Biology Reference
In-Depth Information
to support long-range order (a few tenths of a micron or greater), although short-range order
should be feasible. However, some long-range order, like that observed in the synapse, is
known to exist. Also, a number of agents extrinsic to the membrane can result in clustering
of membrane proteins (see the section on lipid rafts below). Detailed mechanisms explaining
long-range order were not ready to be included in the Singer-Nicolson 1972 Science paper.
Possible discussions on structural coupling between distant lateral sites on the membrane
(cis coupling) and between sites on opposite sides of the membrane (trans coupling) were
also premature.
TABLE 8.1
Salient Features of the Fluid Mosaic Model.
I.
Membrane Lipids:
A.
Molecular structures are amphipathic (membrane lipids are polar).
B.
Most membrane lipids exist in a lipid bilayer matrix.
C.
Some lipids are bound to protein.
D.
Trans-membrane lipid distribution is partially asymmetric.
E.
Membrane lipids exist in a fluid state.
F.
Membrane lipids exhibit rapid lateral diffusion in a membrane.
G.
Membrane lipids exhibit slow trans-membrane diffusion (flip-flop).
H.
Lipids have no catalytic activity.
II.
Membrane Proteins:
A.
Membrane protein molecular structure is amphipathic.
B.
Membrane protein distribution is totally asymmetric.
C.
Membrane protein lateral diffusion rate varies from rapid to immobile.
D.
Absolutely no trans-membrane protein diffusion (flip-flop) occurs.
E.
Membrane proteins exhibit substantial diversity (biochemical activities).
Protein types
1.
Peripheral (essentially globular)
a.
bound to lipid (electrostatic)
b.
bound to protein (electrostatic)
2.
Amphitropic
3.
Integral: Endo, Ecto, and Trans-membrane
a. a
-Helix: Single Span, Multiple Span
b. b
-Barrel
c.
Lipid Linked
i.
Myristoylated, Palmitoylated, Prenylated
ii.
GPI-anchored
4.
Glycoproteins (general type of protein that has attached sugars)
III.
Membrane Carbohydrates:
A.
Totally water-soluble
e
no interaction with membrane interior.
B.
Glycolipids.
C.
Glycoproteins.
D.
Membrane distribution is totally asymmetrical.
E.
Always found on membrane outer surface.
F.
Lateral diffusion is determined by the attached lipid or protein.
G.
No trans-membrane diffusion (flip-flop) occurs.
H.
Carbohydrates have no catalytic activity.
