Chap 10

act as selective barriers

- is involved in cell communication, import/export
of molecules and cell growth/motility

•PM 2% cell membrane
•ER 50%

- embedded protiens
~ 5 nm thick
10,000 PM stacked together to make thickness of paper

•lipid-protein assemblies held together in thin sheet by
noncovalent bonds
•Lipid bilayer is structural framework & barrier
•Proteins carry out most of specific functions
• Often include carbohydrates attached to membrane
lipids & proteins

•Lipid:protein ratio varies greatly depending on
membrane type (e.g., PM vs. ER)
•relates to membrane function

- all amphipathic

•less abundant class of membrane lipids
•derivatives of sphingosine- an amino
alcohol with long hydrocarbon chain
•sphingosine + fatty acid + X
•sphingomyelin -- if X is
phosphorylcholine; a phospholipid
•glycolipid -- if X is carbohydrate

•may constitute 50% of animal cell plasma
membranes
•absent from most plants // bacteria
•smaller and less amphipathic than
phospholipids
•interferes with phospholipid packing; makes
membrane more fluid at lower temps and less
fluid at higher temps

**image

- form a bilayer
in H2O
•10,000 PM stacked
together to make
thickness of 1 sheet
of paper

•each type of cellular membrane has characteristic
lipid composition
•lipid composition:
--determines physical state of membrane
--influences activities of some membrane proteins
--may provide precursors or chemical messengers

- Phospholipids and glycolipids

- added to the
ER cytosolic
membrane
•Scramblases in ER
randomly transfer
some phospholipids to
inner leaflet

- establish and maintain
asymmetric distribution of phospholipids in most
cell membranes
•Flippases in Golgi and
PM move specific
phospholipids (e.g., PS)
to cytosolic leaflet
•Floppases move
specific phospholipids
(e.g., PC) to outer
leaflet

- Some functions of membrane phospholipids in
cell signaling

- is covered with carbohydrates
•Carbohydrate content covalently linked to both lipids & proteins
•2 - 10% PM by weight (<10% in glycolipid & >90% in glycoprotein)

- a fluid-mosaic

- Some functions of PM proteins

•amphipathic (like the lipids near them)
•AA in transmembrane domain forms hydrophobic
interactions with FA tails
•required detergents to solubilize (separate from
membrane)

- Not all transmembrane segments are
entirely hydrophobic (e.g., ion channels)
ex. bacteriorhodopsin

- Methods used to understand
integral membrane proteins
- Red Blood Cell PM disruption with detergent

•amphipathic (polar & nonpolar
ends)
•can substitute for phospholipids
and make hydrophobic proteins
soluble in water

- Solubilization with
detergents
•detergents substitute for
phospholipids in stabilizing
integral membrane proteins
while making soluble in
aqueous solutions
•now amenable to analysis

- usually hydrophobic α-helices

~ 20 AA needed
to span the
membrane

•weak electrostatic bonds with lipids or
integral membrane proteins
•can be solubilized with aqueous salt solutions
•typically dynamic binding to membrane
•may be enzymes, specialized coats, transmit
transmembrane signals or provide mechanical
support

1. GPI (glycophosphatidylinositol)
• external PM face
• covalently linked to glycolipid
• various receptors, enzymes, celladhesion proteins
2. Fatty acid anchor
• internal PM face
• covalently linked to FA
• kinases, etc

- PM fluidity demonstration

•Fluidity is an essential aspect of membrane
function
•Changes in membrane fluidity can alter the
ability of a membrane to function properly
•required for cell movement, growth, division,
intercellular junction formation, secretion,
endocytosis

- Fluidity depends on chain
length and degree of
saturation
- The soft margarine - butter
analogy

•cholesterol disrupts close packing of fatty acids & interferes with
their mobility
•abolish sharp transition temperatures & creates condition of
intermediate fluidity
•it tends to increase durability & decrease permeability of
membrane

internal temp. of most organisms (not
birds/mammals) changes with outside temp.
• organisms respond by altering membrane lipid
composition
At lower temps:
1. Desaturase enzymes single --> double bonds
2. Phospholipases and acyltransferases
reshuffle fatty acid chains to create lipids
with two unsaturated chains

- membrane sidedness
•Phospholipid content
varies between the inner
and outer leaflets of the
membrane
•e.g., phosphatidylserine
(PS) inner leaflet
•glycolipids outer leaflet

- on same membrane side

•Flippases/scramblases
can help lipids switch
leaflets

- diffusion of
membrane proteins after cell fusion

fluorescence recovery after photobleaching

rate of recovery -- direct measure of diffusion rate
• extent of recovery -- direct measure of % of labeled
molecules free to move
• Results --
1. proteins move more slowly in PM than in artificial
lipids
2. large fraction (30-70%) not free to diffuse

- single particle tracking

- have distinct
functions & must have order maintained
•differentiated
functions of the
PM of an
epithelial cell

- human red blood cells
- Red Blood Cell PM cytoskeleton and osmosis

- an example of PM structure
- PM "ghosts"

- forms cell cortex

•cell cortex (proteins on
the inside of the PM)
strengthens PM
•spectrin mutations --
individuals anemic; cells
break in capillaries
What would you see if you FRAP-ed spectrin?