1. Passive Transport
Section 4.1

2. Difference between active and passive transport
Cells maintain homeostasis by controlling the movement of substances across their cell membrane.
No energy needed = passive transport
Energy needed = active transport

3. Passive Transport
Differences in concentration = concentration gradient
Concentration is the same all through = Equilibrium
Passive transport can happen when there is a concentration gradient
Molecules in equilibrium continue to move but the movement is equal in all directions

4. Diffusion – the most common form of passive transport
Molecules move from an area of high concentration to an area of lower concentration
Many substances enter or leave cells by diffusing across the cell membrane.

5. Diffusion

6. Osmosis – a special type of diffusion
The diffusion of water through a selectively permeable membrane = osmosis.
Water goes into or out of a cell by osmosis

7. Osmosis, continued
There are three possibilities for the direction of water movement:
Water moves out - the cell shrinks. A solution that causes a cell to shrink is a hypertonic solution
Water moves in - the cell swells. A solution that causes a cell to swell is a hypotonic solution
Equal water movement – the cell stays the same. A solution that produces no change in cell volume is an isotonic solution

8. Hypertonic, Hypotonic, and Isotonic Solutions

9. Other forms of passive transport
1. Diffusion Through Ion Channels
A transport protein with a pore through which ions can pass.
An ion can cross the cell membrane without touching the interior of the lipid bilayer.

10. Ion Channels

11. Other forms of passive transport, continued
2. Facilitated Diffusion
Carrier proteins - can bind to a substance on one side of the cell membrane, carry the substance across the cell membrane, and release it on the other side.

12. Facilitated Diffusion