1. Membrane Structure and Function

2. What is the Function of The Plasma Membrane? Boundary Must be selectively permeable

3. Phospholipids Recall that phospholipids are amphipathic (both hydrophilic and hydrophobic). The hydrophilic head is due to the polar phosphate group. The hydrophobic tails are the nonpolar fatty acid chains. Phospholipids will form a layer in water:

4. The Bilayer Why are membranes organized into a bilayer? There are two “watery” areas that interact with the membrane, outside of the cell and inside the cell.There are two “watery” areas that interact with the membrane, outside of the cell and inside the cell.

5. Fluid Mosaic Model - 1972 Hydrophobic parts of proteins are embedded within the membrane. Thickness between different membranes is a function of the proteins

6. What evidence is there for membrane fluidity?

7. How is this fluidity maintained? Kinks in unsaturated fatty acid tails of phospholipids. Cholesterol

8. Other membrane components… Interior Protein Network – Intracellular proteins that support the membranes shape such as spectrin in red blood cells Cell Surface Markers- include glycoproteins (carbohydrate + protein) and glycolipids (carbohydrate + lipid); identify cells by sticking up on the cell membrane (cell to cell recognition and immune response) Integral or peripheral proteins – proteins that float on or in the lipid bilayer

10. Protein Functions Transporters- Integral –Single pass anchor- single nonpolar segment which attaches spectrin (inside of the cell) to the plasma membrane. The end facing the outside of the cell may serve as a receptor. –Multiple pass channels and carriers- several nonpolar regions (alpha helices) wind back and forth through the membrane forming a channel –Pores – several nonpolar regions (beta pleated sheets) wind back and forth through the membrane forming a pore * Inside linings of transporters are polar

11. Other Protein Functions Enzymes Cell surface receptors Cell surface identity markers Cell adhesion proteins Attachements to the cytoskeleton

12. What determines the direction of traffic across a membrane? Concentration gradient Particles are always in random motion and naturally want to spread from a higher concentration to a lower Concentration gradient represents potential energy! Membranes move particles with or against their concentration gradients

13. Passive Transport Movement of particles with their concentration gradient from an area of higher concentration to an area of lower concentration –Diffusion –Facilitated Diffusion –Osmosis

14. Diffusion Movement directly through the lipid bilayer by very small particles OR… Movement of cations or anions through ion channels that are hydrated on the interior Channels are specific to the ions they carry

15. Simple diffusion of ions vs. Active Transport

16. Facilitated Diffusion Diffusion of solutes with the help of specific carrier proteins Can carry particles into or out of the cell as long as it is with the concentration gradient The particle binds to the carrier and the carrier releases it on the other side Carrier proteins can saturate because their number is limited

17. Review of passive and active transport:

18. Osmosis The diffusion of water across a semi- permeable membrane Water will follow the solutes! Hypotonic = lower solute concentration Hypertonic = greater solute concentration Isotonic = equal solute concentration

19. Since water passes freely across the membrane, how can the cell control the direction of osmosis?

20. WHY?

22. Do Water Molecules Stop Moving in Isotonic Conditions? No. They continue to diffuse, however there is no net movement! In general, which way does water move? From hypotonic to hypertonic!

23. Water Balance in Cells