Biology Reference
In-Depth Information
the lamellar phase) can exist in a variety of very different states. In prior sections we have
used the terms; bilayer, lamellar, gel, solid, S o ,L
,L
' , ripple, P
' , fluid, liquid crystalline,
b
b
b
L
, liquid disordered, L d , liquid ordered and l o to describe the 'simple' lipid bilayer. But there
is even more, far more, to structures of polar lipids dispersed in water. Depending on the
polar lipid and conditions of the aqueous solvent (e.g. temperature, water content, ionic
strength, polyvalent cations, pH, pressure etc.), lipid dispersions can exist in a wide variety
of additional forms. Multiple long-range structures that amphipathic (polar) lipids can take
when dispersed in water are referred to as 'lipid polymorphism' [18
a
21] . Two of the major
questions about membrane lipids concern 'lipid diversity' and 'lipid polymorphism'. Lipid
diversity addresses the question of why there are so many different lipids in membranes.
Lipid polymorphism addresses the question of why amphipathic lipids form so many
different long-range structures when dispersed in water.
While there are dozens of highly unusual long-range structures that amphipathic lipids
can assume, only four are believed to be stable at biologically high water levels and only
one of these, the lamellar state, predominantly, perhaps exclusively, exists in a living
membrane. The four lipid phases that are stable at high water concentrations [22] are:
e
1. Lamellar
2. Inverted Hexagonal (H II )
3. Cubic Q 224
4. Cubic Q 227
What phase an amphipathic lipid prefers is dependent on the lipid's molecular shape.
Membrane lipids can be roughly divided into the three basic shapes shown in Figure 10.7 :
truncated cone (panel a); cylinder (panel b); and inverted cone (panel c) [23] . Cylindrical
lipids prefer the lamellar phase, while lipids with very truncated or inverted cone shapes
in pure form prefer being in other, non-lamellar structures. All biological membranes are
composed of hundreds or thousands of different lipids of all three basic shapes, yet
membranes are found almost entirely in the lamellar phase. This is particularly perplexing
since most of the world's membrane lipids are not cylindrical. For example, the most prev-
alent membrane lipid found on planet Earth is the photosynthesis-associated lipid monoga-
lactosyldiacylglycerol (MGDG, Figure 5.25), which prefers the H II phase. This suggests that
cylindrical lipids have a stronger influence on overall membrane structure than do the other
(a)
(b)
(c)
FIGURE 10.7 Basic membrane lipid shapes: (a) truncated cone; (b) cylinder and; (c) inverted cone. Circles
represent the polar head group of different sizes [23] .
 
Search WWH ::




Custom Search