1. Exploring Effects of Osmosis
Water Can Kill?
Exploring Effects of Osmosis by
Susan D. Hester
Department of Molecular and Cellular Biology
University of Arizona, Tucson, AZ

2. Membranes! After this unit, you will be able to…
describe the structure and properties of a cell membrane.
describe properties that affect how easily a molecules can pass a pure phospholipid membrane.
predict how easily a molecules will be able to pass a pure phospholipid membrane.

3. Phospholipid membranes are selectively permeable
What are three properties that affect whether an atom/ion or molecule can pass easily through the membrane?
Figure 6.7 from Biological Science, 4th Edition by Scott Freeman, illustrating where common molecules fall on the permeability scale

4. Membrane properties depend on lipid structure
Phospholipid bilayer membranes are fluid.
Figure 6.12 from Biological Science, 4th Edition by Scott Freeman, illustrating phospholipids moving laterally in a phospholipid membrane
A phospholipid can travel over the entire surface of a small cell in a second!

5. Cell membranes are fluid

6. Membrane properties depend on lipid structure
Examples of properties that effect membrane permeability and fluidity:
length of the phospholipid fatty-acid tails
saturation of the phospholipid fatty-acid tails
cholesterol content

7. Membrane properties depend on lipid structure
Long tails = more hydrophobic interactions in the center of the membrane  stiffer, less permeable
Membranes with phospholipids with short fatty-acid tails are more fluid and permeable
Figure of phospholipid membrane from Biological Sciences, 4th Edition, by Scott Freeman

8. Membrane properties depend on lipid structure
Straight tails = more hydrophobic interactions in the center of the membrane  stiffer, less permeable
Membranes with phospholipids with kinked fatty-acid tails are more fluid and permeable
Figure 6.10 from Biological Science, 4th Edition, by Scott Freeman

9. Membrane properties depend on lipid structure
 carbons are saturated with hydrogen=straight tail
Double C=C bond—carbons NOT saturated with hydrogen= kinked tail
Figure 6.8 from Biological Science, 4th Edition, by Scott Freeman

10. Membrane properties depend on lipid structure
Saturated tails = more hydrophobic interactions in the center of the membrane  stiffer, less permeable
Membranes with phospholipids with unsaturated fatty-acid tails are more fluid and permeable
Figure 6.10 from Biological Science, 4th Edition, by Scott Freeman

11. Membrane properties depend on lipid structure
The permeability and fluidity of membranes is determined by the hydrophobic interactions in the middle of the “lipid sandwich” of the membrane.
The more of the hydrophobic fatty-acid “tails” that come into contact with one another, the stiffer and less permeable the membrane.
Figure 6.10 from Biological Science, 4th Edition, by Scott Freeman

12. Cholesterol in cell membranes
Figure of cholesterol inserted into phospholipid bilayer

13. Diffusion and Osmosis After this unit, we will be able to…
define the terms diffusion, tonicity and osmosis.
describe the molecular motions that lead to diffusion.
predict the net movement of solute or water molecules resulting from diffusion and osmosis.
discuss the importance of diffusion and osmosis to cells and multi-cellular organisms.

14. Diffusion
This phenomenon is known as diffusion— solutes tend to spread from areas of high concentration to areas of low concentration.

15. Diffusion
Diffusion is due to the random, undirected motion of the solutes and surrounding water
Particles DON’T “know” they are in an area of high concentration! –They do not actively move away from it.
So how does it happen?

16. Brownian Motion http://www.youtube.com/watch?v=6VdMp46ZIL8
We can see this motion indirectly by watching how the motion of small molecules jostles particles that are large enough for us to see in a microscope.
Individual molecules—including water and solute molecules—are in constant random thermal motion.

17. True or False? An individual solute molecule never moves from an area of low concentration to an area of high concentration
True
False

18. If the solutes can move across the membrane, what will the net movement of solutes be in the situation below?
Into the liposome
Out of the liposome
Neither into nor out of the liposome

19. Diffusion Keep in mind:
Because diffusion is due to molecules being randomly jostled around by the thermal motions of the molecules around them, it takes a l-o-n-g time for particles to diffuse much distance.
When molecules diffuse, they are not “trying” to get anywhere—they don’t move in a straight line towards anything.

20. Osmosis
Osmosis is a special case of diffusion—it happens when there are solutes that cannot quickly pass across the cell membrane and there is a higher solute concentration on one side of the membrane than the other.
When this happens, water will tend to diffuse to the area of higher solute concentration (lower “water concentration”).

21. Osmosis
Tonicity describes the balance (or imbalance) of solutes in and out of the cell.
Hypertonic
Isotonic
Hypotonic
[solute] higher outside
[solute] equal inside
and outside
[solute] higher inside

22. If the solutes cannot move across the membrane, what will the net movement of water be?
Into the liposome
Out of the liposome
Neither into nor out of the liposome

23. Osmosis

24. Membrane Transport Proteins
Not all movement across the cell membrane is through the phospholipid bilayer—otherwise, cells would be at the mercy of their environments.
Membrane transport proteins control passage of certain substances across the membrane.

25. Which of the following would need a membrane transport protein to cross the cell membrane at a significant rate?
H2O O2
Na+
C6H12O6 (glucose)
All of the above
C. and D.

26. Membrane Transport Proteins
We will begin our discussion of cell structures next week by talking about membrane proteins including membrane transport proteins.

27. Water Can Kill?
You will now consider three true stories and discuss/answer questions about what happened.
Please turn in one set of answers per group on a separate sheet of paper.
You have 15 minute to complete Part I, after which we will discuss Part I as a class.