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)
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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 proteinsynthesis, 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 andtheir 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 isplasma membrane
Biological Membranes and Transport
Peripheral membrane proteins - associate with the membrane throughnoncovalent interactions (electrostatic interactions, hydrogen bonding)with hydrophilic domains of integral proteins, easily released
Integral membrane proteins - firmly associated with the membranethrough strong hydrophobic interactions, removable only with agents thatinterfere with hydrophobic interactions (detergents, organic solvents,denaturants)
Biological Membranes and Transport
Lipid-linked peripheral membrane proteins
Biological Membranes and TransportIntegral membrane proteinsStrong attachment because of hydrophobic interactions betweenmembrane lipids and hydrophobic domains of protein
Biological Membranes and TransportStructure of Integral membrane proteinsX-ray crystallography - very difficultHydropathy plot - easier if know amino acid sequence of protein
An α-helical sequence of 20-25 amino acids is just long enough to spanthe thickness (30 A) of the lipid bilayer (length of an α helix is 1.5 A peramino acid)
Biological Membranes and TransportIntegral membrane proteinsIntegrinsInvolved 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 TransportMembrane fusion
Biological Membranes and TransportMembrane fusion
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