Abstract
Osmosis is the diffusion of water through a membrane, and in this lab, osmosis was used
in three different contexts. The movement of water between distilled water and dialysis tubing
filled with an ovalbumin solution was monitored, the effect of solutions on red onion cells was
observed, and the water potential of sweet and white potatoes were calculated. The results
indicated that water diffused into tubing with 5% ovalbumin solution at a rate of .017 grams of
water per minute, that there was no difference between red onion cells treated with distilled
water and those treated with a 1M glucose solution, and that sweet potatoes have a water
potential of greater magnitude than that of white potatoes.
ABSTRACT
In order to analyze osmosis, dialysis tubing, onion cells, and both white and sweet
potato cells were treated with different solutions and the direction of water
measured and explained. The direction of water flow is based on the amount of
free water on both sides of a selectively permeable membrane. The dialysis tube
increased in mass when filled with a 1 M glucose solution and surrounded by a
5% ovalbumin solution. Water entered the red onion cells when they were
surrounded by distilled water, but left them when they were surrounded by the 1
M glucose solution. The results from Part (c), which involved determining the
molarity of different solutions based on their effects on potato cells, were
inconclusive because white and sweet potatoes yielded different results.
Introduction
Water is essential to life. Consequently, the movement of water has very important
ramifications on living organisms. Water, as all substances, will tend to move down its
concentration gradient, from areas of higher concentration to areas of lower concentration,
through a process called diffusion. When water diffuses across a membrane, the process is
termed osmosis.
When cells are located in a solution, there is likely to be a net movement of water either
into or out of the cell. If there is a greater solute concentration (and therefore a smaller free water
concentration) outside the cell compared to inside the cell, water will diffuse out of the cell
through its membrane and the cell will shrivel up (crenation) due to the loss of water. When a
cell is in a more concentrated solution, the solution is said to be hypertonic to the cell. On the
other hand, when a cell is in a hypotonic solution—a solution in which the solute concentration
is lower than that inside the cell—water will diffuse into the cell, causing the cell to swell and
possible even burst (cytolysis). In the case where the solute concentrations inside and outside the
cell are the same, there is no net movement of water and the cell is said to be in an isotonic
environment.
This experiment contains three components. First, the process of osmosis was
investigated using dialysis tubing and two different solutions—distilled water and a 5%
ovalbumin solution in water. The ovalbumin solution was put into the tubing, and the tubing was
placed into the distilled water. It was hypothesized that the tubing and its contents would
increase greatly in weight relatively quickly due to diffusion of water into the tubing.
The second part of the experiment investigated the effect of solutions on cells. Red onion
cells treated with distilled water and then red onion cells treated with a 1M glucose solution were
looked at under a microscope. It was hypothesized that the cells treated with distilled water
would appear very swollen (turgid) while the cells treated with the 1M solution would look less
so.
The third part of the experiment consisted of determining the water potential of both
white and sweet potatoes. Water potential () is a measure of water’s tendency to move from
one area to another and is given by the sum of the pressure potential (P) and the solute potential
(s). The solute potential is given by the equation  = -iCRT where T is the temperature of the
solution in Kelvin, R is the ideal gas constant, C is the molar concentration of the solution, and i
is the number of moles of particles in solution there are for every mol of solute. This part of the
experiment also consisted of determining the molarities of five sucrose solutions (it was known
that there were 0M, 0.2M, 0.4M, 0.6M, 0.8M, and 1.0M solutions, but it was not known which
solution had which molarity). There was no way to predict which solution had which molarity,
but it was hypothesized that the magnitude of the sweet potato’s water potential would be greater
than that of the white potatoes
1. INTRODUCTION
The purpose of this experiment is to analyze the process of osmosis by utilizing three different
types of cells and membranes: (1) dialysis tubing, (2) red onion cells, (3) potato cells (both sweet
potato cells and white potato cells). The experiment was conducted in three parts, one for each
type of membrane being analyzed.
Osmosis is the diffusion of water across a selectively permeable membrane (Campbell et. al.
2008). This process is based on the concentration of a solute on either side of the membrane. In
general, the side of the membrane with a higher solute concentration has less free water available
(Osmosis 2011). Free water is the amount of water not bound to solute molecules. A higher
amount of free water, which occurs on the side of the membrane with a lower solute
concentration, leads to more collisions with the pores of the membrane and, therefore, more
molecules going through the membrane.
Tonicity, or the ability of a solution to cause a cell to gain or lose water, is based on the
amount of free water on either side of a membrane (Campbell et al. 2008). If a solution is
hypertonic to a cell, water molecules will leave the cell because there is more free water in the
cell and less in the surrounding environment. Conversely, a solution that is hypotonic to the cell
will cause water to enter the cell because there is less free water inside the cell. In a solution that
is isotonic to a cell, there is no net movement of water across the membrane because the amounts
of free water are equal (the concentration of solute on both sides of the membrane are equal as
well).
The hypothesis for Part (a) of this experiment, which used dialysis tubing as a selectively
permeable membrane, is that water will enter the tube because there is more free water in the
surrounding ovalbumin solution because, as a protein, it does not dissolve in water and become a
solute. On the other hand, glucose does become a solute, making the solution hypotonic to the
cell.
Part (b) involves treating a sample of red onion cells with two different solutions and
observing their effects on the size of the cells. The prediction is that the cells will appear turgid
in the distilled water because the water will be hypotonic to the cells, but small and plasmolyzed
in the 1 M glucose solution because it is hypertonic to the cells (Campbell et. al. 2008).
The prediction for Part (c) is that the solution with the highest molarity will lead to the biggest
percent decrease in mass, while the solution with the lowest molarity will lead to the smallest
percent decrease in mass.
Discussion
In the first part of the lab, it appears that water did in fact diffuse into the dialysis tubing,
as indicated by the increase in mass from 12.7g initially to 13.3g at the end of 35 minutes.
Accordingly, the rate of diffusion was about .017 grams of water per minute. Although the
increase in mass was expected, the amount of change was much less than expected and the
change occurred much more slowly than expected. The reason for this could be that the
ovalbumin solution was a 5% solution by mass. Because ovalbumin is a protein, it is a very large
molecule and has a very large molar mass (about 42,780g/mol). As a result a 5% ovalbumin
solution has a very small molarity (about 1.17x10-4M). Since the ovalbumin solution was so
dilute, there was not much of a difference in concentration between the distilled water and the
solution, so only a little bit of water diffused into the tubing, and it did so slowly. In the first ten
minutes of recorded masses, the mass of the tubing fluctuated several times within the range of
12.7 grams to 12.9 grams. A possible reason for this is that some times the tubing was dried off
more thoroughly than others, resulting in inaccurate measurements. Since the increase in mass
due to diffusion in this time period was probably small, the added weight of water that was not
dried off from the outside of the tubing would have led to a recorded mass higher than the actual
mass.
In the second part of the lab, the onion treated with water and the onion treated with 1M
glucose solution appeared the same under the microscope, which was not expected. Perhaps this
unanticipated similarity resulted because insufficient time was allowed for a noticeable amount
of water to diffuse into or out of the cells.
In the third part of the lab, it was necessary to determine the molarities of the various
sucrose solutions before it was possible to determine the water potential of white and sweet
potatoes. While the percent changes for each solution varied with the type of potato, in both
cases, the clear solution cause the greatest increase in mass, the green solution caused the
second-greatest increase, then the yellow solution, then the purple solution, then the blue
solution, and then the red solution. An increase in volume indicates that water diffused into the
potatoes, which would show that the solution would be hypotonic to the potatoes. The greater the
increase in mass, the less the concentration of the solution. Accordingly, it can be determined
that the clear solution was the solution of lowest concentration, 0M, the green solution had the
next lowest concentration, 0.2M, the yellow solution’s concentration was 0.4M, the purple
solution was 0.6M, the blue solution was 0.8M, and the red solution was 1.0M. This is supported
by the ranking of the potatoes’ relative texture ranking, in which the hardness of the potatoes
decreased as the concentration of the solution they were in increased. This is because the higher
the concentration of the surrounding solution, the more water diffused out of the potatoes,
making them mushy and not rigid.
To determine the water potential of the white and sweet potatoes, the equation
 = P + S must be used. In this case, the pressure potential P is zero because the cups holding
the solutions and the potatoes experienced atmospheric pressure. Consequently, in this case  =
P + S and since S = -iCRT,  = -iCRT. For both the sweet and white potatoes, i = 1 because
sucrose does not dissociate in solution, T = 295K, and R = 0.0821 Latm/molK.
To find C, it is necessary to find the molarity of the interior of the potato cells. To do this,
the percent changes in mass were graphed against the molarities of the solutions (Figure 5).
The lines of best fit were determined, and then using the equations of these lines, it is
possible to solve for what molarity a solution must have to result in no change in mass (meaning
it is isotonic, or equal in concentration, to the interior of the cell. Using this method, the molarity
of the interior of a sweet potato cell was calculated to be 0.805M and the molarity of the inside
of a white potato cell was calculated to be 0.253M.
Using these values as C for sweet and white potatoes respectively, their water potentials
were calculated. The water potential of sweet potatoes was found be -19.4atm and the water
potential of white potatoes was found to be -6.13atm. According to these results, the hypothesis
that the water potential of sweet potatoes would be greater than that of white potatoes was
correct.
Figure 5