Plasma Membrane: Phospholipid Bilayer

Key points

Overview (2 lectures)

Lecture Outline

Introduction to PM with emphasis on function.

surface membranes of all cells and organellar compartments have the same basic architecture: phospholipid bilayer (with embedded protein), this membrane has remarkable properties unlike any sheet of material in the everyday world

it also allows growth and shape changes w/out loss of continuity
it deforms w/out tearing (if pierced it doesnt collapse or remain torn, it reseals)

composed of lipid and protein approx. 50:50 by weight but this varies w/ function:
myelin 75:25 or Mito 25:75
its structural assembly is essentially non covalent
its properties are essentially that of a fluid,
it is asymetrical, i.e. each surface is characteristically distinct
evidence supporting the universality of the lipid bilayer structure:

EM after osmium tetroxide staining of phospo lipid head groups
railroad track staining of all cells
xray difraction gives characteristic pattern of electron density vs distance
(these expts are in Lodish)

Phospholipid bilayer

Composition of bilayer: the mechanical properties of PM emerge from the phospholipid bilayer
So what are the components? The lipid components are:

the selective advantage of a phospholipid bilayer for cell enclosure will become clear
the phospholids (& sphingolipids) are amphipathic!!!
thus a bilyar structure is energetically favored
this produces:

Remarkably the containing membrane of cells is essentially a 2D fluid: what is meant by "fluid"?

studied in liposomes and planar (black lipid) bilayer
it is clear from these studies that lateral lipid diffusion is very fast at 37°C
exchange rate w/ neighboring molecules (lateral diffusion) very fast
1-2um/sec=> entire length of bacteria every second
rotation rate very high
flip flop rare-> about once every month for each lipid
movement somewhat more restricted in cells, up to 10X
this is probably due to protein/lipid interactions
proteins can also be very mobile in the bilayer or they can be restricted
the restriction is not due to the phospholipid bilayer but to interactions:
inside the cell, eg cytoskeleton
outside the cell, eg ECM or cell/cell contacts
barriers in the membrane, eg tight junctions

Membrane fluidity actually depends on lipid composition and temperature
therefore cells alter composition in response to temp changes to maintain fluidity

lipids actually have a very sharp phase transition from gel to fluid
in general the more order in the bilayer the less fluidity (and the higher its melting temp)
unsaturated acyl chains are kinked-> reduces order, increases fluidity
shorter acyl chains don’t pack as well-> reduces order, increases fluidity
cholesterol (only in animal cells) is more complicated
at 37°C it reduces fluidity by space filling at polar head groups (ie increasing order)
this also causes at marked decrease in permeability
at lower temps it protects against gelation by acyl chain dispersal
thus it significantly broadens the phase transition of the bilayer

experimental example: inactivate desaturating enzyme compare growth rates at diff. temps
actual examples: hibernation, fish in winter ponds, bacteria in thermal vents
Microdomains within PM may have different fluidity characteristics
lipid rafts rich in cholesterol and sphingolipids: "frozen domain in fluid sea"

Another important feature of the bilayer is its asymmetry
topology of cells is such that one can always consider one of two faces

note that release of the PI head group by lipase action is important in cytoplasmic signaling
indeed, lipid asymetry can be documented by exogenous lipase addition (see problem set)
example of function of asymetry

PS important in stop transfer function
PA appearance on outer leaflet signals destruction of lymphocytes
mechanism is flip-flop
spontaneous rate very slow
catalyzed by flippases: highly unfavorable in energetic terms