the change in mass in grams
by weighing the potato chips on a top pan balance (accurate to 2dp)
before the experiment and after the experiment.
CONTROLLED VARIABLES: - are variables I must under all circumstances
keep the same to ensure I am conducting a fair test.
1. The size (length and width) and therefore also the surface area of
the potato cylinders must be kept the same for all the potato
cylinders. This is because if the size of any of the potato cylinders
will vary so will the surface area over which osmosis would occur
which will affect the volume of water the potato cylinders loose or
gain during osmosis. For example if one of the potato cylinders was
1cm longer the surface area of the chip would be larger hence there
would be more area over which osmosis would occur so the potato
cylinder would loose or gain a larger volume of water than the
remaining potato cylinders. I will control this variable by always
using the same size borer (size 5) and always keeping the length of
the potato cylinders constant at 2cm/20mm.
2. The temperature at which the experiment takes place must be kept
constant because the temperature will affect the rate at which osmosis
will occur. For example if the temperature increases at all during the
experiment the molecules will have more kinetic energy meaning there
will be more collisions and the rate of osmosis will therefore be
faster. I will control this variable by carrying out the whole
experiment in the same lab which should be at room temperature.
3. The volume of sucrose solution the potato chips are in must be kept
the same because the volume of sucrose solution will affect the volume
of water the potato cylinders loose or gain during osmosis. I will
control this variable by always using 10cm3 of sucrose solution
measured using a syringe. The sucrose solution must always cover the
potato cylinder because if any part of the potato cylinder is
uncovered it decrease the surface area over which osmosis will occur
therefore decreasing the volume of water the potato gains or looses in
relation to the other potato cylinders.
4. The type of the potato must be kept the same because different
potatoes may absorb at different rates and will contain different
volumes of water (water potentials) that will affect if the potato
looses or gains water and the volume of water the potato looses or
gains. I will control this variable by using the same brand of potato
for all the experiments and treating all the potatoes in the same way
i.e. all have been cut without being washed and peeled.
5. The time the potato cylinders are left in the solution must be the
same for all the chips. This is because the time the potatoes are left
in the solution will affect the volume of water the potato looses or
gains. For example if any of the potato cylinders is left in the
solution longer it will gain or loose more water because it will have
longer to do so than the reaming potato cylinders. Hence I will
control this variable by leaving all the potato cylinders for exactly
50 minutes in the sucrose solutions and I will measure the time the
potato chip is left in the solution with a stopwatch. I will start the
stopwatch as soon as the potato cylinders are put into the solutions
and I will stop the clock as soon as 50 minutes has passed and
immediately removing all the potato cylinders so that no further
osmosis can take place.
6. The light intensity during the experiment must be kept the same,
e.g. no extra light must be shone on the experiment. This is because
light is heat which would increase the temperature around the test
tubes, causing the temperature to increase, which in turn would cause
the molecules to gain more kinetic energy, meaning that they would
move faster and there would be more collisions so the rate of osmosis
will be faster.
7. The same top pan balance (accurate to 2 decimal places) must be
used to measure all the potato chips because the measurements may
slightly vary between scales. I will control this variable by always
using the same top pan balance to measure the mass of my potato chips
before and after the experiment. When using the top pan balance I must
make sure that the balance is reading zero before I begin to weigh
each potato cylinder so that I do not get an inaccurate reading.
For my main investigation I will use the same concentrations of
sucrose solution as for my pilot. There will be a range of 5
concentrations at intervals of 0.5M. The concentrations of sucrose
solutions will be 0.0M (which is distilled water); 0.5 M; 1.0M; 1.5M;
I will repeat the experiment three times so that I obtain 3 sets of
precise and reliable results for each sucrose solution, which I will
then average. In doing this I hope that any anomalous results will
have a chance to show. It is important to repeat any experiments more
than once because if they were conducted only once, then an anomalous
result might be gained, and I wouldn't even know because I wouldn't
have any data to compare it with. Anomalous result need to be repeated
because they are inaccurate results and therefore cause any graphs and
further calculations also to be inaccurate which in turn cause that a
precise conclusion cannot be made. If I get any anomalous results,
which are inaccurate results that don't fit in with the pattern of the
rest of the results I will repeat the procedure again until I get an
accurate reliable set of results. I will include the anomalous results
in my results table, but ring them and not include them in the
average. After I have obtained all my results I will calculate the %
change in mass of each set of results and then further calculate the
average % change in mass which I will use to plot a graph of
concentration of sucrose solution against average % change in mass.
Safety is an important aspect in every experiment, therefore all
solutions and equipment should be used carefully and the whole
experiment should be conducted with care. The points that must be
followed when carrying out this experiment are:
- Handle all glassware (e.g. test tubes) with care because glass is
easily breakable. If breakages to occur, notify the teacher and once
and make sure all the glass is cleared away before continuing with the
experiment. When clearing away avoid handling broken glass.
- Handle the scalpel with care because it is sharp and could easily
cause a wound.
- Act sensibly and do not run in the lab.
- Test tube holders should be used to hold test tubes.
CHEMICALS AND APPARATUS
1. Sucrose solutions of these concentrations: 0 molar (distilled
water), 0.5 molar, 1 molar. 1.5 molar and 2 molar.
2. 2 large hard and healthy potatoes - to act as the plant cell in
3. 5 large beakers - to hold the five different concentrations of
4. 15 test tubes - to hold the potato chips and sucrose solutions
and in which osmosis will occur.
5. 5 test tube racks (3 test tubes to a rack) - to hold the test
6. Cling film - to cover the openings of the test tubes.
7. 10cm3 syringe (accurate to ) - to measure the volume of sucrose
8. Scalpel - to cut the potato.
9. Cutting board - to protect the work surface when cutting the
10. 5 paper towels (1 towel to 3 test tubes/3 potato chips) - to
remove excess water from the potato.
11. Cork borer (size 5) - to bore out potato cylinder from the
12. Glass rod - to push the potato cylinders out after using the
13. Plastic covered graph paper - to measure the length of the
14. Stop watch (accurate to 1/100 of a second) - to time the durancy
of time the potato cylinders are left in the sucrose solution for.
15. Permanent marker - to mark the concentrations of sucrose
solutions on the appropriate test tubes containing these
concentrations so as not to mix the different test tube up.
16. Top pan balance (accurate to 2dp) - to weigh the potato
cylinders before the experiment and after the experiment.
1. Take an averaged sized potato which is hard and healthy, and on a
cutting board so as to protect the work surface cut one end of the
potato off using a scalpel.
2. With care push the borer (size 5) down vertically through the
potato, using a glass rod to push the potato cylinder out. Repeat
this until you have bored three cylinders out of the potato.
3. Cut all the ends of the potato cylinders square and using plastic
covered graph paper measure the potato cylinders to the nearest mm
cutting off any excess potato on a cutting board using a scalpel
so that all three potato cylinders are exactly 2cm/20mm in length.
4. Weigh each potato cylinder on a top pan balance recording the
weight in grams.
5. Place each potato cylinder in a separate test tube.
6. Using a 10cm3 syringe cover each chip with 0.0M sucrose solution
7. Cover all three test tubes with cling film. The test tubes are
covered with cling film so that none of the water in either the
potato or the sucrose solution will evaporate.
8. Leave like this for 50 minutes, timing the time using a
9. After 50 minutes remove all three potato cylinders from their
test tubes wiping off any excess water using a paper towel. The
excess water on the surface of the potato cylinders needs to be
wiped away so that it wont be included in the final mass reading
of the potato.
10. Reweigh each potato cylinder recording the weight in grams in a
11. Now repeat point 1 to 10 using 0.5molar, 1 molar, 1.5 molar and
2 molar sucrose solutions recording all weights.
Concentration of sucrose solution (M)
Initial mass of potato cylinder (g)
Final mass of potato cylinder (g)
Change in mass of potato cylinders (g)
% Change in mass of potato cylinders
0.0 (distilled water)
Concentration of sucrose solution (M)
Average % change in mass of the potato cylinders
0.0 (distilled water)
Using the results I obtained from my main investigation I calculated
the % change in mass of each potato cylinder by dividing the change in
mass of the potato cylinders by the initial mass of the potato
cylinders and multiplying the result by a 100. When I had calculated
the % change in mass for each potato cylinder, I then calculated the
average % change in mass for each set of results for each
concentration and plotted a graph with concentration (the independent
variable) on the x-axis and average % change in mass of the potato
cylinders (the dependant variable) on the y-axis.
The line of best fit on my graph just like I predicted in my
prediction graph is a curve that slopes downwards, it also doesn't not
go through the origin, which means that the % change in mass and the
concentration of sucrose solution are not directly proportional.
From my graph I can see that as the x-axis increases the y-axis
decreases in other words that as the concentration of sucrose solution
increases the % change in mass decreases. I can see this because for
0.5M the change in mass is -7%, then as the concentration increases to
1M the change in mass decrease to 13%, when the concentration again
increases to 1.5M the change in mass further decreases to 18% and
lastly when the concentration increases to 2M, the change in mass
decreases yet further to 22%. This complies with my prediction and
like I stated in my prediction occurs because during osmosis water
moves down a diffusion gradient from high water potential through a
partially permeable membrane to a low water potential. Therefore the
more concentrated the sucrose solution the lower the water potential.
Thus the water will move from the high water potential in the potato
cylinder through a partially permeable membrane to low water potential
in the sucrose solution. The potato cylinder therefore decreases in
mass because it has lost water.
My graph does not support my quantitative prediction because as the
concentration of the sucrose solution doubles from 1M to 2M the
decrease in mass does not double. I think my results didn't support my
quantitative prediction because they weren't entirely accurate, due to
the many errors that occurred during the experiment and also due to
variables not being controlled. However I will discuss this later in
In my prediction I mentioned a limiting factor saying that it may or
may not operate in my experiment because I wasn't sure if I had a
large enough range of concentrations of sucrose solutions. However my
graph did begin to curve slighting at the end, meaning that the
limiting factor was in fact beginning to operate. Therefore if I had
used a wider range of concentrations i.e. 2.5M and 3.0M the graph
would have curved further until it finished in a horizontal line. This
is because the mass of the potatoes cannot just keep decreasing as
concentration increases, because every potato cylinder has only a
certain volume of water to loose. This meaning that eventually there
will be a point at which the potatoes will stop decreasing in mass
even thought concentration is increasing because they will have no
more water to loose.
From the graph I can approximately estimate the concentration of the
potato cylinder as 0.1M, this is because at this point on the graph
the potato is neither increasing nor decreasing in mass. This is known
as the isotonic point and it is when no osmosis is taking place only
random diffusion of particles because both the potato cylinder and the
sucrose solution have the same water potential.
Overall I can confidently say that my investigation was successful
because the experiment was carried out well and produced a good set of
accurate and reliable results which yielded a informative graph from
which I could draw an accurate conclusion.
I believe I did enough repeats (three) for each concentration of
sucrose solution I was using. I left all the potato cylinders in the
sucrose solutions for 50 minutes and this was enough to allow
sufficient osmosis to occur. The range and interval of concentrations
of sucrose solution I used were adequate, but I were to repeat the
experiment I would use a larger range of concentrations and more
concentrations at smaller intervals, so that my results would be more
gradual and show a clearer pattern, but also so I can see clearly when
the limiting factor begins to operate.
Apart from one anomalous result (circled in my results table) all the
remaining results that I obtained are consistent and quite accurate. I
believe they are quite accurate for a three reasons, the first reason
is that I did three repeats for each concentration of sucrose solution
and all the repeats were fairly similar. The second reason is that the
line of best fit in my graph runs through all the plotted points and
the third reason is that both my results and the graph plotted from
them are theoretically supportable therefore meaning they are fairly
correct. The reason why I said the result were 'quite accurate' or
'fairly correct' is because there is a moderately big difference in
values between repeated results, which means the size of error between
these repeated results is bigger than it should be. I believe there
are two things to blame for this; the first thing is errors which
occurred in the experiment and the second thing is variables which
weren't controlled. Both these things affected the accuracy of the
results and the difference in values between repeated results. However
errors and uncontrolled variables will be discussed further on in my
evaluation, but all I can say is that if these errors were reduced and
variables were controlled then there would be only a very small
difference in values between repeated results. Repeated results cannot
be exactly the same as this is a biology experiment involving
I can say that my conclusion is fairly reliable but not as reliable as
it should have been firstly because what I have stated in the previous
paragraph and secondly because the graph I plotted from my results was
not as accurate as it could have been. This is due to the fact that I
plotted my points to the nearest whole numbers and not to 2 decimal
places as in the table. I couldn't include the 2 decimal places
because I was unable to fit in a scale on my graph paper which would
allow me to plot to two decimal places. As I already mentioned earlier
my best-fit line runs through all my plotted points hence I have no
need to consider different best fit curves for points on my graph.
The anomalous result I obtained was for 0.5M sucrose solution and it
was -13.38%. I decided that this particular result was anomalous
because the difference between it and the other results for 0.5M
sucrose solution was much a too large, the other results being-7.46,
-6.91 and -7.27. The reason why this anomalous result occurred is very
simple to see because it is very similar to the results obtained for
1.0M sucrose solution and could easily have been one of the results
obtained for 1.0M sucrose solution. Hence the anomalous result was due
to human error because instead of covering the chip with 0.5M sucrose
solution I must of by mistake covered it with 1.0M sucrose solution.
Which is why the result is much too large for its set and is similar
to the results obtained for 1.0M sucrose solution. After I had
repeated this result using the correct concentration of sucrose
solution I obtained a more suitable looking result. When I plotted my
graph I looked for further anomalous results but found none.
Looking back at the experiment I can see that many errors occurred.
The first error was caused when the potato cylinders were removed from
their test tubes and dried using a paper towel to remove excess water.
I may well have dried some potatoes more thoroughly than others,
meaning that I did not just remove the excess water from the outside
of the potato but also from the inside, which is the water it had
gained during osmosis. Which of course meant that when the potato
cylinders were weighed they weighed less, therefore due to this my
results were inaccurate because they lead me to conclude that the
potato cylinders had lost more than they actually had. Vice versa I
may well have not removed all the excess water from the outside of
some of the potato cylinders, which would have added to the mass of
the potato. Meaning that when the potato chips were weighed they would
weigh more due to the excess water, this again causing my results to
be inaccurate because they lead me to conclude that the potato
cylinders had gained more or lost less than they actually had done.
For this error it is difficult to come up with an improvement, however
one way to reduce the size of error would be to ensure that all the
potatoes are rolled the same amount of times on the paper towels and
as far as possible with the same pressure. If this is done it would
not eliminate the error but reduce it to a minimum
The second error which occurred was due to the fact that I was doing
many sets of potatoes at the same time. Which meant that after the set
time the potatoes were supposed to be in the sucrose solution for was
up ideally I had to remove all the potatoes from their test tubes at
the same times and wipe away any excess water on the outside
immediately so that no further osmosis took place in any of the potato
cylinders. However this was not the case as it was impossible to
remove all the potato cylinders from their test tubes and wipe any
excess water off at the same time because there were just too many
potato cylinders. This means that while I was removing the potatoes
and wiping them the potatoes still left in the test tube waiting their
turn had more time to gain or loose water. Meaning that they had a
smaller or larger mass than they should have had if all the potatoes
had been removed at the same time. This of course made my results in
accurate but only slightly because it was only a matter of 1 minute.
The third error is bound to occur in very experiment where measuring
is concerned and in this experiment small measuring errors when
measuring the sucrose solution using the syringe could have easily
occurred, as I am only human. For example an air bubble could have
become trapped in the syringe and this would affect the measurement.
If any measuring errors occurred they were however only very tiny and
could have only had a tiny or no affect at all on my results. However
I could reduce this error by using a burette (accurate to 0.05cm3) to
measure the volume of sucrose solution required instead of a syringe
because a burette is far more accurate than a syringe.
The forth and last error but also what I think was the largest source
of error in this experiment is to do with the biological variation of
the potatoes. The initial water potential of the potato cylinders
should have been the same for all the potato cylinders. However this
was not the case because the potato cylinders all of the same length
and dimensions were not the same weight, or even close in many cases.
Therefore because the potato cylinders were bored from the outside and
inside of the potato I can conclude that the cells in the center of
the potato have a larger capacity of water than the cells in the
outside of the potato. Meaning that the potato cylinders bored from
the outside had lower potential than the potato cylinders bored from
the inside of the potato. If I were to repeat this experiment to
reduce the error discussed above I would not cut potato cylinders
instead I would cut the potato into thin doughnut shaped discs, so
that I would only be using the out side of the potato, leaving out the
middle where the water potential is higher. Also because the potato
discs would be thin but wide I would be able to cut all the required
potato discs from one potato. Meaning that the water potential was the
same in all the discs, and not slightly varied if I had used a
I managed to control all my variables (but one) because they were all
easily controllable. I left all the potato cylinders in the sucrose
solution for exactly the same amount of time 50 minutes, and to makes
sure this time was timed accurately I used a stopwatch which is
accurate to 1/100 of a second. All the potato cylinders I used were
the same size and had the same surface area. This is because firstly I
used the same size borer (size 5) to bore all the cylinders out which
ensured they all had the same circumference/width and secondly I
measured the potato cylinders using plastic covered graph paper and to
nearest mm so that they were all 2cm/20mm in length. The volume of
sucrose solution I used was kept constant at 10cm3 for all the potato
cylinders and covered all the potato cylinders eliminating and error
in that respect. The light intensity was kept the same because no
classroom or extra lights were shone during or at the experiment. All
the potatoes I used were the same brand and I weighed the potato
cylinders always on the same top pan balance. However if I were to
repeat the experiment I could increase accuracy by weighing each chip
using a more accurate scale, e.g. not to 0.00g but to 0.0000g.
The one variable which was not controlled successfully was
temperature. Temperature was not kept constant for all the experiments
because the experiment was carried out during the summer term over two
lessons all of which were on different days and at different times of
the day. Thus the temperature in the room varied from each lessons
because one day could have been warmer or colder than the other and
mornings were colder than afternoons, which in summer were hot.
Therefore I can conclude that in some sets of experiments the rate of
osmosis either decreased or increased depending whether it was colder
or warmer. This would have an effect on the accuracy of my results.
This investigation can be extended further in the following ways:
· Using different plant cells i.e. different vegetable, perhaps celery
or cucumber and investigating whether osmosis occurs at the same speed
and in the same way in all plant cells.
· The experiment could be carried out using an animal cell, so that
osmosis in plant and animal can be compared and differences observed.
· The experiment could be carried out not only using sucrose solution
but also using other solutions for example salt solution to see if the
solution affects the rate of osmosis and how much osmosis takes place.
· I could extend the range of the independent variable to see at what
concentration the limiting factor would apply.
· The potatoes could be left in the sucrose solution longer, enabling
me to find the saturation point and dehydration point. The saturation
point is when the potato can no longer take in any more water and the
dehydration point is when the potato cannot lose any more water.
· I could extend the experiment to a more exact level by looking at
the potato cylinders under a microscope, then I would be able to see
the cells in greater detail and draw some observational results.
- Encarta DVD
- Biology - Mary Jones and Geoff Jones
- Biology (Key Science) - David Applin