Download Pre-Lab: Diffusion and Osmosis in Model Systems

Pre-Lab: Diffusion and Osmosis in Model Systems
In parts 1 and 2 of this lab, you will have the opportunity to investigate the processes of
diffusion and osmosis in model membrane systems. You will also investigate the effect
of solute concentration on water potential as it relates to living plant tissues.
Part I: Osmosis.
In this part of the lab, you will use dialysis tubing filled with different molarities
of sucrose to investigate the relationship between solute concentration and the
movement of water through a selectively permeable membrane (the process of
osmosis).
When a dialysis bag containing a sucrose solution is placed in fresh water, the
bag accumulates water as a result of osmosis. Because there is a higher concentration
of water outside the bag than inside the bag, water diffuses into the bag. The water
outside the bag is said to be hypotonic to the solution in the bag; the solution inside the
bag is hypertonic relative to the water outside the bag. Water always moves through a
selectively permeable membrane (sucrose cannot move) from hypotonic to hypertonic
solutions.
In theory, if you were to add solute to the water outside the bag, you could
decrease the water concentration out there. It should be possible to add just the right
amount of solute to the water so that the concentration of dissolved substances outside
the bag is the same as the concentration of dissolved substances inside the bag. Such
solutions are said to be isotonic, and no net movement of water will occur. If you were
to continue to add solute to the water outside the bag, you would decrease the
concentration of water outside the bag until the solution outside became hypertonic to
the solution inside. Then, the net movement of water would be in the opposite direction:
from inside to outside, and the bag would shrink.
Part 2: Determining the Water Potential of Potato Cells.
In this part of the lab, you will use pieces of potato tissue placed in different molar
concentrations of sucrose in order to determine the water potential of potato cells. First,
however, let’s explore what we mean by the idea of water potential!
In animal cells, movement of water into and out of the cell is influenced by the
relative concentration of solute on either side of the cell membrane. If water moves out
of the cell, the cell will shrink, (crenulate), and if the water moves into the cell it will
swell, and may even burst (cytolize). In plant cells, the presence of a rigid wall prevents
cells from bursting as water enters the cells, but pressure eventually builds up inside the
cell and affects the process of osmosis.
In predicting which direction water will move through living plant tissues, a
quantity known as water potential is used. Water potential, abbreviated by the Greek
letter  (PSI) refers to the potential energy of water. It has many components, but the
two on which we will focus are osmotic potential which expends on solute concentration,
and pressure potential, which results from the exertion of pressure (positive or negative)
on a solution. We express this as:

=
P
+
S
water potential
= pressure potential
+
osmotic potential
Water will always move from an area of higher water potential (higher potential
energy) to an area of lower water potential (lower potential energy). Stated another
way, water potential measures the tendency of water to leave one place in favor of
another place.
The water potential of pure water in a beaker open to the atmosphere is “0” ( =
0), because both the osmotic and pressure potentials are taken to be zero (S = 0; P =
0). An increase in positive pressure raises the pressure potential, and therefore raises
the water potential. The addition of solute to water lowers the osmotic potential (makes
negative), and therefore lowers the water potential. Therefore, any solution at
atmospheric pressure (P = 0) will always have a negative water potential. For
example, a 0.1 M sucrose solution at atmospheric pressure (P= 0) has an osmotic
potential (S) of –2.3 bars due to the solute, and thus a total water potential of –2.3 bars
(0 + -2.3 = -2.3). A bar is a metric unit of pressure, measured with a barometer,
which is about the same as one atmosphere.
When a solution, such as that inside a potato cell, is separated from pure water
by the selectively permeable cell membrane, water will move by osmosis from the
surrounding area where water potential is higher ( = 0) into the cell where water
potential is lower due to its dissolved solutes ( is negative). In a case where the
solute cannot leave the cell, the movement of water into the cell causes the cell to swell,
and the cell membrane pushes against the cell wall. This creates positive turgor
pressure in the cell.
Eventually, enough positive turgor pressure builds up to oppose the more
negative osmotic pressure of the cell. This will continue until the water potential of the
cell equals the water potential of the pure water outside the cell ( cell =  outside the
cell = 0). At this point, a dynamic equilibrium is reached and NET movement of water
will cease.
If you add solute to the water outside the potato cells, you decrease the water
potential of the solution surrounding the cells. It should be possible to add just the right
amount of solute to the water so that the water potential outside the cell is the same as
the water potential inside the cell. Therefore, net water movement will cease. At this
point, the water potential of the solution is equal to the water potential of the turgid
potato cells. (It is this information that you will use to calculate the water potential of
cells in the potato cores, which have been soaking in different molarities of sucrose.)
Eventually, if enough solute is added to the solution in the beaker, the water
potential of the cell will be greater (less negative) than the water potential of the solution
in the beaker because of the turgor that still exists within the cells. Water will diffuse out
of the potato cells in response to a pressure gradient, from the area of higher water
potential (inside) to the area of lower water potential (outside). If enough water was
lost, the cell membrane would shrink and fall away from the cell wall, and the cell would
become plasmolyzed.
By weighing some potato cubes and immersing them in different sucrose
solutions, then re-weighing, you should be able to pinpoint the sucrose solution which
corresponds to the water potential of the potato tissue!
Pre-Lab Questions
Answer the following questions in your lab notebook. Be sure to write complete
sentences that explain what the question was asking.
1. Define osmosis in your own words.
2. Explain the movement of water molecules in regards to hypertonic, hypotonic,
and isotonic solutions.
3. What is dialysis tubing and why are we using it in this lab?
4. Define water potential in your own words.
5. What is the equation for water potential?
6. How does water move, in respect to water potential?
7. What is the effect of adding solutes to water potential?
8. What does the unit of a bar represent?
9. Define turgor pressure in your own words.
10. What will happen to the mass of a potato that has been placed into a solution
that has the same water potential inside the cell as the solution outside of the
cell?