TA: Yasu
1
Ex 8
Diffusion and Osmosis
Nature is very efficient using their energy, and diffusion and osmosis are two major methods of
transporting materials between living cells. The concepts of two systems (diffusion & osmosis) are often
applied to biological and biochemical experiments. It is important for you to understand mechanisms of
these systems.
Objectives:
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♦
♦
We will observe and learn about Brownian movement, diffusion and osmosis.
Study about differentially permeable membrane and permeable molecules.
Learn more vocabularies often used in biology.
Motion of molecules
It (may be difficult to imagine yet) is the truth that all (even solid materials) molecules display
random thermal motion or kinetic energy. If heat is causing the random movement, it is called Brownian
Movement. These random movement lead molecules to spread evenly.
Look at the figure 8.1 & 2.
Diffusion
It is a passive motion caused by gradient of pressure, temperature (heat), or
concentration. The direction of movement is from higher free energy (high concentration & temperature)
to lower free energy. The speed of movement is depending of size of molecules and temperature of
solution. The free energy is the energy available to a system to do work.
Molecular Weight and Diffusion Rates (Procedure 2, Question 2)
Size of molecules effects speed of diffusion. At the same temperature, pressure, and
concentration, the smaller molecules move faster than larger molecules.
Differentially Permeable Membrane (Semi-permeable membrane)
Membranes surrounding cells and organelles are differentially permeable because they selectively
allow some molecules to pass them through but do not allow others.
Polarity and size of molecules influence permeability to differentially permeable membranes.
Polar molecules have unevenness of electric charges on molecules. Look at the chart below for which
molecules to be permeable or not. We will come back here again at Ex9.
Type of molecules
Permeability
Polar
No / Unless
small
Ions, Amino acids, etc
Yes
Lipids (fats, oils, etc)
Nonpolar
Large
No / or slow
Small
Yes
Examples
Dyes, solids, large organic molecules, etc
Water, ethanol, etc
Dialysis is a technique often used in biology to isolate certain molecules out of other mix chemicals.
Dialysis is the separation of dissolved substances by means of their unequal diffusion through a
differentially permeable membrane. (Observe experiment // Procedure 8.3)
TA: Yasu
2
Osmosis
Osmosis is diffusion of water across a differentially permeable membrane in living organisms.
Think a moment. As concentration of solution increases, does concentration of water in the solution
increase or decrease?
Solute, Solvent, & Solution
Solute is something dissolved in. Solvent is liquid to dissolve. Solution is mixture of solute and solvent.
Hypo-, Hyper-, and Isotonic
Conditions of osmosis
*Conc. of
Direction of water
*Conc. of
cells
surrounding
Low
High
From in to out
Hypertonic
High
Low
From out to in
Hypotonic
Equal
Equal
Equilibrium
Isotonic
* The concentration referred here is concentration of solution, not the concentration of water.
Condition
Biological terms with osmosis (Procedure 8.7 & 8.8)
•
•
Hemolysis (Hem = red blood cells, Lysis = burst):
It occurs when red blood cells are
in hypotonic solution, and cells burst. (Fig. 8.7)
It occurs when plant cells are in hypertonic solution. The
Plasmolysis of Plant cells:
cytoplasm will shrink and there will be open space between plasma membrane and cell wall.
(fig 8.9 &10)
Review table for diffusion and osmosis
Movements
Moving Molecules
Directions
Active / Passive
Brownian
All Molecules
Random
Passive
Diffusion
All Molecules
Higher Æ Lower Conc. Temp.
Or Pressure
Passive
Osmosis
Water
Higher Æ Lower
Passive
Concept of Molecular Movements!
Brownian Movement
Diffusion
Osmosis
Osmosis is a part of diffusion and diffusion occurs because of Brownian movement.
TA: Yasu
3
For Today!!!!
Today, you will run 5 experiments and observations, Procedure 8.1, 2, 3, 4, and 8. Work
with 3-4 people as a group. Follow the procedures on the text unless extra instructions on this
page.
Procedure 8.1 Observe Brownian Movement
• Slides and microscopes are ready for you at bench. Be careful not to dye objective lenses. Do not use
100x objective lens.
Procedure 8.2 Observe Diffusion as Affected by Molecular Weight
• Typing error on the txt; molecular weight of malachite green = 92.9 g/mole.
• Agar plates were prepared, and observe the plates and record in the text.
• Answer the question 2.
You can not fill 10 ml to dialysis tube, so just fill out the tubes.
Procedure 8.3 Diffusion Across a Differentially Permeable Membrane
• Use orange clips instead of strings to seal dialysis tubes.
• One tube should contain water + 3 drops of phenolphthalein.
• The other tube should contain starch suspension.
• Rinse out side of tubes after seal them!!
• Record in color changes on figure 8.5.
• Do Question 3.
Water
+
NaOH
Procedure 8.4 Osmosis
• Use orange clips to seal dialysis tubes.
• Look at the figure 8.6, and prepare tubes accordingly.
• Make sure you will know which tube contains what (procedure 8.4: 3)
• Rinse the out side of the bag!!
• A scale is on each table to weigh tubes.
• Record data in the table 8.1.
• Do Question 4.
Procedure 8.7 Hemolysis
• You won’t perform an experiment, yet you need to understand the concept.
Procedure 8.8 Observe Plasmolysis
• Observe normal cells before add NaCl.
• Celery is demo only.
Be Ready!
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•
•
•
•
Definition of Diffusion, Osmosis, Dialysis, Hemolysis, & Plasmolysis.
• What molecules are moving in which cases? (all or only water)
• Which direction does molecules move? Why?
Remember three elements effect rate of molecule motions, temp, conc., press., and size of molecules.
Characteristics of molecules tend to pass through differentially permeable membrane.
Direction of water movement and concentration of hyper-, hypo-, and isotonic condition.
Results and concepts from all experiments (procedures) conducted during lab period.
• Iodine- Starch test Æ Color of positive / negative reaction
• Phenolphthalein, pH indicator Æ color of the indicator in basic solution.
• Which one of above two chemicals can pass differentially permeable membrane?
• Diffusion rate and molecular weight.
Water
+
Iodine