How Different Concentrations of Sucrose Affect the Mass of a Piece of Potato
To find how different concentrations of sucrose can affect the mass of
a piece of potato left in the solution for a specific amount of time.
I have worked out what would be the easiest and most practical
variable to perform.
I will be changing the concentration of the sucrose. This variable has
been chosen from: changing the mass
of potato, how long it will be
left for, the temperature of the sucrose, the limits of the cell (how
much water it can loose or gain, i.e., how flaccid and turgid it can
become) and the pressure on the sucrose.
I have chosen to change the concentration of the sucrose because I can
find a dynamic equilibrium and I can see how the mass of the potato
changes as the concentration increases. It will also tell me a lot
more about osmosis and how it works under these conditions.
If certain factors are not kept the same throughout the experiment the
results will mean nothing and it would be a waste of time. There are
some factors that fall into this category: the mass of the potato
pieces to start with - (the mass should remain at the required mass
for each piece
of potato in each repeat), time left for, amount of
solution used- (altering the amount of sucrose could alter the rate of
osmosis because if the amount of sucrose is increased, the pressure on
the potato piece will increase and could alter the rate of reaction)
and the temperature of the sucrose.
The process in which water is exchanged in this way is called osmosis.
Osmosis is the movement of water down a concentration gradient through
a selectively permeable membrane, a root of a tree for example. A tree
obtains its water from the soil through osmosis. The concentration of
water is lower inside the root so the water moves from a high
concentration outside to a low concentration inside the root. We can
show how the water moves using a graph:
The concentration gradient will want to reach a dynamic equilibrium
where the solution outside is an isotonic solution and there are equal
amounts of water exchanged. At point 'A' there is a high
concentration. At point 'B' there is a low concentration. The more
concentrated side of the graph could be thought of as having greater
pressure because it contains more particles. This will push the
particles through the selectively permeable membrane and equal out the
amount of particles on each side.
A hyper tonic solution is a solution that has a larger amount of
particles dissolved in it, such as sugar (sucrose) or salt and
therefore has less water and is classed as having a low concentration.
A hypo tonic solution is where there are less particles dissolved into
the water and therefore has more water and it is called a high
concentration. If the concentration were the same it would be an
isotonic solution, there are equal amounts of water and solute.
This is a net movement. It shows that the hyper tonic solution on the
right has less water particles moving in different directions. The
probability of there being more particles moving through the
selectively permeable membrane to the left is lower and so there would
be an equal amount of particles on each side.
In this diagram there is no movement. There is a dynamic equilibrium.
There are equal particle numbers moving each way through the
selectively permeable membrane.
In plant cells the cell membrane is surrounded by a thick cellulose
cell wall. Cellulose is freely permeable to all liquids but it
stretches very little. When the potato cells loose mass the vacuole
will loose water and turgidity and become flaccid. The cell membrane
pulls away from the side and the cells begin to wilt. There is a limit
as to how flaccid or turgid it can become. Plant cells expand until
their cellulose walls can stretch no more, or until pressure from
surrounding cells stops them from expanding. The cells are fully
turgid. The stretched cellulose wall and surrounding cells exerts an
inward pressure, which eventually equals the osmotic pressure of the
cell sap, so osmosis stops. This process is called plasmolysis. There
will be a limit as to how much water can be obtained and lost from the
Using the evidence I have presented, I can make a firm prediction. I
predict that at the start the potato will gain mass because the
concentration of sugar will be higher in the potato, which means that
there was a low concentration of water. Outside the potato in a low
concentration and hypo tonic solution there will be less sugar and
therefore more water, the water will move due to osmosis down the
concentration gradient and into the potato. A dynamic equilibrium will
be reached. In the higher concentrations
and hyper tonic solution the
water concentration will decrease as the sugar level increases and the
water will move out of the potato from the low concentration of water
to the higher concentration outside the potato. I also predict that
there will be a limit as to how much water can be obtained and pushed
out from the potato. There will be a limit to how flaccid and turgid
the potato cells can become.
Beakers x 10
Sucrose (sugar-water solution)
A piece of potato was cut to the specified, equal weight, which was
1.00 grams. 30ml of sucrose was measured out using a measuring
cylinder, the concentrations having already been measured. The
concentrations were then placed into 10 different boiling tubes for
each concentration. 1 piece of potato was added to each of the test
tubes. The boiling tubes were placed in a rack and left for 24 hours.
After this time, each piece was taken out and weighed. The results
Concentration of sucrose
Weight of potato at start (grams)
Weight after 1.5 hours (g)
The percentage increase in mass of the potato in %
Analysis of Preliminary Results:
The experiment tuned out as expected. There were a few anomalies,
which are highlighted in bold, but these were expected. These would
have occurred because we did not do any repeats. In the final
experiment I hope to do at least three repeats. There are some factors
that need to be changed in the method: The amount of solution used was
too high and I will only use 30ml in the experiment instead of 40ml.
24 hours was too long and should be brought down to 1.5 hours. With
these changes the experiment should turn out better and show clearer,
more accurate results.
I also took results from leaving the potato for 24 hours. I gathered
results and they showed no difference to the experiment conducted for
1.5 hours. Here are the results to show this:
Concentration of sucrose in moles/dm3
Start weight of potato (g)
Weight after 24 hours (g)
% increase of potato
Looking at these results we can see that they are not much different
to the ones conducted in 1.5 hours. I have decided to conduct the
final experiment in 1.5 because it will save time and will give the
same results if the experiment was conducted for a longer amount of
A regular sized potato was taken and 10 pieces were cut to 1.00 grams
accurately using an apple corer. 10 boiling tubes were taken and 30ml
of the different concentrations of sucrose were added from 0.2 to 1
mole in 0.2 intervals.
1.5 and 2.0 moles were also measured out and placed in a boiling tube.
The pieces of potato were then added and left without stirring for 1.5
A clock was used to time 1.5 hours. The pieces of potato were then
taken out one by one and weighed accurately on scales.
The results were recorded. Different pieces of potato were then cut up
to the same weight and the concentrations were accurately measured
again and then the experiment could be repeated four times.
I will have to get the measurements and the weights of the solutions
and the potatoes as exact, and as accurate as possible. We will try
and get the measurements of the potatoes as accurate as possible for
every single potato by weighing them and cutting them down with a
knife. We will be using a very sensitive balance so that we can get
the best readings possible.
I should make sure that the potato is fully covered by the solution.
This is because the potato should be fully submerged, and have total
contact with the solution.
When using the balance, we will make sure that the balance is reading
zero each time before I put the pieces of potato on it. This will be
done so that we don't get a false reading.
I will also be reading the measurements of the measuring cylinder by
reading the bottom of the meniscus. This is the correct place to
accurately get the exact measurement.
Carrying out the experiment in a constant temperature for the three
hours of experiment is important. To avoid temperature change, which
might affect the sets of results, we will take the temperature of the
solution into account.
Small things such as a dirty test tube, and a slightly cracked
measuring cylinder could still affect the results, great care will be
taken to check the equipment before using it.
We will be using a very sharp knife, which could injure someone if
it's not handled properly. The scales were kept dry from water as to
not break them or get an electric shock. I will try not to get the
sucrose on my skin and especially not my eyes as it could do some
damage. Care will be taken when cutting the potato with the apple
corer for it is sharp. Care when carrying boiling tubes will be taken
as not to drop them.
Repeats of the same experiment (grams)
1st set of results
2nd set of results
3rd set of results
4th set of results
% increase or decrease
I feel that the experiment carried out was performed accurately and
gave firm, reliable results. We can use the graphs to see how accurate
the results were. Looking at my graph we can see there were no major
anomalies they all followed a trend and most of the points were on the
line of best fit. The graph made a backwards 'S' shape and using the
letters on the graph I can accurately analyse it. At point 'A' the
potato has gained weight from 1.0 grams increasing by 24.2% to 1.242
grams. The line of best fit then slowly decreases to 1.185grams at 0.4
moles. This occurs because the concentration of sugar will be higher
in the potato, which means that there was a low concentration of
water. Outside the potato in a low concentration (hypo tonic solution)
there will be less sugar and therefore more water, the water will move
due to osmosis down the concentration gradient into the potato. Point
'B' on the graph is indicating where there would be equal movement
between the potato and sucrose. This would be the dynamic equilibrium
and the solution of sucrose outside would be isotonic. Looking at the
graph we see that the dynamic equilibrium is at 0.48 moles of sucrose.
The line then plunges down to the negatives where the potato starts to
loose weight. At point 'C' and at 1.0 moles/dm3 the weight of the
potato has decreased by 25.3% to 0.747. This would happen because In
the higher concentrations (hyper tonic solution) the water
concentration will decrease as the sugar level increases and the water
will move out of the potato from the low concentration of water to the
higher concentration outside the potato. The line of best fit then
shallows and then the last point starts to drop off again. Looking at
this point I think that I could be an anomaly because the other points
at 'C' show the decrease in mass to be getting shallower. However the
results all follow the same trend. This trend was shown in my
prediction. From my graph and results I can see that my prediction was
Plant cells expand until their cellulose walls can stretch no more, or
until pressure from surrounding cells stops them from expanding. The
cells are fully turgid. The stretched cellulose wall and surrounding
cells exerts an inward pressure, which eventually equals the osmotic
pressure of the cell sap, so osmosis stops. This process is called
plasmolysis. There will be a limit as to how much water can be
obtained and lost from the potato cells. My graph and results do not
show this and if we had done a larger range of concentrations we could
have found out where these limits were.
Using the evidence I have collected I can form a firm, accurate
As a 1 gram potato piece is added to the low concentrations of 0.0 to
0.4 it will gain mass and then start to loose mass until there is
equal movement between each substance in a dynamic equilibrium. The
mass of the potato will then loose more mass in the higher
concentrations from 0.6 to 2.0. I can also conclude that the isotonic
point is between 0.4 and 0.6 moles/dm3 of sucrose. I know this because
between these two points is where the potato changed from gaining
weight to loosing weight. The graph's line of best fit shows this
point to be a 0.48moles.
The experiment was preformed exactly to my method and the results show
how accurate the experiment was. There were no major anomalies in the
results and they all followed the trend. The percentage increase shows
us exactly how much mass the pieces of potato increased by. Working
out a percentage makes it easier for us to analyse the results and
interpret the results.
Weighing the pieces of potato was the hardest thing to get accurate
along with the cutting of the pieces of potato. I had to trust the
electronic balance I used, they were not the most accurate and only
gave to two decimal places. I did not trust how accurate the last
number was and this not being accurate could have altered my results.
I thought this because the scales did not look like very good quality.
If I were to repeat this experiment I could weigh each potato piece to
a more accurate scale. I could find 0.000 grams instead of 0.00 grams,
which is what I have been finding. This would increase the accuracy of
the results dramatically. However this could be too accurate because
the quality of the equipment used would not be accurate enough to make
it worth while.
If I were to repeat the experiment I would possibly find a machine to
cut the potato as it would ensure that all potatoes would be the same
weight and dimensions. When measuring the amounts of sucrose water I
had to poor it into a measuring cylinder, which was not the best way
to accurately measure the amount of sucrose. I did remember to measure
the amount at the bottom of the meniscus. I could use a burette. This
would ensure that I have an accurate amount of fluid in each test
There were not any anomalies but some were not as close to the line of
best fit as others. This would be caused different factors where there
was a lack of accuracy. When the potato chips were removed from the
test tubes and dried I may well have dried some potatoes more
thoroughly than others and so some would have more excess water, which
would add to the mass. If the experiment were to be repeated I could
find another way to dry the potatoes that would ensure that all were
dried in the same way for the same time. However with all this said I
think that the experiment was successful.
I could use a wider range of concentrations to find the flaccid and
turgid points. This would be useful because I could find out the
limits of the potato piece. I could also find, accurately, the
isotonic solution where there would be equal movement between the
potato and sucrose. This was one of the reasons I chose to change the
concentration but in the range of concentrations that I used I could
not find the isotonic solution. I only know that it was between 0.4
and 0.6 moles of sucrose.