KEY CONCEPT The cell membrane is a barrier that separates a cell from the external environment.

1. KEY CONCEPT The cell membrane is a barrier that separates a cell from the external environment.

2. Objectives for section 3-3 • Describe what cell membranes are composed of and identify a phospholipid bilayer • Explain how chemical signals are transmitted across the • cell membrane.

3. Cell (Plasma) Membrane is a thin, flexible boundary between the cell and its environment • Allows nutrients into and wastes to leave the cell

4. Cell membranes are composed of two phospholipid layers. • The cell membrane has two major functions. • forms a boundary between inside and outside of the cell

5. cell membrane • Cell membranes are composed of two phospholipid layers. • The cell membrane has two major functions. 2. controls passage of materials outside cell inside cell

6. Cell membranes are composed of two phospholipid layers. • Composed of a glycerol backbone, two fatty acid chains, and a phosphate group • Polar (partially charged) head and 2 nonpolar (no charge) tails

7. Phospholipid bilayer

8. Overall picture Lipid bilayer Lipid bilayer makes up the cell membrane Phospholipid molecule

9. Nonpolar tails on inside do not allow water-soluble substances to pass through easily • Allows cell to separate inside environment from the outside • Proteins are held in membrane in a similar fashion

10. Cell membranes are composed of two phospholipid layers. • The fluid mosaic model describes the membrane. • -Bilayer allows other molecules to “float” in membrane Transport protein

11. Fluid Mosaic model

12. Cell membranes are composed of two phospholipid layers. • The cell membrane is selectively permeable -Controls movement and amount of substances entering and leaving the cell

13. Cell membranes are composed of two phospholipid layers. • The cell membrane is selectively permeable. Some molecules can cross the membrane while others cannot.

14. Cell membranes are composed of two phospholipid layers. • The cell membrane is selectively permeable. Some molecules can cross the membrane while others cannot.

15. carbohydrate chain protein cell membrane cholesterol protein protein channel • Cell membranes are composed of two phospholipid layers. • There are other molecules embedded in the membrane.

16. Marker proteins are attached to a carbohydrate on cell’s surface and advertise cell type Me • Will recognize foreign invaders including transplanted organs!!

17. Cholesterol prevents fatty acid tails from sticking together

18. Carbohydrates identify chemical signals

19. Chemical signals are transmitted across the cell membrane. • Receptors are proteins that detect signals and performs and action in response • Bind with ligands and change shape. -Ligands are molecules that a receptor binds to and changes

20. Chemical signals are transmitted across the cell membrane. • There are two types of receptors. 1. intracellular receptor -Intra means “within” -Examples include hormones

21. Chemical signals are transmitted across the cell membrane. • There are two types of receptors. 2. membrane receptor -Bind to molecules that cannot enter a cell -Transport proteins