| Contents in Bag | Initial Mass | Final Mass | Mass difference | % Change in Mass | Class Average |
| Distilled Water |  | | | | |
| 0.2 M | | | | | |
| 0.4 M | | ||||
| 0.6 M | | | | | |
| 0.8 M | | | | | |
| 1.0 M | | | | | |
In this lab, you will observe the process of osmosis and diffusion. You will also
learn how to calculate water potential. If you are not familiar with these concepts,
make sure that you have looked them up in your textbook. If you don't know what
these terms mean, this lab is not going to make sense to you.
OBJECTIVES
EXERCISE
1
Distilled Water 0.4 M sucrose 0.8 M sucrose 0.2 M sucrose 0.6 M sucrose 1.0 M sucrose
4. Rinse the baggie carefully in distilled water to remove any sucrose that may have spilled and carefully blot.
5. Recore the mass of each baggie and record.
6.
Fill six 250 mL beakers two-thirds full with distilled water and place a bag in
each of them. Make sure that you record which baggie is which.
7. Let the bag sit for 20-30 minutes. (While this is running, you may want to begin exercise 3.)
11. Gather the data from the other groups in the class and find the class average for each baggie.
Contents in Bag Initial Mass Final Mass Mass difference % Change in Mass Class Average Distilled Water 0.2 M 0.4 M 0.6 M 0.8 M 1.0 M
1. Explain the relationship between the change in mass and the molarity of sucrose within the dialysis bag.
2. Predict what would happen to the mass of each bag in this experiment if all the bags were placed in 0.4 M sucrose solution instead of distilled water. Explain your response.
3. Why did you calculate the percent change in mass rather than simply using the change in mass?
4. A dialysis bag is filled with distilled water and then placed in a sucrose solution. The bag's initial mass is 20 g, and its final mass is 18 g. Calculate the percent change of mass, showing your calculations
.
Background
In animal cells, the 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. If water moves into the cell, the cell may swell or even burts. In plant cells, the precense of a cell wall prevents the cells from bursting, but pressure does eventually build up inside the cell and affects the process of osmosis. When the pressure inside the cell becomes large enough, no additional water will accumulate in the cell even th though cell still has a higher solute concentration than does pure water. So movement of water through the plant tissue cannot be predicted simply through knowing the relative solute concentrations on either side of the plant cell wall. Instead, the concept of water potential is used to predice the direction in which water will diffuse through living plant tissues.
In
a general sense, the water potential is the tendency of water to diffuse from
one are to another under a given set of perimeters. Water potenial is expressin
in bars, a metric unit of pressure equal to about 1 atmosphere and measured with
a barometer. Water potential is abbreviated by the Greek letter psi 
and
has to major components: solute potential 
,
which is dependent on solute concentration and pressure potential 
,
which results from the exertion of pressure - either positive or negative - on
a solution. We express this as:
In Figure 1.1. A potato cell is placed in pure water. Initally the water potential outside the cell is 0 and is higher than the water potential inside the cell (-3). Under these conditions there will be a net movement of water into the cell. The pressure potential inside the cell will increase until the cell reaches a state of equilibrium.
Contents in Bag Initial Mass Final Mass Mass difference % Change in Mass Class Average Distilled Water 0.2 M 0.4 M 0.6 M 0.8 M 1.0 M
7. Graph the results for both your individiual data and class average on on graph.
In order to do so, the 0 axis line should actually be in the middle of your graph.
The y axis above this line should be labeled % increase in mass while the y axis
below this line should be labeled % decrease. The x axis is the sucrose molarity
within the beaker.
8. Determine the molar concentration of the potato cores. This would be the sucrose molarity in which the mass of the potato core does not change. To find this, draw the straight line on your graph that best fits your data. The point at which this line crosses the x axis represents the molar concentration of sucrose with a water potential that is equal to the potato tissue water potential. At this concentration, there is no net gain or loss of water from the tissue.
1. If a potato core is allowed to dehydrate by sitting in the open air, would the water potential of the potato cells decrease or increase? Why?
3.
