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Lecture 11

BIL 255 Lecture Notes - Lecture 11: Conformational Change, Biological Membrane, Platelet-Derived Growth Factor

Course Code
BIL 255
Diresta Dan

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Chapter 11 Membrane Structure
Barrier, selective, transduce info, mechanically elastic, eukaryotic compartments, variable
composition, bilayer
Membranes form many diff. compartments
Plasma membrane = single bilayer membrane that encloses the cell
Exoplasmic face = facing exterior
Cytosolic face = facing cytosol
Two membranes separated by small intermembrane space:
o Nucleus
o Mitochondria
o Chloroplast
Membrane composition: Lipids and proteins
o Phosphatidylcholine is most common phospholipid
o 5 parts
hydrophilic head (polar)
choline linked to phosphate group (PC)
molecule of glycerol
hydrophobic tails (non-polar)
two hydrocarbon chains
fatty acid
kink in one of them occurs when there is a double bond between
two carbon atoms
o Head group
Synthesis of fatty acids
Growing fatty acid chain linked to coenzyme A
Sequential addition of two acetyl units at a time
Additional enzymes form double bonds
Phospholipid synthesis in ER membrane
Amphipathic molecules
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o Both hydrophobic and hydrophilic
o Last stages of their multistep synthesis take place at the interface between a
membrane and the cytosol and are catalyzed by membrane associated enzymes
1. Two fatty acids from fatty acyl CoA are esterified to the phosphorylated glycerol
backbone, forming phosphatidic acid, whose two long hydrocarbon chains anchor the
molecule to the membrane
2. A phosphate converts phosphatidic acid into diacylglycerol
3. A polar head group (phosphoryl choline) is transferred from cytosine diphosphocholine
(CDP-choline) to the exposed hydroxyl group
4. Flippase proteins catalyze the movement of phospholipids from the cytosolic leaflet in
which they are initially formed to the exoplasmic leaflet
Other membrane lipids
Cholesterol (sterol)
Galactocerebroside (glycolipid)
Phospholipids spontaneously form bilayers
Close in on themselves to form sealed compartments
Energetically favorable stable
Avoids the exposure of the hydrophobic tails to water
Lipid bilayer is a 2-D fluid
Rotation along long axis
Lateral diffusion
Leaflet migration or flip-flop
Because of these movements the bilayer
behaves as a 2-D fluid, in which the
indiv. lipid molecules are able to move
in their own monolayer
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Lipids do’t oe etee oolayers
Fluidity of membrane depends on its composition
The closer and more regular packaging of
tails, the more viscous (less fluid) the
membrane is
o # of hydrocarbons
o # of double bonds
o Cholesterol (stiffens membrane)
Asymmetrical distribution of lipids in plasma membrane
P-inositol: minor
constituent of the plasma
membrane & participate in
cell signaling
Glycolipids: sugars
found exclusivelt in
noncytosolic monolayer
distributed almost equally
in both monolayers
Newly synthesized
phospholipids are added to
the cytosolic side of the ER
membrane and then redistributed by enzymes that transfer them from one half of the lipid
bilayer to the other
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