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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)
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
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
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 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
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 difficult
Hydropathy plot - easier if know amino acid sequence of protein
An α-helical sequence of 20-25 amino acids is just long enough to span
the thickness (30 A) of the lipid bilayer (length of an α helix is 1.5 A per
amino acid)
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
Biological Membranes and Transport
Membrane fusion
Biological Membranes and Transport
Membrane fusion
Biological Membranes and Transport
Membrane fusion