Daiva Kuncaite
SCB 201
Prof. Wejesinghe
10/06/09
Week III
Title: Diffusion and Osmosis.
Objectives: to describe the mechanism and importance of diffusion and osmosis. Define
selective permeable membrane and explain its role in osmosis. Define hypotonic, hypertonic and
isotonic solutions and how the cells behave in each of the solution. Explain the influence of the
cell wall on osmotic behavior in the cell.
Introduction: In order to maintain the homeostasis in the cell the substances inside the cell has
to move out of the cell and substances presented in the extracellular fluids must move in.
Therefore, the movement of the substances can be achieved by different transport methods.
Diffusion is a net movement of particles from an area of high concentration, to the area of low
concentration. Particles move by random motion and using kinetic energy. It occurs due to
Brownian motion and concentration gradient. Selectively permeable membrane allows only
small substances with low molecular weight can pass through by diffusion. Osmosis is a
diffusion of water through a selectively permeable membrane from its region of high
concentration to its region of low concentration. All cells must maintain their osmotic
environment and water uptake. Animal cells are very sensitive to the osmotic pressure and
placed in the hypotonic solution the cell membrane undergo lysis, and when it placed in the
hypertonic solution, the cells shrink or crenate. The plant cell, on the other hand, due to the
structure of cell wall will not burst or crenate.
Procedure: In the first experiment we observe diffusion due to Brownian movement. On clean
slide we put one drop of water. We dipped a tip of a dissecting needle in the dry carmine dye and
added on the drop of water on the slide. Covered with watch glass and observed the movement of
the particles under the microscope.
In the second experiment we determine the permeability of assigned chemical substances using
dialysis tubing, which is an artificial semi permeable membrane. We took dry dialysis tubing and
soaked in the water for few minutes. One end of the tube we tide with the string. In the tube we
added 4 pipettesful of 30 % glucose solution and 4 pipettesful of starch solution. We tide the
opening of the tube with the string that made solution will not spill. Meantime we prepared a 500
ml beaker with 300 ml of water with several drops of I2KI solution until the water become
yellow-amber color. We placed the bag in the solution and left on the table for 30 min. After 30
min. we observed the changes in both solutions. In order to determine the presence of glucose
molecules we performed Benedict’s test. We took 3 test tubes. In first test tube we took a sample
of 2 pipettesful from the dialysis bag, in the second test tube – a sample of 2 pipettesful from the
beaker and the third sample was with water. In each test tube we added 1 drop of Benedict’s
reagent and heated the test tubes in the hot water bath for about 3 min. We recorded our
observations.
Daiva Kuncaite
SCB 201
Prof. Wejesinghe
10/06/09
Week III
In the third experiment we investigated the osmotic behavior of a plant cells placed in different
molar solutions. We took a small sample of the Elodea leaf in distilled water solution and placed
on the slide with the drop of water. We cover it with watch glass and observe the cells under the
microscope. On the second slide we took a sample of Elodea leaf in a NaCl (hypertonic)
solution. We placed the watch glass and observe the cells under the microscope.
Results and Conclusions: In the first experiment we observed the movement of carmine
particles in water. Since the particles were moving randomly, bouncing to each other we can
conclude that we actually observed molecular movement. The movement never stopped. The
smaller particles were moving more rapidly than the large ones. The movement we observed was
driven by the kinetic energy of the molecules. The intrinsic molecular kinetic energy is the
driving force of the diffusion. Diffusion is important in cells life, because there are a lot of
chemical reactions that takes place inside the cell. Some reactions need oxygen which comes by
the diffusion. Carbon dioxide, as a waste, also leaves the cell by the diffusion.
In the second experiment we made a hypothesis that substances that have small molecular
structure will pass easily thorough the dialysis tubing and relatively large molecules will not pass
through the dialysis tubing. Our prediction was that Iodine molecules diffuse into the bag and
glucose molecules will diffuse to the beaker. Starch, as large polysaccharide will remain in the
bag. As a result, the color of the bag from cloudy white turned blue black, which indicated that
iodine molecules have diffused into the bag. The bag color started changing the moment we
placed it in the beaker because iodine molecules started diffusing into the bag. Therefore we
concluded that iodine molecules are very small. After performed Benedict’s test the sample
from the bag turned yellow. We concluded that not all glucose molecules had diffused through
the dialysis membrane and the remains of the glucose molecules showed positive Benedict’s test
results. The sample from the beaker after Benedict’s test turned orange, which indicated the
presence of glucose. Therefore, glucose had diffused through the dialysis membrane. Our
prediction and hypothesis was confirmed.
In the third experiment:
Elodea in distilled water solution
Elodea in NaCl (hypertonic) solution
We observed the cell of the Elodea leaf in distilled water solution that contained central vacuole
and the chloroplasts against the cell wall. The water is hypotonic solution toward the cell, but the
Daiva Kuncaite
SCB 201
Prof. Wejesinghe
10/06/09
Week III
cell wall made out of the cellulose prevents of the excess intake of water into the cell. It exerts
osmotic pressure that opposes water movement to the plant cell and prevents it from bursting.
The cell of Elodea leaf in NaCl solution looked much different. The cell wall was visible and
intact, but inner organelles of the cell shrank. That indicated that cell was placed in hypertonic
solution and water came out from the cell. However, the cell wall remained intact and only
plasma membrane detached from the wall and shrank. It is different from the animal cells,
because animal cell don’t have cell walls made out of cellulose, therefore the cells crenate placed
in the hypertonic solution. We try to reverse the process in the plant cell by soaking Elodea leaf
in the water for 10 min. and that observing if any changes occurred. We noticed that plasma
membrane expend a little bit and chloroplast dispersed. That concluded that reverse process in
the plant cells also can occur by placing the cells back to the hypotonic solution. The pond water
is hypotonic to the Elodea cells. The cell wall show rigidity and resistance to slight ionic
gradient.
Homework:
1. Describe plant wilting in terms of turgor pressure.
When water moves into plant cells by osmosis it makes the cell expand. However, the cell walls
are very strong and cannot expand much. So the pressure rises inside the cell – it becomes tightly
filled it reaches the greatest turgor pressure. This makes the cells rigid and helps to support
plant’s leaves. If the plant looses too much water or runs short of water in the soil, the plant will
die because of the water loss.
2. The emergency room intern, Jim W. Dooley, treated a patient by administering fluids
intravenously. The patient died as a result of using the wrong IV fluids. What kind of
osmotic solution would have resulted in the patient’s death? Why?
The administration of hypotonic or hypertonic solution would have resulted in the patient’s
death. Human cells placed in hypotonic solution will undergo lysis and cells placed in hypertonic
solution will undergo crenation.
3. Tori, an ER volunteer and amateur botanist, is shocked by the death of Dooley’s patient.
The next time she waters her flower garden she wonders why applying pure water does not
kill plants, but makes them thrive?
Pure water for the plans is hypotonic to the cell, however, the turgor pressure and presence of
cell wall made out of cellulose prevents of the excess intake of water into the cell. It exerts
osmotic pressure that opposes water movement to the plant cell and prevents it from bursting.
Daiva Kuncaite
SCB 201
Prof. Wejesinghe
10/06/09
Week III
4.The pond water samples you observed probably contained a variety of multi-cellular,
colonial, and single-celled organisms. Some of these may have had cell walls, but others
were lacking cell walls. What adaptation for osmoregulation is found in single-celled
organisms, such as the Amoeba, and multi-cellular organisms that lack cell wall but live in
hypotonic environment?
These organism have contracting vacuole that gather the excess water and expel it to the outside
of the cell.