Biological Membranes and Transport

1. Biological Membranes and Transport Functions of membranes Define cell boundaries, compartments Maintain electric and chemical potentials Self-sealing (break and reseal) Selectively permeable to polar solutes (retain charged species within membranes) Actively transport specific molecules Cell surface has transporters, receptors, adhesion molecules Within cell membranes organize cellular processes (lipid and protein synthesis, energy transductions in mitochondria and chloroplasts) Membrane structure Based on lipid bilayers Contain transmembrane proteins Bind other proteins at their surface Are thin, fluid and flexible (shape changes as cell grows and moves) Can fuse with or pinch off sections (vesicles)

2. Biological Membranes and Transport Membrane Architecture - Fluid Mosaic Model Membranes are impermeable to most polar or charged solutes Membranes are permeable to nonpolar compounds 5 to 8 nm thick, appear trilaminar Proteins imbedded at irregular interval Pattern of lipid and proteins is constantly changing Membrane mosaic is fluid because interactions within it are noncovalent

3. Biological Membranes and Transport Why do Lipids aggregate? Lipids are amphipathic, they are usually dissolved in aqueous systems Need a way to allow their hydrophobic regions to contact each other and their hydrophilic regions to interact with surrounding water Phobic in contact with surrounding water Unstable, forms liposome

4. Biological Membranes and Transport Membrane proteins - span lipid bilayer How did scientists figure out protein spanned bilayer?

5. Biological Membranes and Transport Membrane proteins - span lipid bilayer How did scientists figure out protein spanned bilayer? Use reagents that cannot cross membranes Use human erythrocytes (red blood cells) because only membrane is plasma membrane

6. Biological Membranes and Transport Peripheral membrane proteins - associate with the membrane through noncovalent interactions (electrostatic interactions, hydrogen bonding) with hydrophilic domains of integral proteins, easily released Integral membrane proteins - firmly associated with the membrane through strong hydrophobic interactions, removable only with agents that interfere with hydrophobic interactions (detergents, organic solvents, denaturants)

7. Biological Membranes and Transport Lipid-linked peripheral membrane proteins

8. Biological Membranes and Transport Integral membrane proteins Strong attachment because of hydrophobic interactions between membrane lipids and hydrophobic domains of protein

9. Biological Membranes and Transport Structure of Integral membrane proteins X-ray crystallography - very difficult Hydropathy plot - easier if know amino acid sequence of protein An -helical sequence of 20-25 amino acids is just long enough to span the thickness (30 A) of the lipid bilayer (length of an  helix is 1.5 A per amino acid)

10. Biological Membranes and Transport Integral membrane proteins Integrins Involved in adhesion & platelet aggregation, role as receptor, signal transducer Mutations linked to cancers Cadhedrins Interact with other cadhedrins on adjacent cells Immunoglobulin-like proteins Interact with other Ig-like proteins or integrins on adjacent cells Selectins Ca2+-dependent binding

11. Biological Membranes and Transport Membrane fusion

12. Biological Membranes and Transport Membrane fusion

13. Biological Membranes and Transport Membrane fusion