Biology Reference
In-Depth Information
Fatty acids
Phospholipids
CH2
N (CH2)3
Palmitic
Stearic
Hydrophilic Head
CH2
O
P
O
OH
OH
Special region
O
C
O
C
Collapsed monolayer
O
O
Phosphate
CH
2
CH
2
H
2
C
H
2
C
CH
2
CH
2
CH2
OO
COO
CH2
CH
CH2
H
2
C
H
2
C
CH
2
CH
2
H
2
C
H
2
C
Fatty acid tail
CH
2
CH
2
CH2
H
2
C
H
2
C
CH2
CH2
CH2
CH2
CH2
CH2
CH3
CH2
CH
CH
2
CH
2
H
2
C
H
2
C
CH
2
CH
2
CH
H
2
C
H
2
C
CH
2
CH
2
CH2
CH2
CH2
CH3
H
2
C
H
2
C
60
CH
3
CH
2
Hydrophobic Tail
H
2
C
CH
3
50
Condensed
phase
Solid
S
S
40
Liquid Condensed
30
LC
Fluid phase
Liquid Expansed
20
LE-LC
LC
Gaseous phase
10
LE
Gaz
G
G
50
100
Å
2
/molecule
FIGURE 11.2
Pressure-area (
P
-A) isotherms for a simple, saturated fatty acid on the left and a heterochain phos-
pholipid on the right. Note that the phospholipid isotherm is far more complicated than that observed for the simple
fatty acid. Phases are: G, quasi, two-dimensional gas state; LE, liquid-expanded phase; LC, liquid-condensed phase; S,
solid phase; and finally the Collapse Point. The sequence of states that a phospholipid monolayer goes through upon
compression is: G
G-LE
LE
LE-LC
LC
S
Collapse Point. Reprinted with Permission
[56]
.
/
/
/
/
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in a liquid-like phase throughout the transition. Very steep
P
-A curves indicate poor
compressibility (e.g. long chain di-saturated phospholipids or cholesterol) while shallow
curves indicate the lipid is compressible (e.g. lipids containing polyunsaturated chains).
A variety of important physical parameters can be derived from
P
-A isotherms. These
include the area/molecule as a function of lateral pressure, Collapse Point (maximum lipid
packing density and therefore the minimal area/molecule), effect of polar head groups on the
area/molecule, effect of acyl chains on the area/molecule, 'condensation' between two or
more lipids, surface elasticity, and molecular 'squeeze out' of a component from a membrane.
A few of these properties are briefly discussed below.
Area/Molecule
The area/molecule for membrane lipids at different lateral pressures or at different
temperatures can be directly read off
P
-A isotherms. One interesting application of these
