Biological Membranes

1. Biological Membranes Today we will closely examine the nature of the cell membrane and the various mechanisms for transporting molecules across it.

2. Today’s Guiding Question • What properties of real biological membranes cause them to be semipermeable?

3. Electron micrograph of the cell membrane (plasma membrane)

4. Cell membranes are made mostly of phospholipid molecules Polar “head” that is attracted to water Nonpolar “tails” that are not attracted to water but are attracted to other phospholipid tails

5. On a piece of scratch paper… • Sketch a diagram of how phospholipids might group together if they are mixed with water. Remember the “rules”: • The “heads” face towards water • The “tails” face away from water and are attracted towards tails of other phospholipids How many different arrangements will work?

6. (…draw on board…) • Micelle • Bilayer • Monolayer on water surface

7. Phospholipids make lipid membranes

8. Water molecules Polar head faces out towards water because of the attraction Nonpolar tails face inward so they are protected from water Water

9. Movement of molecules through membranes made only of phospholipids (phospholipid bilayer)

10. Movement of molecules through membranes made only of phospholipids (phospholipid bilayer)

11. A GOOD ANIMATION http://www.johnkyrk.com/cellmembrane.html

12. Exploring Cellular Transport • To this point, we have discussed osmosis and simple diffusion. • Today we will expand our discussion of cellular transport. • The following animation will help us visualize these processes: Shockwave Animation

13. Facilitated diffusion The movement of molecules across a cell membrane from an area of higher concentration to an area of lower concentration with the assistance of a carrier protein. This transport does NOT require extra cellular energy to happen.

14. Active transport The movement of molecules across a cell membrane from an area of low concentration to an area of higher concentration (“up” or “against” the concentration gradient). Active transport requires the use of cellular energy (ATP).

15. Passive transport Passive transport happens spontaneously across the cell membrane. It does NOT require any cellular energy (ATP) because molecules move DOWN their concentration gradients (from high to low concentration). There are two types of passive diffusion: • Simple diffusion: molecules pass directly through the lipid bilayer. • Facilitated diffusion: molecules are too big to pass through the lipid bilayer. They bind to a carrier protein to diffuse across the membrane.

16. Simple diffusion Facilitated diffusion Passive transport Concentration Gradient Carrier protein

17. 3 steps in facilitated diffusion 1. Particle binds to one side of carrier protein 2. Carrier protein changes shape 3. Particle diffuses to other side of membrane

18. Think about it… • If a molecule requires a carrier protein to cross the membrane, does it necessarily require active transport? • When can you be sure that active transport will be required for a molecule to cross the membrane?

19. Electron micrograph of cell membrane

20. Active Transport • REQUIRES ENERGY • There is more than one variety of active transport: Shockwave Animation • Animation of sodium / potassium pump

21. Na+/K+ pump (Na+/K+ ATPase) • Pumps sodium and potassium up their concentration gradient (from low to high concentration) • Requires ATP (energy) • Pumps 3 sodium ions OUT and 2 potassium ions IN for each molecule of ATP used • Important for keeping the cell in isotonic conditions to prevent cytolysis

22. Other Active Transport • EXOCYTOSIS- (moving out of cell) • ENDOCYTOSIS- (moving into cell)

23. Compare and Contrast • Compare and Contrast (handout): • Simple Diffusion • Facilitated Diffusion • Active Transport

24. Diagram Cell Membrane • Homework: Draw and label a diagram of a section of the complete cell membrane (also known as plasma membrane). Include phospholipids, transport proteins, and substances transported through simple diffusion.