Biology Reference
In-Depth Information
FIGURE 8.6
The Singer-Nicolson Fluid Mosaic model for membrane structure. Reprinted with permission
[29]
.
were attached to proteins (glycoproteins) and lipids (glycolipids), but were found only on the
outer membrane surface. Integral proteins were described as being 'globular and amphi-
pathic' and 'having a significant fraction of its volume embedded in themembrane'. This early
description of protein structure was later modified to include trans-membrane
a
-helical and
b
-barrel spans (discussed in Chapter 6). Membrane components were distributed in a 'mosaic
structure' where both proteins and lipids were dispersed inhomogeneously resulting in lateral
and trans-membrane patches called domains
[12]
.
A second important element Singer and Nicolson added to their model can be loosely
described as 'membrane dynamics'. The Danielli-Davson-Robertson model was essentially
a frozen picture. The Fluid Mosaic model recognized for the first time that a biological
membrane is dynamic, fluid, and ever changing. Although identifying the vast diversity of
membrane lipids, proteins, and carbohydrates is by itself a difficult task, it is membrane
dynamics that presents the most challenges. Dynamics involves how the various membrane
components interact with one another. Avariety of techniques are required to cover the vast
size and time domains encountered in membrane studies. Various aspects of membrane
dynamics are discussed in Chapter 9.
The original Fluid Mosaic model predicted totally free lateral diffusion of both lipids and
proteins. This concept was later modified by proposing limited diffusion for some proteins
due to their interactions with the essentially immobile cytoplasmic proteinaceous mesh
known as the cytoskeleton
[13]
. The Fluid Mosaic model was the first to recognize both
membrane fluidity and lateral mobility. The existence of an extensive lipid bilayer matrix
and free lateral diffusion in the bilayer presented a dilemma. A bilayer should not be able
