1. What can we compare this to in our bodies?
KEY CONCEPT The cell membrane is a barrier that separates a cell from the external environment.
What can we compare this to in our bodies?

2. The cell membrane has two major functions.
1. forms a boundary between inside and outside of the cell
2. controls passage of materials
cell membrane
outside cell
inside cell

3. Attracted to other tails
Head
The head is polar
interacts with water
Hydrophilic
Attracted to water
The tail is non-polar
repelled by water
hydrophobic
Attracted to other tails
Tail

4. *Phospholipids arrange themselves like a sandwich into two layers.
Contain:
Proteins: help to transport substances through the membrane
Cholesterol: strengthens the cell membrane
Carbohydrates: serve as ID’s so cells can tell each other apart
protein
cholesterol
carbohydrate
chain
protein channel
cell membrane

5. The “Fluid Mosaic Model” was developed from two cell membrane characteristics:
The cell membrane is flexible, not rigid…it is fluid and able to move.
The molecules embedded in the cell membrane make it appear like a mosaic (a colorful tile arrangement).

6. The cell membrane is selectively permeable.
Some molecules can cross the membrane while others cannot.

7. Selective Permeability
Three things determine which molecules can cross the cell membrane.
The molecule’s size. (How big is the molecule?)
The molecule’s polarity. (Is it polar or nonpolar?)
The concentration of substances inside and outside a cell. (How many molecules are inside the cell versus outside the cell?)