Biology Reference
In-Depth Information
72.8 dynes/cm at 25
C. Phospholipids reduce this to ~7 to 15 dynes/cm. Proteins further
reduce the surface tension down to that of a biological membrane (~0.1 dynes/cm). The
conclusion was that proteins must play an important role in membrane structure. However,
by 1935 the only proteins that were understood to any extent were water-soluble globular
proteins. Nothing was known about membrane proteins.
By 1935, questions about overlap between the related fields of biochemistry, protein struc-
ture and function, and membranes were starting to emerge. Although it was clear that
different membranes supported different biochemical functions, it was not known whether
the structures of different membranes were completely different, or if they had important
features in common. In other words, are the various membranes more similar or different?
It was clear that membranes were 'semi-permeable', being permeable to some things while
impermeable to others. How was semi-permeability accomplished? Any proposed model
for membrane structure would have a lot of things to consider
[2]
.
The first realistic model of a biological membrane was that of James Danielli and Hugh
Davson, initially proposed in 1935
[3]
. Their Pauci-Molecular model was subsequently
tweaked and altered as membrane science progressed
[2]
. The history of membrane models
will be discussed around some major milestones, starting with the Danielli-Davson model
and progressing through the Robertson Unit Membrane model, to the Singer Fluid Mosaic
model, and finally to Simons' Lipid Raft model. Two misdirected models that do not employ
a lipid bilayer, and so clearly fall off this major track, the Benson and Green Lipoprotein
Subunit models, will also be discussed.
B. TH
E DANIELLI-DAVSON PAUCI-MOLECULAR M
ODEL
In 1935 Danielli and Davson proposed their model for a biological membrane
[3]
. Their
Pauci-Molecular model was the first to employ a lipid bilayer. While the seminal experiment
of Gorter and Grendel in 1925
[4]
was the first to demonstrate the possibility of a lipid bilayer
barrier (membrane) surrounding a cell, these authors did not actually propose a membrane
model. Instead, ten years later Danielli and Davson used the lipid bilayer as the foundation
for a new membrane model. Ironically the original 1935 Danielli-Davson paper did not even
mention the Gorter-Grendel work! They realized that a membrane was not just lipid, as sug-
gested by Gorter-Grendel, and somehow proteins had to be incorporated. Since at that time
only globular proteins were known, Danielli and Davson simply attached globular proteins
loosely to the lipid bilayer surface in the form of a protein
protein sandwich
(
Figure 8.1
a). Note their original model had no membrane-penetrating protein. Later,
Danielli, realizing the membrane had to be semi-permeable, added thin, peptide-lined
trans-membrane channels (
Figure 8.1
b). One important feature of this simple model is that
it was proposed to be the basic foundation of all biological membranes. In addition, at the
heart of their model was the lipid bilayer. These concepts have remained intact through all
subsequent revisions of the Pauci-Molecular model. The major problems with this first model
were the failure to incorporate proteins into the membrane bilayer interior and the total lack
of dynamics in the model. The Pauci-Molecular model is static. The model predicts that all
membranes would be identical and fails to indicate how variety would be achieved, nor
does the model take into account membrane asymmetry. Also, the globular protein coat
lipid
e
e
