Biological Membranes and Transport

Functions of membranesDefine cell boundaries, compartmentsMaintain electric and chemical potentialsSelf-sealing (break and reseal)Selectively permeable to polar solutes (retain charged species within membranes)

Actively transport specific moleculesCell surface has transporters, receptors, adhesion moleculesWithin cell membranes organize cellular processes (lipid and protein synthesis, energy transductions in mitochondria and chloroplasts)

Membrane structureBased on lipid bilayersContain transmembrane proteinsBind other proteins at their surfaceAre thin, fluid and flexible (shape changes as cell grows and moves)Can fuse with or pinch off sections (vesicles)

Biological Membranes and Transport

Membrane Architecture - Fluid Mosaic Model

Membranes are impermeable to most polar or charged solutesMembranes are permeable to nonpolar compounds5 to 8 nm thick, appear trilaminar

Proteins imbedded at irregular intervalPattern of lipid and proteins is constantly changingMembrane mosaic is fluid because interactions within it are noncovalent

Biological Membranes and Transport

Why do Lipids aggregate?Lipids are amphipathic, they are usually dissolved in aqueous systemsNeed 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 waterUnstable, forms liposome

Biological Membranes and Transport

Membrane proteins - span lipid bilayer

How did scientists figure out protein spanned bilayer?

Biological Membranes and Transport

Membrane proteins - span lipid bilayerHow did scientists figure out protein spanned bilayer?

Use reagents that cannot cross membranesUse human erythrocytes (red blood cells) because only membrane is plasma membrane

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)

Biological Membranes and Transport

Lipid-linked peripheral membrane proteins

Biological Membranes and Transport

Integral membrane proteins Strong attachment because of hydrophobic interactions between membrane lipids and hydrophobic domains of protein

Biological Membranes and Transport

Structure of Integral membrane proteins X-ray crystallography - very difficultHydropathy 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)

Biological Membranes and Transport

Integral membrane proteinsIntegrins Involved in adhesion & platelet aggregation, role as receptor, signal transducerMutations linked to cancers

CadhedrinsInteract with other cadhedrins on adjacent cells

Immunoglobulin-like proteinsInteract with other Ig-like proteins or integrins on adjacent cells

SelectinsCa2+-dependent binding

Biological Membranes and TransportMembrane fusion

Biological Membranes and Transport

Membrane fusion

Biological Membranes and Transport

Membrane fusion