Laboratory Exercise 5: Cell Physiology: Diffusion and Osmosis
Cells are dependent on their environment for nutrients and oxygen and for removal of waste products to meet
their metabolic demands. Cells exchange these materials with the environment surrounding them through the
cell membrane.
The cell membrane regulates the materials that enter or leave the cell. The cell membrane to function effectively
must be selectively or semi-permeable. It allows certain molecules and ions to pass and others not to pass.
A. Diffusion
Due to the energy molecules and ions possess they are constantly moving randomly. When a difference in
concentration exists, the molecules and ions move faster from a region of higher concentration towards a
region of lower concentration. Diffusion is a physical process of substances moving down a concentration
gradient from a region of higher concentration to a region of lower concentration due to the energy that the
molecules and ions possess. Diffusion is a passive process, it requires no energy input, it requires only the
energy that the molecules and ions themselves possess. A dynamic equilibrium is established when all the
molecules and/or ions are equally distributed throughout the system.
Factors Effecting Diffusion
How fast particles diffuse depend on:
1. The concentration gradient;
2. Molecular weight;
3. Temperature;
4. Electric charge on the particles.
B. Osmosis
Osmosis is diffusion of solvent (H2O) molecules across a semi-permeable membrane. The membrane
permits free passage of the H2O molecules but will prevent certain solutes from passing.
Osmometer
In living systems, H2O passes into and out of the cells freely by a passive process solely dependent on the
concentration gradient of the H2O molecules. Water will move from where there is more of it to where
there is less. When two solutions differ in solute concentrations and water content are separated by
semi-permeable membrane, water molecules will move down their concentration gradient from lower
concentration of solution to higher concentration of solution. As the water crosses the membrane and
enters a compartment (such a cell) it exerts a force in that compartment known as osmotic pressure.
Osmotic pressure is directly related to solute concentration: the greater the number of solute particles, the
greater the tendency to attract water, the higher the osmotic pressure.
Water will move osmotically until the rate of water molecules moving across the membrane in both
directions are equal. At this point, hydrostatic pressure (weight of the water forcing water molecules out)
equals the osmotic pressure (the tendency of the solute molecules to draw water in).
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Osmotic Changes in Model Cells
The osmotic movement of water molecules depends on the relative concentrations of solutions on both
sides of a semi-permeable membrane. Solutions are described in relative terms reflecting their solute
concentration (tonicity).
A solution hypertonic to the cell has a greater solute concentration and less water than inside the cell
(intracellular fluid).
A solution hypotonic to the cell has a lower solute concentration and more water than inside the cell.
If the intracellular and extracellular fluids contain solutions of equal concentration, they are isotonic.
The Effect of Hypertonic, Hypotonic and Isotonic Environments on Red Blood Cells (Erythrocytes)
Red blood cells' shape (biconcave disk) can be altered by environments that are not isotonic to their cytoplasm.
If the cells are placed in a hypertonic environment, there is shrinkage or crenation of the cells resulting in
rounded projections on their surface. If the cells are placed in a hypotonic environment, there is rupture or
hemolysis of the swollen cells resulting in the loss of the red hemoglobin pigment from the cells.
The Effect of a Semi-Permeable Membrane on the Movement of Solutes Down Their Concentration Gradients
The separation of a mixture of solute molecules of different sizes in solution by a semi-permeable membrane is
called dialysis.
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Procedure
D. Osmotic Changes in Model Cells
1. Working in groups of 4 at each table.
Obtain: 3 tube-shaped membranes, knot each membrane at one end,
3 100 mL beakers;
String for tying the open end of the tubing,
Balance for weighing simulated cells.
2. Pipette 8 mL of 20% NaCl solution into each bag, tie a tight string knot around the open end of each bag.
3. Dry "cells" thoroughly with paper toweling. Weigh each "cell" to the nearest 0.1 gram.
4. Position each "cell" vertically in an empty 100 mL beaker. Pour just enough solution into each beaker to
exceed the fluid level of each "cell". Add 10% NaCl solution to beaker 1; 20% NaCl solution to
beaker 2; and 30% NaCl solution to beaker 3. Do not get the strings wet or an inaccurate weight will
be obtained when weighing the “cells” later.
5. Leave the "cells" in each beaker for 60 minutes. After 60 minutes, dry each "cell" and reweigh.
6. Record the new weights of each "cell", weight changes and % change from the initial weights.
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Qualitative Analysis
Test For:
Chloride
AgNO3 + Cl  AgCl2  white ppt
Glucose
glucose + oxidized Cu  reduced Cu
(Benedict's test)
blue
(Benedict's reagent)
red
Starch
starch + I2  starch-I2 complex
red-yellow
blue-black
Protein
(Biuret reaction)
protein + Cu  protein-Cu complex
blue
violet
(Biuret.
Cu in the Biuret reagent reacts with the peptide
reagent)
bonds of the protein
H O
 
NC
peptide bond
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