Experiment I: Nature of the semi permeable membrane.
Introduction
Diffusion involves the movement of particles and molecules from an area of high
concentration to low concentration via a concentration gradient. Diffusion is a key process in
metabolic transport because it results in the intermingling of substances due to the kinetic energy
of random motion. The concept of diffusion is central to the understanding of many scientific
disciplines including; physics, chemistry and, biology and is accordingly used quite often. The
movement of particles in diffusion is a result of thermal agitation with no net input of energy.
Many types of diffusion can be identified namely; passive diffusion (simply passive movement
down a concentration gradient) facilitated diffusion, which is movement down a concentration
gradient with the aid of a specific carrier protein.
A special (and highly important) type of diffusion is the diffusion of water through a
semi-permeable membrane from a region of high solute concentration to low. Osmosis is a key
component of many biologic systems and examples of its use within nature include; plant water
absorption and water re-absorption in the distal and proximal tubules of the nephron kidney.
Osmosis is the process by which the cell membrane regulates permeability (osmoregulation).
The current view of the cell membrane is known as the fluid mosaic model meaning, the plasma
membrane is a lipid bilayer composed of a mosaic of macromolecules including; integral
proteins, glycoproteins, glycolipids, and cholesterol.
In the following experiment, dialysis tubing will simulate a semi-permeable cell
membrane. The permeability of certain substances (amylase, iodine, and glucose) to the dialysis
bag will be investigated. Through the experiment, a better understanding of an actual plasma
membrane’s selectivity can be deduced. Many factors affect substance permeability such as,
concentration, solubility, and weight. Our hypotheses for the three solutions are as follows:
iodine will easily diffuse through the dialysis bag membrane as it is the lightest of the three,
starch however, will not be permeable to the membrane. Finally, glucose will also be permeable
to the makeshift membrane.
Materials:
String or rubber band or dialysis clamp (2)
Wax pencil or sharpie
30% glucose solution
Starch solution
I2KI solution
Benedict’s reagent
stir plate (1)
500-mL beaker (1)
Handheld test tube holder
Standard test tubes (3)
Disposable transfer pipettes
400-mL beaker (2)
30-cm strip of moist dialysis tubing
DI water
Methods:
1. Prepare a dialysis bag with initial solutions:
a. Fold over 3 cm at end of a 25- to 30- cm piece of dialysis tubing that has been
soaking in water for a few minutes, pleat the folded end “accordion style”, and
close the end of the tube with a string or rubber band, or a dialysis clip forming a
bag. Make sure it is secure so that no solution can seep through.
b. Roll opposite end of bag between your fingers until it opens, and add 4 pipettes
full of 30% glucose into bag. Then add 4 pipettesful of starch solution to glucose
in bag.
c. Hold bag closed and clap or tie off.
d. Mix its contents.
e. Record the color of the dialysis bag.
f. Carefully rinse outside of bag in tap water.
g. Add 300 mL of water to a 400- to 500-mL beaker.
h. Add several droppersful of I2KI solution to the water until it is visibly yellowamber. Record the color of the H2O + I2KI solution.
i. Place the prepared dialysis bag in the beaker containing the iodine solution. Do
not allow liquid to spill out of bag.
2. Leave bag in beaker for at least 30 minutes.
3. After 30 minutes, carefully remove bag and stand it in a dry beaker.
4. Record in Table 1 the final color of the solution in the bag and the final color of the
solution in the beaker.
5. Perform Benedict’s test for the presence of sugar in the solutions.
a. Label three clean test tubes: control, bag, and beaker.
b. Put 2 pipettesful of water in control tube.
c. Put 2 pipettesful of bag solution in bag tube.
d. Put 2 pipettesful of beaker solution in beaker tube.
e. Add 1 dropperful of Benedict’s reagent to each tube.
f. Heat tubes in a boiling water bath for 5 minutes.
g. Record results in Table 1.
Results:
Table I
Solution
Source
Original Contents Original Color
Final Color
Color After
Benedict’s Test
Bag
Beaker
Control
Starch & Glucose
Water & Iodine
Benedict’s Water
Blue
Yellow/Orange
Blue
Cloudy yellow
Brick Red
Blue
Cloudy white
Orange/yellow
Blue
Table 1: Results of Experiment Investigating the Permeability of Dialysis Tubing to Glucose, I2KI, & Starch
Conclusion
In conclusion, the purpose of this experiment was to investigate the nature of a semipermeable membrane and it’s selectivity in what is allowed to diffuse across the membrane. I
accepted my first two hypotheses which states that Iodine and glucose are permeable and will
diffuse. This was supported by the data that I collected proving that they both were diffused. I
accepted my third hypothesis that states that starch will not be permeable because the data
proved that starch did not diffuse out of the bag into the solution in the beaker because it was too
big to diffuse out of the bag.
Discussion
We discovered that the mass and size of a solution will affect its ability to permeate in
and out of the cell wall; shown by starch’s inability to permeate through the dialysis bag. The
special feature of the dialysis bag renders it to be a good example of the cell wall which I found
to be both surprising and interesting because it illustrates perfectly the semi-permeability ability
of the cell wall.
One thing I believe that I could change to improve my experiment is to reinforce the
dialysis bag to prevent leakage resulting in calculation errors. The work and results obtained in
the experiment demonstrate what occurs in a real-life system; that is the cell membrane. The cell
membrane is selectively permeable to many substances and a number of factors affect this
permeability. Some factors that affect permeability are pressure, concentration, temperature and
solubility.
The next experiment that I would like to test is what type of effect temperature would
have on permeability and I would test that by using the same compounds with the dialysis bag
while testing the permeability while changing the temperature.
Experiment II: Investigating the Osmolarity of a Plant Cell
Question 1: Define what is meant by isotonic point
Isotonic point is the point where the solution is equal in the amount of concentration it
contains compare to its environment. The isotonic point can also be defines as when no osmosis
is taking place due to equal water concentration inside and outside the cell. The water potential is
equal on both sides of the semi-permeable membrane and only random diffusion of particles
occurs.
Question 2: What is the isotopic point for each vegetable?
The isotonic point of a solution is said to occur when there is no net movement of water
across the plasma membrane because an equal concentration of solution inside and outside the
cell exists. The isotonic point of the vegetable can be determined by the graph of percent change
in weight by concentration. Once the graph was derived, a trendline of linear best-fit was set to
determine the isotonic point or, the point at which the line crosses the x-axis. The isotonic point
was found to be 0.285M.
Approximate time in solutions:
Sucrose
0.0 M
0.1 M
0.2 M
0.3 M
0.4 M
0.5 M
0.6 M
Molarity
1.2
1.21
1.2
1.17
1.08
1.03
1.03
Final weight (g)
1.07
1.14
1.15
1.16
1.13
1.12
1.18
Initial weight
(g)
-0.13
-0.07
-0.05
-0.01
0.05
0.09
0.15
Weight change
(g)
12.14953 6.140351 4.347826 0.862069 -4.42478 -8.03571 -12.7119
% change in
weight
15
Weight Change vs. Concentration
% Weight Change
10
5
0
0
0.2
0.4
0.6
-5
-10
y = -39.896x + 11.73
R² = 0.9897
-15
Concentration
Figure 1: Graph of the percent change in weight by concentration.
0.8
Isotonic Point
is 0.285M