Noémie Pauwels
BIOLOGY COURSE WORK
Finding the isotonic point for potato tissue
- Raw results table
- Processed results table
- Example calculations
For 0.0 M :
- Difference in mass
First trial : Starting mass = 2.90g
Final mass = 3.05g
Difference in mass = final mass – starting mass
= 3.05-2.90
= 0.15g
- Percentage change in mass
First trial : Percentage change =
=
= 5.17%
- Mean percentage change in mass
Mean =
=
= 5.41%
- Standard deviation of percentage change in mass
S.D. = √
= √
= 0.82%
- Discussion :
Description :
- The isotonic point of the potato stick is 0.31 M.
- Between 0.3 M and 0.0 M, the mass of the potato stick increases as the concentration of sucrose solution decreases.
- Between 0.32 M and 1.0 M, the mass of the potato stick decreases as the concentration of sucrose solution increases.
Explanation :
A : At 0.31 M, the potato sticks do not change
in mass. This is because the concentration of
the sucrose solution and the concentration of
the solute in the cell cytoplasm is the same,
they are isotonic. There is no net movement
of water particles into/ out of the cells by osmosis.
B : Between 0.3 M and 0.0 M the potato sticks
gain mass as the sucrose solution became more
dilute. The water particles entered the cells by
osmosis from the more dilute ...
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Explanation :
A : At 0.31 M, the potato sticks do not change
in mass. This is because the concentration of
the sucrose solution and the concentration of
the solute in the cell cytoplasm is the same,
they are isotonic. There is no net movement
of water particles into/ out of the cells by osmosis.
B : Between 0.3 M and 0.0 M the potato sticks
gain mass as the sucrose solution became more
dilute. The water particles entered the cells by
osmosis from the more dilute sucrose solution
to the more concentrated solution of the cell
cytoplasm. With additional water, the potato
cylinders gained mass.
C : Between 0.32 M and 1.0 M, the potato sticks
lose mass as the sucrose solution became more
concentrated. The water particles leave the cells
from the more dilute cell cytoplasm to the more
concentrated sucrose solution by osmosis.
The net movement of water particles goes out of
the cells.
D : Between 0.1 M and 0.0 M, the graph
begins to level off as the potato sticks
reached their maximum mass.
The potato sticks are fully turgid.
As the concentration of water is lower
inside the cell than outside the cell, the
water particles enter the vacuole of the
cell by osmosis. The vacuole of the cell
pushes on to the cell wall and exerts turgor
pressure on the cell wall. While the potato
sticks reached their maximum mass, they can
still take in water because the cell wall
prevent the cell to burst and so the mass of
the cell stays the same.
E : Between 0.7 M and 1.0 M, the graph begins
to level off as the potato sticks reached their
minimum mass. As the concentration of water
is higher inside the cell than outside the cell,
the water particles diffuse out of the vacuole
of the cell. The vacuole then shrinks pulling
the cytoplasm away from the cell wall but
the mass of the cell stays the same.
This process is called plasmolysis.
- Evaluation
Sources of error
- PROBLEM: No control of temperature
EXPLANATION: The experiment was done on 2 different days so there could have been a change of temperature. On the graph, we draw a line of best fit to show that there could have been a change in temperature.
Temperature affects the rate of osmosis. When the temperature is higher, the water particles have more kinetic energy. This means that the rate of osmosis would be faster and that the potato sticks would increase or decrease in mass faster.
TYPE OF ERROR: This is a procedural and significantly major error because the temperature is not even measured.
IMPROVEMENT: Heat the sucrose solution for 20 minutes in a boiling tube in a water bath set to 25 °C and check the temperature of the water with a thermometer before adding the potato pieces.
- PROBLEM: Difference of potato sticks
EXPLANATION: The cells of the potato sticks have different concentrations of solutes because more than one potato was needed to obtain enough samples of potato. Cells with a greater concentration of solutes would absorb more water and gain more mass when soaked in 0.0 M sucrose solution. The potato sticks are not from the same potato. This means that the cells have a different solute concentration, a different concentration gradient and so the rate of reaction would be different.
TYPE OF ERROR: This is a procedural and significantly major error. This factor limits the reliability of the method.
IMPROVEMENT: Check the solute concentration of the potato cells and adapt the concentration of the sucrose solution to reach a same concentration gradient.
- PROBLEM: Timing
EXPLANATION: For this experiment, a time watch was used.
It is not very accurate because the level of precision is ± 0.01 seconds.
TYPE OF ERROR: Although it is not the most accurate way, this factor is a systematic and minor error because the level of precision of the time watch is very little.
IMPROVEMENT: Use a more accurate time watch with a higher level of precision.
- PROBLEM: Surface area
EXPLANATION: The potato sticks were made with a measuring cylinder. The level of precision is ±0.5 ml.
This method is not the most accurate; there can be little changes in the surface area of the different potato sticks. The surface area is a factor limiting cell size and is important in the rate of osmosis.
TYPE OF ERROR: This factor is a systematic and minor error because the level of precision is very little and the error is due to material. It is also a random error because the potato sticks can be O.5 mm shorter or longer than 6 cm. Longer potato sticks would have a bigger surface area and gain more water. Shorter potato sticks would haver a smaller surface area and gain less water.
IMPROVEMENT: Use a micrometer to check that each potato stick has the same length and diameter.
- PROBLEM: Drying method
EXPLANATION: For this experiment, the potato sticks need to be dried because it can affect the rate of osmosis. The potato sticks were not dried evenly and we used the same tissue for different potato sticks. This means that some potato sticks were dried with a humid tissue which results that some potato sticks were wetter than others.
TYPE OF ERROR: This factor is a procedural and significantly major error because the rate of osmosis can be very different and so the results can differ hardly.
IMPROVEMENT: Each time a potato stick is “dried”, use a new piece of paper tissue and roll the potato stick for 20 seconds.
- PROBLEM: Electronical balance and mass
EXPLANATION: We weighted the potato sticks on an electronical balance.
This is not the most accurate method; there can be little changes in the mass of the potato sticks.
The level of precision is ±0.01g.
TYPE OF ERROR: This factor is a systematic and minor error because the level of precision of the electronical balance is very little and the error is due to material.
IMPROVEMENT: Use a new balance with a level of precision of ±0.001g.
- Anomalies
- The 3rd repeat at 0.2 M is greater than the other repeats. This may have happened if the potato stick was not dried properly after soaking, so than the final mass was higher than expected.
- The 6th repeat at 0.4 M seems to have lost more mass than the other repeats. This may be because the starting mass was too high. It may also be because the potato stick was not dried properly before soaking.
- The 4th repeat at 0.6 M has a lower percentage loss of mass. This may be because the potato stick was too short and had a lower surface area. The rate of osmosis might have been lower so less water would have left the cells.
- The 1st repeat at 1.0 M has a lower percentage loss of mass. This may also be because the potato stick was too short and had a lower surface area. The rate of osmosis might have been lower so less water would have left the cells.
- Reliability
The method is reliable because most of the results are generally satisfying. Although, the results do not follow a significant pattern, the graph is decreasing. This means that we can trust the data. Except the anomalies, most repeats were also very similar. The graph is also easy to draw; there is no problem to find the line of best fit and the graph always matches between the error bars. The error bars of the mean percentage change in mass of O.O M, O.2 M an 0.4 M don’t overlap. The difference between the mean percentage changes in mass are significant; there is a significant change in mass of potato sticks because the water particles are either entering or leaving the cell.
However, the error bars of the mean percentage changes in mass of 0.6 M, 0.8 M and 1.0 M do overlap. The difference between the mean percentage changes in mass is not significant because the change in mass of the potato sticks is very little. The cells of the potato sticks become plasmolysed an so their mass remains the same.
From the graph, we can conclude that the more hypertonic the potato sticks are to the sucrose solution, the more the potato sticks will increase in mass. On the other side, when the potato sticks are more hypotonic to the sucrose solution, the mass of the potato sticks will decrease more.
- Glossary
- concentration gradient : a gradual change in the of in a
- cytoplasm : living part of the cell, bound by the plasma membrane, excluding the nucleus.
- dilute solution : a solution that contains a small concentration of solute..
- hypertonic solution : a more concentrated solution (one with a less negative water potential) than the cell solution.
- hypotonic solution : a less concentrated solution (one with a more negative water potential) than the cell solution.
- isotonic : being of the same osmotic concentration and therefore of the same water potential.
- kinetic energy : energy in movement.
- micrometer : A device for measuring very small distances
- osmosis : passive movement of free water particles from a more dilute solution to a more concentrated solution across a partially permeable membrane along a concentration gradient.
- plasmolysis : withdrawal of water from a plant cell by osmosis.
- solute : a dissolved in another .
- turgid : having high internal pressure
