Phil Than
Per 4
9.30.09
Osmosis and Diffusion Lab
Osmosis Part A: Potato Cores
Data collection and processing
Aspect 1:
(See papers turned in)
Aspect 2:
Sucrose Molarity
Distilled Water
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M
Potato Core Results
Percent Change In Mass (%)
Period 4
Period 6
Group 1 Group 2 Group 3 Group 4
13.8
18.88 ----------20
5.66
7.53 ----------11.1
6.38
12.96 -----------3.7
-21.8
-21.6 -----------30.9
-25.9
-21.2 -----------25.5
17.9
-27.4
-32.7
-30.2
Expected
Group 5
7.2
9.89
0
-13.9
-25.56
-30.7
21.4
6.9
-4.5
-12.8
-23
-23.5
Aspect 3:
Percent Change In Mass at Different Molarities of Sucrose Averages
Sucrose Molarity
Period 4
Junior Class
Expected
Distilled Water
16.34
14.97
21.4
0.2 M
6.6
8.55
6.9
0.4 M
9.67
3.91
-4.5
0.6 M
-21.7
-21.95
-12.8
0.8 M
-23.55
-24.54
-23
1.0 M
-20.62
-14.1
-23.5
Junior Class Potato Core: Percent Change In Mass
at Different Molarities of Sucrose Averages
20
16.34
10
9.67
6.6
0
-10
Distilled
Water
0.2 M
0.4 M
0.6 M
-20
0.8 M
-21.7
1.0 M
-14.1
-23.55
-30
Percent Change (%)
20
14.97
10
8.55
3.91
0
-10
Distilled
Water
0.2 M
-20
Sucrose Molarity
30
21.4
10
6.9
0
-10
-20
-30
Distilled
Water
0.6 M
-21.95
Expected Potato Core: Percent Change In Mass at
Different Molarities of Sucrose Averages
20
0.4 M
-30
Sucrose Molarity
Percent Change (%)
Percent Change (%)
Period 4 Potato Core: Percent Change In Mass at
Different Molarities of Sucrose Averages
0.2 M
-4.5
0.4 M
0.6 M
0.8 M
-12.8
-23
1.0 M
-23.5
0.8 M
-24.54
1.0 M
-20.62
Students used and shared the same materials (beakers, solutions, droppers,
funnels, flasks, etc) to carry out their labs which could have resulted in cross
contamination of said materials. Another error is the inaccurate measurements of the
students when adding solution into the beaker. Since students used different materials for
actually measuring the volume of solution added, the volumes were different from other
groups’ volumes. The inaccuracy of the scale could have skewed the results of the mass
of the potatoes. The amount of time the potato was in the solution could have been an
error because some of the potato cores had more exposure to the solution than others,
possibly skewing the results.
Conclusion and evaluation
From the data, it can be concluded that osmosis occurred between the potato cores
and the sucrose solutions. Osmosis is the movement of water down its concentration
gradient. In regards to the experiment, when the solution in the beaker was hypotonic to
the hypertonic nature of the potato cores, water diffused from the solution to the potato
cores; thus, increasing the mass of the potato cores. This can be seen from the data
observed from the experiment. In distilled water, the potato cores gained the most mass,
about a 16.34% and 14.97% increase, because water moved from the outside into the
potato cores, because the distilled water was very hypotonic in comparison to the potato
cores. Still, at 0.2 M of sucrose solution, the solution is still more hypotonic than the
potato cores, so water diffused into the potato cores, increasing the mass about 6.6% and
8.55%, to balance the solute concentrations, in and out of the potato cores. At 0.4 M of
sucrose solution, the expected result was that the potato core would lose about -4.5%
mass, due to the hypertonic sucrose solution. What really occurred was that the potato
cores still gained mass, around 9.67% and 3.91% increase, in Period 4 and the junior
class, respectively. Only at 0.6 M of sucrose solution, does the potato core, in both data
sets, actually lose mass; in period 4 around -21.7% change and in the junior class a 21.95% change in mass. This decrease in mass shows that water diffused out of the
potato core and into the surrounding solution, because at 0.6 M of sucrose solution, the
solute concentration in the potato core was lower than the outside; therefore, water had to
diffuse out in order to balance the concentrations.
A limitation of this lab was the time and the resources used. There was not
enough to perform the lab entirely as an individual or in groups, so the splitting up of
procedure limited the ability to obtain accurate results. The resources was a limiting
factor because we had to rely on other’s results which also limited the ability to obtain
results because the more students performing experiment, the more an error is likely to
occur. The lack of accurate measurements also limited the ability to have a control of
variables such as the amount of solution in the beakers.
Improvements to this lab include more time to perform the experiments, more
accurate materials (scales, flasks, beakers), and more materials (i.e. potatoes). Another
improvement to this lab is to eliminate the sharing of materials, giving each group their
own materials, including solutions, so as to rid the possibility of cross contamination.
Osmosis Part B:
Data collection and processing
Aspect 1:
(See papers turned in)
Aspect 2:
Normal Onion Cell
Normal, healthy looking
cells. Purple.
With NACL
Cells are smaller/shrunken.
Cytoplasm seems nonexistent. Traces of purple.
Flooded with Water
Cells regain some of their
original structure. But still,
shrunken. Traces of purple,
still.
Aspect 3:
Errors included the amount of NACL and Water added. Excess NACL and Water
could have been added, or not enough NACL and water were added, skewing the results.
Microscopy errors could have resulted and problems with the onion skin could have
skewed the results. Another error could be an error in preparing the slides.
Conclusion and evaluation
Based on what we observed under the microscope, it can be concluded that
osmosis occurred when the NACL was added to the slide and when the onion skin was
flooded with water. When the NACL was added, the cytoplasm of the cells diffused out
in order to balance the concentrations and when water was flooded into the slide, some of
the water diffused back into the cells. The shrunken cells after the NACL was added was
proof that the cytoplasm diffused out and the increase in size when the water was added,
shows that water really did diffuse back into the cells.
Again, a limitation when doing this lab was the time. The short amount of time
did not allow the wait for the salt water and the water to work at their full potential. As
soon as the NACl was added, the slide was put back under the microscope, and same with
the water. If more time was allotted, the exposure of NACL and water could have been
increased, allowing maximum results. Another limitation was the ability to obtain a
piece of purple onion skin. Students tore the onion up, disallowing latter groups to obtain
a good piece of skin to observe. Another limitation is the microscopes used. The
microscopes used were only able to magnify up to 40x, not the recommended 100x. This
means that it limited the ability to be able to observe the occurrences happening in, out,
and to the cell. The onion itself was a limitation because many students touched the
onion which could’ve damaged the onion, yielding undesirable results.
An improvement to this lab would be to increase the time allotted to perform the
experiment. This would decrease the amount of errors and could yield better results.
Better materials like microscopes would help observe the cell more in-depth to really see
what is happening to the cell. Another improvement is to have more than one onion, in
order to get better skin samples, and reduce the risk of damaging the onion which could
lead to bad results.
Osmosis Part C
Data collection and processing
Aspect 1:
(See papers turned in)
Aspect 2:
Dialysis Bag Results
Period 4
Period 6
Solution
Concentration
Distilled Water
0.2 M
0.4 M
0.6 M
0.8 M
1.0 M
Group 1
-4.16
0
7.69
9.09
10.7
14.8
Group 2
0
4
8
6.8
11.1
19.05
Group 3
--------------------------------------------------8.21
Group 4
0
4.4
8.63
11.22
16
12
Expected
Group 5
-12.2
1.53
11.1
----------16
20
1.2
3.1
7.7
11
14.8
18.2
Aspect 3:
Dialysis Bag Percent Change In Mass (%)
Solution
Period
Junior
Expected
Concentration
4
Class
Distilled Water
-2.08
-4.09
1.2
0.2 M
2
2.48
3.1
0.4 M
7.85
8.86
7.7
0.6 M
7.94
9.04
11
0.8 M
10.9
13.45
14.8
1.0 M
14.02
14.81
18.2
Period 4 Dialysis Bag Percent Change In Mass
14.02
10.9
10
7.85
20
18.2
15
14.8
11
10
7.7
5
0
1.2
Distilled
Water
3.1
0.2 M
7.94
0.4 M
0.6 M
0.8 M
1.0 M
Solution Concentration
5
2
0
-5
-2.08 0.2 M
Distilled
Water
0.4 M
0.6 M
0.8 M
1.0 M
Solution Concentration
Junior Class Dialysis Bag Percent Change In
Mass
Percent Change In Mass
(%)
Percent Change(%)
15
Percent Change (%)
Expected Dialysis Bag Percent Change In
Mass
20
15
13.45
10
8.86
14.81
9.04
5
0
-5
2.48
Distilled
-4.09 0.2 M
Water
0.4 M
0.6 M
-10
Solution Concentration
0.8 M
1.0 M
A big, possible error in the experiment was the results that came out to be a
negative or 0% change in mass. This could be due to a leakage in the dialysis bag which,
leaked solutions in and out of the bag. Another error was the sharing of materials and
solutions which could have led to cross contamination, which could have skewed the
results in a negative way. Inaccurate measurements of volume and mass could be another
error.
Conclusion and evaluation
In this experiment, an orange solution of 15% glucose and 1% starch was added
into semi-permeable dialysis bags, which were then placed into a beaker containing H2O
+ IKI, which was amber colored. Because the dialysis bags were semi-permeable, it
allowed select solutions and solutions to diffuse through. In this case, the iodine was
small enough to permeate through and diffuse through the bag, reacting with the starch
producing a black precipitate at the bottom of the bag. In contrast, the starch was too big
to go through the semi-permeable membrane, so it stayed in the dialysis bag, which led to
the iodine reacting with it. This experiment mimics the semi-permeable membrane of
cells in that the cell membrane is selective on what goes through. When distilled water
was in the beaker, the expected result was a increase of mass of 1.2%. But instead, a
negative change occurred, ending in a 2.08% average change in period 4 and a -4.09%
change in the junior class average. As expected, as the concentration of glucose in the
bags increased, the percent change in mass also increased. This is due to the fact that
more solution needed to diffuse through to the bags to balance the concentration gradient
difference between the two surroundings.
A limitation of this experiment was the fact that we had to rely on other students’
results. The more people that perform the labs increases the amount of possible errors
that can occur. Time was also a limitation because some bags did not have a precipitate
on the bottom because of the time constraints, and not letting the experiment run for as
long as we wanted to.
The main improvement to this lab would be to increase the amount of time to do
perform the lab. Increasing the time would allow the reactions to take place and allow
the iodine to diffuse through. Also, more time would allow the students to perform the
whole lab in their groups, and not in sections. This will allow more results which can
produce a better and more accurate data series. Another improvement would be to give
students their own separate materials, like glucose solution, measuring instruments, etc,
in an attempt to eliminate the possibility of cross contamination, which could’ve altered
the results.