The Death of Mila the Camel Driver
Introduction:
Before we explore the conditions surrounding Mila’s death, let’s review some basic facts.
We define diffusion as movement of materials from a region of higher concentration to a region of lower concentration. This movement is due to kinetic energy of particles. An example of this can be observed by opening a bottle of perfume. Soon you will smell the perfume in distant parts of the room. It diffuses from the high concentration of molecules in the bottle to a lower concentration of molecules in the air.
Osmosis is a type of diffusion
. It involves movement of water through a differentially permeable membrane from a region of high concentration (of water molecules) to a region of lower concentration (of water molecules).
We define 3 types of solutions based upon osmotic activity. In an isotonic solution, the solute concentration is the same on both sides of the membrane so there is no net movement of water. (See Figure 1)
(
C
ell
placed
in isotonic solution stays the same
“Normal Cell”
No change
)
(
Figure 1
:
Effects of an Isotonic
S
olution
)
In a hypotonic solution, solute concentration is higher inside the cell than outside. So water moves from a high concentration outside the cell to a lower concentration inside the cell. The cell swells and may burst from the excess water. This is what occurs when we soak salad ingredients in ice water to make them crisp! (See Figure 2)
(
Cell placed
in hypotonic solution swells and perhaps bursts
!
)
(
“Normal Cell”
)
(
Cell
S
wells
)
Figure 2: Effects of a Hypotonic Solution.
In a hypertonic solution, the solute concentration is higher outside than inside the cell so water moves outward from the high concentration (of water) inside the cell to the lower concentration (of water) outside the cell. Thus the cell shrinks! (See Figure 3)
(
“Normal Cell”
C
ell
placed
in hypertonic solution shrinks
Cell
S
hrinks
)
Figure 3. Effects of A Hypertonic Solution.
Case Study:
Mila the camel driver lost her way in the desert. After several days of wandering with her trusty camel Richard, she found himself on a beach. She carefully searched the area for a spring but found no fresh water. Finally, Mila became so thirsty that she decided to drink seawater. A few days later, a ship spotted Mila’s signal fire and landed on the beach. The ship’s captain found a delirious Mila who died the next day. Let’s investigate the cause of Mila’s death.
We know that all living things are composed of cells. Since our camel driver is living, we can use stalks of celery, also composed of cells, to represent his body. We will use water and salt to investigate the effects of various concentrations of salt water on the cells. This should give us some clues to the cause of Mila’s demise.
Movement of water through a selectively permeable membrane from a region of high concentration of water to an area of lower (water) concentration is termed osmosis. We will investigate the effects of three concentrations of salt solution on the celery stalks (Mila). One solution is hypertonic, that is, it contains more solutes and less water than the cytoplasm of the cells. One solution will be hypotonic. This solution will contain fewer solutes than the cell cytoplasm. The third solution will be isotonic and will contain a solute concentration identical to the cell cytoplasm.
By observing the effects of these solutions on the celery stalks, we should be able to determine what happened to Mila’s cells when she drank seawater.
Materials:
1 celery stalk and table salt
A knife
Measuring spoons and measuring cup
Water (tap water will work unless your water has a high concentration of dissolved solutes) Distilled water works best.
3 containers for holding the solutions and celery stalks
Some type of covering to close the containers
Procedure:
1.
Cut 4 two inch long slices of celery (try to keep them the same length and thickness.)
2.
Make 4 vertical cuts 2/3 of the length of each piece. Try to space them evenly.
(
Figure 4: Make 4 vertical cuts in
each piece of
celery.
) (
Celery
)
3.
Label 3 containers A, B and C
4.
Add 1 cup of water to each container
5.
Add the following amounts of salt to each container:
a.
A – no salt
b.
B – ½ teaspoon salt
c.
C – 2 teaspoons salt
6.
Stir to dissolve the salt.
7.
Immerse one celery stalk in each container and reserve one celery stalk as a control.
8.
Cover the containers and wrap the “control stalk” in plastic wrap or place in an air tight container.
9.
Leave the celery stalks in the container 6 hours or overnight.
10. Remove the stalks from the containers and bend each stalk
11. Record your description of each stalk in the table in the Observation section.
Observations:
Solution | Flexibility As Compared To the Control Stalk | Did the Water Move Into the cells or out of the cells? |
A | ||
B | ||
C |
1. Based upon your observations of the flexibility of each stalk, which solution would you describe as isotonic, hypertonic or hypotonic?
Solution | Tonicity |
A | |
B | |
C |
2. Which solution was the closest approximation of seawater?
3. What happened to Mila’s cells as a result of drinking seawater?
4. What symptoms do you think that Mila presented before her death?
5. How would you treat someone in the condition in which the captain found Mila?
6. Give three possible sources of error in your experiment.
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