1. CELLULAR TRANSPORT
Diffusion and Osmosis

2. Our goal today: To describe how OSMOSIS happens.
Osmosis is the diffusion of water across a membrane.

3. Key Concepts
To understand osmosis, you must first understand the following:
Diffusion
Properties of a cell membrane
Effects of solute concentration on water molecules
Source Citation: Text excerpts from Miller and Levine, 2006, Biology, Pearson, Prentice Hall.

4. DIFFUSION
Particles move constantly. They bump into each other and spread out randomly. As a result, the particles tend to move from an area where they are more concentrated, to an area where they are less concentrated. This is called DIFFUSION.
Dye evenly dispersed in water
Drop of dye
Pure water

5. THE CELL’S ENVIRONMENT
The cytoplasm of a cell contains a solution of many different substances in water. Recall that a solution is a mixture of two or more substances. The CONCENTRATION of a solution is the mass of the solute in a given volume of solution.
Same volume
Less concentrated
More concentrated

6. EQUILIBRIUM
Dye evenly dispersed in water
Drop of dye
Pure water
When the concentration of the solute is the same throughout a system, the system has reached EQUILIBRIUM. Keep in mind, however, that even in this state, the molecules are still in motion.
Back to “Key Concepts”

7. THE CELL MEMBRANE
One of the most important functions of the cell membrane is to regulate the movement of dissolved molecules from the liquid on one side of the membrane to the liquid on the other side.
Source Citation: "Kimball, John W. 'Transport Across Cell Membranes.' Kimball's Biology Pages. August 17, (January 2003)." Science Resource Center. Thomson Gale. 15 April 2007

8. Because cells are surrounded by liquid, and have liquid inside of them, diffusion of molecules dissolved in the liquid is constantly occurring across the cell membrane. The direction in which they move depends on the concentration gradient. Not all molecules can diffuse across the membrane; some are too large, some are too strongly charged.

9. But molecules such as water, oxygen, and carbon dioxide (which are small and not strongly charged) can easily pass across the bilayer. For this reason, cell membranes are called SELECTIVELY PERMEABLE.
Lipid-soluble
molecules,
O2, CO2, H2O
(outside)
(inside)
Back to “Key Concepts”

10. Click here to return to objective.
OSMOSIS
Click here to return to objective.
Image from Miller and Levine, 2006, Biology, by Pearson Prentice Hall
When water diffuses across the membrane, it is called OSMOSIS. Water will move across a membrane until equilibrium is reached. Click to see osmosis in action (X-out when you are done).
here

11. THE CONCENTRATION OF WATER
The concentration of water can change by adding solutes to a fixed volume of liquid.
For example, if we keep the volume of a solution CONSTANT, and increase the amount of sugar or salt, then we decrease the concentration of water.
More solute, less water
Less solute, more water
Volume does not change

12. THE CONCENTRATION OF WATER
Likewise, if we keep the volume of a solution constant, and decrease the amount of sugar or salt, then we increase the concentration of water.
Volume does not change
More solute, less water
Less solute, more water

13. If cells are placed in a solution that is low in solutes (or high in water), it causes the cell to swell. If cells are placed in a solution that is high in solutes (or low in water), it causes the cell to shrink.
Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates
Click here to
learn key terms.
X-out when you are done
Back to “Key Concepts”

14. Elodea leaf in salt water
Demonstration
Watch what happens when a plant cell is placed into a solution of salt water. Click the button below.
Elodea leaf in salt water
X-out when you are done

15. Did you miss something?
Check one or all of the interactive links below to help you fill in any “blanks” you might have:
All about Cell Transport
Diffusion and Osmosis (stop at “Calculating Water Potential)
Osmosis
Key terms
X-out of each activity to return to this Power Point

16. APPLICATION
How can we apply what we now know about diffusion to our bodies?
An important part of respiration is the uptake of oxygen and release of carbon dioxide by our cells. These gases are exchanged in small structures in our lungs called alveoli.
Click here to see an animation of a human lung
Click here to see a close-up view of gas exchange

17. APPLICATION Where else does diffusion and osmosis happen?
Our kidneys have the important job of removing wastes such as urea from our bodies. Urea is excreted in urine. Urine is a product of diffusion and osmosis.
Click here to get a closer look at kidney function
Clickhttp:// here to see how dialysis works in patients with kidney failure

18. HSA Review
Which of these is the process by which water moves across a selectively permeable membrane?
osmosis
transpiration
capillary action
active transport A B C D

19. Try again…
You are looking for the process that describes the movement of water down its concentration gradient. It does not require the cell to use any energy.

20. Osmosis is the diffusion of water across a cell membrane.
You’re CORRECT!
Osmosis is the diffusion of water across a cell membrane.

21. HSA Review
The exchange of oxygen and carbon dioxide between the body and the air occurs in the lungs. This exchange of gases takes place at the cellular level. What part of the cell is primarily responsible for this exchange? F
the cell membrane
the nucleus
the cell wall
the ribosome G H J

22. Try again…
You are looking for the part of the cell that is in direct contact with the cell’s environment and is permeable to various molecules.

23. You’re CORRECT!
The cell membrane is like the gatekeeper of the cell; its structure allows certain molecules to move into and out of the cell.