Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
Lugol’s solution (essentially iodine)
*Binds to helical structure of starch (iodines are the
faded purple spheres above), turning the substance
or solution a blue/black, but only binds because
structure is helical. Therefore, will it bind to
glucose? explain
No, it isn’t helical and therefore the
solution will not turn blue-black.
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
Lugol’s solution (essentially iodine)
What is lugol’s iodine solution?
Conclusion: Lugol’s solution is a starch indicator
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
1. Making the model “cell”
Dialysis tubing (see below)
A synthetic membrane that has microscopic pores (holes) that only let molecules of a
certain size or smaller to pass. The one we are using only allows monomers or smaller
molecules to pass.
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
1. Making the model “cell”
Dialysis tubing
A synthetic membrane that has microscopic pores (holes) that only let molecules of a
certain size or smaller to pass. The one we are using only allows monomers or smaller
molecules to pass.
If you soak it in water, you will
be able to open up the tubing
to put solution inside. Before
you can do this, however, you
must…
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
1. Making the model “cell”
Dialysis tubing
A synthetic membrane that has microscopic pores (holes) that only let molecules of a
certain size or smaller to pass. The one we are using only allows monomers or smaller
molecules to pass.
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
1. Making the model “cell”
Dialysis tubing
A synthetic membrane that has microscopic pores (holes) that only let molecules of a
certain size or smaller to pass. The one we are using only allows monomers or smaller
molecules to pass.
Glucose, starch, water
Then put
glucose and
starch solution
in bag
Artificial Cell
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
2. Place the model cell into a solution containing lugol’s
(iodine).
Predict what should happen:1. The glucose should diffuse out down its concentration
I-
II-
Glucose, starch, water
I-
gradient and the iodine should diffuse in down its
concentration gradient since both can pass through the
membrane pores. The starch should stay in – too large
to pass through pores. Water should diffuse in and out –
direction will depend on how much solute you dissolved
in the water.
I-
2. How will you observe that the iodine went in?
The iodine should bind to the helical starch structure causing
the solution inside the dialysis tubing to turn blue-black.
I-
3. How will you observe that the starch did not
The
outside should remain amber (the color of lugols) since
diffuse
out?
I-
there is no starch present to react with it.
4. How do we observe that glucose diffused out?
Artificial Cell
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
Use Benedict’s solution
- Benedict’s is a molecule that reflects blue light that will react with
monosaccharides, lactose and maltose and convert into a molecule that reflects
yellow/orange light. The reaction is endergonic. How we can we get it to happen?
You must heat the Benedicts in the presence of the sugars. Input ENERGY!
1. Add a few
drops of
benedict’s to the
solution
2. MUST HEAT!!
3. Turns orange
if
monosaccharide
s, lactose or
maltose is
present
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
2. Place the model cell into a solution containing lugol’s
(iodine).
How do we observe that glucose diffused out?
I-
II-
Glucose, starch, water
I-
Take a sample of the outer solution into a clean test
tube, add a few drops of benedicts, and heat it up. If
it turns orange then it diffused out.
I-
I-
Time to do it!
I-
Artificial Cell
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
I-
II-
Glucose, starch, water
Q. You have a solution containing 1mM catalase. The pH of the solution is around 6.8 and the
salt (NaCl) concentration is around 400 mM (milli Molar). However, you have previously
determined that this protein functions best at a pH of 5.7 and a salt concentration of 250 mM.
Explain how you can get your protein into the desired conditions without changing the
concentration of the enzyme much.
Take a sample of the outer solution into a clean test
tube, add a few drops of benedicts, and heat it up. If
IIit turns orange then it diffused out.
I-
Time to do it!
I-
Artificial Cell
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
Q. You have a solution containing 1mM catalase. The pH of the solution is around 6.8 and the
salt (NaCl) concentration is around 400 mM (milli Molar). However, you have previously
determined that this protein functions best at a pH of 5.7 and a salt concentration of 250 mM.
Explain how you can get your protein into the desired conditions without changing the
concentration of the enzyme much.
Diffusion Through a Membrane State Lab
Part 1 – Build a model cell and demonstrate diffusion across a
selectively permeable membrane
Q. You have a solution containing 1mM catalase. The pH of the solution is around 6.8 and the
salt (NaCl) concentration is around 400 mM (milliMolar). However, you have previously
determined that this protein functions best at a pH of 5.7 and a salt concentration of 250 mM.
Explain how you can get your protein into the desired conditions without changing the
concentration of the enzyme much.
Catalase solution with
pH 6.8 and 400mM
NaCl
Aqueous solution with pH
5.7 and 250 mM NaCl
A. Use a dialysis tube. Put your protein solution into
the dialysis membrane and tie it off. Place the
membrane into the desired solution. Salt will diffuse
out and H+ will diffuse in. Protein will stay inside as
it is too large to fit through pores…
This is called Buffer
Diffusion Through a Membrane State Lab
Part 2 – Demonstrate diffusion of water across a real membrane
(osmosis)
You want to observe the effect of salt on
osmosis across the plasma membrane
of red onion cells under the light
microscope. How will you do this?
You look at the cells in water first. This
control group
is your…______________.
All of the cells should be
_____________.
What will you do next?
Red onion cells in water (turgid)
turgid
Add salt to the solution that the cells are
in. What would you expect to happen
and why?
Diffusion Through a Membrane State Lab
Part 2 – Demonstrate diffusion of water across a real membrane
(osmosis)
Add salt
(NaCl)
Red onion cells in water (turgid)
plasmolysis
Osmosis (diffusion of water) should occur from inside the cell [high] to outside [low] since the salt
CANNOT cross the membrane and will cause the water concentration outside the cell to now lower than
inside…you should observe plasmolysis. How do we make these cells turgid again?
Diffusion Through a Membrane State Lab
Part 2 – Demonstrate diffusion of water across a real membrane
(osmosis)
Remove salt
(NaCl)
Red onion cells in water (turgid)
plasmolysis
Put them back into tap water (get rid of the salt).
Diffusion Through a Membrane State Lab
Part 2 – Demonstrate diffusion of water across a real membrane
(osmosis)
Remove salt
(NaCl)
Red onion cells in water (turgid)
plasmolysis
Remember: This is a real cell membrane (phospholipid bilayer with integral membrane protein),
which will only allow small hydrophobic molecules to pass (you should know why). Sodium chloride
(Na+, Cl-) is charged and hence hydrophilic.