not change for the 0.6M external sugar solution because the internal and external
concentrations were equal. For the 0.8M and 1.0M solutions, the mass of the visking
tubing decreased because the external sugar solution was greater and water molecules had
passed out of the visking tubing by osmosis. In conclusion, there was a greater change in
mass when the difference between the external and internal concentrations was higher
because there were more free water molecules in the less concentrated solution that were
able to pass though the membrane by osmosis.
Graph of Results
In the preliminary experiment, the visking tubing acts like the cell membrane and
cell wall of a plant cell combined, since it is partially permeable and allows water
molecules to diffuse in and out by osmosis. Therefore by using evidence from the
previous experiment, I predict that in the experiment on potato chips, the results will form
a similar graph. Where the concentration of the external sugar solution is weaker than the
internal concentration of the potato chip, there will be an overall net movement of water
molecules passing into the chip by osmosis, so the chip will gain in mass. Where the
concentration of the external solution is stronger than the internal concentration, more
water molecules will pass out of the chip, so it will lose mass.
I predict that the results will form a graph like the one below, which can be used to work
out the internal concentration of a potato cell:
The internal concentration of the potato chip is the point at which the line of best
fit crosses the x-axis on the graph because there is no increase or decrease in mass of the
potato chip. Therefore the concentration inside the potato chip must be equal to the
external sugar solution, so this is the internal concentration of a potato cell.
Method:
Apparatus: Sugar solutions - 0.2M, 0.4M, 0.6M, 0.8M, 1.0M
Distilled water - 0.0M solution
Boiling tubes
Balance
Paper Towels
Scalpel
Cork Borer
3 Large Potatoes
Cut out 12 potato chip cylinders of equal circumference using the cork borer. Cut
the chips to the same length of 4cm so the surface area remains the same on each chip and
osmosis can occur over an equal area. Take off any potato skin using the scalpel because
the skin is not selectively permeable so it may effect the results by reducing the surface
area over which osmosis occurs. Dry the chips using the paper towels, to remove surface
moisture caused by cell sap which is released when potato cells and damaged. This is
done because the surface moisture may cause a greater mass, or contaminate the solution
it is going in to.
Take the mass of all the potato chips on the balance. Fill the boiling tubes with
the 6 different solutions of 0.0M, 0.2M, 0.4M, 0.6M, 0.8M and 1.0M. Place the chips in
the boiling tubes in a specific order, to avoid error on the time length for which they are
in the solutions. Make sure the solution covers the whole potato chip. Leave the chips in
the solution for 30 minutes. Take the chips out of the solution in the order they were
placed in. Dry the chips again on the paper towels and record the mass. Calculate the
change in mass of each potato chip. The whole experiment should then be repeated and
average results obtained.
Fair Test:
To make the experiment fair, key factors need to be taken into account and
controlled. These factors affect the rate at which osmosis occurs:-
* pressure
* temperature
* surface area
* concentration gradient
* size of molecules
Osmosis occurs more quickly at a higher temperature or pressure, so these factors
are controlled as the experiment will be done under the same conditions throughout, at
room temperature. The surface area is controlled because more osmosis will be able to
take place over a greater surface area. The potato cylinders will be cut out using the cork
borer, and then cut to 4cm in length. Potato skin will be removed because it is not a
partially permeable membrane, so osmosis cannot occur across it. If the potato chip floats
to the surface of the solution in the boiling tube, this will be noted, because less surface
area will be exposed to the solution.
The concentration gradient is the variable. The size of potato molecules may vary
but this is difficult to control, although the same species of potato will be used throughout
the experiment. The same type of sugar (glucose) will be used in the solutions for all the
boiling tubes of differing sugar concentrations. The reliability of evidence will be
improved and anomalies will be reduced by repeating the results and obtaining the
average for each result.
Conclusion:
The graph shows the general trend that as the concentration of the external sugar
solution increases, the percentage change in the masses of the potato chips decreases, so
that it becomes negative. This negative correlation shows that the concentration of the
external solution is inversely proportional to the percentage change in mass, which is the
same pattern as the sketch graph I predicted in the hypothesis. In my prediction I stated
that “where the concentration of the external sugar solution is weaker than the internal
concentration of the potato chip... the chip will gain in mass” and that “where the
concentration of the external solution is stronger than the internal concentration... it will
lose mass”. My results support this prediction because the chips submerged in 0.2M
solution showed an average percentage increase in mass of 2.01%, whereas the chips
submerged in 0.4M solution showed an average percentage decrease in mass of 0.68%.
Therefore I can determine from these results that the internal concentration of a potato
cell is between 0.2M and 0.4M.
As I predicted, there was a percentage increase in mass of the potato chip when
the concentration of the external sugar solution was lower than the internal concentration
of the potato chip, because water molecules passed into the chip by osmosis. This occurs
because water molecules diffuse “from the dilute to the concentrated solution”, as there
are more free water molecules in a dilute glucose solution which can pass through the
pores of a partially permeable membrane. Sugar molecules are larger due to the water
molecules clustering around them, so they pass through the pores more slowly. So there
is a steady net movement of water molecules flowing from the weaker solution to the
stronger solution until there is an equal concentration on either side. As the water
molecules passed into the potato cells by osmosis, it caused the vacuole to expand,
pressing the cytoplasm against the cell wall. The potato cells swelled up so they could
contain a greater mass, and became turgid.
When the concentration of the external sugar solution was higher than the internal
concentration of the potato chip, the chip decreased in mass because there was a net
movement of water molecules passing out of the potato chip by osmosis. As water
passed out of the cells, the potato cells decreased in mass and became flaccid. The potato
chips gave a greater change in mass when the difference between the concentrations (i.e.
concentration gradient) was higher because there were more free water molecules in the
less concentrated solution, which were able to diffuse through the cell membranes of the
potato cells by osmosis.
In my hypothesis, I predicted that the graph can be “used to work out the internal
concentration of a potato cell”. It will be the point at which the line of best fit crosses the
x-axis because there is no percentage increase or decrease in mass of the potato chip. No
percentage change in mass occurs because there is equal amount of water molecules
moving in and out of the potato cells by osmosis, so the concentration inside the potato
chip is equal to the external sugar solution. Therefore, there is no net movement of water
molecules passing in or out of a potato chip which is submerged in an external glucose
solution of this concentration. In conclusion, according to the results on my graph, the
internal concentration of a potato cell is about 0.34M.
Evaluation:
The procedure used for this investigation was adequate because it enabled fairly
accurate evidence to be obtained, from which conclusions could be drawn. I think the
results were quite reliable and precise because the points plotted on the graph formed a
close correlation around the line of best fit. There is one slightly anomalous result on the
graph, for the average change in mass of potato chips in 0.8M sugar solution. The
percentage change is 1% lower than the line of best fit. Inaccuracies may have been
caused by human error, problems with apparatus or the procedure.
A potential problem with the procedure is that the time which the chips remained
in the solution may have varied slightly, because it was impossible to put all six potato
chips into the boiling tubes at exactly the same time. I tried to reduce this source of
inaccuracy by placing the chips in the solutions and removing them in a certain order, but
the reliability of evidence could be improved further by using a different stop-clock for
each solution. The surface area of the potato cylinders may have differed slightly because
it was difficult to cut the ends at an exactly vertical angle. This would affect the results
because if the end of a potato chip is cut at an angle, the chip will have a greater surface
area over which osmosis can occur. There will be more exchange of water molecules
which leads to a greater change in mass. Reliability can be improved by using a
guillotine to cut the ends of the potato cylinders at a completely vertical angle.
Anomalous results may also have occurred because some of the potato cylinders
floated on the surface when placed in the glucose solution. I have noted on the results
table that all the chips placed in 0.6M, 0.8M and 1.0M solutions floated to the surface.
These potato chips had a smaller surface area exposed to the glucose solution over which
osmosis could occur, so there would be less exchange of fluid and the potential change in
mass would be reduced. This problem could be overcome by using weights to prevent
the chips from floating, which would improve the accuracy of the experiment.
Inaccuracies may have been caused by drying the chips with a paper towel to
remove surface moisture, because you cannot be sure the chips were completely dry or
that each chip had been dried equally. Any remaining surface moisture could affect the
results by adding mass to the potato chips, or contaminating the external glucose solution
in which the chips were placed by altering the concentration. If the experiment were to be
repeated, a blow-dryer could be used to ensure that the chips were evenly and thoroughly
dried to remove all surface moisture.
Another factor which may have affected the results is the age of the potatoes. If
the potatoes used were of different ages, this would affect the results because the aging
process may reduce the cells ability to absorb and retain water. This problem could be
overcome by making sure the cylinders used were cut from the same potato. The potatoes
used in my experiment were all from the same species of potato. The size of potato cells
may also vary but this factor is difficult to control.
Although the evidence obtained in this experiment is sufficient to support my
hypothesis, it is not enough to prove it. To provide additional evidence for the
conclusion, it would be necessary to extend the enquiry. A further investigation could be
conducted to find out if the internal concentration of a potato cell is indeed 0.34M, and to
see if this internal concentration is the same for different types of potato cells. This could
be done by carrying out the following experiment:
Aim: To find out if all types of potato cells have an internal concentration of 0.34M.
Diagram:
Method:
Cut out potato cylinders of equal circumference, from different species of potato,
using the cork borer. Use a guillotine to cut all the potato chips to a length of 4cm, so the
surface area is the same for each. Dry all the chips using a blow-dryer and weigh on a
balance. Place the chips in test-tubes filled with 0.34M glucose solution and leave them
in the solution for 30 minutes. Remove the chips, dry them again using the blow-dryer
and reweigh. Calculate the percentage change in mass if any has occurred. If there is no
change in mass, the internal concentration of that type of potato cell is 0.34M.