NAME & section #:
PRELAB EXERCISE 1 (2002-2003)
CELL MEMBRANE PERMEABILITY
(Seeley 40-41, 61-73)
READ CAREFULLY AND COMPLETE BEFORE THE LAB
1. Define the following terms (Seeley p. 40):
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
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lower, regardless of the reason for the water potential difference.
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.
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3. Distinguish between the following terms: (Seeley p. 66-68)
Diffusion
Osmosis
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.
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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
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.
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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).
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 (Seeley p. 68-69):
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
Solution A has a lower concentration of osmotically
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red blood cells
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 nonpenetrating 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.
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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 (Seeley p. 66, 70-74):
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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