Chapter 8 Membrane Structure

Chapter 8
Membrane Structure
Cell membranes act as selective barrier
The plasma membrane is involved in cell communication,
import and export of molecules,
and cell growth and motility
Receptor proteins
Transport proteins
Membranes form the many different compartments
in a eucaryotic cell
Two membranes
A cell membrane can be viewed in a number of ways
 50 atoms
Lipid bilayer
2-D
3-D
A typical membrane lipid molecule has a hydrophilic head
and hydrophobic tails
H2O
Electrostatic
attractions
Hydrogen
bonds
Charged atoms
or
Polar groups
Water-loving
Amphipathic
Uncharged atoms
and
Nonpolar groups
Hydrocarbon
Water-fearing
Phosphatidylcholine is the most common phospholipid
in cell membranes
Phosphatidylcholine
Originate as
fatty acids
Double
bonds
Hydrocarbon
Phosphatidyl
Different types of membrane lipids are all amphipathic
Hydrophilic head
A hydrophilic molecule attracts water molecules
A hydrophobic molecule tends to avoid water
Hydrophobic
Fat molecule are hydrophobic, whereas
phospholipids are amphipathic
Animal fats
Plant oils
Phospholipid
Amphipathic phospholipids form a bilayer in water
Heads
Tails
Heads
Phospholipids bilayers spontaneously close in
on themselves to form sealed compartemnts
Hydrophobic hydrocarbon
Self-sealing
> 25 nm
Pure phospholipids can form closed, spherical liposomes
Closed spherical vesicles
25 nm – 1 mm
A synthetic phospholipid bilayer can be formed across
a small hole (about 1 mm in diameter) in a partition
that separates two aqueous compartments
Phospholipids can move within the plane of the membrane
(2 m/sec)
Needs facilitate from proteins
(30,000 revolutions/min)
Hydrocarbon tails
short
unsaturated
Fluidity ↑
Cholesterol tends to stiffen cell membranes
Lipid raft
Phospholipids and glycolipids are distributed
asymmetrically in the plasma membrane lipid bilayer
(Noncytosolic half of the bilayer only)
Phosphatidylserine
Glycolipid
Sphingomyelin
Cholesterol
Phosphatidylcholine
Phosphatidylethanolamine
Phosphatidylinositol
Flippases play a role in synthesis the lipid bilayer
Membranes retain their orientation even after transfer
between cell compartments
budding
fusing
Membrane-based vesicle transport
Exocytosis
Endocytosis
Plasma membrane proteins have a variety of functions
Protein 50%
Membrane components
Lipid + Carbohydrate 50%
Na+-K+-ATPase
Membrane proteins can associate with the lipid bilayer
in several different ways
Integral membrane proteins
Single  helix
Peripheral membrane proteins
Multiple  helices
Rolled-up  sheet
( barrel)
Hygrophilic
Hygrophobic
Amphipathic
 helix
Covalent
Noncovalent
The peptide bonds that join adjacent amino acids together
in a polypeptide chain are polar and therefore hydrophilic
(Hygrophilic)
A segment of helix crosses a lipid bilayer
20 a.a.
Hygrophilic
A transmembrane hydrophilic pore can be formed
by multiple  helices
Hygrophilic
side chains
Hygrophobic
Porin proteins form water-filled channels in the
outer membrane of a bacterium
Hygrophilic
side chains
Hygrophobic
side chains
16-stranded  sheet
SDS and Triton X-100 are two commonly used detergents
Displace lipid molecules
Displace lipid molecules
Unfold the proteins
A single
Hygrophobic
tail
Hygrophilic
Ionic
Nonionic
Small, amphipathic, lipidlike molecules
Membrane proteins can be solubilized by a mild detergent
such as Triton X-100
protein-detergent
complexes
lipid-detergent
complexes
Bacteriorhodopsin acts as a protein pump
+ H+
Polar
amino acid
side chains
7 transmembrane
 helices
The photosynthetic reaction center of the bacterium
Rhodopseudomonas viridis captures energy from sunlight
electron carrier groups
light
chlorophy II
five transmembrane
 helices
one transmembrane
 helix
Human red blood cells have a distinctive flatted shape,
as seen in this scanning electron micrograph
A spectrin meshwork forms the cell cortex
in human red blood cells
The cell cortex
Formation of mouse-human hybrid cells shows that
plasma membrane proteins can move laterally
in the lipid bilayer
Fluid-mosaic model
Photobleaching techniques can be used to measure
the rate of lateral diffusion of a membrane protein
Fluorescence recovery after photobleaching
Proteins show different patterns of motion
Single-particle tracking (SPT)
Corralled within a small
membrane domain by other proteins
Free to diffuse randomly
Tethered to the cytoskeleton and
hence is essentially immobile
Mild detergents can be used to solubilize and reconstitute
functional membrane proteins
The lateral mobility of plasma membrane proteins
can be restricted in several ways
Cell cortex
Extracellular matrix molecules
Diffusion barriers
Surface proteins
A membrane protein is restricted to a particular domain
of the plasma membrane of an epithelial cell in the gut
(basement membrane)
Asymmetric distribution of membrane proteins
Eucaryotic cells are coated with sugars
The noncytosolic surface side
Glycolipids
Glycoproteins
Proteoglycans
(1) Protect the cell surface from mechanical & chemical damage
(2) Absorb water  slimy surface
(3) Cell-cell recognition & adhesion
Glycoprotein & Proteoglycan
Protein glycosylation in the ER and Golgi
The recognition of the cell-surface carbohydrate
on neutrophils is the first stage of their migration
out of the blood at sites of infection
Extravasation of leukocyte rolling to migrate
into the tissues