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The aim of this investigation is to discover the effect of water
concentration on the mass of potato tissue, and also to investigate
the movement of osmosis through potato tissue.
When a substance such as a sugar dissolves in water, the sugar
molecules attract some of the water molecules and stop them moving
freely. This, in effect, reduces the concentration of water molecules.
The water potential of a solution is a measure of whether it is likely
to lose or gain water molecules from another solution. A dilute
solution, with its high proportion of free water molecules, is said to
have higher water potential than a concentrated solution, because
water will flow from the dilute to the concentrated solution (from a
high potential to a low potential). Pure water has the highest
possible water potential because water molecules will flow from pure
water to any other aqueous solution, no matter how dilute.
When two such solutions (one strong, one weak) are separated by a
semi-permeable membrane the water will move from the side with more
water to the side with less until both sides are equal (have reached
This can be seen in all living cells. The cell membrane in cells is
semi-permeable and the vacuole contains a sugar/salt solution. So when
a cell is placed in distilled water (high water concentration, lower
water potential) water will move across the semi-permeable membrane
into the cell (lower water concentration, higher water potential) by
osmosis, making the cell swell to carry the extra water. This cell is
now referred to as turgid. The opposite of this is where the cell
becomes flaccid, where the cell membrane actually can break away from
the cell wall. If this process were done with the potato cells I would
expect them to increase in length, volume and mass due to the extra
If these potato cells were placed in a solution of low water
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the cell into the solution. In extreme cases, the cell membrane would
break away from the cell wall and then the cell is referred to as
plasmolysed. The potato cells would have decreased significantly in
length, volume and mass.
The greater the concentration of water in the external solution the
greater the amount that enters the cell via osmosis, and visa versa.
However there will be a point where the water on the inside and
outside of the cell is equal (isotonic). At this point there will be
no change in the length, volume and mass of the potato, as the net
movement of water will be zero, no more osmosis has occurred except to
keep the levels of water approximately balanced. Osmosis will still
occur, but in a very small amount, simply to keep the levels on both
sides balanced, or isotonic.
I have decided to vary the concentrations, to see what affect if any
it will have on the mass, size and weight of the potato cores.
Diagram of Osmosis:
This 'time/water concentration per potato' graph shows the predicted
shape for the stages in the experiment. The red line shows where the
levels of water on each side of the semi-permeable membrane are equal.
This therefore is where the potato and the solution have reached its
isotonic point (become equal on both sides, so that no more can be
gained or lost).
I will keep all the variables the same except concentration.
* PH - The more acidic the solution, the slower osmosis will occur.
The more alkali the solution, the faster osmosis will occur.
* Temperature - The warmer the solution, the quicker osmosis will go.
This is because the molecules will be moving more quickly. Careful
though, if you heat something like a potato chip too much, you will
just kill the cells and no osmosis will occur.
* Difference in concentrations - This will speed up the rate of
osmosis because there will be more or less particles to move.
*Quantity of each concentration - If there is more of a solution in
one beaker than another the experiment will not be fair because there
would be more molecules to move from a beaker with 80cm3 than in a
beaker with 40cm3.
*Surface area of the Potato - This will speed up the rate of osmosis
because there will be more space on the potato fro osmosis to take
* Concentration of water in the potato - The higher the concentration
of water already in the potato, the quicker osmosis will occur because
less water will have to diffuse into the potato to even the amounts of
water inside and outside of the potato cells.
*Duration of the experiment - I am not sure how this will affect my
results, therefore I will try to leave my actual experiment for longer
to see if there were any changes noticed.
*The weighing scales - The weighing scales may not be exact, so I must
leave a leeway of approx. 0.4/0.5 on each result due to the (perhaps!)
inaccuracy of the scales, to produce more precise results.
* I will be using a sharp knife, which could injure someone if not
handled properly. Therefore I must not rush with the equipment and I
must handle everything carefully.
* I will ensure the lab is safe around me as I am working; removing
baggage and unnecessary books from my work area, pushing stools under
the desks and stand as I do the experiment in the interest of safety.
* In order to keep the benches clear and clean I will lay paper towels
around where I am working, because even though the substances are
relatively harmless, I will be working with glass and sharp
instruments, so it is safer to do so.
* I will also wear an apron to prevent any of the substances damaging
my clothing as I work.
* I will keep all the other variables the same and constant to make my
* I will keep the PH constant so that my results are not affected by a
change in acidity.
* I will use the same amount of solution in each boiling tube,
measuring different quantities of sucrose in ration to distilled water
to create the correct molarity solutions.
* I will make sure the potato pieces are the same size and have
approximately the same surface area to make sure that osmosis doesn't
occur more due to a larger surface area (see variables).
* I will make sure I use the same size of boiling tube and the same
size corer to make sure the depth of the liquid and the surface area
of the potato piece is approximately the same.
* The variable I will alter in my experiment is the concentration of
* I will make sure the room temperature stays approximately the same
and I will try not to vary it too much if it can be helped.
* I will use a set of digital weighing scales to make sure that my
results are as accurate as possible.
* I will use a measuring cylinder to measure out the liquids to make
up the solution. If I had access to a more exact method of measuring
out the solution, I would do so, but I only had limited equipment
* I used a ruler to measure each potato core to make sure it was the
correct length, or as near as it was possible (never more than 1 mm
* My measurements will therefore be very precise. Below is the
preliminary work I have carried out to determine some of the work
above. I have used secondary sources in my scientific knowledge; a
list is shown below;
Encarta Encyclopaedia '98, Britannica Millennium Edition, DK
Chronicles of Science, GCSE Science (Collins Study Guides), Various
The preliminary work allows you to make hypotheses and check that the
test can be done completely fairly, with reasonable measurements. I
also cut one piece of potato into mm strips, but this was very time
consuming. However this did show that the surface area of the potato
does make a difference to the results.
My first results were not as good as I had hoped on my first
preliminary, so I altered my method and recorded far more accurate
results. Here is a short list of the alterations I made:
* I realised I had to be more careful during the weighing process,
because if we weighted the same piece twice, occasionally the results
would be slightly different each time.
* I was going to do all the six different molarities with five repeats
in different beakers, however I realised this would use up a lot of
equipment and solution, so I decided to do all the five repeats in one
beaker, and multiply the amount of solution by five so I would still
have a fair test.
* I am still going to do five repeats for three different molarities,
because that will produce more accurate results. I will still use the
molarities in the range of 0.0 to 1.0M. I am taking several readings
for all the results to allow me to find an average of results.
* I was going to use test tubes, but I realised the amount of solution
I wanted to use would not fit into one test tube, having multiplied
the amount of solution, so I used small beakers instead.
Weak solution - 0.2 moles
Strong solution - 0.8 moles
Diced in weak solution - 0.2 moles
Osmosis is the passage of water molecules from a weaker solution to a
stronger solution, though a partially permeable membrane. In this
case, the tin holes in the membrane of the potatoes will allow the
water molecules to pass through in and out of the solution and the
potato, depending on the concentration gradient of the two substances.
I predict that as I increase the concentration of sucrose then the
amount of water moving in or out of the potato will change. A low
concentration of sucrose in ratio to water will make the potato
increase in length and mass.
I can support this prediction by looking back at my scientific
knowledge. Water will move into the concentrated potato cells because
of the high concentration gradient. I am expecting therefore to have a
high water potential in the high concentration (of sucrose) solutions.
I am expecting the opposite for the low concentration (of sucrose)
solutions. In a very concentrated sucrose solution (high water
potential) water will move out of the potato and into the sucrose
solution. I therefore predict that the length and mass of the potato
will decrease as the levels of sucrose increase.
I am going to put six pieces of potato at 2cm long in a beaker with
80cm3 of the sucrose/water solution. The levels of sucrose in ratio to
water will be measured in moles, rising in 0.2 molarity each time. The
range I will use is 0.0 (only distilled water), 0.2, 0.4, 0.6, 0.8 and
1.0 (only sucrose solution).
*As was stated previously, I have to keep all the different
non-variables the same, to make sure that none of them affected the
experiment in any way.
*Whilst cutting the potato, extreme care was needed to make sure it
did not harm anybody.
*The measurements of the solutions and the lengths and thickness of
the potato had to be exact as to not change the outcome of the
* I had to ensure that each time I handled the potatoes my hands were
clean and dry. This was to stop any contamination and made sure I
didn't pass on any excess water or sugar from my hands.
* Cork borer - diameter of 8mm
* Distilled Water
* Sucrose solution - 1 molarity
* Measuring Cylinder
* Cutting tile
* Potato - of one type to make the experiment fair (see fair testing)
* 6 beakers
* Pen - to label the beakers
* Ruler - to measure the potato cores
* Scalpel - to cut out the potato cores to the correct size
* Paper towels - to help keep the work area clear and clean
* Digital scales - more accurate than a manual balance
* 2 Pipettes - one for the sucrose solution, one for the distilled
water so that they do not become contaminated.
Sucrose Solution (cm3)
1. I took one average sized potato, checking it was hard and healthy.
2. Using a borer (size 8) I cut 30, 2cm lengths of the potato.
3. Taking 6 beakers, I labelled each one 0.0, 0.2, 0.4, 0.6, 0.8 and
4. Using the measuring cylinder, I measured out each of the correct
levels of sucrose solution to distilled water (see molarity table
above) and I placed each of the mixtures into the correct beakers.
5. Then I weighed all the potato chips on an electronic balance (see
results) and recorded the results.
6. I placed 5 pieces of potato into each beaker and left them for
approx 36 hrs.
7. After this time I drained out the solutions from the beakers and I
carefully placed them in order of molarity on a paper towel.
8. I dried the potato chips gently and then weighed each potato piece
and recorded the results.
9. As I had extra time I made a second experiment and also recorded
Starting Weight (g) (Results 1)
Final Weight (g)
Percentage Change (%)
Start Weight (g) (Results 2)
Final Weight (g) (Results 2)
Percentage Change (%) (Results 2)
These values clearly support my prediction, and even though there are
some anomalous results, there is an overall negative trend across the
whole set of results, proving them accurate.
Analysis of Results:
The sucrose concentration of the solution into which the potato tissue
is placed affects to what degree it grows or shrinks. As you can see
from the graph the results show a clear negative correlation, a very
obvious inversely proportional trend. From this a conclusion can be
drawn. When the water concentration is high, the potato gains water,
as seen by the 0.0 molarity solution. When the water concentration is
low, the potato loses weight and therefore decreases in mass, as seen
in the 1.0 concentration solution. This proves my hypothesis correct.
The results were quite widely ranged, as seen on the graph, with quite
a few anomalous results, especially results no. 2. The results show
that the line of best fit should be slightly curved, to accommodate
for the 0.4 and 0.6 results. I have drawn the graph with % change,
because it is more accurate. I have used the equation shown below to
calculate the % change.
A = Starting weight of potato core
B = Final weight of potato core
[IMAGE]% Increase = (B - A)
(A) All * by 100
As shown in my scientific knowledge, this experiment proves that the
water potential in the potato cores ranging from 0.0 to 0.4 is high,
decreasing gradually towards the 0.4 concentrations. The potato cores
in the 0.6 to 1.0 solutions have the lowest water potential, gradually
decreasing in potential towards the 1.0 concentration, and visa-versa.
The potato cores in the 0.0 to 0.4 solutions can absorb more water,
because water will move from an area of high concentration to an area
of low concentration of water molecules. This also means that the 0.6
to 1.0 potato cores cannot absorb as much and therefore osmosis occurs
in the opposite direction, the potato core losing water to the
solution. This is shown on the graph by the points below the 0% line.
The results fully support my prediction, except for the few anomalies
on the graph. The possible reasons for the anomalies are outlined
below, in the Evaluation and Conclusion sections.
I believe we gained sufficient enough results, shown on the graph and
the tables that conclude the experiment, and to prove my hypothesis. I
obtained a good amount of fairly accurate results, from which I was
able to make informative graphs. I believe I took enough repeat
results for the number of concentrations I was using, and I believe
the time I used was sufficient to prove that osmosis had occurred. My
final results were very reliable, due to the precautions I took to
make this a fair test. However there were some anomalous results. I
believe these were caused by some of the reasons highlighted below and
also the fact that I could not control any heat variations or the
evaporation levels of the liquid within the lab. I could have
controlled this by placing the beakers in a water bath kept at a
constant temperature and by covering the beaker in cling film to
prevent moisture escaping.
Even though there were anomalous results, and that I would have to
repeat those results more to prove completely conclusively that the
graph is completely correct, there is definitely a negative downward
trend across the graph, proving my conclusion correct. I believe that
my procedure was suitable for my experiment and I would improve it
with the previous factors mentioned above and the list of factors
mentioned below. I believe that overall my results were fairly
accurate, and a firm conclusion can be drawn from them.
Looking at the overall experiment, I have thought of a number of
things that would have helped to make the experiment more accurate.
v The potato cores all had different levels of moisture at the start
of the experiment, meaning that some potato cores would need less
osmosis to reach equilibrium within the same molarity solution. I
could have regulated this by using the same potato all the way through
v I could have used more solutions of different molarities, e.g. 0.1,
0.15, 0.2, 0.25 etc. This would have meant that I could have found out
the isotonic point far more accurately than I did.
v The concentrations were measured out using a measuring cylinder,
which was not completely accurate. The experiment would have been far
more exact if there had been some form of exact measuring device,
perhaps digital. Some solutions probably had a tiny amount more
solution in the beaker than others due to human error.
v I will make sure the beaker's size is kept the same size and that
all the potato pieces are fully submerged to the same depth at the
start of the experiment.
v I could have increased the time length of the experiment, to allow
more osmosis to occur and to perhaps find the saturation point of the
v When the potato pieces were dried after the experiment, to remove
the surface liquid, it was not necessarily done the same with each
potato piece. The experiment could have been more exact by creating a
uniform drying method for each of the pieces.
v At the end of the experiment I noticed that the diameter of the
centre of the potato increased or decreased according to the
concentration of the solution. It would have helped to explain the
results by measuring the centre diameter of each core before and after
v The experiment would have been more exact if I had found a way of
removing and measuring all the sores at the same time to reduce the
time difference between weights being measured. This could have
affected the results slightly. Otherwise it would have been better to
do each core one by one and measure them after an exact space of time.
However this is unpractical due to the probable slight variations in
time and measuring of the substances due to human error.
v Using more molar solutions would have helped to obtain more accurate
results and produced a far more accurate graph.
v I could have also varied the tissue type, for example, to use
carrot, or apple or even animal cells, to prove that osmosis occurs in
all living plant/animal cells. I could have also varied the potato's
age, to see if age affects the rate or levels of osmosis. I did try to
keep the cells the same, but this was very difficult, as I had no way
of telling their exact age. I also tried to use the cells from the
same potato, however the potato didn't have enough cells to cut enough
cores from for all the results and therefore I was forced to use two
v I could have also altered the length and diameter of the cores. This
however, would only change the results due to a larger or smaller
surface area, and I would obtain the same sort of results as I did
from the chopped core in my preliminaries. I could have also diced the
potato core, like I did in my preliminaries, because that would
increase the surface area.
v The scales I was using gradually became sticky due to all the potato
cores, therefore more weight may have been added later because of the
excess water and sugar.
v The potato pieces were all cut by hand with a ruler and scalpel, so
the cuts were often not exact and often the potato cores were slightly
longer or shorter than they should have been. For the next time, I
would make some sort of template or machine to cut all the cores to
exactly the same length.
v The anomalies in the graph may have been caused by the fact that the
chips may not have been fully submerged in the solution, because the
potato cores had a tendency to float on the surface, limiting the
surface area, and therefore meaning that some of the cells were not in
the solution and were not able to carry out osmosis. I could have made
some sort of fabric sieve or cover over the pieces to make sure that
they were properly submerged.
v I could repeat the experiment more to gain even more reliable
results for each molarity.
To conclude this experiment I can say that the movement of water
through a semi-permeable membrane (the process of osmosis) is affected
by the concentration of glucose solution, or by any other solution.
The higher the concentration of glucose solution, the more the potato
chip will lose its weight, width and length. All the evidence I have
obtained supports my hypothesis. I have been able to collect
sufficient results by repeating the experiment five times. However
there are improvements that could be made (see Evaluation).
Because water molecules have a form of kinetic energy, they are always
moving around in either a gaseous, solid or liquid state, randomly
from one place to another. The greater the concentration of water
molecules in a solution, the greater the total kinetic energy, and the
higher the water potential will be. This means that as the
concentration of glucose molecules increases in a solution, the
concentration of water decreases, lessening the solution's water
potential, and decreasing a solution's ability to move between
solutions due to osmosis. Therefore as the concentration of glucose
increases in each solution, the water in that solution is less able to
move to the potato, causing water from the potato to move into the
solution, decreasing the potato's length, mass and width. An osmotic
system is set up when a semi-permeable membrane is placed between two
solutions. There are many examples in the biological world, in many
plants and animals.
The plasma membranes of the cell decide the permeability of the
membrane. A semi-permeable membrane occurs when some substances with
small molecules can pass through, e.g. Water, and some substances
cannot pass through the membrane because they have larger molecules,
e.g. Sucrose. The permeability of a membrane varies with certain
conditions, and can vary due to temperature, and even due to hormonal
impulses, e.g. the loop of Henley in the kidney.
My investigation shows that, in concentrations above 0.6M, there
appears to be no further water loss, suggesting that the cell is
completely plasmolysed. However, it is important to realise that this
is only an estimate because potato cells will not be uniform in their
concentrations. I have enjoyed doing this experiment and I believe it
has been very informative.