I.A. # 10 ESTIMATING THE WATER POTENTIAL OF POTATO TUBER CELLS
Date: January 30, 2025
NAME: ______________________________________ GROUP: _______
Skills: AI, MM
Intro: If pieces of potato tuber tissue (made up of closely packed cells) are placed in different concentrations of
sucrose solutions, the potato cylinders will gain or lose mass, depending on the water potential of the solution. If
there is no change in the mass of the tissue, then the sucrose solution has a water potential which is the same as
that of the tissue itself. In this investigation, you will establish the mean water potential of the potato cells. [60m]
Preliminary action: Based on your knowledge of osmosis and plant cells, record a reasonable hypothesis for this
experiment in terms of predicted gain or loss of mass of the potato cylinders with sucrose concentration. [1]
Hypothesis:
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Identify the variables in this experiment: [2]
Independent variable:
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Dependent variable:
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State THREE variables that should be standardised in this experiment – known as controlled variables (factors that
should remain constant throughout the experiment, so that results can be valid). [3]
1.
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2.
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3.
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PART 1: Creating a dilution series.
Provided with a 1M sucrose stock solution (1 mol dm-3), create a range of four different concentrations
(0.2M, 0.4M, 0.6M, 0.8M) making up a total of 20 cm3 of each. Distilled water is provided.
N.B. To prepare a dilution series, you first must calculate the volume of the stock solution needed to make
up each dilution, using the equation:
𝑟�𝑒�𝑞�𝑢�𝑖�𝑟�𝑒�𝑑��
𝑐�𝑜�𝑛�𝑐�𝑒�𝑛�𝑡�𝑟�𝑎�𝑡�𝑖�𝑜�𝑛��
x final volume required
𝑐�𝑜�𝑛�𝑐�𝑒�𝑛�𝑡�𝑟�𝑎�𝑡�𝑖�𝑜�𝑛��𝑜�𝑓��𝑠�𝑡�𝑜�𝑐�𝑘��𝑠�𝑜�𝑙�𝑢�𝑡�𝑖�𝑜�𝑛�
You can then calculate the volume of distilled water to add to the stock by subtracting the volume
calculated in the first step from the final volume required.
e.g. if 20 cm3 of 0.9 mol dm-3 sucrose solution was required, using a 1 M stock solution:
Step 1:
x 20 = 18 cm3 of stock solution
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Step 2: 20 – 18 = 2 cm3 distilled water
Complete the table below. [4]
TABLE 1: MAKING UP A SUCROSE DILUTION SERIES
FINAL CONCENTRATION OF
SOLUTION/ mol dm-3
Volume of 1M sucrose solution
added/cm3
Volume of distilled water
added/cm3
0.0
0
20
20
0
0.2
0.4
0.6
0.8
1.0
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Label six boiling tubes appropriately, and to each add a different sucrose solution.
PART 2: Preparing the potato cylinders.
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You have been provided with 6 potato cylinders. Standardise the length to the shortest one, trimming
carefully on the tile, using the razor. Gently blot each dry, gently removing any surface moisture (do not
squeeze).
Weigh the first cylinder on filter paper, record the mass, and place it in the 0M Petri dish. Repeat this
procedure with the remaining tubes. [Ensure that you first weigh the filter paper so that this can be
subtracted to give the accurate mass of the potato]. Record your initial measurements in the table.
Leave the cylinders in the solutions for 30 minutes, having stoppered them firmly with the bungs.
After 30 minutes, remove the cylinders one at a time (in the same order that you inserted them); gently
blot the surface of the potato and reweigh individually, recording the final mass of each.
RESULTS:
TITLE:_________________________________________________________________________________________
Concentration of sucrose
solution/mol dm-3
Initial mass of
cylinder/ ____
Final mass of
cylinder/ ____
0.00
0.20
0.40
0.60
0.80
2
Change in
mass/____
Percentage change in
mass/%
1.00
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Complete the table, ensuring that you include the units in the headings. [1]
Calculate the percentage change in mass (divide the difference by the original mass). Indicate,
using + or -, whether it is positive (an increase) or negative (a decrease). Go to 2 decimal places. [6] ●
Show ONE sample working of the percentage change in mass in the space below. [2]
DATA ANALYSIS:
Plot a graph of the percentage change in mass against the molar concentration of the sucrose solutions.
Give your graph a suitable title and label your axes appropriately. Allow for increase & decrease on the
vertical axis. Draw a LINE of BEST FIT through the points. [6]
Use the graph to determine the molar concentration that is equal to the water potential of the potato
tissue. Give a reason for your answer, stating clearly what is happening to the osmotic movement of
water, through the cell membrane, at this concentration.
Concentration of sucrose solution: __________________ [2]
Reason: [2]
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The graph below shows the relationship between the molar concentration of a sucrose solution and its
water potential. Use the graph and your answer above to estimate the solute potential of the contents of
the potato cells. [2]
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Estimated water potential of the contents of the potato cells: _____________________
Define the term ‘osmosis’. [2]
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Explain in terms of water potential, the percentage change in mass that occurred in distilled water. [2]
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Describe and explain, in terms of water potential, the changes in mass that occurred in the different
sucrose solutions. (It would be useful to group the solutions in terms of the direction of movement and to make
reference to the rate or extent of movement in each instance). [6]
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Given the general trend observed, do your results align with your initial hypothesis? Explain your answer
briefly. [2]
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State any two precautions that were taken in the procedure. [2]
1____________________________________________________________________________________
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2___________________________________________________________________________________
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What are two limitations that were encountered? Indicate briefly how each might impact the results
obtained. [2]
1____________________________________________________________________________________
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2___________________________________________________________________________________
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Another way to perform this experiment is to measure the length of each potato stick, rather than its
mass. Suggest why measuring a change in mass is more likely to give a true value for water potential of
potato cells than you would get by measuring the change in length. [2]
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Suggest two ways in which you could improve the procedure to make your results more reliable. [2]
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2___________________________________________________________________________________
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Given the images of potato cells below, in light of your results, label each with an appropriate sucrose
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concentration. Give a clear summary statement in each case, indicating the net water movement and
where the water potential was higher or lower, inside or outside of the cell, using appropriate symbols
(e.g. Ψw inside cell > Ψw outside cell, therefore net movement of water was IN/OUT.) [9]
CONCENTRATION________________________________
NET WATER MOVEMENT__________________________
SUMMARY_____________________________________
CONCENTRATION________________________________
NET WATER MOVEMENT__________________________
SUMMARY_____________________________________
CONCENTRATION________________________________
NET WATER MOVEMENT__________________________
SUMMARY_____________________________________
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