1. Diffusion and Physical Transport
Membranes, Materials, and Movement

2. Membranes Cell membranes are selectively permeable
Most biological membranes are permeable to small or lipid-soluble molecules
Water molecules may pass the lipid bilayer

3. Membranes
Gases, small polar molecules and a few other substances may also pass
Other molecules move through special channels, primarily through membrane transport membranes

4. Diffusion
Diffusion is a process by which molecules move from an area of higher concentration to lower concentration
Some substances may diffuse through the cell membrane

5. Diffusion Random motion of particles leads to diffusion
Atoms and molecules above absolute zero exhibit random motion

6. Diffusion
Molecules move from an area of higher concentration to one of lower concentration, ultimately reaching equilibrium

7. Diffusion
The rate of diffusion depends on temperature, the size of the molecules, electrical charges, and the concentration gradient

8. Diffusion
Diffusion rates increase as temperature increases

9. Diffusion
Molecular motion slows at low temperatures, hence diffusion slows

10. Diffusion Diffusion increases with decreasing molecular size
2 chemical dyes with different molecular weights
Diffusion increases with decreasing molecular size

11. Diffusion
Large molecules move more slowly (makes sense!), hence diffuse a smaller distance in the gel

12. Diffusion
The rate of diffusion increases with a greater concentration gradient

13. Diffusion
Diffusion slows as concentration gradient lessens over time

14. Osmosis
Osmosis is the diffusion of water across a selectively permeable membrane

15. Osmosis
Two solutions may be isotonic to each other, or one may be relatively hypertonic and the other relatively hypotonic

16. Osmosis
Human cells are isotonic with a 0.9% sodium chloride (salt) solution
Human cells are hypotonic compared to sea water

17. Osmosis Human cells are hypertonic compared to distilled water
How would you state the comparison of distilled water to human cells?
Of this IV fluid?

18. Osmosis Fluids in the human body are approximately 0.9% salts
An IV bag is balanced to that salt balance, termed physiological saline
The fluids in this bag are isotonic to human body fluids

19. Osmosis in animal and plant cells

20. Osmosis
Animal cells placed in a hypertonic solution tend to shrivel and die

21. Osmosis
This is seen in the red blood cell at the top; normal red blood cell at bottom

23. Osmosis
Plant cells and others with cell walls placed in a hypertonic solution tend to plasmolyze
The cell membrane shrinks around the cell contents

24. Plasmolyzed cells
Normal cells

25. Plasmolysis

26. Osmosis
Animal cells placed in a hypotonic solution tend to swell and burst
How can we still drink distilled water and live?

27. Osmosis
Plant cells placed in a hypotonic solution tend to become turgid (“full”)
Turgor pressure is the internal water pressure usually present in cells with walls (plants, fungi)

28. Osmosis
Turgor pressure not only provides pressure to each cell, but gives the plant pressure to stay erect, holding up stems and leaves

29. Osmosis
Turgor pressure is maintained by water contained in the central vacuole of mature plant cells
Central Vacuole

30. Plant Cells and Turgidity

31. Osmosis
Turgor pressure provides structural support in non-woody plants
Wilting occurs when plants are underwatered, resulting in low cellular water pressure

32. Osmosis
Wilting can be reversible, or irreversible

33. Osmosis
Wilting is of critical importance in agricultural research

34. Diffusion; the End