NAME & section #:
PRELAB EXERCISE 1 (2002-2003)
CELL MEMBRANE PERMEABILITY
(Martini 41-43, 90-96)
READ CAREFULLY AND COMPLETE BEFORE THE LAB
1. Define the following terms (Martini p. 41):
Solution:
Solute:
Solvent:
Potential energy: energy objects have because of their position and internal structure in relation to each
other. It is stored or inactive energy that has the potential (or capability) to do work but is not presently
doing so.
Differences in potential energy of substances in solution can be caused by differences in entropy (disorder),
pressure, temperature, concentration of solute particles, etc...
2. Solute particles move from one place to another because of differences in their potential energy. Solute
particles move from a region where their potential energy is greater to a region where their potential
energy is lower, regardless of the reason for the potential difference.
In the lab exercises, the gradient of potential energy of solute will be caused by their gradient of
concentration. As the concentration of solute particles increases, their potential energy increases.
Thus, solute particles will move from the region of higher solute particle concentration (where their
potential energy is higher) to region of lower solute particle concentration (where their potential energy is
lower).
Like solute particles, water molecules also move from one place to another because of differences in their
potential energy. This is usually referred to as the water potential. Water moves from a region where
water potential is greater to a region where water potential is lower, regardless of the reason for the
water potential difference.
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In the lab exercise, the gradient of water potential will also be caused by a gradient of concentration of
solute particles.
Water potential is affected by the concentration of dissolved particles of solutes and as the concentration
of solute particles increases, the water potential decreases. Why?
As the dissolved solute particles interrupt the ordered three-dimensional interactions that normally occur
between water molecules, entropy (or disorder) of water will increase and thus water potential will
decrease).
We know that water molecules move from regions of high water potential to regions of lower water
potential. This means that water moves from regions of low concentration of solute particles (low entropy
and high water potential) to regions of high concentration of solute particles (high entropy and low water
potential).
permeable membrane
A
B
The concentration of solute particles is higher in region A than in region B. This means that the
potential energy of the solute particles is higher in A than in B and thus the solute particles will move
from A to B. This also means that the water potential in A is lower than in B and thus the water will
move from B to A.
3. Distinguish between the following terms: (Martini p. 90-93)
Diffusion
Osmosis
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4. Distinguish between
Penetrating solutes: solutes that can pass through the cell membrane.
Non-penetrating solutes: solutes that cannot pass through the cell membrane.
However, notice that:
Solutes whose diffusion inside is as fast as their active transport to the outside behave like
non-penetrating solutes.
AND
Solutes that are disposed off by the cell as fast as they diffuse inside, behave also like non-penetrating
solutes.
5. Differentiate between MOLARITY, OSMOLARITY and TONICITY.
Molarity represents the concentration of solute molecules in the solution. It is expressed in mole/liter.
A number of different ways exist to describe the concentration of a solution. The simplest way is to express
it in weight of the solute per unit of volume of solution, for example in grams or milligrams per liter of
solution (150 mg/L of glucose) or per 100 ml of solution (15 mg/100 ml - or 15 mg%).
However, it is frequently more advantageous to express concentration in terms of molarity.
These measurements actually represent the number of molecules of solute per unit of volume of
solution.
One mole of any solute = 6.02 x 1023 molecules of solute. A one molar (1M) solution contains 1 mole of
solute molecules per liter of solution.
Solutions with equal molarities have equal number of solute molecules, no matter what their solutes
are.
This means that a 2 molar (2M) glucose solution contains the same number of solute molecules as a 2
molar (2M) sodium chloride solution. One mole of solute is equal to the molecular weight of the solute in
grams. The molecular weight of NaCl is 58.5g and the molecular weight of glucose is 180g. In order to
make a 2M glucose solution you will have to dissolve 360g (2 x 180g) of glucose in 1 liter of water. To
make a 2M solution of NaCl, you have to dissolve only 117 g (2 x 58.5g).
OSMOLARITY, ON THE OTHER HAND, REPRESENTS THE TOTAL CONCENTRATION OF
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ALL OSMOTICALLY ACTIVE SOLUTE PARTICLES IN THE SOLUTION.
It is expressed in osmol/l.
Saying that a solution has an osmolarity of 1 osmol/L is equivalent to saying that it has a total of 1 mole of
osmotically active particles, no matter what these particles are.
When in solution, some solutes have molecules that dissociate into several particles.
For example: potassium chloride molecule (KCl) dissociates into two particles (K+, Cl-),
sodium chloride molecule (NaCl) also dissociates into two particles (Na+, Cl-),
calcium chloride molecule (CaCl2) dissociates into three particles (Ca+, Cl-, Cl-)
The osmolarity of solutions containing these solutes can be calculated from the following formula:
Osmolarity = Molarity x Number of Particles Dissociated.
For example: the osmolarity of a 2M solution of NaCl is: 2x2 = 4 osmol/L.
the osmolarity of a 1M solution of CaCl2 is 1x3 = 3 osmol/L.
You can also use this equation to calculate the osmolarity of solutes that do not dissociate such as glucose.
The osmolarity of a 4M solution of glucose is 4x1 = 4 osmol/L. In the case of solutes that do not dissociate
in solution (such as glucose, urea, or glycerol), the osmolarity of their solutions is equal to their molarity.
Complete the following table:
SOLUTE
MOLARITY
(mole/L)
NaCl
urea
EXPLANATION
4
3
CaCl2
CaCl2
OSMOLARITY
(osmol/L)
0.3
0.1
TONICITY REPRESENTS THE ABILITY OF A SOLUTION TO CHANGE THE SHAPE OR
TURGIDITY OF CELLS BY ALTERING THEIR INTERNAL WATER VOLUME ( = the ability of
a solution to grab water from the cells).
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Every cell including the red blood cell contains within the boundary of its cell membrane a certain
number of non-penetrating solute particles (0.3 osmol/L) that will not diffuse out of the cell.
Explain the following terms by completing the blanks:
Solution A is isoosmotic to red
blood cells
Solution A is hyperosmotic to red
blood cells
Solution A has a higher concentration of osmotically active
solute particles than the red blood cells (osmolarity of the
solution A is higher than the osmolarity of red blood cells)
Solution A is hypoosmotic to red
blood cells
Solution A has a lower concentration of osmotically active
solute particles than the red blood cells (osmolarity of the
solution A is lower than the osmolarity of red blood cells)
Solution A is isotonic to red blood
cells
Solution A is hypertonic to red
blood cells
Water is drawn out of the cell by this solution and the red
blood cells shrink or crenate.
Solution A is hypotonic to red
blood cells
Water moves into the red blood cells, the cell volume
increases and cells can even burst. When red blood cells
rupture it is called hemolysis.
6. Make sure that you understand the following mechanisms:
Hemolysis of red blood cells
The cells are ruptured. When cells are placed in a hypotonic solution, there is a net flux of water going
from the outside solution into the cells. Inside these cells, the pressure exerted by the water on the cell
walls increases. The cells become distended. When the limit of elasticity of the cell walls is reached, the
cell walls break and thus hemolysis occurs.
Crenation of red blood cells
The cells shrink. When cells are placed in a hypertonic solution, there is a net flux of water going from
inside the cell to the outside solution. Inside the cells, the hydrostatic pressure decreases. The cell walls,
being elastic, will collapse and the cell will have a shrunken appearance.
7. To answer the following two questions, you have to remember that every cell, including the red blood
cell, contains within the boundary of its cell membrane a certain number of non-penetrating solute
particles (0.3 osmol/L), which will not diffuse out of the cell.
Consider different solutions to which red blood cells have been added. Do you agree with the following
statements? Explain.
All isosmotic solutions of non-penetrating solutes are isotonic to the red blood cells.
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All isosmotic solutions of penetrating solutes are isotonic to the red blood cells.
8. The membrane that surrounds a cell not only forms a boundary between the cell and its environment,
but also serves to regulate the movement of materials from the cell to the environment and vice versa.
Membrane permeability (= the ease with which it permits substances to pass through it) is an important
factor in the normal functioning of a cell. In general, cell membranes are characterized as being selectively
permeable. This means that a membrane will allow some substances (e.g. water) to readily pass across the
membrane, while other substances (e.g. protein) are prevented from doing so.
The ability of substances to cross a cell membrane depends on the following:
1) their solubility in lipid;
2) their size;
3) their charge;
4) the presence of channels and transporters in the cell membrane.
Use a separate page, ONLY ONE. Make a labelled diagram showing the molecular structure of the
plasma membrane and explain how each of the following classes of compounds passes through this
membrane (Martini p.71, 90-100):
1) water;
2) ionized solutes (Na+, Cl-, etc.);
3) small polar solutes (urea);
4) moderately large polar solutes (glucose);
5) non-polar compounds (O2, fatty acids, steroid hormones);
6) very large molecules (proteins).
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