1. Diffusion and Osmosis
Section 1.2

3. The Cell Membrane The cell membrane is:
Selectively permeable
Permeable = Pass through (Latin)
Cell membrane = Gate-Keeper that determines what can and can’t enter the cell.

4. The Gate Keeper: The Cell Membrane
Who comes in for the party?
Small molecules like water, carbon dioxide, or oxygen can easily pass through the pores of the membrane.
Who gets bounced?
Large molecules like glucose
Charged molecules like salts.

5. Active Transport (Senior Science)
Molecule Movement
Recall cells are small because they need to obtain sufficient nutrients and dispose of their wastes.
This is done by the cell membrane by three means:
Diffusion
Osmosis
Active Transport (Senior Science)

6. Definitions.
Solution = A homogenous, liquid mixture of two or more substances.
Solvent = The dissolving agent of a solution. Water is the most versatile solvent known.
Solute = Substance that is dissolved in a solution.

7. Diffusion
The process by which molecules tend to move from an area of higher concentration to an area of lower concentration.
Concentration= amount of substance in a given volume.
Think of the perfume example (high low)
Think of the food-dye example (high low)

8. Why Diffusion? Kinetic molecular theory- Molecules are always moving
Collisions occur between molecules, which causes the molecules to move away from one another.
If there is space these molecules want to spread out evenly.

9. Diffusion Demonstration

10. Diffusion and the Cell
Movement of molecules by diffusion is how the cell fulfills its needs.
Nutrients: Low concentration in the cells, high concentration outside of the cells.
What direction will the nutrients diffuse?
From outside the cell to inside the cell.
High to Low Concentration!!

11. Diffusion and the Cell
Wastes: High concentration in the cell, low outside of the cell.
What direction will the wastes move?
From inside the cell to outside the cell.
High to Low concentration

12. How Diffusion Works

13. A special case of diffusion: Osmosis
Water moves through the pores of the cell membrane like other molecules.
The movement of water from a high concentration to a low concentration (thru a semi-permeable membrane) is called: Osmosis
Osmosis = movement of water (the solvent)
Diffusion = movement of dissolved materials (the solutes)

14. Osmosis is the movement of water (red dots) through a semipermeable membrane to a higher concentration of solutes (blue dots).

15. How Osmosis Works – Video

16. Osmosis – a cure for the flu?
Who here has taken penicillin to combat sickness?
Penicillin weakens bacterial cell walls
Water is at higher conc. outside the cell…which way will water move?
With a weak cell wall, the bacterial cell swells and…POP!

17. **Summary**
Both osmosis and diffusion involve the movement of molecules from an area of high concentrations to a area of low concentrations.
Osmosis = movement of water (the solvent)
Diffusion = movement of dissolved materials (the solutes)

18. Stop Here

19. Water Balance
The cell membrane doesn’t actually control the movement of water.
The environment around the cell determines the direction of water movement.
For example, a cell is placed in a salt water tide pool. The sun evaporates some water.
The concentration of water inside the cell is higher than outside.
What happens?
Water leaves the cell.
Possible shrinking of the cell.

20. Water Balance On the contrary imagine it rains.
A higher concentration of water outside the cell is the result.
What happens?
Water moves into the cell
Possible swelling of the cell.

21. Water Regulation
Without other mechanisms the cell would burst or shrivel.
Plant vacuoles store water even when the environment is dry.
The cell wall is rigid to prevent bursting or shriveling.
Animals can increase or decrease their amount of water intake and outtake (Urine concentration/amount of water that is drank)

23. Hypertonic Solution – More water inside cell than outside
Plasmolysis
Plasmolysis
Cells shrink and die

24. Iso-osmotic Solution – same concentration of water inside and outside cell
Animal cell
Plant cell
No net gain or loss of water

25. Hypotonic Solution – more water outside cell than inside
Cytolysis – cell swells and bursts
Builds up turgor pressure – cell becomes stiff, keeping plant upright

26. Cell’s are about 90% water.
What would happen to a cell placed in pure water?
90% water
10% solute
100% water
Human Red Blood Cell Before:
After

27. Cell’s are about 90% water.
10% solute
What would happen to a cell placed in a 20% salt solution?
80% water
Human Red Blood Cell Before:
After