Membrane Structure and Function

1. Chapter 7 Membrane Structure and Function

2. Cell/Plasma Membrane • Selectively permeable and amphipathic • some substances can pass easily (hydrophobic/nonpolar) • hydrocarbons, CO2, O2 • other substances cannot pass easily (hydrophilic/polar) • amino acids, nucleic acids, carbohydrates, proteins, ions • Small molecules like glucose, H2O and some ions flow through the membrane constantly AND slowly due to the hydrophobic layer of fatty acids slowing them down

3. A Phospholipid Bilayer • The plasma membrane is made of phospholipids with a phosphate head and 2 fatty acids tails with varying saturation (some have kinks while others don’t) • FYI - The ability of some organisms to regulate the fluidity of their cell membranes by altering lipid composition is called homeoviscous adaptation. • Also has proteins and carbohydrates held together by weak interactions = FLUIDITY • FLUID MOSAIC MODEL

4. Fluidity • Cholesterol reduces fluidity by reducing movement but at low temps it hinders solidification by preventing the “packing” of phospholipids

5. Fluid Mosaic Model

6. Besides the phospholipids… • Proteins • Integral Proteins are embedded in the membrane and span across it (transmembrane) • Peripheral Proteins are loosely bound to the membrane surface

7. 6 functions of proteins in the membrane

8. Besides the phospholipids… • Carbohydrates • Used in cell to cell recognition (important for immune function and developing organisms)

9. How does everything get through the membrane? • Hydrocarbons, CO2, O2 can easily pass • Ions and polar molecules usually pass through transport proteins that span the membrane • H2O pass through the membrane slowly AND through aquaporins (hydrophilic channels) quickly = up to 3,000,000,000 H2O molecules/second

10. Passive Transport • Diffusion of a substance across a membrane with NO energy required (generally moving DOWN the concentration gradient from HIGH to LOW concentration) • Osmosis – the diffusion of water • Facilitated Diffusion – ions and polar molecules can pass through transport proteins that provide: • A hydrophilic channel for molecules to pass through OR • The bind loosely to molecules and carry them through the membrane

12. Active Transport • Movement of solutes across a membrane AGAINST the concentration gradient (LOW to HIGH) = REQUIRES ENERGY • Ex – the sodium/potassium pump – pumps Na+ out of the cell and K+ into the cell against the gradient for nerve function

13. Passive v. Active Transport

14. Cotransport • After pumping the H+ against the gradient, the H+ can do work when they move with the gradient across the membrane • May bring sucrose with it though it is moving against the SUCROSE gradient • Like pumping water uphill and having it do WORK as is comes back down

15. Bulk Transport • Large molecules are moved across the membrane by endocytosis and exocytosis • This requires vesicles entering or leaving the cell • Ex- phagocytosis (eating) and pinocytosis (drinking)

16. Receptor-Mediated Endocytosis • Specific substances bind to receptors on the cell membrane causing a vesicle to form around the substance and move into the cell