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Biological Membranes and Transport - Coggle Diagram
Biological Membranes and Transport
Lipids Aggregate into Structures in Water
Three major structures are observed:
micelles
have larger, more polar head than tail
concentration dependent
bilayers
Forms when lipids with polar head groups and more than one lipid tail are in aqueous solution
phospholipids
sphingolipids
liposomes(Vesicle)
Small bilayers will spontaneously seal into spherical vesicles in a concentration-dependent
artificial carriers of molecules
Structures formed depend on:
type of lipid
concentration
Functions
boundaries
Allow import and export
Retain metabolites and ions within the cell
Provide compartmentalization within the cell
Produce and transmit nerve signals
Store energy as a proton gradient
Support synthesis of ATP
Features
Sheet-like flexible structure, 30—100 Å (3—10 nm) thick
Main structure is composed of two leaflets of lipids (bilayer)
Form spontaneously in aqueous solution and are stabilized by noncovalent forces, especially hydrophobic effect
Protein molecules span the lipid bilayer
Asymmetric
Fluid structures: two-dimensional solution of oriented lipids
Fluid Mosaic Model of Membranes
Lipids form a viscous, two-dimensional solvent into which proteins are inserted and integrated more or less deeply.
Proteins can either be embedded in or associated with the membrane
The Composition of Membranes
Ratio of lipid to protein varies
type of phospholipid varies
abundance and type of sterols varies
lack of sterols in prokaryotes
cholesterol predominant in the plasma membrane, virtually absent in mitochondria
galactolipids abundant in plant chloroplasts but almost absent in animals
Asymmetry
Phosphatidylserine outside has a special meaning
platelets: activates blood clotting
other cells: marks the cell for destruction
Archaea Membrane
Unique glycerol chirality in phospholipids
Unique fatty acids
Unique linkages
monolayer in some archaea
Proteins in Membranes
Functions
Receptors: detecting signals from outside
Channels, gates, pumps
Enzymes
Classifications
Peripheral (non-GPI linked) membrane proteins
Associate with the polar head groups of membranes
loosely associated
Removed by disrupting ionic interactions either with high salt or change in pH
Amphitrophic and GPI-linked proteins
covalent interaction with lipids or carbohydrates attached to lipids
Biological regulation results in attachment to, or cleavage from, lipids.
integral membrane proteins
Tightly associated with membrane
Removed by detergents that disrupt the membrane
Purified integral membrane proteins still have phospholipids associated with them.
Lipid-linked Membrane Proteins
Palmitoyl group on internal Cys(or Ser)
N-Myristoyl group on amino-terminal Gly
Farnesyl(or geranylgeranyl) group on carboxyl-terminal Cys
傳訊
CaaX
Farnesyl transferase
GPI anchor on carboxyl terminus (外)
Lipid Anchors
contain a covalently linked lipid molecule
long-chain fatty acids
isoprenoids
sterols
glycosylated phosphatidylinositol (GPI)
reversible
Hydropathy Plots Can Predict Transmembrane Domains
Membrane Phases
Depending on their composition and the temperature
At higher temperatures, cells need more long, saturated fatty acids.
At lower temperatures, cells need more unsaturated fatty acids.
liquid-ordered state (i.e., "gel phase")
liquid-disordered state (i.e., "fluid phase")
More fluid membranes require shorter and more unsaturated fatty acids.
Sterols and Hopanols
cholesterol in animals
phytosterols in plants
ergosterol in fungi
aerobic prokaryotes contain hopanols
Membrane Dynamics:
Lateral Diffusion (快)
Fluorescence recovery after photobleaching (FRAP)
Transverse Diffusion
(慢)(ATP)
Flippase(外往內)
Floppase (內往外)
Screamblase(內外翻)
Membrane Rafts (固定)
Caveolin Forces Membrane Curvature
more ordered
Membrane Fusion
Neurotransmitter Release