Year 12 Osmosis Practical
Carry out a practical investigation with supervision
This resource book contains:
-
Practise practical
Investigation plans
Investigation checklist
Model answers
Note: Evaluate means to justify the conclusion in terms of the method used. This will involve,
where relevant, consideration of the:
 reliability of the data and validity of the method
1
Year 12 Biology – Osmosis Practical Practise
Water Balance in Living Apple Cells
INTRODUCTION
In living cells the cell membrane is selectively permeable, which means that while water molecules are able to
diffuse through it from high to low concentration, larger molecules may not. If the water concentration within
the cell is lower than that in the surrounding medium, water will diffuse in. Likewise, a higher concentration
inside the cell will result in water diffusing out. No net movement occurs if the water concentrations on both
sides of the membrane are the same. Osmotic equilibrium may exist in such a case.
In the following experiment, apple/swede/carrot/potato cylinders are weighed, placed in sugar solutions of
various concentrations, and re-weighed the next day. Their weight gain or loss will enable us to determine the
strength of the solution which just balances that inside the apple cells.
AIM
To investigate water movement in and out of living plant cells.
MATERIALS (per groups)
1 apple/swede, 5 mm cork borer, 5 containers with lid, scalpel, paper towel or newsprint, accurate balance
reading to 0.1 g, sucrose solutions made up as follows:
0.7 M =
23.9 g sucrose dissolved in 100 ml water
0.5 M =
17.1 g sucrose dissolved in 100 ml water
0.3 M =
10.3 g sucrose dissolved in 100 ml water
0.1 M =
3.4 g sucrose dissolved in 100 ml water.
(These will have already been prepared for you)
METHOD
1
Use a cork borer to cut 5 cylinders from the apple/swede. Trim these to an equal length – about 4-5
cm. (Alternatively you could use a scalpel to cut the apple into rectangular “chips”.)
2
Label the five containers and add the four sucrose solutions to four of the containers and water to the
fifth.
3
Weigh each apple/swede cylinder and record the results on a table like the one below. Pat dry.
4
Place a piece of apple in each cylinder. Leave overnight.
5
Next day, lightly dry and re-weigh the cylinders, recording their new weights on the table.
6
Calculate the change in weight and work out the percentage weight change. (This enables a fair
comparison to be made between cylinders of slightly different size).
Difference in weight x 100
% Weight Change =
Original weight
2
WRITTEN REQUIREMENT
1
Write the title and aim of the experiment.
2
Summarise the method (you may use a diagram).
3
Results
a
Rule up and complete a table like the one below.
0.7 m
Sucrose
0.5 m
Sucrose
Results Table
0.3 m
0.1 m
Sucrose
Sucrose
Distilled
Water
Final
weight
Original
Weight
Difference
In weight
Average %
Weight
change
b
Plot your results on graph paper. Labelling the axes as shown below.
Loss
%Weight
change
of
apple
Gain
Graph of Weight Change in Apple in Sucrose Solution
Of Different Concentration
+10
+5
0
-5
10
0
4
Sucrose concentration
0.7 m
Conclusion
a
Is there any relationship between the changes in cylinder weight and the concentrations of the
solutions? Explain your results in terms of osmosis.
b
What concentration of sucrose would appear to be closest to the internal concentration of
dissolved materials in the apple cell cytoplasm?
3
Investigation Plan Sheet for Osmosis Practical
1.
Purpose of investigation (this may also be in the form of an aim, testable question, prediction or
hypothesis)
2.
Which variable will be changed? (This is the independent variable)
Give a suitable range of values for this variable
3.
Which variable do you think will have to be measured or observed in order to get some data or
information from the investigation? (This is the dependent variable)
How will it be measured or observed?
4.
What other variables do you think are important in the investigation and will have to be controlled to
make it a fair test?
Variable
What influence could this have on your investigation and how will this variable be
kept the same (controlled) or measured?
1. How will you ensure that your results are reliable?
6. List the equipment you will need:
4
Use the information questions 1 to 7 to write a method for carrying out your investigation.
• Your method must have enough detail to enable another student to repeat the experiment.
• Draw diagrams if appropriate.
5
Student Checklist for Investigation Plan
Use the checklist to see if your investigation plan meets the requirements of a workable one. If you find you
cannot tick a given requirement then you will need to change or add to your plan.
YES
NO
Has a prediction, aim or hypothesis been stated and is it clear?
Has the dependent variable been identified and have you
specified how you will measure it?
Has the independent variable and its range been identified
and its range and units
Are all other variables been identified and have you explained
how and why you are going to control them?
Are any sources of bias in your experiment been identified?
Have you specified how you are going to collect enough data
to make your experiment valid?
Could your METHOD be followed by another person with a
reasonable knowledge of science equipment?
Has your data been presented in a well constructed stand
alone table?
Has your data been processed in some way?
Are any graphs completely self explanatory?
Has a CONCLUSION been written that links the purpose of the
investigation with the processed data?
Does your DISCUSSION link your findings with the process you
are investigating?
Does your EVALUATION consider the reliability of the data and
the validity of the method used?
Example Results
Sugar Conc.
Original weight
(g)
0
9.2
Final weight (g)
Difference in weight
(g)
% weight
change
13.7
0.1
9.3
12.3
0.3
9.5
11.2
0.5
9.1
9.2
0.7
8.7
7.7
6
Water Balance in Living Apple Cells: Model Answers
AIM: To investigate water movement in and out of living plant cells.
Apparatus: 1 apple, 1 knife, 5 containers, towel, electronic balance, sucrose solutions made up as follows: 0.7
M, 0.5 M, 0.3 M, 0.1 M, These were already prepared
METHOD
The knife was used to slice the apple up into small pieces of approximate equal size. These then had their
mass measured and recorded. They were then placed into separate containers where they were submersed
in a sugar solution.
The containers were left overnight and re-weighed the following day.
% Weight Change =
Difference in weight x 100
Original weight
RESULTS
Final
weight
(g)
Original
Weight
(g)
Difference
In weight
(g)
% Weight
change
0.7 m
Sucrose
0.5 m
Sucrose
0.3 m
Sucrose
0.1 m
Sucrose
Distilled
Water
3.801
4.038
4.060
4.150
4.241
3.980
4.100
4.000
3.990
4.020
-0.179
-0.062
0.06
0.16
0.221
-4.49%
-1.5%
1.5%
4.01%
5.49%
Loss
%Weight
change
of
apple
Gain
Graph of Percentage Weight Change in Apple pieces in Sucrose Solutions
Of Different Concentration
+10
+5
0
-5
10
0
0.7 m
Sucrose concentration (mol-1 l-1)
Conclusion
7
Is there any relationship between the changes in cylinder weight and the concentrations of the solutions?
Explain your results in terms of osmosis.
Yes, there is a relationship between the final weight of the apple and the concentration of the sucrose in the
solution. The graph shows that as the concentration of the sugar increased the percentage weight change
decreased. As the concentration of the sugar in the solution increases the water concentration decreases,
this creates an osmotic gradient between the piece of apple and the solution. If the water concentration
outside the apple is greater than that inside the piece of apple then water will move into the apple by osmosis
(water moving from an area of high concentration to an area to low concentration). If the water
concentration inside the apple is greater than that outside the piece of apple then water will move out of the
apple by osmosis.
What concentration of sucrose would appear to be closest to the internal concentration of dissolved
materials in the apple cell cytoplasm?
If the internal concentration of the piece of apple is the same as the external concentration then there would
be no osmotic gradient, so there would be a zero level of osmosis. So from the graph the zero level for
percentage weight change is 0.4M.
Try these, some have questions, some have animations but all have some explanations
1.
2.
3.
4.
5.
6.
7.
8.
9.
http://en.wikipedia.org/wiki/Osmosis
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm
http://www.purchon.com/biology/osmosis.htm#definition
http://www.vivo.colostate.edu/hbooks/cmb/cells/pmemb/osmosis.html
http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf
http://edtech.clas.pdx.edu/osmosis_tutorial/
http://www.mun.ca/biology/Osmosis_Diffusion/tutor2.html#q1
http://biog-101-104.bio.cornell.edu/BioG101_104/tutorials/osmosis.html#anchor44430
http://science.nhmccd.edu/biol/Osotutor.html
Excellence Exemplar
Purpose: (can be in the form of an aim)
Purpose stating what
will happen to the chips
in respect to osmosis
1.
To investigate the osmotic effect of changing sucrose
concentrations on potatoes
2.
When chips are placed in solutions with high concentrations of sugar ie
0.7 mol L-1, the potato chips will lose mass as the chip will lose water
because of osmosis.
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Method:
1
2
3
4
5
6
7
8
9
10
L-1,
L-
Collect distilled water and sugar solutions; 0.1 mol
0.3 mol
1, 0.5 mol L-1, 0.7 mol L-1.
Fill 3 test tubes with 10 mL 0.1 mol L-1 solution. Clearly label
each test tube.
Repeat with each of the other solutions
Cut 15 chips from the same potato to the same size and shape
using a cork borer.
Blot each chip with a paper towel.
Weigh each chip and place each one in each of the test tubes.
Record the weight of the chip and the test tube its in.
Cover each test tube with gladwrap.
Place all the test tubes in the same place in the fridge so that the
temperature remains the same.
Leave all chips in the solution for 12 hours.
Remove a chip, blot it dry with a paper towel, when reweigh it.
Record the final weight. Use the same electronic scales to
ensure consistency.
A method includes:
 valid range IV - five
conc including distilled
water plus 0.3 mol L-1
or below and above 0.3
mol L-1 (step 1);
 DV – mass of chips
before (step 6) & after
(step 10);
 Repeats (step 2 & 4)
 OV - volume of solution
(step 2), same potato
(step 4), same
shape/size chips (step
4), blotting dry before
weighing (steps 5 &
10), cover to stop
evaporation (step 7),
stated time (step 9),
same temperature
(step 8), same scales
(step 10).
Observations:
The chips in the water (0 mol L-1 sugar concentration) were slightly larger and felt firm, turgid. The chips in the
0.7 mol L-1 were soft, flaccid. The chips in the 0.3 mol L-1 were much the same as they started.
Sugar Conc. Mol
L-1
Original mass (g)
Final mass (g)
Difference in mass (g)
% mass change
3.0
4.0
1.0
33
O
3.0
4.1
1.1
37
mol L-1
3.3
4.2
1.1
33
3.0
3.5
0.5
17
0.1
3.2
3.6
0.4
13
mol L-1
2.9
3.3
0.4
14
3.0
3.0
0
0
0.3
2.9
3.0
0.1
3
mol L-1
3.2
3.2
0
0
3.2
2.8
-0.4
-13
3.0
2.6
-0.4
-13
3.1
2.7
-0.4
-13
3.1
2.2
-0.9
-29
3.3
2.4
-0.9
-27
3.0
2.0
-1.0
-33
0.5
mol
L-1
0.7
mol
L-1
Mean % mass
change
34
15
1
-13
-30
9
Mean % change in mass of chips in different sugar
concentrations
Sufficient data
(valid range, 5
conc’s and repeats)
recorded and
processed to show
trend.
40
30
Mean % mass change
20
Trend shown by
accurate calculation
of % mass change
OR by the
appropriate graph.
10
0
0
0.2
0.4
0.6
0.8
-10
-20
-30
-40
Sugar concentration (mol L-1)
Valid conclusion
reached based on the
processed data in
relation to the purpose
of the investigation
Conclusion:
My results showed that the chips gained mass in water and low sugar
concentrations but lost mass in high concentrations of sugar.
This is what we predicted in higher sugar concentrations because in the these
solutions the water will move out of the potato cells by osmosis.
Discussion
When the chips were put in distilled water they gain mass because the
chips gain water from the surrounding solution due to osmosis. The
process of osmosis causes a net flow of water, across the semi
permeable membrane, along the decreasing water potential gradient
from a solution with a high water-potential to one with a lower water
potential. The chips gain water because the distilled water has a higher
water potential than the chips.
In solutions with a sugar concentration less that 0.3 mol L-1 the water
potential of the solution inside the cell is still higher than that of the
surrounding sugar solution so water moves into the potato cells by
osmosis. In the 0.3 mol L-1 solution the water potential of the solutions
inside and outside the cell are very similar so there is very little net flow
of water into the potato cells as can be seen by the relatively small gain
in mass of the potato chips.
Discussion uses
knowledge of the
process of osmosis
to explain the trend
or pattern in the
results:
 gained mass,
gained water,
due to osmosis
(water potential,
semi permeable
membrane, net
flow of water,
water potential
gradient)
 small mean %
change in mass,
small net water
flow
 loss of mass,
water loss, due
to osmosis
10
As the concentration of the sugar solution increases from 0.3 to 0.7 mol L-1 the
potato chip loses more and more mass because the difference in water potential
between the inside of the potato cells and the surrounding solution is
increasingly different. This means that when the chips are placed in the higher
concentration solutions more water will move out of the chips as the result of
osmosis.
EVALUATION:
My results showed that the chips gained mass in low sugar
concentrations but lost mass in high concentrations of sugar. This
conclusion is justified because I use a number of ways to ensure I
used the best method possible to make my investigation a fair test.
For example I used a cork borer to make sure all the chips used were
cut as close to exactly the same shape as possible. This meant that
each chip had the same surface area available for diffusion of water
across the semi permeable membranes of the potato cells. The
repeats used showed similar results to each other.
Evaluation of the
investigation by
justification of the
conclusion in terms of
the method used, such
as sufficient data,
appropriate range of IV,
appropriate processing
using % mass change,
minimization or removal
of sources of errors,
limitations, bias.
A change made to my original method was to make the chips slightly
smaller than planned. This was because I had to get all 15 chips from
the same potato so that I could control the variable of water potential in cells in
different potatoes.
The method was carried out in a way that variables that could have changed the
rate of osmosis, such as evaporation and temperature, were controlled by
covering the test tubes with gladwrap and keeping them in the fridge for 12
hours.
11