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Coordinated Science Laboratory Work Report
Title of the Investigation: Osmosis and Potato Strips
Candidate’s Name: Daniel Njuguna
Date of the Investigation: 10/01/2011
Candidate Declaration: I confirm that this laboratory report is my own work, and is the final
version. I have acknowledged ALL other peoples’ ideas that I have used in carrying out the
experiment and writing this laboratory report.
Candidate Signature: Dnlnjuguna
Date: 20/01/2011
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Aim: To investigating the Effect of Different Concentrations of Sucrose Solution on the
Length of Potato Strips.
Background Information:
Osmosis is the diffusion of water molecules through a partially permeable membrane from a
region of high water potential to a region of low water potential until the water potential is
equal on both sides of partially permeable membrane. Osmosis occurs in all plant and
animals cell (Education 2003).
When potato cylinders are put in sucrose solution, cells will either take in water or lose by
osmosis depending on water potential of the solution in which they are put in. When potato
cells absorb water by osmosis, they will swell up, become firm, heavier and bigger in size but
they will not burst because the cell membrane is surrounding by a cell wall. The cells will
become turgid. On the other hand, when a potato cells lose water by osmosis, they shrink
and become flaccid i.e. they lose the firmness they had (Jones 2010).
Research Question: How does changing the concentration of sucrose solution from 0.2 to
1.0 mol dm-3 affect the length of potato strips?
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Hypothesis: if the sucrose solution concentration is increased from 0.2 to 1.0 mol dm, then
the length of potato strips will decrease. Increasing the sucrose concentration will lower the
water potential of the solution thus making water molecules to move from the potato strips to
the sucrose solution by osmosis leading to decrease in length of potato strips.
Variables:
Table 1.1 Identified variables
Independent variable
Dependent Variable
Controlled Variables
Concentrations of
sucrose solutions (0.2,
0.4, 0.6, 0.8 & 1.0 mol
dm-3)
Length of potato strips
after immersion in
sucrose solution
(i) surface area of potato strips
(ii) volume of sucrose solution used
(iii) temperature of the solution used
(iv) duration in which the strips are kept
in sucrose solution
Requirements:
(a)
Materials;
(i)
(ii)
(iii)
1X 10 paper towels
1X 3 large Irish potato
1X1 scalpel blade
(b)
Reagents;
10 ml of: 0.2 mol dm–3; 0.4 mol dm–3; 0.6 mol dm–3; 0.8 mol dm–3 & 1.0 mol dm–3 sucrose
solution.
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(c)
Apparatus;
(i)
(ii)
(iii)
(iv)
(v)
1X10
m measuring cylinder
1X5 Petri-dishes
1X150
transparent ruler
1X 5 mm diameter cork borer
1X1white tile
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Safety:
Take care when using a sharp blade to cut potato strips
Don’t hold the potato in your hands when cutting with a cork borer!
Use a white tile.
Procedure:
(i)
Push a cork borer through a large potato tuber and remove a strip of tissue from the
borer as shown in the diagram below. Repeat this procedure to obtain a total of ten
potato strips.
Source: (Mackean 2002)
(ii)
Chip off one end of each potato strip as shown in the figure below.
Source: (Education 2003)
(iii)
Starting from the chipped end, measure exactly 50mm length and cut the potato strip.
Repeat this for the other nine strips.
(iv)
Divide the potato strips into five groups of two strips each and place each group in
petri-dishes marked: 0.2 mol dm–3 ; 0.4 mol dm–3 ; 0.6 mol dm–3 ; 0.8 mol dm–3 & 1.0
mol dm–3 sucrose solution.
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(v)
Add 5 cm3 of respective sucrose solution to each of the five petri-dishes. Leave the
potato strips in their respective solution for 40 minutes.
(vi)
After 40 minutes, carefully remove the potato strips from each petri-dish and gently
dry them using paper towel.
(vii)
Re-measure the length of each potato strip and record these values in raw data
table.
(viii)
Gently feel the potato strips from all the five petri-dishes and squeeze them inbetween your finger and thumb. Record your observations in a data table
(ix)
Process the raw data as fully as possible, present the processed data using a line
graph and make conclusion based on the results obtained.
Control of variables:
Table 1.2 Ways of controlling or monitoring constant variables
Controlled variable
surface area of potato
strips
volume of sucrose
solution used
temperature of the
solution used
duration in which the
strips are kept
How it will be controlled
All potato strips will be cut using the same cork borer of the same
diameter (0.5 mm) and will be trimmed to the same length (50
mm) keeping the surface area constant
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In all five petri-dishes, 5 cm of sucrose solution will be used
Sucrose solution used in all petri-dishes will be at room
0
temperature (28 C).
All potato strips will be kept in sucrose solutions for 40 minutes,
thus keeping the time allowed for osmosis to occur constant.
Observations:
(a)
Quantitative Observations
Table 2.1 Length of potato strips before and after being immersed into different
concentrations of sucrose solutions
Sucrose solution
concentration
/mol dm–3
0.2
0.4
0.6
0.8
1.0
Length of potato strips
initial length of strips
50.0
50.0
50.0
50.0
50.0
final length of strip 1
50.0
51.0
49.0
50.0
48.0
0.5 mm
final length of strip 2
53.0
50.0
49.0
46.0
47.0
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(b)
Qualitative Observations
Table 2.2 Descriptive observations of potato strips after being immersed into different
concentrations of sucrose solutions
Sucrose solution
–3
concentration /mol dm
0.2
0.4
0.6
0.8
1.0
Qualitative observations
The potato strips became stiff and rough to touch
The potato strips became stiff and rough to touch
The potato strips became floppy, soft and slimy to touch
The potato strips became floppy, soft and slimy to touch
The potato strips became floppy, soft and slimy to touch
Data processing:
1.
Calculate the mean [average] length of the potato strips by adding the final length of
strip 1 to that of strip 2 and dividing by two:
Mean length =
e.g.
2.
= 51.5 mm.
Calculate change in mean length of potato strip by subtracting initial length of strip
from mean length of potato strip after immersion
Change in mean length = mean length - initial length
e.g. 51.5 – 50.0 = + 1.5 mm
All calculated values are recorded in table 2.3
Presentation of processed data:
Table 2.3 Mean length and mean change of potato strips after being immersed into different
concentrations of sucrose solutions
Length of potato strips
sucrose
solution
initial length
concentration of strips
/mol dm–3
final
length of
strip 1
final
length of
strip 2
0.5 mm
mean length
of potato strips
after immersion
change in mean
length of potato
strips
0.2
50.0
50.0
53.0
51.5
+ 1.5
0.4
50.0
51.0
50.0
50.5
+ 0.5
0.6
50.0
49.0
49.0
49.0
- 1.0
0.8
50.0
50.0
46.0
48.0
- 2.0
1.0
50.0
48.0
47.0
47.5
- 2.5
6
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Conclusion:
As the sucrose concentration increases from 0.2 to 1.0 mol dm–3, there is a decrease in
mean change in length of potato strips. Sucrose concentration of 0.44 mol dm–3 is isotonic to
the cell sap of potato cells because there is no net change in length i.e. the amount of water
moving inwards is equal to amount of water moving outwards (Education 2003). In sucrose
concentration below 0.44 mol dm–3 there is increase in length of potato strips due to
movement of water by osmosis along the water potential gradient into potato strips. In
sucrose concentration above 0.44 mol dm–3 there is decrease in length of potato strips due
to movement of water out of potato strips by osmosis along the water potential gradient
(Jones 2010).
Evaluation:
(a)
Limitations and Sources of error:
Only two potato strips were used per sucrose concentration. This increases the chances of
experimental error.
All strips were not from same potato thus there was variation in the initial water potential in
the potato strips leading to variations in the rate of osmosis.
Petri-dishes were not covered; there were chances that evaporation of solvent took place
altering the amount and concentration of sucrose solutions.
Due to uneven edges of the potato strips, it is possible that the degree of accuracy in
measuring final length of the strips was not the same in all potato strips. This affects the
reliability of the results obtained.
(b)
Suggestions for improvements:
The following measures could be taken to improve the experiment so as to make the results
more reliable:
(i)
use more chips per sucrose solution and then calculate the average, this increases
the accuracy and hence the reliability of the results;
(ii)
cut all the strips from same potato to eliminate variation in the initial water potential;
(iii)
cover the petri-dishes to reduce evaporation of solvent hence ensuring the
concentrations of sucrose solution remains the same throughout the experiment;
(v)
measure the mass of potato strips instead of length or repeat measurements to
constant values. This could increase the degree of accuracy.
References:
Education, Ministry of. Secondary Biology Form One student's Book. Nairobi: Kenya Literature
Bureau, 2003.
Jones, Mary Jones and Geoff. IGCSE Biolgy Coursebook Second edition. Cambridge: Cambridge
University Press, 2010.
Mackean, DG. IGCSE Biology Second Edition. London: Hodder education, 2002.