The Effects Of Various Salt Concentrations On Osmosis And Weight Fluctuations In Potato Tissue: Laboratory Report

Abstract:

This experiment seeks to demonstrate the effects of osmosis in potato tissue when exposed to various sodium chloride solutions and how this process can cause weight fluctuations. It is expected that osmosis would occur in an inwards direction when placed in distilled water causing weight gain and in an outwards direction in salt solutions causing weight loss. The weight loss is expected to increase with concentration with 5% solution expected to have highest decrease. Multiple core samples of potato were placed for 24 hours in various solutions and weighed before and after to record mass increase or decrease. In order to account for various experimental inconsistencies a percentage mass gain or loss was used and then tabulated and graphed for ease of interpretation. The resulting data showed an increase in weight for distilled water (0% salt) and a decrease in weight for all salt solutions (1%, 3%, 5%). The highest decrease was expected to be 5% however 3% had more weight loss which is further explored in terms of mass transfer of sodium chloride during osmosis, errors and experimental redesign options. This data can be used to find the concentration of salt in cells based on equilibrium with cell concentration and clearly demonstrates the difference in osmosis of water entering and leaving the cell and the effect on mass.

Introduction:

Osmosis is the process by which water moves across a cell membrane by diffusion. Diffusion takes place when the molecules of a substance move from areas of higher concentration to areas of lower concentration. The cell membrane, however, is semi-permeable, and some substances do not readily cross the cell membrane, particularly if they are large, hydrophilic and/or charged (Reece et al. 2014). In these cases, the difference in solute concentration drives the movement of water across the cell membrane instead from the hypotonic side (the least concentrated) to hypertonic side (the most concentrated) until both sides are isotonic (equally concentrated) (Reece et al. 2014). This is the process of osmosis.

Osmosis must be tightly controlled by living cells, otherwise they die (Reece et al. 2014). For example, a red blood cell placed in pure (distilled) water will quickly take up water until it bursts (Figure 1a). That explains why plasma, the liquid portion of our blood, is made of water with proteins and salts dissolved in it, preventing unnecessary and dangerous gain of water by blood cells (Reece et al. 2014).

In plants, controlling osmosis is just as important as in animal cells. Plants with too little water wilt. This happens when water moves out of the plant cells by osmosis. Without this water there is too little hydrostatic pressure inside the cells and the plant cell can no longer support itself against the pull of gravity. Eventually, this leads to plasmolysis (Figure 1b). After watering the plant, cells become “reinflated” with water and the plant again stands upright.

Figure 1. Effect of immersion is hypotonic, isotonic and hypertonic solutions with a) animal cells (such as a red blood cell) that do not have a cell well, and b) plant cells that do. Arrows indicate the next water movement after the cells are first placed in the solution. Source: Fig 7.12 in Reese et al. (2014).

This study investigated the effects of osmosis on plant cells. Specifically, the aim of the study was to determine the change in weight of potato cores after overnight incubation in solutions of varying salt concentrations. It is hypothesised that potato cores placed in a “free water” environment (distilled water) would gain weight as the cells take on the water, and in salt solutions the cores would lose weight increasingly with concentration as cells give off water to the solution via osmosis.

Materials and methods:

Experimental methods

A metal cork-borer was used to create 12 cylinders of identical diameter from a potato (purchased from a nearby supermarket). Any skin remaining on the cylinders was trimmed off, and a blade was used to cut the cylinders into equal lengths (of approximately 3 cm). The cylinders were split into four group of three, blotted dry on a paper towel, weighed to the nearest 0.01 g, and inserted into a small plastic container. They were then submerged in 4 different solutions: distilled water (0% salt), and a 1%, 3% and 5% salt solutions (made by dissolving salt to the required concentration in distilled water). After 24h incubation in the solutions, the potato cylinders were removed, blotted dry, and again weighed to the nearest 0.01 g. Observations were made of their texture, colour and flexibility.

Data analysis

Before and after weights of each group of potato cylinders were entered in a data processor (Microsoft Excel) for every group in the class. Due to variability in available potato sizes and shapes and differences in corer diameter, the weights of the different groups of potato cylinders were different. Therefore, changes in weight were expressed as a percentage of weight increase/decrease to account for size variability amongst groups. The dataset was then analysed using excel to asses whether any outliers were present. Using colour classification, 8 groups were flagged and shown to only have made their initial measurement and not their final measurement. These results were deemed inaccurate as the percentage change was unable to be obtained. Once those results were removed, the test was run again to show 3 major outliers with above average percentage increases. These were included in the data set as the measurements were complete and could not be removed without creating an untrue and biased report however careful note was made of these.

Results:

Data obtained consisted of two measurements, one taken before 24 hours in various solutions, and one measurement taken after. The results obtained from the experiment were converted into percentage weight gain/loss and tabulated along with maximums, minimums, means and standard deviations and displayed in table 1 below. The results showed distilled water samples gained weight with an average class percentage of 21.2% weight increase. Our personal groups percentage was 15.9% weight gain, below average but still a significant increase as hypothesised. The lowest concentration salt solution showed 3.3% weight loss (shown by negative value) for the class, and 11.5% weight loss for our group. The 3% salt solution showed 21.4% decrease in class weights and 22.5% decrease in group weights. The strongest of the solutions (5% salt) showed a decrease of 18.8% for the class weights and 18.1% for the group weight, although this showed a decrease as expected, it is still less decrease then the results for 3% salt solution weights. Standard deviations from the samples are displayed showing similar standard deviations for all four solutions. Max and min percentages show increases for all solutions, a significant increase in distilled water which may suggest some error to be examined and discussed.

Table 1: Tabulated group weight change percentages, minimum and maximum class weight change percentages, class average weight change percentages and class weight standard deviation percentages for potato cores after 24 hours in various concentrations of salt solutions.

• Distilled (0%) / Salt solution (1%) / Salt solution (3%) Salt solution (5%)
• Group percentages 15.9% -11.5% -22.5% -18.1%
• Class Max 137.2% 51.4% 47.9% 53.4%
• Class Min -10.1% -20.8% -45.1% -45.8%
• Class Mean 21.2% -3.3% -21.4% -18.8%
• Class Standard Deviation 20.7% 13.7% 16.0% 17.3%

To interoperate these results a visual representation is used in figure 2. The column graph can position all percentages and standard deviations (as shown in error bars) neatly next to each other so we can more clearly see any abnormal results. As shown the error bars are quite large for the samples showing how scattered class measurements were and this should also be discussed. The trend line can show the relationship explored more clearly.

Figure 2: Effects of Osmosis on weights of potato cores and their average mass percentage changes in varying concentrations of salt solutions. (this graphical representation was chosen for comparison of all four solutions and error bars are displayed to account for standard deviation)

Overall, we can see that the results showed an increased weight for the distilled water solution only and he salt solutions caused decreases in weight at all three concentrations.

Discussion:

When examining the results for our experiment we noticed that all data matched our hypothesis however one solution when compared to other solutions was unexpected. The data for both class and group showed the only solution to cause a mass percentage gain was the distilled water (0% salt). As expected with osmosis, the distilled water would have a higher percentage of free water when compared to potato cores causing water to flow into the cells, causing a weight increase (Reece et al. 2014). When looking at the maximum and minimum results however it seems as though the outliers mentioned earlier may have strong effect on these results. Possible explanations for these measurements include incorrect group methods for preparation and measurement, faulty equipment or some other error we can not be sure of. Since we can not confirm the reason the data was abnormal, we cannot remove it from the data.

All salt solutions, as expected, showed a decrease in weight which was mostly because osmosis causes free water to flow out of the cell towards the salt concentration, decreasing cell weight (Reece et al. 2014). When the experiment was conducted it was hypothesised that with increasing percentage of salt concentration, weight loss would also increase. When we examined the results, we saw that while 1% solutions and 3% solutions showed this trend, solutions containing 5% for both group and class data showed a slight decrease when compared to 3%, meaning 3% salt solution suffered the greatest weight decrease. Possible reasons behind this were examined and there is high possibility of outliers effecting these results. The error bars show that the results had a high range of standard deviation, showing the high possibility in the data of error. Completing the experiment with a higher rate of controlled variables would be suggested, having the same people take the measurements, with the same equipment multiple times would eliminate some error but an experimental redesign examining solutions of 1%, 2%, 3%, 4% and 5% for example also may be able to either pinpoint an abnormality or eliminate the possible error to determine the cause of the unexpected weight decrease.

It is also important to examine other factors that may have been the underlying cause for the unexpected values of the 5% solution compared to the 3% solution. When examining literature, it was apparent that osmosis and cell mass changes have multivariable factors which may explain the results achieved. The special distribution of solids in an osmotic process depends on several factors including solute type, solution concentration, time spent undergoing osmosis and the temperature at which osmosis took place (Lenart & Flink, 2007). Major factors in this experiment that may have affected the potatoes include the temperature of the room, the 24-hour period of osmosis and the type of solution used.

The mass associated with osmotic dehydration shows a multicomponent mass transfer, which leads to loss of water (osmotic dehydration) as well as other mass related components (Shi & Le Maguer, 2002). There is a strong possibility that in the 5% solution, there was a significant water weight loss due to the osmotic process of free water exiting the cell, but a mass increase as some salt was transferred by the semipermeable layer into the cell, increasing its weight. This may show why the 5% solution resulted in less overall loss, but this is likely because some gain was experienced in other factors.

To reduce the possibility of mass transfer into the cell we can examine possible experimental redesigns. An experiment performed by Lenart and Flink in 2007 was able to produce a range of osmosis distribution curves showing great differences between uptake of salt solids when compared to sucrose. Osmosis in salt solutions shows significant salt transport where as in sucrose tests this was dramatically reduced (Lenart & Flink, 2007). Preparing the osmotic solutions from sucrose instead of sodium chloride would be ideal as this would cause less mass transfer. Another experiment conducted to record salt induced changes in potato tissue showed that increased salting time leads to an increase in the amount of salt found in the cells. It was concluded that longer exposure time to saturated salt solution causes an increased uptake of salt (Straadt, Thybo & Bertram, 2008). A shorter time spent in solution should be considered to reduce the effects of this.

Sodium absorption in potatoes was studied using salt osmotic solutions to determine quantitatively the sodium mass transfer coefficient. In this experiment factors that affect the quantity of salt uptake were examined and showed that simple changes in method can lead to significant increase in salt within the cell, one of these major variables being osmotic temperature and saturation time (Hinkley, Pandya, Kinchela & Decker, 2015) There are many uncontrolled variables such as time, temperature and solution suitability that must be reexplored to obtain the most accurate and reliable results from this experiment.

An isotonic solution strength can also be determined by examining the point of the graph shown in figure 2 at which the potato weights did not increase or decrease to determine internal salt concentration. We see that the trend line passes 0 weight gain/loss just before the 1% solution mark. As potatoes have different salt concentrations, we can not know for sure the exact concentration, but we know approximately 0.9% salt solution is isotonic to potato tissue. Isotonic solutions cause neither loss or gain as they reach equilibrium with the cells, the water doesn’t flow from one to the other as both concentrations are the equal. This means the potato would have the same concentration as the isotonic solution, meaning potato salt concentration was close to 0.9%.

Osmosis can relate to everyday life, and when explaining osmosis, we are able to understand things such as “why do they spray water on fresh vegetables at the grocery store?”. To answer this, we refer to what happens to plant cells when fresh (free) water is added. When a cell (with a salt concentration) encounters a solution with less salt concentration, it causes the solution with most free water to enter the cells and cause weight gain. In the grocery store this helps keep vegetables fresh, plump and increases weight for purchase.

Conclusions:

This experiment aimed to demonstrate weight changes caused by osmotic water flow between solutions of various sodium chloride concentrations and potato tissue. It was hypothesised that the solution containing more free water (0% salt) would result in a weight gain as water flows from water into cell via osmosis, and a weight loss for all salt solutions increasingly as water flows from the cell to solution. As hypothesised, the distilled water solution caused an average of 21.2% increase in weight. The salt solutions (1%, 3% and 5%) showed a decrease of 3.3%, 21.4% and 18.8% respectively. All salt solutions caused a decrease in mass. however, 5% solution was expected to have the most weight loss which did not follow trend, 3% solution showed overall greatest weight loss. Reasons for this could not be definitively concluded, it was possibly resulting of human and experimental design error which were explored in depth. In order to obtain more accurate results, the experiment should be redesigned and performed again.

References:

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