1.3 Revision notes
MEMBRANE STRUCTURE
Phospholipid bilayer
- Amphipathic: part hydrophobic and hydrophilic
- Polar hydrophilic head: glycerol & phosphate molecule
o Face outwards (interstitial and cytosolic fluid)
- Two non-polar hydrophobic tails: fatty acid (hydrocarbon)
chains
o Face inwards
- Bilayer is held together by weak hydrophobic forces between tails
- Hydrophilic/hydrophobic layer restricts flow of substances
- Phospholipids can move in bilayer, allowing membrane fluidity
- Allows for breaking and reforming of membrane (endocytosis / exocytosis)
Davson-Danielli Model
- First model of cell membrane
- Viewed under transmission electron Cell 1
microscope
- “Trilaminar” appearance = 3 layers (2 dark
outer layers and 1 inner lighter layer)
- Two protein layers flanked central
phospholipid bilayer Cell 2
- Dark segments wrongly identified as two
protein layers
Problems with Davson-Danielli Model
- Assumed all membranes had constant lipid-protein ratio
- Assumed all membranes had symmetrical internal and external surfaces
- Did not account permeability of substances
- Temperatures at which membrane solidified did not correlate with predictions of model
MEMBRANE STRUCTURE
Phospholipid bilayer
- Amphipathic: part hydrophobic and hydrophilic
- Polar hydrophilic head: glycerol & phosphate molecule
o Face outwards (interstitial and cytosolic fluid)
- Two non-polar hydrophobic tails: fatty acid (hydrocarbon)
chains
o Face inwards
- Bilayer is held together by weak hydrophobic forces between tails
- Hydrophilic/hydrophobic layer restricts flow of substances
- Phospholipids can move in bilayer, allowing membrane fluidity
- Allows for breaking and reforming of membrane (endocytosis / exocytosis)
Davson-Danielli Model
- First model of cell membrane
- Viewed under transmission electron Cell 1
microscope
- “Trilaminar” appearance = 3 layers (2 dark
outer layers and 1 inner lighter layer)
- Two protein layers flanked central
phospholipid bilayer Cell 2
- Dark segments wrongly identified as two
protein layers
Problems with Davson-Danielli Model
- Assumed all membranes had constant lipid-protein ratio
- Assumed all membranes had symmetrical internal and external surfaces
- Did not account permeability of substances
- Temperatures at which membrane solidified did not correlate with predictions of model
