This lab report summarizes an experiment on osmosis in potato cells. Potato slices were placed in distilled water, tap water, and a salty solution to test how cell mass changes with solution concentration. The slice in salty water lost 9.09% mass, while slices in distilled and tap water gained 54.4% and 17% mass respectively, showing that water moves from lower to higher solute areas. However, limitations include a lack of control and uneven potato sizes. The conclusions are that solution concentration affects water movement through cell membranes, but the missing control prevents validating the initial hypothesis.

1. IB1 Biology SL
Petra Smith
September 17th / 19th, 2010
Will increasing the salinity of the surrounding solution (hypertonic to the cells’ content)
decrease the mass of the potato slide? Comment [al1]: Your research question
should include a prediction whenever
possible.
Hypothesis: If the surrounding solution is hypertonic to the cell’s content, then the mass of Comment [A2]: An “if… then” sentence
the potato slice will decrease by at least 25% because water moves from areas of high water that clearly shows cause and effect.
potential to areas of low water potential.
Background: Comment [A3]: 1) significant and
relevant to the activity, 2) Sources are
We will measure the change in mass of three potato slices after they had been in given, 3) Toxicity of reactants and products
different aqueous solutions for 48 hours. The function of the plasma membrane is to control is included as needed, 4) ecologically
sound waste disposal is described
the passage of materials in and out of the cell. One example of this controlled transport
through the cell membrane is osmosis. According to Brown, osmosis is the movement of Comment [A4]: One way to cite… use
italics for author’s last name or institution.
water molecules from an area of high water concentration to an area of low water
concentration across a semi-permeable membrane. The main purpose of this lab is to test
how the concentration of the solution affects the mass of the potato slices:
1. The slice placed in distilled water is expected to become heavier. The reason for this is
that the region of high concentration of water is the distilled water as there are no
impurities. Therefore the solution would be hypotonic to the cells and the mass of the
slice of potato is expected to increase (Purchon). Comment [A5]: A second way to cite
work…
2. The slice kept in tap water should not change much in mass even though osmosis would
take place because the amount of water entering the cell would be the same as the amount
of water going out of the cell.
3. The slice kept in salty water should loose mass. The reason for this is that the
surrounding solution would have a higher solute concentration and water should move out
of the cells.
Through the change in mass of the potato slices, we can find out the direction of Comment [A6]: Notice: use “we” ,
“one”… but not “I”
movements of the materials which passes through the plasma membrane. The plasma
membrane is semi-permeable membrane, which means that it allows some molecules to pass
and blocks other molecules if not needed (Nelson).
Toxicity Comment [al7]: As / when needed
Chemical Toxicity Precaution Disposal
Variables:
a) Independent: Composition of the solution (%)
b) Dependent: Mass of the potato slice (g)
c) Controlled variables: Comment [A8]: Identify 1) dependent
variable w/ unit. 2) independent variable
w/ unit. 3) Controlled variables w/
Variables that need to be controlled / reason How are these variables controlled in the explanation.
procedure
The source of tap water must be kept constant from Use only one pre-selected faucet
one trial to the next; different taps in the schools
might have different impurities and or salts which
will affect bulk transport.
1

2. The water temperature must be kept the same Always use the same water source; keep
because different temperatures will affect osmotic insulated; measure the temperature to double
rate and our experiment is time sensitive. check
The relative thickness and size of the potato slices Use a ruler to cut exact sizes.
should be kept constant between trails so that the
surface area for osmosis to take place is not
affected.
Comment [A9]: Only included in a lab
report when YOU design the activity
Comment [A10]: Only included in a lab
Materials: report when YOU design the activity.
Refer to handout
If so: Develop a method that allows for the
collection of sufficient relevant data. The
method should include explicit reference to
how the control of variables is achieved. If
Procedure: the control of variables is not practically
Refer to handout possible, some effort should be made to
monitor the variable(s).
Comment [A11]: Relevant / carefully
organised raw data in DT
Data Collection:
Comment [al12]: Indicate dependent
Osmosis in potato slices vs. independent variables
Mass Comment [A15]: Qualitative data: a
sample size of at least five is needed if error
(DV) Initial Mass Final Mass analysis is to be carried out.
Qualitative data
[Sol] (g) ±0.01 * (g) ±0.01 *
(IndV) Comment [A13]: Columbia University
tutorial: Estimating errors:
The color of the water is brown http://phys.columbia.edu/~tutorial/estimatio
Edges of potato are brown / The n/tut_e_2_1.html
Salt water, 50% 1.43 1.30
slice remained almost the same, it
Comment [A14]: Variable, unit,
felt a bit stiffer. uncertainty as heading to a column
The water is blurry / The slice Comment [A16]: Centered quantitative
Distilled water 1.69 2.61 became soft and seemed to have raw data with proper number of sig figs as
contents of a column
shrunk.
Some white particles on the surface Comment [A17]: Suitable
Tap water 1.94 2.27 presentation (spreadsheet, table,
/ The slice had become harder.
* Uncertainty rationale: for electronic devices, we use the smallest reading possible. graph, chart, flow diagram, and so on)...
clear, unambiguous headings for
Data Processing calculations, tables or graphs. Graphs
need to have appropriate scales,
1. Calculate the change in mass of the potato slices labeled axes with units, and accurately
(Final mass – Initial mass) = change in mass plotted data points with a suitable best-
a. In salt water: fit line or curve (not a scattered graph
1.30g (± 0.01) – 1.43g (± 0.01) = - 0.13g ±0.02 with data-point to data-point connecting
lines)... metric/SI units are expected…
b. In distilled water: correct number of significant figures...
2.61g (± 0.01) – 1.69g (± 0.01) = 0.92g ±0.02 uncertainties associated with the raw
data must be taken into account.
c. In tap water: Comment [A18]: Uncertainty as stated
2.27g (± 0.01) – 1.94g (± 0.01) = 0.33g ±0.02 in DC
Comment [A19]: Columbia University
tutorial: Calculating with errors:
http://phys.columbia.edu/~tutorial/propagati
on/
2

3. 2. Calculate the percent of water gained or lost
(Initial mass / 1 = mass change / x) * 100
a. In salt water:
(1.43 / 1 = 0.13 / x) * 100 = - 9.09%
b. In distilled water:
(1.69 / 1 = 0.92 / x) * 100 = 54.4%
c. In tap water:
(1.94 / 1 = 0.33 / x) * 100 = 17.0%
3. Graph: Comment [A20]: Whenever possible
include a graph. Link: Making a graph with
Excel:
% water gained / lost by potato slices after 48 http://qrc.depaul.edu/StudyGuide/MakingG
hours raphsWithExcel.htm Make sure to include
title, variables, and units.
60 For discrete variables (as in this example):
use a bar graph (with columns NOT
touching)
50
For continuous variables: Use a best-fit
40
Water movement (%)
line graph
30
20
10
0
Salty Sol Distilled water Tap water
-10
-20
Type of solution
Weaknesses and limitations: Comment [A21]: Comment on design
and method as well as the quality of the
Problem Type of error Significance of weakness Suggestions for data.... not only list the weaknesses but
(effect) improvement must also appreciate how significant
Data collected Random error Small number of values do data collection would the weaknesses are. Comments about
data for three not allow for elimination of benefit from having the precision and accuracy of the
different those other values that do five points of data (five measurements are relevant here. When
solutions only not “fit the line.” different
evaluating the procedure used, look at
concentrations) and
the processes, use of equipment and
five trials per
concentration time management
absence of a Design error The lack of a control limits By creating “potato
control trial our ability to quantify the juice” using a blender The modifications proposed should be
results for the hypotonic and and cheese cloth we realistic and clearly specified. It is not
hypertonic solutions could have made a sufficient to state generally that more
hypotonic solution; precise equipment should be used.
placing a potato slide in
this “juice” would have
provided us with a
control.
3

4. Problem Type of error Significance of weakness Suggestions for
(effect) improvement Comment [al22]: When your chart is
divided between two pages, include the
Uneven size of Systemic error: procedure instructed us to using a ruler to cut heading again…
the potato produced by the use a knife or scalpel to cubes with exactly
quality / cut “similar” potato slices the same size, 2x2 cm
calibration of the for example, will
equipment used. keep the contact area
between the potato
and the solution
constant
Conclusions: Comment [A23]: States a conclusion,
Different concentrations of solute do affect the net movements of water molecules with justification, based on a
through the plasma membrane in potato cells. However, the absence of a control group does reasonable interpretation of the data...
not allow us to validate our hypothesis as stated. include comparisons of different graphs
Our data shows that water moved from lower solute concentration to higher solute or descriptions of trends shown in
concentration. Hypertonic solutions, which have higher solute concentration and therefore graphs. The explanation should contain
lower water concentration, cause the cells to loose mass and shrivel because water moves out observations, trends or patterns
of the cell through the plasma membrane. This was the case with the slice left in salty where revealed by the data.
the sample lost 9.09% of its original mass.
Hypotonic solutions, on the contrary, have lower solute concentration and higher
water concentration, and cause the potato cells to expand because the water moved into the
cells. In this case, the distilled water and the tap water samples were hypotonic to the potato
cells and hence the potato slices gained 54.4% and 17% mass respectively. These results also
indicate that the distilled water was more hypotonic than the tap water compared to the potato
cells as the water mass gained by the potato slice was considerably higher in distilled water.
Our conclusions are supported by Riter who states that the composition of the solution Comment [al24]: Include support
sources
affects the direction water moves across the plasma membrane.
Works cited: Comment [AL25]: Offers at least 2
MLA style reputable bibliographic sources
(no dictionaries or Wikipedia); entries are
Baker, Mallinckrodt. "Material Safety Data Sheet." 26 Sept. 2007 indented and alphabetized.
<http://www.jtbaker.com/msds/englishhtml/S3338.htm>.
Brown, Terry. "Osmosis." 1999. 26 Sept. 2007
<http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm>.
Geibel, Greg. "The Cell Membrane." Menloschool. 18 Sept. 2007
<http://sun.menloschool.org/~cweaver/cells/c/cell_membrane/>.
Purchon, Nigel D. "Osmosis." 1997. 26 Sept. 2007
<http://www.purchon.com/biology/osmosis.htm>.
Ritter, Bob. Biology 11. Toronto: Kevin Martindale, 2002.
Comment [A26]: Others:
. 1)Pages are numbered
2) Manipulative skills are assessed
summative: see IB guide, p. 28
4