Ch 7 Membrane Structure and Function
Cellular membrane
-overall functions
- Boundary between intracellular compartments,
living cells, and abiotic environment
– Selectively permeable
– Some molecules cross membranes more readily than others
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
LE 7-2
Organization of membrane phospholipids
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
Mosaic: something made of small pieces
LE 7-3
Mosaic: Proteins dispersed among phospholipids in membrane:
Hydrophilic region
of protein
Phospholipid
bilayer
Hydrophobic region of protein
• 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
Extracellular
layer
Proteins
Knife
Plasma
membrane
Extracellular layer
Cytoplasmic
layer
Cytoplasmic layer
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)
Movement of phospholipids
Flip-flop
(~ once per month)
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?
LE 7-5b
Degree of saturation of fatty acid tails
Fluid
Unsaturated hydrocarbon
tails with kinks
Membrane fluidity
Viscous
Saturated hydrocarbon tails
Steroid cholesterol
also component of membranes
• 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
LE 7-5c
Cholesterol
Cholesterol within the animal cell membrane
Movement of membrane proteins
Can drift within the bilayer
– Proteins much larger than lipids--> move more slowly
• Cell fusion studies support fluidity of membrane
proteins
LE 7-6
Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed
proteins
after
1 hour
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
LE 7-7
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE