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Osmosis lab Page1
Osmosis Lab
BOT 106
(Adapted from Nadler, 1998)
Cell or plasma membranes in plants can be thought of as semipermeable membranes. They
allow water to pass freely through the membrane but exclude most other molecules from
doing so. Plant cell membranes are elastic and inflate or deflate depending on the amount of
water in them. The structure of a plant cell is defined by a plant cell’s wall. Plant cell walls
provide a rigid enclosure for a cell, and set the boundaries for plant cell membranes. When a
cell is in a turgid state, the cell membrane is inflated with water such that it is pushing up
against the cell wall at all points. This is the normal and health state of most plant cells. A
cell that is flaccid is one whose plasma membrane has peeled away from its cell wall. Plant
cells that are flaccid are stressed and may not be functioning properly.
Water moves in and out of cells, but in many cases it is important for the plant to have the net
flow of water between the cell and its surroundings to be directed into the cell and in other
cases it is important for the net flow of water to be directed out of the cell. For researchers to
decide which way the net flow of water was moving, they would have to explore the concept
of water potential. If the water potential of a cells’ surroundings (Ψsur) is equal to the water
potential of the cell (Ψcell), then there is no net movement of water*. However, if the water
potentials of cell and surroundings are not equal, then there will be a net movement of water
to correct that imbalance. The water potential of a cell is defined by the sum of the osmotic
potential of a cell (Ψ) and the pressure potential (Ψp). If the water potential of the
surroundings is lower than the water potential of the cell (Ψsurr < Ψcell), then water will flow
out of the cell, releasing it of some of its pressure (Ψp will decrease so that Ψsur will equal
Ψcell). On the other hand, if the water potential of the cell is less than the water potential of
the surroundings (Ψcell < Ψsur), then water will flow into the cell, increasing the pressure in
the cell (Ψp will increase so that Ψsur will equal Ψcell). Remember that the plasma membrane
is a semipermeable membrane and will not allow other compounds, called solutes, in or out
of the cell without assistance, thus the osmotic pressure will remain constant.
When the pressure potential of a cell reaches zero, the cell membrane is no longer pushing up
against the cell wall and becomes flaccid. The term used to describe this condition is called
plasmolyzed. The process of plasmolysis occurs just as the plasma membrane pulls away
from the cell wall. At this point, the pressure potential is zero, and the water potential will
equal the osmotic potential, which will equal the water potential of the surroundings.
*
No net movement of water means that water will move in and out of the cell at random, not that water is prevented from
moving in or out.
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Water potential is inversely related to the concentration of solutes in a cell ( [S] ). Thus
when solutes are at a high concentration, the water potential of that cell is low. And a
general rule of water potential is that water flows toward cells with low water potentials.
This makes sense if you think of simple diffusion. Molecules that are at high concentrations
will diffuse or redistribute themselves randomly. If there is a high concentration of
molecules in a cell, more water coming into that cell will create more space for those
molecules to diffuse into.
Water
To summarize:
Ψcell = Ψ + Ψp
If Ψcell = Ψsur, there is no net movement of water
(also called equilibrium state).
If Ψcell < Ψsur, water moves into the cell, Ψp .
If Ψcell = Ψsur, water moves out of the cell, Ψp .
When plasmolysis occurs: Ψp = 0,
Ψcell = Ψ + 0,
Ψsur = Ψcell = Ψ
Ψ = (1/ [S] )
To visualize these process more clearly, we will conduct an experiment on the water
potential of potatoes.
Procedure, Day I:
1. Using a cork borer, make 30 potato cylinders: cut the ends of a potato evenly, stand the
potato on one of its flat ends and plunge the borer into the potato. Try not to allow the
path of the borer to cross the trek of an earlier core. Lastly, cut all your cores to a
standard length. Record this length as LI.
2. Weigh your standardized cores in groups of five to be placed in one of six treatments.
Record those weights as WI.
3. Construct your experimental setup by labeling six petri dishes with the six treatments:
0.0M, 0.1M, 0.2M, 0.3M, 0.4M, and 0.5M. Each treatment in this list has an increasing
solute concentration. (What can you say about each treatment’s water potential?)
4. Add some of the correct treatment solution to each of your petri dishes.
5. Add one group of five potato cores to each treatment
6. Parafilm the petri dishes closed so no evaporation will occur (What would happen to the
solute concentration and the water potential if evaporation occurs?)
7. Stack your petri dishes, rubber band them together, and label your stack with your group
name.
Procedure, Day II:
1. Look at your treatments: have any of the cores gotten longer or fatter?
2. One dish at a time, take out the cores and measure length and weight of them
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a. Length: line up the cores end to end, take the length of the whole row and
divide by five to get the average length of the cores after the treatment.
Record this length as LF.
b. Weight: blot the cores gently and weigh. Record this weight as WF.
3. Find the change in weight (W) and the change in length (L) by subtracting the initials
from the finals:
a. W = WF - WI
b. L = LF - LI
4. Free hand graph the data in the space provided.
5. At what molar are the potato cells plasmolyzed?
Weight
Treatments
0.0M
0.1M
0.2M
0.3M
0.4M
WI
WF
W
Length
LI
LF
L
Graph the changes in length and weight:
Weight:
W
Ψ of potato cells is ___________
0
Molarity
0.0
0.2
0.4
Length:
L
Ψ of potato cells is ___________
0
0.0
0.2
0.4
Molarity
0.5M