1. Substances cross the cell membrane without the cell expending energy
Passive Transport
Substances cross the cell membrane without the cell expending energy

2. Simple Diffusion
Substances spread out by moving from areas of high concentration to areas of lower concentration
Moves “downhill”

3. Facilitated Diffusion
Substances diffuse across the cell membrane through special channels in proteins
Proteins are specific (fit a certain substance)

5. Osmosis
The diffusion of molecules through a selectively permeable membrane
Water moves “down” the concentration gradient – from an area of high water concentration to an area of lower water concentration

6. The force exerted by osmosis
Osmotic Pressure
The force exerted by osmosis
Due to water moving from a more dilute solution into a more concentrated solution (ex: water moving from an area of very little salt to an area with a lot of salt)

7. Remember, water can easily pass through the cell membrane
This can cause problems for a cell

8. To Control Osmotic Pressure
Bacteria & plants use a cell wall – prevents cell from expanding too much
Pump out water using the contractile vacuole
Bathe cells in blood – cells do not come in direct contact with water (in our bodies)

9. The concentration of water is equal inside and outside the cell
Isotonic Solution
The concentration of water is equal inside and outside the cell
“equilibrium” – no net movement of water
Water molecules are still moving!

10. There is a higher concentration of water outside the cell
Hypotonic Solution
There is a higher concentration of water outside the cell
More water moves in than out
The cell expands (explodes)

11. There is a higher concentration of water inside the cell
Hypertonic Solution
There is a higher concentration of water inside the cell
More water moves out than in
The cell shrinks

12. Substances use energy to cross the cell membrane
Active Transport
Substances use energy to cross the cell membrane
Substances move AGAINST the concentration gradient (upstream) as they move from areas of LOW concentration to areas of HIGH concentration

13. Methods of Active Transport
Membrane proteins can act as PUMPS

14. Endocytosis vs. Exocytosis
Can take in large molecules
3. Exocytosis
The opposite of endocytosis