Biology Reference
In-Depth Information
25
˚
wide separated by a light zone ~25
˚
wide'. According to the Unit Membrane model, all
membranes have the same basic structure. They are composed of a lipid bilayer with protein
monolayers attached covalently to both sides, very similar to the Danielli-Davson model.
Whereas the Danielli-Davson model was non-committal concerning the thickness of
a membrane, Robertson's model was not.
As with the Danielli-Davson model, Robertson's Unit Membrane model had its advan-
tages and disadvantages. Robertson's EM (electron microscope) pictures allowed for a precise
membrane measurement. Indeed, Robertson's EM images were the first direct observations
of membrane structure. Robertson found the same tri-laminar structure ('railroad tracks') in
the plasma membranes from a wide variety of cell types as well as from internal cellular
membranes including those of the mitochondria, endoplasmic reticulum, and the nuclear
envelope. Unfortunately the same flaws that eventually doomed the Danielli-Davson model
also doomed the Robertson model. The Unit Membrane model is a static model and fails to
appreciate either the dynamic aspects of membranes or the asymmetric nature of membrane
structure. Membrane dynamics would have to await further advances in technology to
address these problems. The eventual replacement of Robertson's use of KMnO
4
by gluter-
aldehyde and OsO
4
indicated that the intra-cellular membranes are not directly connected
to one another, as Robertson predicted, but instead are connected by tiny vesicles. The newer
methodology also showed that the plasma membrane has ties to the internal cytoskeleton.
The Unit Membrane model was so similar to the Pauci-Molecular model that the two are
often melded into one Danielli-Davson-Robertson membrane model.
D. BEN
SON AND GREEN'S LIPOPROTEIN SUBUNIT M
ODELS
Throughout the 1960s membrane science was still in its infancy and the most important
missing aspects involved membrane asymmetry and dynamics. It should not be surprising
that even the venerable lipid bilayer was in question by some. Two well-disseminated
membrane models replaced the lipid bilayer with lipoprotein subunits (depicted in
Figure 8.3
). The Lipoprotein Subunit models were proposed by Andrew Benson (
Figure 8.4
,
for photosynthetic thylakoids,
[8]
) and David Green (for mammalian mitochondria,
[9]
).
These models, depicted in
Figure 8.3
, received considerable attention due to the stature of
their inventors. Andrew Benson was a world-renowned plant physiologist from his early
FIGURE 8.3
The Benson-Green Lipoprotein Subunit model membrane. Proteins are depicted as large, open
sack-like structures while phospholipids are the small, black spheres with attached acyl chains. Note the complete
absence of a lipid bilayer.
