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Exercise 8: The Plasma Membrane
Skill Check Worksheet
NAME:______________________
LAB SECTION_________________
Activity 1: Diffusion of Molecules through Selectively Permeable membranes.
Color of glucose/starch solution in dialysis bag at start of experiment
Color of solution in beaker at start of experiment golden
no color-clear
yellow
Part A: Pre-Experiment Questions
1. Which molecules do you think will move through the dialysis bag? Why?
2. Which molecules are least likely to move through the dialysis bag? Why?
Solution
Original
Contents
Original Color
Final Color
Glucose
Present final
(dipstick
results)
Bag
Glucose and
starch
clear
Dark
Brown/black
Positive for
glucose
Beaker
IKI and water
Golden
yellow
Golden
yellow
positive for
glucose
3. Describe the results of your tests with reference to the colors of the final solutions.
4. Explain what these results mean in terms of the movement of the various molecules (starch,
glucose and IKI)
5. From your results, predict the size of IKI molecules relative to the starch and glucose
molecules.
6.What results would you have expected if the experiment had started with glucose and IKI
inside the bag and starch in the beaker? Explain why.
Activity 2: The Plasma Membrane of Red Blood Cells
DI
0.45
0.90
1.50
2.00
Undiluted
Opacity
Reading
clear
clear
Opaque
Slightly
cloudy
Slightly
cloudy
Opaque
Description
of Cells
No cells
visible
No cells
visible
Biconcave
cells
Starshaped
cells
Starshaped
cells
Biconcave
cells
Drawing of
Cell shape
No cells
visible
No cells
visible
7.Were there any differences between the opacity of the undiluted tube when compared with any
of the other sample tubes? If there were differences, why were they different?
8.Describe any differences in the microscopic shapes of the cells in each of the tubes compared
to the undiluted control. If there were differences, why were there differences? How do the looks
of the cells relate to the conditions in the tubes? Were you able to find cells in all of the samples?
9.What is the relationship between the opacity data and the microscopic characteristics of the
cells? What does this tell you about what occurred with the cells in the tubes?
Activity 3: Cell Homeostasis Virtual Lab
10. Describe what happened in each of the dialysis tubes, Tube B, C, D and E, and explain why the tubes
either shrank, increased in weight or stayed the same. Explain the results using the proper osmosis
terminology when you can, ie, include the terns hypertonic, isotonic, hypotonic.
Note: hypertonic and hyperosmotic mean the same thing, so do isotonic/isosmotic, hypotonic/,
hypoosmotic. These two sets of terms are both used. Just be consistent in which set of terms you use.
Don’t mix them.
Exercise 8: The Plasma Membrane
Introduction
The healthy existence of cells is dependent upon their ability to maintain an internal steady-state known
as homeostasis, This steady state is achieved, in part, by the selective movement of materials through a
series of semi-permeable membranes, which includes the plasma membrane that surrounds the cells,
and the various organelle membranes such as those surrounding the mitochondria or chloroplasts.
Additionally, materials are moved through the cytoplasm of the cell, facilitating molecular interactions
within the cell and with the external environment.
The cytoplasm of the cell and the external environment surrounding cells is composed of many organic
and inorganic molecules, the solutes, which are dissolved in a solvent, water. The behavior of these
molecules is predictable, and can be explained by laws of chemistry and physics. The movement of some
of the dissolved substances across a membrane may require an expenditure of energy by the cells and is
known as active transport. On the other hand, the cell membrane is permeable to water and to some
dissolved substances without an expenditure of energy. This passive process occurs via a mechanism
known as diffusion, a physical process in which molecules move down a concentration gradient, from an
area of high concentration to an area of lower concentration. In the case of diffusion, it is the intrinsic
kinetic energy found in all atoms and molecules that drives the movement of the molecules across a
membrane or throughout a cell. A dissolved substance will continue to diffuse until the system reaches
equilibrium as long as nothing occurs to stop the process. Diffusion allows cells to acquire food, and to
exchange waste products from the environment.
Osmosis is a name given to the diffusion of water molecules through a selectively permeable membrane.
The molecules will follow the basic rule of diffusion that states that molecules move from a region of high
concentration to a region of lower concentration.
Activity 1: Diffusion of Molecules through Selectively Permeable Membranes( Video on Diffusion)
The cell membrane is selectively permeable. This means that small, hydrophobic solute molecules, water,
and other very small polar, uncharged molecules can move freely through the membrane. But larger
molecules and small charged ions may pass more slowly or not at all.
Solutes that diffuse through a selectively permeable membrane always move from the solution that
contains more of the solute to the solution containing less solute. If two solutions contain equal
concentrations of a solute, no net movement of the solute will occur despite the fact that molecules will
continue to move in both directions.
In a laboratory, it is possible to separate molecules of different sizes by a process called dialysis.
Artificial membranes can be made which act as semi-permeable membranes. Dialysis tubing is an
artificial membrane made of regenerated cellulose fibers formed into a flat tube. This tubing can be
formed into tube-shaped bags, known as dialysis bags. The size of the pores in the dialysis tubing
determines which substances can pass through. Molecules larger than the pore size cannot pass
through the membrane either into or out of the bag, while smaller molecules and ions diffuse through the
pores in the membrane. In each case, molecules will move from an area of higher concentration to an
area of lower concentration.
On a practical note, dialysis is often used as part of the process of purifying proteins. A partially purified
protein solution can be placed in a dialysis tubing bag made from dialysis tubing that has a pore size that
will allow the passage of small molecules (
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