To investigate the impact of varying the concentration of sucrose solution has on the mass of potato.
Plan
---------------------------------------------AIM-------------------------------------------------
To investigate the impact of varying the concentration of sucrose solution has on the mass of potato.
---------------------------------------BACKGOUND----------------------------------------------
SUCROSE
Sucrose is a non-reducing, disaccharide sugar used in sweetening, being obtained from the juice of the sugar cane and from sugar beet. Sucrose (C12 H22 O11) is formed by a condensation reaction between Fructose and Glucose.
POTATOES
Potatoes are natural forms of carbohydrate, which are rich in starch.
OSMOSIS
Osmosis is the movement of a solvent (usually water in biological systems) through a differentially permeable membrane from a solution that has a high water concentration to one with a low water concentration and high solute concentration.
Diagram 1: osmosis.
WATER POTENTIAL
Water potential is the tendency of a cell to draw in water from outside by osmosis with the water moving from a higher to a lower potential. Since pure water at one atmosphere has a water potential of zero, cells drawing in water have a water potential of less than zero. Thus, zero cells drawing in water have a water potential of less than zero. Thus water potential is measured as a negative value.
------------------------------------------HYPOTHESIS -----------------------------------------
With the increase in the concentration of sucrose there will be a net movement of water from the potato to the surrounding solution providing the potato has a higher water concentration (potential) than the surrounding solution. When there is a decrease in the concentration of sucrose there will be a net movement of water from the potato to the surrounding solution.
--------------------------------------OTHER VARIABLES---------------------------------------
¨ Water potential of the potato
¨ Size of potato - Alters the mass and surface area.
¨ Light intensity - (can alter the energy of particles and speed up osmosis.)
¨ Mass of potato
¨ Volume of solution the potato is in
¨ Type of potato
¨ Time left in solution
¨ Surface area of potato
¨ Temperature - (can alter the energy of particles and speed up osmosis.)
FAIR TEST?
If a fair test is going to be conducted, then these other factors have to be controlled. Doing all the tests at one temperature will control the temperature. I am going to conduct all the experiments at room temperature.
To eliminate the effects of the variable of the water potential of the potato, the pieces will be cut from the same potato.
The size of the potato is very important. This is because if the size of the potato varies, the mass and surface area are altered. I will cut all the pieces of potato, to the same size.
The light intensity will be kept the same because all experiments will be conducted and left in the same part of the room on the same test tube rack where it is unlikely that the light intensities can be different (no extra light will be shone on the experiments, nor will there be any experiments kept in the dark).
The mass of the potato is a dependent variable, and this means that it will be measured throughout the experiment. I will measure the mass in grams. The potato will be measured before it is put in ...
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The light intensity will be kept the same because all experiments will be conducted and left in the same part of the room on the same test tube rack where it is unlikely that the light intensities can be different (no extra light will be shone on the experiments, nor will there be any experiments kept in the dark).
The mass of the potato is a dependent variable, and this means that it will be measured throughout the experiment. I will measure the mass in grams. The potato will be measured before it is put in the solution, and after. This will allow us to see whether osmosis has taken place, and to what extent.
The type of potato to be used kept the same, because different potatoes may absorb at different rates. For this experiment a single potato will be used.
The time the potato is left in the solution must be kept the same in each experiment. This is because more or less solution may be absorbed depending on time. The potato pieces will be kept in each solution for 24 hours, in test tubes.
The surface area of the potato will be kept the same through the use of a cork borer.
This will ensure a constant diameter. It will then be easier to control the surface area. The surface area must be kept the same because the amount of surface area exposed to the solution will affect the rate of osmosis.
The potato will be lightly rolled on the filter paper to remove water from the surface only.
To weigh the potato the same balance will be used each time. This is because the measurements may vary slightly between scales.
------------------------------------------PREDICTION------------------------------------------
It is being said that the higher the concentration of sucrose that the potato is exposed to, the more water will move out making it flaccid and reducing its' mass to a greater extent.
It is predicted that the higher the concentration of salt solution the more water will move out of the potato. Where the concentrations are equal there will be no change of mass and where the potato has a higher water potential than the solution the mass will increase.
The percentage change of mass will be proportional to the water potential of the solution, and therefore will be inversely proportional to the sucrose concentration of the solution.
It is known that osmosis is the flow of one solvent, (water) of a solution through a membrane while the other constituents are blocked and unable to pass through the membrane. Experimentation is necessary to determine which membranes permit selective flow, or osmosis, because not all membranes act in this way. Many membranes allow all or none of the constituents of a solution to pass through; only a few allow a selective flow. As the potato is a plant cell, it contains a vacuole and a cytoplasm. The cell membrane is also partially permeable i.e. it lets some substances in but not all substances. This means that water particles can diffuse into cells by osmosis if the cells are surrounded by a weak solution. (Osmosis is the movement of water molecules from a high area of water potential to an area of low water potential through a selectively permeable membrane.) If the cells are surrounded by a stronger solution, e.g. salt water, the cells may loose water by osmosis.
Obtaining Results
-----------------------------PRELIMINARY INVESTIGATION------------------------------
A preliminary experiment was conducted to acquire knowledge of the best quantities of potato and sucrose solution would be best to use in the final experiment. The conclusions of the preliminary investigation will be used to write a suitable method for the final experiment.
The results of the preliminary experiment proved that concentrations above 1m, should not used because they do not give accurate enough results and there is not enough water within the potato to be diffused outwards when the sucrose solution is very high.
------------------------------------ACTUAL INVESTIGATION------------------------------------
APPARATUS
* Ruler
* Cork Borer
* Knife
* Cutting tile
* Filter Paper
* Balance
* Test tubes x 5
* Measuring Cylinder
* Sucrose Solutions: 0m (distilled water), 0.1m, 0.25m, 0.5m, 1m
* Potato
*
METHOD
The diameter of the cork borer was measured and recorded. The cork borer was then used to cut five cylindrical pieces of potato each five centimetres long.
A cylinder of potato was taken and cut into five pieces of length 1cm. These were weighed, their mass was recorded and they were added to a labelled test tube containing 20ml of distilled water. The test tube was then bunged and the time was recorded.
This process was repeated with the other potato cylinders but sucrose solutions of 0.1m, 0.25m, 0.5m and 1m were used in place of the distilled water.
The potato pieces were left in the solutions for 24 hours, they were removed and the time was recorded. They were rolled lightly on filter paper to remove water from the surface and were weighed and the mass recorded.
SAFETY
To ensure safety, the test tubes were kept in a test tube rack so that they couldn't be knocked over. The potato pieces were cut on a white tile with a scalpel so that no one was cut.
DIAGRAM
Results
TABLULAR FORM
Solution
Mass of potato at start (g)
Time started
Time Finished
Mass of potato
At end (g)
Percentage mass change
0m
2.67
09:05
09:05 (next day)
3.24
+ 21.3%
0.1m
2.72
0:00
0:00 / \
2.60
- 4.4%
0.25m
2.57
0:10
0:10 / \
2.18
- 13.5%
0.5m
2.76
0:20
0:20 / \
2.20
- 24.8%
m
2.73
0:25
0:25 / \
.70
- 37.7%
CALCULATIONS
Surface area: Although surface area of the potato did not need to be calculated because it was constant, the surface area can be found through the following formula:
Where 'd' is the diameter of the cork borer, 'r' is the radius (diameter / 2), 'l' is the length of each piece (1cm) and '5' is the number of pieces.
Because the diameter of the cork borer was 0.8cm the surface area for each experiment was 17.59cm2 (to 2.d. p)
Percentage mass change: Percentage of change in mass was calculated through the formula:
00(M2 / M1) where M1 and M2 are 'the mass at start' and 'the mass at end.'
Analysis
The evidence obtained was accurate. The scatter-graph shows that the results taken were accurate and the results were all quite close to the line proving there were no anomalies. The prediction that the higher the concentration of salt solution, the more water will move out of the potato and the mass will decrease was correct.
However, the results did not conform to a straight line of best fit, as predicted but to a curve, which means that the relationship between the two measured factors is not proportional - but there is an inverse relationship.
The point at which the line crosses the x-axis indicates the concentration of a sucrose solution that has a water potential equal to that of the potato, hence there was no change in mass because water is not moving. This value gives us an idea of the water potential of a potato.
The graph would have been more accurate if a solution of 75m (unobtainable) was tested as well.
Conclusion
Osmosis, the movement of water molecules from an area of high water potential to an area of low water potential through a selectively permeable membrane takes place when a potato is submerged in sucrose solution.
Osmosis, in such a situation, is affected by the light intensity, temperature, surface area, type of potato and the water potentials of the solution and the potato itself.
Because the concentration of the sucrose in the solution is the opposite of the water potential increasing it has the adverse effect of increasing the water potential. Increasing the concentration cases more water to diffuse out of the potato and a higher percentage loss in mass.
When the concentration is zero, the potato has a lower water potential causing water to be drawn in to the potato, increasing its' mass.
The factors, 'percentage of increase in mass in the potato' and 'the concentration of the solution' have a negative correlation that is NOT proportional as predicted but follows a curve.
Evaluation
The chosen method gave good results that conformed nicely to a curved line of best fit. The prediction was incorrect where it stated that the relationship between the factors would be inversely proportional.
When the mass of potato was measured at the start, different results were obtained for each set of pieces, as the volumes were equal it suggests that the density of the potato is higher in some places and not constant throughout the potato. This means that the water potential was also varied through the potato, which suggests that potatoes aren't the most ideal substances to be used for such an experiment.
The accuracy of the balance my have been poor, however because the same balance was being used throughout the investigation this was not a problem.
Water removed from surface
Although much effort was made to keep the surface area constant, surface area was lost where the potato rested against the sides of the test tube. This is the same for the potato samples that float in which case they are not fully exposed to the solution. It is impossible to prevent such an occurrence, although one idea is to attach the potato to string somehow so it can be lowered or anchored but such a method is unscientific.
To increase accuracy the process could have been repeated twice and average results taken Ant the line of best fit could be made smoother if smaller intervals of molar values of the sucrose solution had been used.
To expand this experiment other variables in the experiment could be changed for example instead of changing the weight of the potato the type of the potato could be changed. For example 'New potatoes', 'King Edwards' etc. could be compared.
Temperature could also be changed for example the samples could be placed in different water baths and brought up to different temperatures to see if temperature played its part in the osmosis of potatoes. 5 sets of 5 potatoes could be placed in water baths at 10oC, 20oC, 30 oC, 50 oC and 60 oC. Then leave them for 24 hours making sure all the variables in the first experiment still apply however just using one molar solution. Then after 24 hours the samples would be re weighed and results recorded. I would expect that at high temperatures the potato samples would osmose the most. This is because at high temperatures the solutions water molecules would move faster and therefore equal the concentration faster. A preliminary experiment could be set up beforehand to find out how long the experiment should be kept going because if the concentration of the potatoes equalises then the weight of the potatoes will be almost exactly the same.
Other solutions could be used as well, e.g. glucose solution.
I was not disappointed with my investigation because it achieved accurate results
