1

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

2

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?

Hypothesis: if the sucrose solution concentration is increased from 0.2 to 1.0 mol dm,-3 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) 1X 10 paper towels

(ii) 1X 3 large Irish potato

(iii) 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.

3

(c) Apparatus;

(i) 1X10 m3 measuring cylinder

(ii) 1X5 Petri-dishes

(iii) 1X150 transparent ruler

(iv) 1X 5 mm diameter cork borer

(v) 1X1white tile

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.

4

(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 in-

between 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

How it will be controlled

surface area of potato strips

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

volume of sucrose solution used

In all five petri-dishes, 5 cm3

of sucrose solution will be used

temperature of the solution used

Sucrose solution used in all petri-dishes will be at room

temperature (280 C).

duration in which the strips are kept

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

Length of potato strips 0.5 mm

initial length of strips

final length of strip 1

final length of strip 2

0.2 50.0 50.0 53.0

0.4 50.0 51.0 50.0

0.6 50.0 49.0 49.0

0.8 50.0 50.0 46.0

1.0 50.0 48.0 47.0

5

(b) Qualitative Observations

Table 2.2 Descriptive observations of potato strips after being immersed into different

concentrations of sucrose solutions

Sucrose solution

concentration /mol dm–3

Qualitative observations

0.2 The potato strips became stiff and rough to touch

0.4 The potato strips became stiff and rough to touch

0.6 The potato strips became floppy, soft and slimy to touch 0.8 The potato strips became floppy, soft and slimy to touch 1.0 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.

= 51.5 mm.

2. 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

sucrose solution concentration

/mol dm–3

Length of potato strips 0.5 mm

initial length of strips

final length of strip 1

final length of strip 2

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

7

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.