MEASURING THE RATE OF OSMOSIS USING “DESHELLED” CHICKEN
EGGS
(Effect of Solute Concentration Upon Rate/Degree of Osmosis in Chicken Eggs).
Introduction
If a cell is to perform its functions, it must maintain a steady state in the midst of an everchanging environment. This constancy is maintained by the regulation of movement of
materials into and out of the shell. To achieve this control, cells are bounded by a
delicate membrane that differentiates between different substances, slowing down the
movement of some while allowing others to pass through. Since not all substances
penetrate the membrane equally well, the membrane is said to be differentially
permeable.
The external and internal environment of cells is an aqueous solution of dissolved
inorganic and organic molecules. Movement of these molecules, both in the solution and
through the cell membrane, involves a physical process called diffusion – a spontaneous
process by which molecules move from a region in which they are highly concentrated to
a region in which their concentration is lower.
A special kind of diffusion is the phenomenon of osmosis. Simply defined in biological
systems, osmosis is the diffusion of water through a differentially permeable membrane
from a region in which it is highly concentrated to a region in which its concentration is
lower. More often, however, osmosis is defined in terms of the effects that solutes have
on the thermodynamic activity of water (i.e., the activity of the water molecule due to the
kinetic energy of motion). For example, the addition of a solute to water tends to
decrease the activity of the water. In other words, as more water molecules are displaced
by solute molecules, the activity of the water goes down. Thus, in thermodynamic terms,
water diffuses across membranes from a region in which the thermodynamic activity of
water is high (low solute concentration) to one in which the thermodynamic activity is
low (high solute concentration).
We use the terms hypotonic, hypertonic and isotonic in referring to the relative
concentrations of solute particles of different solutions. Below are the definitions of the
three terms according to Curtis, 1983.
Hypotonic
(Gk. hypo, under + tonos, tension): Of two solutions of different
concentration, the solution that contains the lower concentration of
solute particles; water moves across a semi-permeable membrane
into a hypertonic solution.
Hypertonic
(Gk. hyper, above + tonos, tension): Of two solutions of different
concentration, the solution that contains the higher concentration
of solute particles; water moves across a semi-permeable
membrane into a hypertonic solution.
Isotonic
(Gk. isos, equal + tonos, tension): Having the same concentration
of solutes as another solution. If two isotonic solutions are
separated by a semi-permeable membrane, there will be no net
flow of water across the membrane.
It should be noted that the number of solute particles is the thing that affects the relative
activity of the water, not the kind of particles.
Heat increases the motion of molecules. Therefore, we expect an increase in temperature
to speed up the rate of osmosis, regardless of which direction the solvent is moving. This
is diagrammatically shown below.
A.
Water enters cell
B.
Water leaves cell
0.1M solute
0.9M
solute
particles
particles
(i.e., sucrose)
(i.e.,
sucrose)
0.5M solute
particles
(i.e., proteins)
0.5M solute
particles
(i.e., proteins)
H2O
H2O


(H2O activity
lower than
outside)
(H2O activity
higher than
outside)
(H2O activity
(H2O
higher than
lower
inside cell)
inside
activity
than
cell)
HYPOTONIC
HYPERTONIC
THE EXPERIMENT
Part I: Effect of Solute Concentration
Each group of students will be given 6 chicken eggs from which the shell has been
dissolved away. The remaining membrane (the shell membrane) is differentially
permeable. We will assume that each egg has approximately the same concentration of
solute in this membrane, and based on the rate of osmosis, will attempt to determine what
this concentration must be.
Weight each egg separately to the nearest 0.1g and record the weights in Table 1 at time
“0”. Place eggs 1, 2, 3, 4, 5, and 6 into separate beakers containing solutions of distilled
water, 10% sucrose, 20% sucrose, 30% sucrose, 40% sucrose and unknown sucrose
solution respectively. At 15 minute intervals, that is after 15, 30, 45, 60, and 75 minutes,
remove the eggs from the beakers; carefully wipe off all excess water; and again weigh
each egg separately. Record the weight in Table 1. Plot the changes in weight of each of
the eggs against time on a piece of graph paper.
Which solutions would you say were hypotonic to that of the eggs? Which of them were
hypertonic? Isotonic? What would you expect to happen if an egg was put into a sixth
beaker containing a 50% sucrose solution?
Table I: Weight Change of Eggs (g) vs Time (minutes)
Time (Min.)
0% suc
10% suc
20% suc
30% suc
40% suc
Unknown
40% suc
Unknown
0
15
30
45
60
75
Table II: Weight Change of Eggs (g) vs Time (minutes)
Time (Min.)
0
15
30
45
60
75
0% suc
0
10% suc
0
20% suc
0
30% suc
0
0
0
Table III: Total Weight Change of Eggs (g) vs Sucrose Concentration (%)
Sucrose Conc. 0%Suc
Tot.
Wt.
Change
10%Suc
20%Suc
30%Suc
40%Suc
In order to fill out table 3, extract data on the 75th minute row of table 2 and place them
on table 3.
Lab Report for Movement of Materials Across Cell Membranes
To assist you in your understanding of this laboratory, you are asked to prepare the
following graphs using Microsoft excel and answer the questions at the end of each
section.
1.
Prepare a graph in which you plot the weight change of the six (6) eggs (use data
from table 2), which have been placed in solutions of varying solute (sucrose)
concentrations, as a function of time. You will have six (6) lines on this graph
(one for each egg). This graph indicates the weight change of eggs (g) vs time
(minutes)
(+)
Weight Change
of Eggs (grams)
(-)
0
0
15
30
45
60
75
a. What conclusion can you draw from the data in this graph?
b. Which solutions were hypotonic? Hypertonic? Isotonic?
2. Prepare a graph in which you plot total weight change of the eggs (g) placed in
varying solute (sucrose) solutions against sucrose concentration (%). You, in effect,
will be constructing a type of standard curve. Use the data in table 3 to construct this
graph.
(+)
Total Weight
Change (grams)
0
(-)
0
10
20
30
40
Sucrose Concentration (%)
Using this graph, you should be able to:
1.
Determine the isotonic point of the contents of a chicken egg. (Hint: where
your curve crosses the 0 line, read down to the concentration axis and record
the value you obtain.)
2.
Determine the concentration of the unknown solution. (Hint: locate on the
total weight change axis, the value representing the total weight change of the
egg placed in the unknown solution. Read across to your standard curve and
then down to the concentration axis and record the value you obtain.)