Ch 7 Membrane Structure and Function

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• Ch 7 Membrane Structure and Function

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Cellular membrane -overall functions

• - Boundary between intracellular compartments,
  living cells, and abiotic environment
• Selectively permeable
• Some molecules cross membranes more readily than
  others

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Membrane Structure

• Predominant constituent phospholipids
• Amphipathic molecules hydrophobic AND hydrophilic

Dispersed protein components
Membrane organization and properties described
by Fluid Mosaic Model
Singer and Nicolson 1972
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LE 7-2
Organization of membrane phospholipids
WATER
Hydrophilic head
Hydrophobic tail
WATER
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Mosaic something made of small pieces
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LE 7-3
Mosaic Proteins dispersed among phospholipids in
membrane
Hydrophilic region of protein
Phospholipid bilayer
Hydrophobic region of protein
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• 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

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LE 7-4
Extracellular layer
Proteins
Knife
Cytoplasmic layer
Plasma membrane
Cytoplasmic layer
Extracellular layer
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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

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LE 7-5a
Lateral movement (107 times per second)
Flip-flop ( once per month)
Movement of phospholipids
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Effects of Temperature on membranes

• 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?

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LE 7-5b
Degree of saturation of fatty acid tails
Viscous
Fluid
Unsaturated hydrocarbon tails with kinks
Saturated hydro- carbon tails
Membrane fluidity
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Steroid cholesterol also component of membranes

• Tends to moderate effects of temp. on membrane
  state
• At warm temperatures (such as 37C), restrains
  movement of phospholipids
• At cool temperatures, maintains fluidity by
  preventing tight packing

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LE 7-5c
Cholesterol
Cholesterol within the animal cell membrane
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Movement of membrane proteins

• Can drift within the bilayer
• Proteins much larger than lipids--gt move more
  slowly
• Cell fusion studies support fluidity of membrane
  proteins

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LE 7-6
Membrane proteins
Mouse cell
Mixed proteins after 1 hour
Human cell
Hybrid cell
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Membrane Proteins and Their Functions

• Proteins determine most of the membranes
  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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LE 7-7
Fibers of extracellular matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR SIDE OF MEMBRANE
Cholesterol
Peripheral proteins
Microfilaments of cytoskeleton
Integral protein
CYTOPLASMIC SIDE OF MEMBRANE