ch 7 membrane structure and function. - boundary between intracellular compartments, living cells,...
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- Boundary between intracellular compartments, living cells, and abiotic environment
– Selectively permeable– Some molecules cross membranes more readily than others
Cellular membrane-overall functions
Predominant constituent: phospholipids• Amphipathic molecules: hydrophobic AND hydrophilic
Membrane Structure
Dispersed protein components
Membrane organization and properties described by:
Fluid Mosaic Model
Singer and Nicolson 1972
LE 7-3
Hydrophilic regionof protein
Hydrophobic region of protein
Phospholipidbilayer
Mosaic: Proteins dispersed among phospholipids in membrane:
• Freeze-fracture studies of the plasma membrane
• Frozen membrane split along the middle of the phospholipid bilayer using a knife
• Imaged by EM
Supports mosaic part of model
LE 7-4
Knife
Cytoplasmic layerExtracellular layer
Cytoplasmic layer
Plasmamembrane
Extracellular layer
Proteins
The Fluidity of Membranes
• Phospholipids move laterally within the bilayer
• Some membrane proteins also drift laterally
• Rarely does a phospholipid flip-flop transversely across the membrane
LE 7-5a
Lateral movement(~107 times per second)
Flip-flop(~ once per month)
Movement of phospholipids
• Cool temp: membranes switch from fluid to more solid state
• Solidification depends on type of lipid
• What property of lipids would favor liquid versus solid state?
Effects of Temperature on membranes
LE 7-5b
ViscousFluid
Unsaturated hydrocarbontails with kinks
Membrane fluidity
Saturated hydro-carbon tails
Degree of saturation of fatty acid tails
• Tends to moderate effects of temp. on membrane state
• At warm temperatures (such as 37°C), restrains movement of phospholipids
• At cool temperatures, maintains fluidity by preventing tight packing
Steroid cholesterolalso component of membranes
Can drift within the bilayer– Proteins much larger than lipids--> move more slowly
• Cell fusion studies support fluidity of membrane proteins
Movement of membrane proteins
Membrane Proteins and Their Functions
• Proteins determine most of the membrane’s specific functions
• Peripheral membrane proteins – not embedded– attached to extracellular or cytoplasmic
surface
• Integral membrane proteins– penetrate the hydrophobic core of bilayer– often span the membrane
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