Download Biological Membranes

Biological Membranes
薛雅薇
中央大學物理系
Cell Membrane
Membrane Structure (I)
Two primary building blocks :
- Protein
- Lipid, or fat . Lipids form a bilayer.
The glycocalyx carbohydrate network on the outside surface:
- responsible for cell–cell recognition and adhesion to other cells.
Functions of cell membranes
1. Supporting and retaining the cytoplasm (inner cell)
2. Being a selective barrier:
Get nutrients in and waste products out.
non-polar molecules and some small polar molecules can cross.
Most polar compounds, e.g. amino acids, must be specifically
transported across the membrane by proteins (Transport).
3. Communication (signaling, via receptors)
Information Flow Through the Plasma Membrane
via a Membrane Receptor
4. Recognition
Structure and function are related.
Structure of Lipid Molecule
Lipid molecule contains both hydrophilic
and hydrophobic regions.
Polar headgroup : hydrophilic (i.e. water-loving)
O – CH2 – CH2 – N+(CH3)3
O¯
H
O
H
H
C
C
O
O=C
P=O
CH2
O
C=O
CH2
H2C
CH2
H2C
CH2
H2C
CH2
H2C
H2C
H2C
H2C
CH2
polar
non-polar
CH2
CH2
CH
H2C
CH2
CH
H2C
CH2
H2C
CH2
H2C
CH2
H2C
CH2
H2C
CH3
H2C
CH2
H2C
CH3
Tail (or acyl chain): hydrophobic (i.e. water-fearing)
60:40 DPPC/Chol bilayers
In liquid crystalline phase
Lipid motion:
- Lateral
- Rotational
- nodding
-…
Membrane Structure (II)
Consists of specialized domains of different compositions and
properties.
These specialized domains (called lipid rafts) are enriched in
cholesterol (膽固醇) and sphingolipids (high melting lipids)
Lipid rafts are postulated to be very important in signal transduction
in cells (as signaling platforms, for example).
Size of domain: 0 – 10m
It has found that many diseases, such as
Alzheimer's, Huntington's, Parkinson's
disease, AIDS..etc are related to lipid rafts.
Fluorescence image
(1:1 brainPC/brainSPM)+25mol%Chol
AFM images
(1:1 DOPC/SPM)+25mol%Chol
Crane & Tamm 2004
Two bilayer thicknesses
Kruijff et al. 2001
Probing Membrane’s structure & dynamics
Use model membranes
Nuclear magnetic resonance (NMR, 核磁共振)
Differential scanning calorimetry (DSC)
Fluorescence microscopy
Atomic force microscopy (AFM, 原子力顯微鏡)
Principles of NMR (Nuclear Magnetic Resonance)
For a bare spin in the presence of H0:
Hzeeman =  nH0• I
H0 : Static Magnetic Field
H1(t):
Alternating Field
with frequency 
For 2H nuclear spin, I = 1  = E,  resonance occurs
(absorption of energy)
m = 1

0
1
L
For a spin in the membranes
m = 1
0
1
Em =  nH0m

H = H zeeman + HQ
L
info: local environments, mol. dynamics
L

Solid-state NMR spectrometer (固態核磁共振儀)
Multi-component lipid mixtures, with one lipid labeled
O – CH
O¯
- Probing specific site by
choosing proper L
- Sample in any form
- Non-invasive
2
P=O
O
H
H
CH
H
C
O
O=C
CD2
D 2C
CD2
D 2C
CD2
D 2C
CD2
2
C
O
C=O
H 2C
CH
2
CH
2
CH
2
H 2C
H 2C
H 2C
CH
D 2C
CD2
CH
D 2C
CD2
H 2C
D 2C
CD2
2
CH
2
CH
2
CH
3
H 2C
D 2C
CD3
CH
H 2C
H 2C
– CH 2 – N + (CH 3 ) 3
Characteristic 2H NMR spectra for liquid phases
lo
Liquid-disordered
ld
(disordered chain)
lo
x10
-3
Liquid-ordered
(highly ordered chain)
Cholesterol-associated
Chemical composition, fraction of each domain, lipid-lipid &
lipid-protein interactions, dynamics, mechanical properties …
Probing phase transition
Pure POPE-d31
gel
o
17 C
Intensity
gel + ld
o
22 C
Lattice melting & chain melting
ld
o
25 C
-100
-50
0
kHz
50
100
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