The Volume Of The Sodium Chloride Solution

Published Date: 02 Nov 2017

ABSTRACT

This investigation was undertaken to observe the effects of the SA:V ratio on the osmotic activity of the potato. The experiment was conducted effectively to test the hypothesis: The larger the surface area of the potato, the quicker, the rate of osmosis, thus, greater % change of mass. Potato cubes of different dimensions were submerged in a 10% sodium chloride solution to calculate the percent of mass difference. The results indicate that the solution was hypertonic as water moved from high water concentration to low water concentration due to osmosis. From the results, it can be concluded that the smallest cube with the largest surface area to volume ratio experienced the greatest %mass loss because the efficiency in water transportation increases with the SA:V ratio. Therefore, the results obtained from this investigation support the hypothesis.

INTRODUCTION

BACKGROUND KNOWLEDGE

Cells are the structural and functional unit of life, which not only make the bodies of living things, but also conduct their life processes, such as osmosis. All cells are surrounded by a selectively permeable membrane composed of a bi-phospholipid layer with proteins penetrating through which acts as a barrier between the internal and external environment of the cell. The carbohydrates attached to the proteins act as receptors to enable cells to recognise each other. Its primary functions include, the regulation of the passage of materials into and out of the cell to maintain homeostasis, while providing support and protection for the cell. Since cell membranes are selectively permeable, only allow certain particles are permitted to move in and out of the cell. Such particles must be small, uncharged and soluble in lipids. Otherwise, channel proteins are required to assist the movement of substances such as glucose, amino acids and ions along the concentration gradient.

The microscopic size of the cell accounts for the fact that as the cells gets larger, the surface area to volume decreases, which in turn, decreases the efficiency in which materials can be exchanged and transported via the cell membrane. Small cells can communicate more rapidly as there is less distance required for information to pass from the nucleus to the rest of the cell. Thus, if the cell grows beyond a certain limit, insufficient amount of material will be able to enter the membrane at a rate that accommodates the increased cellular volume. Some nutrients, including, glucose and oxygen and waste products, such as, urea and carbon dioxide are able to diffuse in and out of the cell. As the size of the cells get smaller, its surface area to volume ratio increases, thus, these substances are able to move more rapidly to allow the cell to function properly.

Osmosis is a special case of diffusion, where water, a vital substance required for the proper functioning of a cell, is moved from a region of higher water concentration to a region of lower water concentration across a semi-permeable membrane to achieve a state of equilibrium. It is a passive process, where the expenditure of energy by the cell is not required for the diffusion of water, as the molecules are moving along the concentration gradient. Vegetables such as potatoes have a cell wall in addition to its cell membrane. Therefore, preventing it from bursting when placed in a hypotonic solution. The cell wall resists any further intake of water, thus, the cell is turgid. In a hypertonic solution, such as the 10% NaCl solution, the cell undergoes plasmolysis as the water moves from a low solute concentration to a high solute concentration. The cell would shrink, thus, the cell membrane is pulled away from the cell wall, making the cell flaccid.

AIM

The purpose of this practical is to investigate the possible effects of the SA:V ratio on the rate of osmosis in potatoes.

HYPOTHESIS

The larger the surface area to volume ratio of the potato the quicker the rate of osmosis, thus, greater % change of mass.

VARIABLES

Independent- Size of the potato cubes, i.e., its surface area to volume ratio

Dependent- Percentage difference in mass for the potato cubes after being in 10% NaCl solution for 20 minutes.

FACTORS TO BE KEPT CONSTANT:

The volume of the sodium chloride solution the potatoes are placed in must be the same as the amount of water molecules will affect the rate of osmosis. By ensuring that all three potato cubes are placed in a 100mL solution of sodium chloride solution, this variable is controlled.

The concentration and pH level of the solution in which the potato cubes are placed in are to be kept constant. The greater the concentration gradient of a solution on either side of a membrane, the faster osmosis occurs. As the amount of H+ increases in the solution, so does the acidity, this in turn will damage the proteins found in the cell. This factor is kept constant by placing all three potato cubes in a 10% NaCl solution.

The species and batch of potatoes used need to be the same, as the rate of osmosis is likely to differ if a different variety of potato cells are used. The rate of osmosis is likely to differ if potatoes aren’t used from the same batch as potato cells may be damaged or be starting to decay. This variable is controlled by using the same batch and species of potatoes for this experiment.

The potato cubes should be left in the solution for the same duration of time, otherwise, there will be a greater or lesser percentage of change in mass. This factor is kept constant by placing the three potato cubes in the solution for twenty minutes.

The environmental and atmospheric situation of the room must be kept at a constant as it will affect the rate of osmosis in the potato cells. As the temperature increases, so does the kinetic energy of the molecules, thus, there will be more collisions which will increase the rate of osmosis. This variable will controlled by conducting the entire experiment in the same room.

The light intensity throughout the entire experiment must be kept the same as light exerts heat which in turn would increase the temperature of the solution, thus, the molecules gain more kinetic energy to move at a faster rate. This will increase the rate of osmosis of the potato cells.

MATERIALS

2 large fresh potatoes of the same species

100mL 10% NaCl solution

250mL beaker

Spoon

Ruler

Knife

Chopping Board

Scalpel

Electronic balance

Paper towels

Stopwatch

METHOD

Use a ruler and a sharp knife or scalpel to measure and a cut the peeled potato cubes with the dimensions 1cm3, 2cm3 and 3cm3, as accurately as possible.

Place the potato cubes on an electronic balance, and determine the mass of each as accurately as possible. Record this value on the results table.

Fill the beaker with 100mL of 10% NaCl solution and place the cubes simultaneously so that it is completely submerged. Leave the cubes in the beaker, ensuring that it is completely covered in the solution, for twenty minutes.

Using a spoon, carefully remove the cubes as simultaneously as possible and place them on a paper towel.

Gently dry the damp potato cubes using the paper towel to remove excess water.

Place these cubes on an electronic balance and the record the mass again on the results table.

Clean all utensils used for this experiment, and discard the solution and potatoes. Clear the workbench.

Produce a results table that include the following for each dimension of the cube tested; actual surface area, initial mass, final mass, difference in mass and %change in mass.

Collate the results of the entire class and find the average for each observation

Using the data in the results table, produce a graph using the axes provided.

RESULTS

DIMENSIONS (cm)

SA:V RATIO

AVERAGE INITIAL MASS (g) [A]

AVERAGE FINAL MASS (g) [B]

AVERAGE DIFFERENCE IN MASS (g) [C] [B-A]

% DIFFERENCE [C/A x 100]

3 x 3 x 3

2:1

31.3g

28.3g

-3.00g

9.55%

2 x 2 x2

3:1

9.80g

8.41g

-1.39g

14.2%

1 x 1 x 1

6:1

1.30g

5.07g

-1.45g

20.71%

Table 1: Average investigation results of the class

Graph 1: Percentage change in mass for different cube measurements

DISCUSSION

The percentage difference of mass was averaged throughout the class by using the resultant masses of the potato cubes. These results were presented in a table and plotted on a graph for an easier analysis of the data obtained.

By analysing the graph, there is a slightly positive co-relation between the surface area to volume ratio and percentage difference in mass. This implies that the potato cells undergo a greater rate of osmosis, that is, diffusion of water, as the surface area to volume ratio increases due to the decreasing size of the potato cell.

A pattern can be observed in the results table where there is a consistent decreasing trend of the difference in mass and percentages loss as the cubes get bigger, though some figures differ slightly from others. This is due to errors occurred during the experiment which will be explained.

Due to the smaller size of the potato cubes, there is less distance required for the small cells to communicate, thus, information is passed from the nucleus to the rest of the cell more rapidly. The smaller cubes will provide proportionally more surface area per unit of volume for exchange of materials with their environment. This accounts for the hypothesis which states that as the surface area to volume ratio of the potato increases, the quicker, the rate of osmosis, thus, greater % change of mass. Therefore, it can be seen from Table 1, the smallest cube of 1cm3, with a surface area to volume ratio of 6:1, showed the greatest difference in mass with a percentage of 20.71%. The 2cm3 cube showed a lesser change in mass, with a difference of 14.2% followed by the 3cm3cube which showed the least mass change of 9.55%.

As seen in Table 1, the final mass of all three potato cubes were lesser than their initial mass. This implies that the potato cubes were placed in a hypertonic solution, in which the solute concentration of the solution is higher than that of the potato cell. Since osmosis is the diffusion of water from a low solute concentration to a high solute concentration, water is drawn out of the potato cube, thus undergoing plasmolysis which decreases its mass to become flaccid.

EVALUATION

In any practical investigation, there will be factors which may influence the final results. These errors include random, systematic and even human mistakes, which also accounts for slight variances in the results between the five groups.

One of the strengths of this experiment is the use of the same species and batch of potatoes across all groups. This increases the experiment’s reliability as all groups are exposed to the same variable that is being tested. However, a negative aspect to this is that it cannot actually be proved that the potatoes used were from the exact batch, therefore, some potatoes maybe older than others, which in turn affects their rate of osmosis due to decay which decreases it ability to diffuse as well as the others. Another strength was the use of the results from all five groups instead of using just one set of results. This ensures that a larger sample size was tested to reduce the effect of variations due to random errors. The results of the five groups were collated so that an average can be obtained; this ensures that the overall picture is seen with the simplicity of the data. However, the presence of extreme values can give an inaccurate representation of the data collected as the average value will be either higher or lower than it should be, this in turn, will affect the outcome of the experiment. By looking at the appendix table, it can be seen that there isn’t much scatter, hence implying that the results are similar to one another, with minimal deviation from the average result.

Some of the weaknesses of this investigation was the small sample size used, as there were only five groups. Though the results of the five groups were combined to gain an average, the sample size is still too small and needs to be tested many more times, so an even more reliable average can be attained. In addition to this, a large variation of sizes of potato cubes weren’t used. Therefore, it’s difficult to come an accurate conclusion, whilst if there were more results, more accurate results could be drawn.

Random errors

Many of the weaknesses in this investigation were also sources of random errors in the investigation. A random error that occurred in this experiment is the exposure to direct sunlight for groups 1, 2 and 3 during this experiment, as they were on the different side of the room to groups 4 and 5. Therefore, the presence of sunlight sped up the rate of osmosis for the groups exposed to sunlight as light exerts heat which in turn would increase the temperature of the solution, thus, the molecules gain more kinetic energy to move at a faster rate. This accounts for the fact that groups 1, 2 and 3 experienced a slightly greater difference in mass as seen in the raw data due to conducting the experiment in a different environmental situation. Therefore, the results obtained are reliable and valid to a lesser extent due to uncontrolled variables that affects its accuracy and reliability. Though the results of the five groups of results were combined to gain an average, the sample size is still too small and needs to be tested many more times, so an even more reliable average can be attained.

Another possible random error is the air conditioner that may have been blowing air into one of the group stations, which in turn affects the temperature to which the apparatus was exposed to. Therefore, this will affect the rate of osmosis experienced by the potato cells as heat increases the rate of osmosis, while a cooler a temperature may decrease its rate. Exposure to different environmental conditions may be one of the reasons as to why each group had slight variances in their results. The experiment might have taken place at a wrong temperature as most equipment and methods have an ideal temperature where the readings are more accurate.

Another potential error includes the use of unclean beakers in the experiment, hence, impurities may have been present in the solution which may have hindered the process of osmosis due to contamination. E.g. the solution may have contained a substance other than the sodium chloride from a previous experiment, thus, the difference in mass for the potato cubes will differ from its correct value. Since the potatoes were cut to particular dimensions, the cells may have been damaged. The smaller cubes would have a greater proportion of damaged cells, hence they are at a disadvantage to the larger cubes. Therefore, there would be fewer cells to lose water in the smaller cubes, thus, affecting the rate of osmosis.

A random error that occurred in this experiment is the cutting and measuring of the cubes to its accurate dimensions. Due to a blunt knife and scalpel used in this investigation, the cubes may be of an inaccurate measurement, thus, not giving an accurate representation of its actual dimension, which in turn affected its accuracy. Therefore, the validity of the experiment is affected as incorrect measurements may have been used. Since the cubes were cut from different parts of the potato, this could affect the rate of osmosis, depending on the density and how much water is contained in each part of the potato.

The knife and scalpel was not cleaned prior to use, therefore, it was probably contaminated with a substance that will either increase or decrease the rate of osmosis. Inconsistency in drying the potato cubes was also a random error. Some potato cubes may have been dried more thoroughly than other, thus, not only was excess water removed from outside the potato, but the inside in which water was gained during osmosis. So when the potato cubes were weighed, they weighed less, therefore, the results obtained may have been inaccurate as it leads to a conclusion that the potato cubes had lost more mass than they actually had.

The excess water from the potato cubes may not have been removed completely, thus adding to the mass of the potato. These data obtained as a result of improper drying gives an inaccurate representation of the actual percentage difference in mass of the potatoes. A possible random error that could have occurred is the varying times in which the potato cubes were placed in the solution. As one potato cube was taken to remove excess water, other potatoes still left in the beaker waiting had more time to gain or lose water. Therefore, their masses are smaller or larger than they should have been if all the potatoes had been taken from the beaker at the same time. The temperature of the solution wasn’t tested before and after the investigation, therefore, it is unknown if there was a temperature change that may have affected the rate of osmosis.

Systematic errors

A systematic error occurred when the potato cubes were placed in the solution; only five of its sides were exposed to the solution as they were floating. Therefore, water was being drawn out of only those five sides. Another random error may have been the potato skin that may have not been peeled, thus, acting as a barrier for the water molecules to go through which will decrease the rate of osmosis. The electronic balance may have not been accurately calibrated and it was placed in front of the air conditioner, thus, the mass recorded may be slightly deviated from the potato cube’s actual mass. Since the timers used are electronic, they cannot be controlled, thus making room for incorrect calibration. Therefore, the potato cubes were probably in the solution for an unknown amount of time. Another systematic error is the concentration of the NaCl solution. Though the solution may have been labelled as a 10% NaCl solution, it may not have been exactly 10%. Thus if the practical is to be repeated, it is not certain that the concentration of the solution will be the same.

The original method was effective and valid, as it achieved the aim of investigating the possible effects of the SA:V ratio on the rate of osmosis in potatoes. However, it could have been improved to increase its validity, by including a detailed list of materials, more steps rearranged in a chronological order (e.g. rinsing all the equipment required in the beginning) so that the chance of human mistakes made is reduced.

Other improvements to the actual practical to increase its validity would be to perform the investigation in a temperature controlled room where all groups are exposed to the same variables, such as sunlight and air to ensure that the results were more accurate. The accuracy of glassware is limited, and not all beakers are created equally. Hence, another improvement to increase the chance of the class finding similar results would be to ensure that everyone uses similar equipment. It can be ensured that everyone uses a strategic approach to removing the excess water from the potato cubes by sieving, rolling and patting the potatoes using the paper towel for same number of times and with the same pressure. Though this may not eliminate the error, it can still be reduced which will improve the accuracy of the experiment.

Since the potatoes floated, the potatoes can be systematically turned throughout the twenty minutes to ensure each side has even exposure to diffusion. This will also ensure that the sodium chloride is evenly dispersed within the solution. If different potatoes were used in such an experiment, they could be pre-soaked in a hypotonic solution to ensure that their turgid. This will ensure the concentrations within the potato won’t differ too much, hence decreasing the rate of osmotic activity. Since it was hard to be accurate with the measuring and cutting the cubes, moulds or borers can be used to increase the uniformity of the cubes and increase the straightness of all the sides.

The variations in temperature can be minimised if a water bath was used to place the beakers on, this ensures that the beakers are exposed to a constant temperature during the twenty minutes of the experiment.

The reliability of this investigation can be increased by using a larger sample size of a bigger class and group which will reduce the effect of the variations due to random errors. It reduces errors because a larger sample size of the investigation corresponds to a smaller difference in the results obtained. Not only should the number of groups be increased, but also a greater range of dimensions should be used such as 1.5cm3, 7cm3, 9cm3, this ensures that the hypothesis can be tested and supported across many situations without it being confined to a certain range of dimensions. This in turn, will increase the reliability of the experiment in that in can be repeated again and generalised across all situations. Only one trial was conducted, therefore, more trials can be done in the future to increase the reliability. By repeating the experiment, systematic errors can be identified and rectified for the future.

This experiment can be improved and extended in other ways. The size of the cube can be kept constant while the concentration of solution can be varied. In doing so, the concentration of the solute within the potato cell can be discovered. This way, it can be investigated whether the size of the potato or the difference in concentration has a larger effect on the rate of osmosis. Another species of potato can be used, like washed, unwashed, sweet and red. By using other vegetables, such as cucumber or carrot, it can be investigated whether the same trend as the potato occurs in the rate of osmosis. A different solution can be used, such as sucrose, hence, the two sets of results can be compared to observe any differences in its osmotic activity and investigate the reason behind it. Different concentration of salt solutions can also be used for this investigation in order to obtain the isotonic point of the vegetable, i.e., where there is an osmotic balance as the concentration of water is the same in the internal and external environment of the potato cell.

CONCLUSION

In conclusion, the hypothesis is generally supported by this investigation. The trends seen in the table and graph indicate that the rate of osmosis increases in proportion to the surface area to volume ratio due to the smaller sizes of the potato cubes. The smaller sizes enable the cell to transport materials in a more efficient manner as there is less distance required for information to be passed from the nucleus to the rest of the cell.