1. The Permeability of the Plasma Membrane
The plasma membrane is selectively permeable.
Macromolecules cannot pass through because of size, and tiny charged molecules do not pass through the nonpolar interior of the membrane.
Small, uncharged molecules pass through the membrane, following their concentration gradient.
Molecules move passively from and area of high concentration to an area of low concentration, following their concentration gradient.

2. How molecules cross the plasma membrane
The small, curved arrows indicate that these structures cannot cross the plasma membrane, and the large arrows indicate that these substances can cross the plasma membrane.

3. Movement of materials across a membrane may be passive or active.
Passive transport does not use chemical energy; diffusion and osmosis are both passive.
Exocytosis and endocytosis are included as methods of active transport.

4. Diffusion
Diffusion is the passive movement of molecules from a higher to a lower concentration until equilibrium is reached.
Gases move through plasma membranes by diffusion.

5. Process of diffusion
The solute (thing that is going to diffuse around) is placed in the solvent.
When crystals of dye are placed in water, they are concentrated in one area.

6. The solute spreads out to the area of lower concentration.
The dye dissolves in the water, and there is a net movement of dye molecules from higher to lower concentration within the container.

7. Eventually the solute is evenly spread out all through the solvent
At equilibrium, the water and the dye molecules are equally distributed throughout the solution.

8. Gas exchange in lungs occurs by diffusion
Oxygen from the lungs diffuses into the blood and carbon dioxide from the body diffuse into the lungs to be breathed out.
Oxygen diffuses from the alveoli into the capillaries because there is a higher concentration of oxygen in the alveoli than in the blood of the capillaries.

9. Osmosis
The diffusion of water across a selectively permeable membrane due to concentration differences is called osmosis. In osmosis only the water moves, the solute can’t get through the membrane
In osmosis the water
always moves from
where there is a low
solute concentration
to where there is a
to higher concentration.
Water enters cells due to osmotic pressure within cells.

10. Osmosis demonstration
A thistle tube, covered at the base with differentially permeable membrane, contains a 10% sugar solution.

11. The solute (green particles) cannot pass through the membrane, but the water (blue particles) passes through in both directions. There is a net movement of water into the thistle tube, where there is a lower percentage of water molecules.

12. Due to the incoming water molecules, the level of solution rises in the thistle tube.

13. Osmosis in cells
A solution contains a solute (solid) and a solvent (liquid).
Cells are normally isotonic to their surroundings
“Iso” means the same, and
“tonocity” refers to the strength
of the solution.
So isotonic means the same
strength
So the solution of the cell is the
same strength as the surrounding
solution
A 0.9% solution of sodium chloride is isotonic to red blood cells, therefore, intravenous solutions have this tonicity.

14. In isotonic solutions the same amount of water goes in and goes out
Osmosis in plant and animal cells
In isotonic solutions the same amount of water goes in and goes out
Arrows indicate the direction of movement of water. In an isotonic solution, there is no net movement of water, and the cell neither gains nor loses water.

15. Hypotonic solutions cause cells to swell and possibly burst.
“Hypo” means less than.
So hypotonic means less strong
Animal cells undergo
lysis in hypotonic solution.
Increased turgor pressure occurs in plant cells in hypotonic solutions.
Plant cells do not burst because they have a cell wall.

16. In hypotonic solution, a cell gains water
In hypotonic solution, a cell gains water. The animal cell may undergo lysis (burst). In the plant cell, vacuoles fill with water, turgor pressure develops, and chloroplasts are seen next to the cell wall.

17. Hypertonic solutions cause cells to lose water.
“Hyper” means more than;
hypertonic means more strong.
Animal cells undergo crenation
(shrink) in hypertonic solutions.
Plant cells undergo plasmolysis, the shrinking of the cytoplasm.

18. In a hypertonic solution, a cell loses water
In a hypertonic solution, a cell loses water. Animal cells shrivel (undergo crenation). Plant cell vacuoles lose water, the cytoplasm shrinks (plasmolysis), and chloroplasts can be seen in the center of the cell.

19. Summary
The structure of the plasma membrane allows it to be differentially permeable.
The fluid phospholipid bilayer, its mosaic of proteins, make possible many unique functions of the plasma membrane.
Passive and active methods of transport regulate materials entering and exiting cells.