Diffusion and Osmosis Lab
APBiology
Period n
NAME____________________________________
Abstract:
In this lab we studied the movement of water across the cell membranes of potatoes in an effort to determine the
effective water potential of potato cell cytoplasm. Potatoes cubes or cored cylinders were placed in sucrose solutions
varying in concentration from 0 to 1 molar and allowed to reach dynamic equilibrium. Since sucrose cannot cross the
plasma membrane, dynamic equilibrium was reached by osmosis, or diffusion of water across the semipermeable
plasma membrane. Potato sections were massed, bathed in solution overnight, and massed again. We found that
potatoes in 0 and 0.2 M sucrose gained mass, while those in 0.4 to 1 M sucrose lost mass. Our results indicate that
solutions below and including 0.2 M sucrose are hypotonic, while solutions with sucrose concentrations 0.4M and higher
were hypertonic. The isosmotic point lies in between the transition from hypotonic to hypertonic. Thus, we estimate
that the osmolarity of potato cell cytoplasm is approximated by 0.3 M sucrose.
Introduction:
The kinetic energy of solutions causes molecules to move around and bump into each other. Diffusion is one
result of this molecular movement, and results from random movement of molecules from an area of higher
concentration to areas of lower concentration. Osmosis is a special kind of diffusion where water moves through a
selectively permeable membrane (a membrane that only allows certain molecules to diffuse though). Diffusion or
osmosis occurs until dynamic equilibrium has been reached. This is the point where the concentrations in both areas
are equal and no net movement will occur from one area to another.
There are three classifications of solutions based on their relative solute concentrations. If two solutions have
the same solute concentration, the solutions are said to be isotonic. If the solutions differ in concentration, the area
with the higher solute concentration is hypertonic and the area with the lower solute concentration is hypotonic. Since
a hypertonic solution contains a higher level of solute, it has a high solute potential and low water potential. This is
because water potential and solute potential are inversely proportional. A hypotonic solution would have a high water
potential and a low solute potential. An isotonic solution would have equal solute and water potentials. Water
potential (ψ) is composed of two main things, a physical pressure component, pressure potential (ψp), and the effects
of solutes, solute potential (ψs). A formula to show this relationship is ψ = ψp - ψs. Water will always move from areas
of high water potential to areas of low water potential.
The solute potential ψs = i MRT, where
, M = molarity and T = absolute temperature in
Kelvin. i is the van 't Hoff factor, which depends on the manner in which a solute dissolves. i = 1 for nonelectrolytes,
but it is approximately equal to the number of ions generated during dissolution for electrolytes. For example i for NaCl
is 2, but for sucrose, i = 1.
In this study we wished to determine the effective osmolarity of potato cell cytoplasm. In order to do so we
placed potato pieces in solutions ranging from 0 to 1 M sucrose overnight and examined their change in mass. Since
sucrose cannot likely cross the plasma membrane of the potato cells, the volume, and hence mass of the potatoes will
change to reflect water movement required to reach dynamic equilibrium. We hypothesized that the osmolarity of
potato cell cytoplasm is equivalent to a sucrose concentration between 0 and 1 M, and that some solutions would be
hypertonic while others would be hypotonic. We indeed found that solutions could be grouped accordingly. Analysis of
the results showed that there was a transition from hypotonic to hypertonic as sucrose concentration was increased
from 0.2 M to 0.4 M, indicating that the isotonic point lay in between these two values.
Materials and Methods:
Potatoes were sliced into 1 cm3 pieces and massed on a 3 beam balance. Four pieces were massed together and placed
into a 250 ml beaker containing ~50 ml of the test sucrose solution. Twenty four pieces in all were placed in six beakers
containing 0 (tap water), 0.2, 0.4, 0.6, 0.8 and 1 M sucrose. After overnight incubation, the potato pieces were removed
then lightly blotted dry and massed again. Percent changes in mass were calculated and data was combined from 6
groups to find the average change for each sucrose solution. We expected potato cell membranes to be impermeable to
sucrose; thus, changes in mass likely reflected changes in water content. Lower masses after incubation indicated that
water left the cytoplasm due to bathing in a hypertonic solution. Higher masses indicate water entering the cytoplasm
due to bathing in a hypotonic solution.
Results:
First we present data from our group and then combined data from all groups in both AP classes. In Table 1, the original
masses of each of the six sets of 4 potato pieces were recorded. The masses of the pieces after overnight incubation are
also shown. The percent change was calculated as
𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐶ℎ𝑎𝑛𝑔𝑒 =
Table 1.
Water
10.9 g
16 g
5.1 g
46.8%
massorig
massafter
Change
% change
0.2
8g
8.5 g
0.5 g
6.25 %
0.4
14 g
12 g
-2 g
-4.3 %
𝑚𝑎𝑠𝑠𝑎𝑓𝑡𝑒𝑟 − 𝑚𝑎𝑠𝑠𝑜𝑟𝑖𝑔
𝑚𝑎𝑠𝑠𝑜𝑟𝑖𝑔
0.6
9g
7.4 g
-1.6 g
-17.8 %
0.8
8g
6g
-2 g
25 %
1.0
12.5 g
9.5 g
-3 g
24 %
Positive percent changes in mass indicate the potatoes were bathed in a hypotonic solution. Negative percent changes
in mass indicate the potatoes were bathed in a hypotonic solution. Note the transition from positive to negative percent
changes between 0.2 M and 0.4 M sucrose.
We also present data from 9 groups across two AP biology classes in Table 2. Most groups recorded results similar to
ours. Again, there was transition from positive to negative percent changes between 0.2 M and 0.4 M sucrose.
Table 2.
Column1 0
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 1
Group 2
Group 3
Mean
0.2
15.09
0
23.1
46.8
23.74
13
15
21.4
6.89
0
13.4
6.25
2.54
-9.3
9
38
11
19.76625 8.642222
0.4
0.6
0.8
1
-10.28
-23.08
-30.77
-27.27
-3
-6
-11
-3
-5.8
-10.8
-9.1
-15.4
-14.3
-17.8
-25
-24
-15.38
-23.52
-36.25
-32.24
-13
-20.9
-27.7
-34
-16
-26
-34.5
-28
-15
-24
-30
-28.5
-9
-16
-29
-22
11.3067 18.6778 25.9244 23.8233
When we examine plots of the percent change vs. sucrose concentration for all groups (Figure 1). The transition from
hypotonic to hypertonic occurs between 0.2 and 0.4 M, and the average line for all groups’ data crosses the line
corresponding to zero percent change at approximately 0.3 M.
Figure 1.
60
50
40
Series1
Series2
30
Series3
Series4
20
Series5
10
Series6
Series7
0
0
-10
-20
-30
0.2
0.4
0.6
0.8
1
Series8
Series9
Series10
Series11
MEAN VALUES
-40
-50
Discussion:
In this study we examined the effects of overnight incubation of potato pieces in varying concentrations of sucrose to
determine the concentration of sucrose that most closely matches the water potential of the potato cell cytoplasm.
Loss or gain of water was measured as changes in mass. We felt confident that the changes in mass reflected water
movement rather than solute movement because potato cells are filled with starch and sucrose is a large molecule for
which no cell membrane transporters have been identified. The solutions that were hypotonic to potato cell cytoplasm
were the 0 and 0.2 M sucrose, whereas solutions of 0.4 M sucrose and greater were hypertonic. The transition from
hypotonic to hypertonic occurred between 0.2 M and 0.4 M. Since the water potentials of the six solutions were 0, -496,
-991, -1487, -1982, and -2478 N. Thus the water potential of potato cell cytoplasm lies between -496 and -991 N.
Judging by the plots in Figure 1, linear interpolation between these the two points for 0.2 and 0.4 M sucrose gives a
potato cell cytoplasm water potential of -743 N, which is equivalent to 0.3 M sucrose. Figure 1 also demonstrates that
potato cells can absorb more water than they can release. Since the concentration dependence of percent mass
changes do not fall on a straight line, but tends to diminish with increasing sucrose concentration, one could argue that
the potatoes used in this study did not exhibit the maximum turgor pressure until placed in distilled water. For future
study, perhaps potatoes could be incubated for varying durations to determine the time course of osmotic flow.