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?

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

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

*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

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).

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

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?)