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Aim: To determine the concentration of the cell sap in potato storage
tissue. By using Osmosis, determine what the sugar concentration of
cell sap is.
I predict that the potato segment in the distilled water will
definitely gain in weight because the solution outside it has a much
higher concentration of water then in the cell sap meaning Osmosis
will occur and the potato segment take in water. I predict that 0.2M
sugar solution will also gain weight because it still has quite a high
concentration of water outside the potato. The potato in the 0.4M
solution will gain weight but the potato in the 0.6M and 0.8M
solutions will lose weight. The glucose concentration of the cell sap
will be between 0.4M and 0.5M.
This experiment is based on the concept of Osmosis. Osmosis is the
diffusion of water molecules from a region of high water concentration
to a low water concentration through a semi permeable membrane (in
this case, the cell potato cell membrane). The cell walls of the
potato cells are semi permeable meaning that water molecules (which
are small) can fit through but other bigger molecules such as glucose
cannot pass through. The water molecules can flow both ways through
the membrane, letting molecules both in and out.
The Kinetic theory states that all molecules of every substance have
energy which causes them to move about for example a chemical reaction
can be accelerated through heating because the molecules gain more
energy efficiently causing harder and more effective collisions of the
molecules. In water the molecules have energy which makes them move,
it is this energy which powers Osmosis and allows water molecules to
pass through the semi permeable membrane.
The experiment uses Osmosis to make an estimation of the concentration
of the cell sap in the potato storage tissue. The cell sap is a
solution mainly of water, sugar, starch and salts.
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potato in a created sugar solution we can use osmosis to determine
which solution has a higher or lower concentration of water, the cell
sap or the solution outside the potato. This is because if the
solution outside the potato piece had a lower concentration of water
i.e. had less water than the solution inside the potato storage
tissue, water would move outside the potato into the artificial sugar
solution as if to dilute it. Like it wants to 'balance out' the water
concentration on either side of the cell walls. This is what Osmosis
aims to do, allow water to flow in and out in order to achieve an
equal concentration of water on either side of the semi permeable
membrane. Once it has done this it is called Net flow, or a dynamic
equilibrium. This is basically when the same amount of water molecules
coming in is the same as the number of water molecules going out. If
the solution outside the potato had a higher water concentration then
that inside the potato, water would move into the potato making it
larger. By monitoring the size of the potato we can determine what
solution had the higher concentration. If no change is made then the
concentration of the solution outside the potato is identical to the
concentration of the solution inside the potato.
Osmosis is used in the human body and in all plants. It is how water
ü 10 boiling tubes
ü 10 identical pieces of potato (peeled)
ü 0.2 m sugar solution
ü 0.4 m sugar solution
ü 0.6 m sugar solution
ü 0.8 m sugar solution
ü distilled water
ü Measuring cylinder
ü Cork borer
ü Chopping board
ü Absorbent tissue
ü Test tube rack
1. Measure out 30 ml of distilled water using measuring cylinder and
pour into boiling tube.
2. Measure out 30 ml of 0.2m sugar solution using measuring cylinder
and pour into boiling tube.
3. Measure out 30 ml of each sugar solution and pour into individual
and labelled boiling tubes in a test tube rack.
4. Use cork borer to remove 10 skinless lengths of potato from a whole
5. Cut each potato piece to the same length (2 cm) using scalpel and
6. Use balance to weigh each piece of potato making sure that all were
the same weight to one decimal point. Record weight of each piece of
7. Place one piece of potato in each boiling tube (make sure all are
submerged) and leave overnight.
8. Remove potato pieces from solutions and allow to dry on tissue.
9. Weigh each piece of potato and record new weight.
10. Find difference between start and finish weight for each piece of
potato in each solution.
Many factors could affect the fairness of the experiment. For example
temperature may affect the rate of Osmosis. Here's what I will do to
make it a fair test:
ü Each potato piece will start at the same weight, cut by the same
cork borer in order to achieve the same surface area also. Surface
area would affect the rate of Osmosis by allowing more area for the
water to enter or exit. The weight of the potato will effect the
results because if one potato piece is larger then another it will
have more water inside it.
ü Each sugar solution will be measured to the same amount for each
boiling tube (30 ml).
ü All boiling tubes will be kept in the same area so that no effects
are made by differentiating environments.
ü All water used in the experiment will be distilled and without any
impurities that may effect the rate of Osmosis.
ü I will repeat the experiment twice and use 5 different sugar
solutions to achieve a good set of results that with facilitate my
ü The range of sugar solutions used i.e. 0.2M, 0.4M etc.
ü Start mass of Potato segments
ü Measurement of sugar solution potato segments are put in (30 ml).
This experiment is not very dangerous, but the sugar solutions are
sticky and messy. I must be careful using the cork borer and the
Scalpel, as they are a potential hazard.
Start mass (g)
Finish mass (g)
Start Mass (g)
Finish Mass (g)
Average Difference (g)
Graphs of Results
From the graph I can clearly see that Distilled water has a much
larger concentration of water then the cell sap in the potato tissue.
I can make this conclusion because the results show a big increase in
weight of the potato piece. Meaning that water has moved from outside
the potato into the potato due to Osmosis, proving that the distilled
water had a higher concentration of water, as water moves from a high
concentration to a low concentration.
I can also see that the glucose solution of 0.8M has a much lower
water concentration then the cell sap in the potato tissue. I know
this because the potato in the 0.8M solution decreased in weight by
0.5g. This means that due to Osmosis, water moved from inside the
potato i.e. the higher concentration of water, to outside the potato
i.e. the lower concentration of water.
From the graph I can see that both the 0.2M and 0.4M solutions are
close to the concentration of the cell sap as they are both closest to
0. However I can find more accurately what the genuine glucose
concentration of the cell sap is by looking where the line of best fit
crosses the 'X' axis. The glucose concentration of the potato cell sap
is 0.38M meaning my prediction was relatively close but incorrect. If
I put a potato segment into a glucose solution of 0.38M, after 24
hours I would find no change. This is because the concentration inside
and outside of the potato is the same so no Osmosis occurs. Instead
the same number of water molecules flow in and out of the potato cells
achieving 'Net Flow' or a 'Dynamic Equilibrium,' which is what Osmosis
aims to do.
My investigation appears to have been completed fairly well. However,
my results did not show a perfect pattern meaning there must have been
errors in my method or most likely how I carried out the method.
Errors or mistakes I could have made might be that there could have
been over or just under 30ml of solution in one of the boiling tubes,
or an error was made during the weighing of the potato segments.
Also the time I left the potato segments was not timed. If I had timed
the experiment I could have been able to repeat it and therefore
compare results, however, to do this accurately room temperature would
have had to be to be monitored also as this could effect the Osmosis
process. Temperature could speed up or slow down the process because
more heat would mean that the water molecules would have more energy
and so would move quicker and with more force.
All of my results lie fairly close to the line of best fit meaning
that they are all fairly accurate. However, they are not directly on
the line of best fit and so obviously small but significant errors
were made. These could include slight measuring or weighing mistakes,
especially involving the already prepared sugar solutions.
On my hand drawn graph I have a hand drawn line of best fit. This line
of best fit determines my finding of what the exact concentration of
sugar potato cell sap is. However, because my results are not all
perfect this may have affected my findings.
My hand drawn graph may be inaccurate, but by creating a computer
generated graph I can compare the too. Looking at the computer
generated graph I can see that my hand drawn graph is fairly accurate,
although the lines of best fit are slightly different. This alters my
found Glucose concentration of the cell sap but still leaves me with a
fairly accurate result. Improvements could be made to obtain a far
more reliable result.
In order to have made my investigation more accurate I could have
spent more time on making the potato segments more equal in weight, I
could also have used a larger and more informative set of glucose
solutions: 0.25M, 0.35M, 0.45M, 0.55M. By using a closer and narrower
range of solutions, the concentration of the cell sap could be found
more accurately. I could have monitored room temperature and any other
environmental changes that could also affect the experiment such as
light intensity. I could have done this by keeping the light intensity
constant by using an electric lamp. By repeating the experiment a
greater number of times my averages would be made that little but more
Also to extend the experiment, because the Glucose concentration of
the cell sap that I found is not entirely accurate, I could repeat the
experiment but this time using a range of glucose solutions that the
concentration may be. For example 0.32M, 0.34M, 0.36M, and so on. This
would offer me a more accurate and reliable result to the experiment.
I also hypothesise that the experiment is still accurate if all
constants are kept constant, and that if an environmental change
occurs, as long as it occurs to all of the solutions in equal amounts,
the results would not be altered. I could extend the experiment by
proving this hypothesis.